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2016-06-12_06-35-12

Reigate May 2016 summary statistics

  • Tmax 27.4C
  • Tmin 0.3C
  • Tav 13.8C (UK 11.3C)
  • total rainfall 42mm (town) 45mm (Hartswood)
  • max gust 36mph
  • average wind direction NNE
  • sunshine 181.7 hours (May 2015 161 hours)

Whilst there were fortunately no severe weather events in Reigate and few across the UK in May, the weather we experienced more widely could be linked tenuously to climate change.  Of course, caution is required with such speculative statements but attribution studies on the May floods in Paris, not so far away, have concluded that they were made 90% more likely due to climate change. The same stalled low pressure system delivered our easterly winds so we were influenced, albeit on the edges, by the same blocked weather pattern.

People attempting to climb Snowdon in North Wales in May were lucky to experience sunnier-than-usual conditions for much of the month (south wales had more thunderstorms which reduced the sunshine totals there).  Meanwhile, in Surrey, we experienced occasionally warm conditions with an unusual mean monthly wind direction from the NNE.

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Reigate pressure rose hesitantly mid-month

Pressure fell across the UK to start May but then rose mid-month, especially to the North, bringing a relatively unusual easterly flow into Reigate and the south.  Whilst there were few severe weather events during the month, this post briefly explores some of the wider factors that may have contributed to this Easterly flow and the possibility of it being linked to climate change.

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Whilst mostly dry for the UK as a whole, occasional showers, some thundery, brought Reigate rainfall totals to just above average at around 42-45mm as recorded from our two weather stations respectively in and out of town.  SE England as a whole recorded rainfall at 111% of normal rainfall, mostly falling in thundery showers, more common in SE wind regimes.

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With relatively dry Easterly winds, sunshine totals for the UK were accordingly above average given the relatively high pressure overall.  Reigate experienced 182 hours of sunshine in total for the month.

Unusually, the sunniest places in the UK were in the North and West given the easterly winds bringing occasionally cloudier conditions off the North Sea to the south and east.  May 24 shows a typical scenario with the higher pressure to the North dragging in E/NE winds across the southern part of the UK with cloud across eastern areas and clearer conditions to the west.

Some great sunny days were recorded in the mountains of Wales, Cumbria and Scotland!

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Sunny Snowdonia with kelvin-helmholtz type wave clouds trying to form over Ogwen Valley

The higher than average rainfall patches shown below in the south were associated with showers on occasionally unstable warm and humid SE winds.  Reigate reached a Tmax of over 27C in this warm flow.

The wider pressure pattern across the northern hemisphere was characterized by anomalously high heights over the Arctic and LOWER than normal pressure in mid-latitudes including Europe. This situation is called “northern blocking” and in winter could cause cold conditions in mid-latitudes.  In Spring, as the continent rapidly warms up in stronger sunshine, easterly winds can be warm or even hot for the UK.

northern blocking over Pole

northern blocking over Pole

Northern Hemisphere pressure patterns are measured by the Arctic Oscillation which, as can be seen below, remained unusually negative through much of April and May showing high pressure persisting over the Arctic relative to low pressure in the mid-latitudes.  This pressure pattern turned winds from the usual westerlies into easterlies in the UK and Europe.

negative arctic oscillation Spring 2016

negative arctic oscillation Spring 2016

The causes of this reversal of the usual mid-latitude zonal westerly wind set-up have been linked to low sea ice extent in the Arctic, especially the Kara and Arctic Gateway seas. Warmer influxes of air into the Arctic builds air pressure which then links to higher chances of Easterly winds in mid-latitudes.

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low Arctic sea ice cover March 2016

The very low sea ice extent this year was brought about by much warmer-than-usual conditions during the Polar winter, where monthly average temperatures in the Arctic (>60N) were at times 3.5C or more above average during the cold season of 2015-16. This Arctic amplification is widely accepted as being caused by human induced climate change.

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It turns out that Spring Arctic sea ice extent is some of the lowest recorded in the 38 year satellite series.

 

So, unusual sunshine in North Wales, a warm NNE mean wind direction in Reigate and cloudy conditions on the east coast can be linked to the above tele-connecting weather patterns which, in turn, can be linked to climate change in the far flung Arctic.

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Meanwhile, the strong 2015-16 El Niño declined rapidly through May and ENSO conditions were neutral by early June. Models suggest the chance of La Niña (cool Pacific) conditions by Autumn 2016 are as high as 60%.  Some forecasters bring La Nina through the summer.  La Nina, and the warmer SSTs of the tropical Atlantic, are associated with more frequent hurricanes in the Atlantic basin.  In turn, high hurricane accumulated energy transfered to the North Pole during such seasons can build Polar heights in Northern Hemisphere winters, warming the Arctic and further melting sea ice.  Whilst this is just outrageous long term amateur speculation, it is nevertheless interesting to ponder the potential for feedbacks to accelerate further climate change in the near future.

The turning down of the vast heat engine of the El Nino might be linked to the slightly lower May global average temperature, though confirmation from expert sources has not verified this as yet.

 

Local data for May and all months stretching back to 2012 can be found on our data page here

http://www.metoffice.gov.uk/climate/uk/summaries/2016/may

 

2016-04-27_18-39-27

Today we put up a new weather station at our sports ground at Hartswood outside Reigate. This location will complement our established town weather station located at Reigate Grammar School. Hartswood is an out of town location with more exposure from all wind directions. It is already recording different conditions to the town (see links below).

The new weather station is a robust self-contained Davis Vantage Vue automatic weather station (AWS).  This model was chosen for its ability to cope with exposed sites and it has a reputation for being relatively maintenance free for longer periods.  It is commonly put on masts on rooves, as we have done here.

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This AWS is unusual because it uses the new Vantage Connect system.  The Connect system uses the mobile phone network to transmit data at 15 minute intervals to the Weatherlink website where it is pushed onwards to other websites, such as Weather Underground.  There is also a handy local live read out of weather on a console in the Tea Hut window.

The Vantage Vue weather station is simple to set up being a single housed unit. Attaching the anemometer and wind vane involves tightening screws with a tiny allen key. The Connect System is also easy to set up. Insert and connect batteries, start both systems up and they will endeavour to discover each other with little intervention.

The console unit also discovers the AWS and starts displaying data almost immediately with little user input.

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Once the systems are working and data is uploading reliably to the internet then assembly and fitting onto the roof is the next step.  An aerial expert was employed for this bit.

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Orientation to the South is important for both the Vantage Vue and Connect systems. Not only do they both use solar panels to maintain battery power (used at night of course) the Vantage Vue also requires a southerly orientation to ensure that wind direction readings are accurately recorded by the wind vane.  This is all explained in the manuals.

 

Roof top sites for AWS are popular but they have pros and cons.  Whilst wind readings benefit greatly from a clear wind run at height (so long as the mast exceeds a metre or so above the roof line to avoid eddies and turbulence), the accuracy of rain recordings can sometimes suffer with greater wind speed rendering totals somewhat less reliable than traditional ground based rain gauges (although ground based AWS often do not entirely satisfy strict meteorological conditions for rain gauge placement either).  Roof locations benefit from better security and connectivity.  Overall, with single-unit compact weather stations a roof top location is a good compromise and the most effective use of this technology.  Our Vantage Pro 2 AWS in town allowed us to divide the rain gauge and temperature sensor units on the ground from the anemometer on the roof, a better solution.

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“Live” weather data from Hartswood can now be viewed on the internet in these locations:

www.wunderground.com

weatherlink.com

Weatherlink summary

It is hoped the data will prove to be useful for checking the weather conditions before matches for staff, students, players and spectators preparing for their match or visit.

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Eventually a ground frost sensor can be added to issue alarms when ground temperatures fall to near zero. This will save some guess-work and early visits to check if pitches are frozen or not. Data will also be useful for students doing weather studies in urban micro-climate and the data can also be used by computing and maths students amongst many other applications.

RGSweather will also be able to compare data between town and edge of town locations.

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El Nino: massive Pacific heat engine

El Niño has no obvious or strong effect on UK winter weather.  Historically, El Niño years have coincided with both mild/wet and cold/dry winters in the UK.  By itself, El Niño does not directly drive our winters in any single, simple direction. For example, El Niño winter 2009/10 was the coldest winter for 30 years with a notable “Big Freeze”, while the El Niño winter 2006/2007 was the second warmest winter on record.  The strongest recent “Mega” El Niño in 1997-98 turned out to be a stormy and mild December in the UK, with just two minor snow events and then a notably mild January and February, with Tmax even reaching 17C on occasions.  However, despite weak and ambiguous El Niño signals for UK winters, when other weather drivers and teleconnections are combined with El Niño there is some research to suggest that stronger impacts such as stormy early winters and cold dry late winters are possible. The current El Niño could turn out to be one of the most powerful in 50 years (though recent measurements still show it 3rd in the league table of Mega-El Niños).  Of course, wintry weather can also occur completely independently of any El Niño event, such as the snow of January 2013.

“In Britain, the impact of El Niño is nowhere near as marked as in other parts of the world. But it does tip the balance a little bit more in favour of wet and windy weather. It makes it more probable,”

Jeff Knight, a climate modeller at the Met Office’s Hadley Centre, Exeter.

Read on for more details on how this fascinating and topical weather story might or might not impact our winter weather!

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Some drivers, teleconnections and indicators of UK winter weather

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Winter 2015-16 most powerful El Nino in 50 years?

El Niño: What is it?

El Niño is a natural change in the atmospheric pressure and wind patterns and flow of ocean currents in the Equatorial Pacific.  In normal “non-El Niño” conditions, a 200 metre deep pool of the world’s warmest sea surface water builds up in the West Pacific.

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Average Pacific Ocean sea surface temperatures non-El Nino conditions

This is known as the West Pacific Warm Pool and it is formed by intense insolation, a piling up of warm water driven by the easterly trade winds and low evaporation in light winds found round Indonesia.  Moist warm air over the West Pacific Warm Pool creates an area of instability and convergence in low pressure systems where air rises forming deep tropical clouds, heavy rain and thunderstorms and sometimes typhoons.  A contrasting cold pool exists in the East Pacific where upwelling of deep ocean water reaches the surface off the coast of Peru courtesy of the cold Humboldt current bringing Antarctic water up from the ocean depths . Cool dry air subsiding over the cold pool in the East Pacific condenses moist air and forms low cloud and fog that acts as a feedback loop by reducing insolation and creating cooler conditions. Normally, brisk Easterly Trade Winds drive this cool tongue of ocean water west and, on it’s journey along the Equator, the sea surface warms up.  This normal Pacific pattern is known as the “Walker Circulation”.

Normal Pacific Walker Circulation

Normal Pacific Walker Circulation (RGSweather diagram)

Strong El Niño episodes result in a reversal of the normal pattern of Pacific Ocean wind and ocean currents and dramatically changes the sea surface temperatures across the Pacific.

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Pacific Ocean sea surface temperatures during warm El Nino phase

The reversal of winds and currents causes the West Pacific Warm Pool to move to the Central and East Pacific where there is normally cold ocean water, hence El Niño are known as “warm phases”.  Strong El Niño phases produce a tongue of above average sea surface temperatures extending 13,000 km long and 1000 km wide across the Equatorial Pacific and this has a major impact on weather patterns across parts of the world, especially during the Northern Hemisphere winter when El Niño usually reaches a peak of intensity.

El Nino phase Pacific Ocean

El Nino phase Pacific Ocean (RGSweather diagram)

During El Niño episodes the Pacific trade winds weaken, the subtropical jetstream can reverse and strengthen and wind driven upwelling slackens.  As a result the Equatorial ocean current reverses as warm water starts moving to the east. Whilst it is not known what causes an El Niño, a key change is in the pressure pattern across the Pacific basin.

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Tahiti sea level pressure correlates negatively with Darwin i.e. when one is high, the other is low. ENSO warm phase yields HIGH pressure over Darwin.

ENSO: El Niño Southern Oscillation or Pacific Pressure See-Saw

In El Niño phases the normally LOW pressure measured over Darwin, Australia changes to higher pressure and the reverse goes for pressure over the east Pacific, measured in Tahiti, where pressure falls.  The changing fortunes of these pressure cells is known as the Southern Oscillation.  The reversal of pressure gradient weakens or reverses the trade winds and allows the West Pacific warm pool to “slosh” east across the Pacific towards South America.  The resulting thermal expansion and the reversal of ocean currents, actually raises the sea level in the East Pacific.  This all takes months and the coupling of the ocean currents and atmospheric winds is critical in creating a complete El Niño Southern Oscillation (ENSO). This video explains the phenomenon well:

So during an El Niño event, the easterly trade winds converging across the equatorial Pacific weaken.  This in turn slows the ocean current that draws surface water away from the western coast of South America and reduces the upwelling of cold, nutrient–rich water from the deeper ocean, flattening out the thermocline (boundary between deep cold water and surface warm water) and allowing warm surface water to build in the eastern part of the Pacific.  Once the ocean currents and atmospheric winds “couple-up” then a positive feedback loop is established which causes further sea surface warming in the East Pacific.  Here the air is warmed above, becomes more buoyant and rises, lowering pressure so further drawing in more westerly winds.  These changes transport enormous amounts of heat and energy to the East Pacific which alters the subtropical jetstream which transfers changes in the atmosphere further “downstream” to other parts of the world.

El Nino

El Nino sea surface temperature anomalies October 2015

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El Nino sea surface temperature anomalies November 5th 2015 (courtesy xmetman wordpress)

Significant global Impacts

Due to the release of immense amounts of heat from the Pacific Ocean, El Niño years often become record-breakers for global average temperature.  The energy and moisture released flows “downstream” into the global circulation and has significant impacts on weather elsewhere.  El Niño reaches a peak around Christmas, hence the name “Christ Child” bestowed on the phenomenon by Peruvian fishermen who suffer from the collapse of their fisheries during warm episodes as the upwelling of nutrient rich bottom waters are capped by the invasion of the nutrient poor warm pool.  This causes a temporary collapse in sea life in the East Pacific.  El Niño occur periodically but irregularly over a cycle of 3 to 7 years, they differ in strength and are sometimes followed by a corresponding reversal to a strengthened “normal” flow called La Nina. The last mega-El Niño was 1997-1998 and our 2015-2016 El Niño looks like matching that strength or possibly exceeding it (update November: not so likely now* see Xmetman blog post in refs at foot of page)

The effects of El Niño around the Pacific and neighbouring continents are the most obvious and well correlated with the event, for example wetter and stormier conditions in South America, drier drought conditions with more wildfires in Indonesia and Australia and NE Brazil and a weaker SE Asian monsoon and wet winters in SE USA.  El Niño years also correlate with 44% fewer Atlantic hurricanes due to the enhanced subtropical jetstream shearing the heads off developing thunderstorms and enhanced Pacific hurricanes due to warmer SSTs e.g. Patricia October 2015.  Some of these effects have already occurred in the 2015 El Niño with hurricane activity correlating well with expected changes and an Indian heatwave with reduced monsoon.

The chart below shows a composite of analogue surface temperature anomalies for October in El Niño years (source JMA) compared to the actual conditions measured for mid-October 2015.  The patterns match surprisingly well, especially for the more significant and more strongly correlated locations.  This hints at how patterns for this El Niño might expect to map out as expected. Note the lack of any significant impact in NW Europe.

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Around the Pacific, very roughly, places that are normally wet and stormy become drier and more settled but can also suffer drought and fires e.g. Indonesia and Australia, while those places which are normally dry become stormy and wet and suffer from flash floods and landslides e.g. Peru and California. The most extreme weather impacts occur during the cold winter season of each El Niño in the Pacific but the knock-on effects can last into the following summer and link with places over great distances. The charts below show some of the recognised El Niño impacts. Note the complete absence of any reliable or linear teleconnections in Europe recognised by NOAA.

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Impacts of El Nino

Weak UK and European Impacts…

Impacts on the weather further away from the Pacific mostly consist of weaker signals that are often reversible due to other stronger weather drivers.  The impact of El Niños on European weather, especially the UK, fits into this category because there are no strong, reliable impacts based solely on El Niño episodes on UK weather.

“There is really no effect in the U.K. that we can say is definitely caused by El Niño” AccuWeather Meteorologist Tyler Ros states.

Other drivers of weather become more significant because the UK is located further downstream and along way from action in the Pacific.  Research by Judah Cohen, Atmospheric and Environmental Research (AER), suggests El Niño warm years overall bring warmer winters to the Northern Hemisphere. However, other research has picked up on some weak “teleconnections” between El Niño events and colder European winters. Some of the connections are illustrated below:

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El Nino connections to European winter weather (diagram RGSweather)

…But some possible El Niño signals for UK and Europe?

In Europe, research shows that any El Niño signals are “strongest” in middle and late winter and they approximate to a negative North Atlantic Oscillation.  A negative NAO corresponds with higher pressure over Iceland and a weaker meridional (wiggly) jetstream.  This situation can lead to cold outbreaks for the UK as a sinuous jet can provide chances for Arctic air to leak out of the Poles. In addition, El Niño is associated with low temperatures and decreased precipitation over NE Europe, connected with higher than normal pressure here. This provides the UK with the risk of cold North Easterly winds coming from Russia… so called “Beast from the East”. Some modest El Niño signals emerging from research for European winter weather are listed below but it is important to point out that these are weak signals and other research finds no reliable El Niño winter signals at all!

  • Atlantic storm tracks shifted south taking storms over Mediterranean
  • More cyclonic weather patterns over Central Europe
  • Pressure over Scandinavia HIGH; or western Russia anticyclone expanded over Europe: this would increase the chances of cold Easterly winds
  • High sea level pressure over Iceland across to Scandinavia and NE Europe
  • NE Baltic cold impact: but not in very strong El Nino events when warm impact may occur (UK MetOffice)
  • LOW sea level pressure across central Europe and Western Europe: higher precipitation
  • Some El Ninos have cold winters in NE Europe and enhanced precipitation in Central Western Europe
  • Positive NAO in Nov-Dec : this would mean a stronger jetstream with milder conditions for much of Europe, especially NW
  • Negative NAO late winter into Spring: this would mean a weaker more meridional jetstream with the possibility of blocked patterns and potential Arctic outbreaks or easterlies (other things coming into play)
  • High rainfall in the Mediterranean and decreased precipitation over NW Europe and Scandinavia
  • Frequency of upper troughs over central Europe was very high
  • Temperatures and precipitation over Turkey are high.
  • Israel high rainfall

There are mixed messages regarding the El Niño signal for European weather. Overall, the signal is most consistent in late winter and resembles the negative phase of the North Atlantic Oscillation which itself links to higher chances of cold winter episodes with northern blocking.  The prolonged 1940–1942 El Niño was accompanied in northeastern Europe by three of the coldest winters of the 20th century. In early winter the signal is almost the opposite with a positive NAO, stronger jetstream which brings milder stormier conditions to Europe.  Variability between El Niño impacts on Europe is also large and range much larger than the impacts themselves. Some research shows such variability might be due to volcanic eruptions in the tropics prior to El Niño events.  There is some evidence that pronounced El Niño impacts on European weather follows major volcanic eruptions e.g. El Chichon 1982, Pinatubo 1991.

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Calbuco Volcano erupted April 2015 but is this eruption big enough to trigger stronger El Nino teleconnections this winter?

Links between El Niño  and other atmospheric drivers

In the 20th Century all three of the strong El Niño events followed major volcanic eruptions. Even so, the signals were not consistent between these events.  Some studies also show a connection of El Niño events to stratospheric conditions. Warming of the stratosphere (sudden stratospheric warming) and subsequent weakening of the Polar Vortex have been linked to increased chances of cold winter weather in Europe (due to a weakening of upper westerly zonal winds propagating down into the Troposphere, allowing cold easterlies to break out into Europe).  Some research finds an increased frequency of such stratospheric events, especially in late winter, during El Niño years. Additionally, volcanic eruptions might also play a role in warming the lower stratosphere and encourage SSW (sudden stratospheric warming) events but further research is needed to establish any firm connection.

In addition to volcanic activity and stratospheric behaviour, other drivers and atmospheric behaviours can have significant influences on UK winters and these might enhance or reduce any El Niño signal or overwhelm it completely. Examples of some drivers / indicators and teleconnections that seasonal forecasters use include:

  • Solar activity: low sunspot numbers connect to northern blocking.  Currently low.
  • Atlantic hurricane activity: more hurricane activity injects heat to the Poles that increases the chance of northern blocking and cold winters. 2015 season very low hurricane activity.
  • October Siberian snow cover: high and rapid expansion of Eurasian snow cover in October links to increased chance of sudden stratospheric warming later in the winter which can cause cold late winters. Current Siberian snow cover is more than more recent recorded years.
  • October weather patterns: recent research shows that an anticyclonic October in the UK (dry) can link to cold winters with LOW pressure in Europe and northern blocking at high latitudes. This enhances a negative NAO.
  • Quasi-Biennial-Oscillation: westerly upper tropical wind pattern surpresses chances of cold outbreaks in mid latitudes. Currently westerly QBO.
  • Atlantic sea surface temperatures: tripole of warm/cold/warm pattern hints at potential for -ve north atlantic oscillation in winter.  Currently no tripole but cold pool anomaly in central North Atlantic could cool NW flow a little more than usual.

How the different teleconnections work together is complicated and many are at the cutting edge of climate long range forecasting and research.

The search for ENSO / winter correlation

Reanalysis of groups of strong El Niño years can be correlated with years exhibiting current atmospheric patterns from the list above and these show interesting results for UK winters illustrated below.  The following recent twitter chat is an example of such reanalysis widely undertaken by weather experts and enthusiasts:

//platform.twitter.com/widgets.jsThese can then be rolled forward to see how things pan out through the winter. Here is an interesting example from consultant meteorologist Anthony Masiello.  He has reanalysed El Niño winters with years with November positive Arctic Oscillations (as now).

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December enso: westerly influence, positive NAO

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January enso: with Siberian snow cover: Atlantic blocking with -Ve AO and potential for Arctic incursions

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February enso: Siberian snow cover years yield a beast from the east

The results above seem to match the idea of warmer unsettled Atlantic driven conditions before Christmas and colder blocked patterns after Christmas i.e. January and February have a decidedly blocked patterns to the north with low pressure to the south… as predicted in El Niño years as a possibility. HOWEVER.. if you check the number of years represented there are only 10 for El Niño years with +NAO and only 5 with widespread October Siberian snow cover. This is therefore not a significant finding, as Anthony himself points out on twitter.

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Too much inter-event variability for headline news

Globally, there is great inter-event variability between different El Niño years. The charts above, from JMA, show composite impacts of El Niño events and their significance over several decades. These appear to show some warmer and wetter than average winter conditions in Europe which agrees with Cohen et al but is in contrast to other findings.  To complicate matters further there are also different types of El Niño such as Central Pacific (Modoki) events that are correlated with different impacts on global weather e.g. colder winters in USA.  The latest 2015-16 El Niño appears to be turning out as a “standard” East Pacific mega-El Niño event with a long continuous tongue of warm SST anomalies stretching across the east Pacific.  Even so, no two El Niño events are the same.

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annual nonsense headlines from Daily Express about winter weather

One thing is for sure, recent newspaper reports touting confident headlines suggesting certainty over severe winter weather impacts in the UK and Europe “caused” by El Niño are not based on the findings of climate research or historic precedent which show only tentative and conflicting connections with our winter weather. It might be more accurate to suggest that no one really knows how El Niño  mixing with all the other connections will play out this winter!  Nevertheless, this should not stop the efforts of scientists trying to find clues for long range forecasts.

The last word should go to the UK MetOffice who state the following for the UK winter outlook with regard to El Niño 2015:

What does El Niño imply for the UK this winter?

Unlike some parts of the world, the effect of El Niño on Europe is relatively subtle. In El Niño years there is a tendency for early winter to be warmer and wetter than usual and late winter to be colder and drier. Despite this, it is just one of the factors that influence our winters, so other influences can overwhelm this signal – it is relatively straightforward, for example, to find years where these general trends were not followed.

El Niño moderately increases the probability of the positive phase of the North Atlantic Oscillation (NAO) in late autumn and early winter and the negative phase of the NAO in late winter. (In winter) the positive phase of the NAO is associated with milder- and wetter-than-average conditions, whilst the negative phase is associated with colder- and drier-than-average conditions.

useful references:

https://bobtisdale.wordpress.com/2014/02/22/el-nino-and-la-nina-basics-introduction-to-the-pacific-trade-winds/

http://www.vox.com/cards/el-nino-2014/why-is-everyone-talking-about-el-nino

http://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/ENSO-impacts

http://onlinelibrary.wiley.com/doi/10.1029/2006RG000199/full

http://www.weatherwest.com/archives/3566

http://blog.metoffice.gov.uk/tag/el-nino/

http://www.metoffice.gov.uk/media/pdf/n/b/A3_plots-temp-NDJ_v2.pdf

http://news.sky.com/story/1552290/what-is-an-el-nino-and-how-will-it-affect-us

http://www.accuweather.com/en/weather-news/godzilla-el-nino-not-to-blame-for-uk-cold/53117537

https://xmetman.wordpress.com/2015/11/07/latest-el-nino-still-only-3rd-strongest/

winter 2006 – 7 http://news.bbc.co.uk/1/hi/sci/tech/6401063.stm

winter 2009-10 http://www.metoffice.gov.uk/about-us/who/how/case-studies/winter09-10

winter 2013 http://www.metoffice.gov.uk/about-us/how/case-studies/january-2013-snow

http://www.independent.co.uk/news/science/el-nino-how-the-phenomenon-triggers-major-changes-in-weather-patterns-across-the-world-a6725806.html?utm_source=&utm_medium=&utm_campaign=

http://www.telegraph.co.uk/news/weather/11999259/British-winter-could-feel-longer-and-colder-because-of-El-Nino.html

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Forecasting confidence decreases over time

If we are not certain about the weather next week, how can we be confident about climate change predictions 50 or 100 years into the future? The charts below show two predictions about the temperature in Europe.. one was forecast for a day in June 2015, some 198 hours ahead, the other is a prediction for European July temperature by 2100, some 876,000 hours ahead.  If computer model forecasts beyond 120 hours are known to become significantly more unreliable pushed further into the near future, how confident can we be in similar computer models predicting atmospheric conditions some 876,000 hours into the distant future?

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Two temperature anomaly predictions for 198 hours and 876,000 hours ahead.

Of course, weather and climate are different things: weather is short term atmospheric conditions over days and weeks and is usually forecast at high resolution over small areas, while climate describes long term weather over decades or more for larger regions or the whole planet.  Nevertheless, much like the computer models that broadcast meteorologists use to issue short range weather forecasts, climate models use equations of fluid motion and thermodynamics to determine the behaviour of the atmosphere and ocean and to project predictions of Earth’s climate into the future. Both short range and long range predictions require powerful super-computers to run similar complex models that ingest millions of real time observations and perform trillions of calculations to produce predictions of atmospheric conditions on a huge three dimensional grid across different surfaces and altitudes, including the oceans.

The atmospheric system is essentially unpredictable.   Even the most powerful super-computer short range weather predictions can become quite unreliable beyond about 120 hours.  This is perhaps why the MetOffice still only widely publish short term forecasts out to 5 days.  It’s also why charts beyond 300hours are sometimes called “Fantasy Island”… they are so unreliable and not to be taken seriously as a forecasting tool on their own.

In the medium range, accurate ten day forecasts are still something of a holy grail and longer range seasonal forecasts arguably remain largely experimental as they are based on the unreliable time-frame of most numerical model output and the application of complex teleconnections and /or the extrapolation of observations from historic analogue patterns. The MetOffice have even put their seasonal long range predictions into obscure parts of their website after notable public failures in past seasonal forecasts, where they still languish today despite huge investment in computer power.  The charts below show examples of MetOffice contingency planner long range forecast information.

Even with a “perfect model”, weather and climate prediction will always suffer from uncertainties due to the immense complexity of the climate system, the chaotic nature of the atmosphere and the simplifications and approximations necessarily built into models themselves.  The ensemble charts below illustrate this growing uncertainty over just a short time scale of a few weeks.  Note the increasingly wide range of possible outcomes from the individual members showing the growing uncertainty as time progresses, and this is in a relatively settled period of weather.

So, if we really cannot be not certain about the weather next week, how can we be confident about related computer model predictions of climate for 50 or 100 years ahead?  To answer this we need to outline the three different types of uncertainty over climate change prediction:

There are three main types of uncertainty over climate change predictions:

  1. Model uncertainty: climate models have to approximate and estimate feedbacks and processes, they do this slightly differently.
  2. Internal variability: weather is chaotic partly due to uncertain internal forcings, like volcanic eruptions.
  3. Scenario uncertainty: future estimates of human behaviour and emissions of greenhouse gases in particular are uncertain

So, do these uncertainties increase over longer time scales thus rendering computer model predictions of climate in 100 years completely unreliable?  The answer is “No”!  Or more precisely, most of these uncertainties actually decrease over time making model predictions of climate in 100 years possibly more reliable than a seasonal forecast for next summer! Time scale and geographic scale are two reasons that can explain why this happens:

  1. Time scale: climate models deal with longer timescales better than short. The chart below shows lines indicating different model output for temperature change between 1850 and 2010. Note the various models (shown as different lines) all reaching a similar overall temperature increase of +0.8C by 2010.
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temperature change more certain over longer time scale

Over the long timescale shown above, the model runs all agree on an overall temperature rise of +0.8C due to initial changes in radiative forcing linked to combinations of, for example, increases in emissions of greenhouse gases, changes in solar radiation and frequency of volcanic eruptions etc.  Models confidently handle these changes over long time scales because climate system and model uncertainty both decrease over time..

Now… if we zoom into one part of the chart above the predictive capability of the model is shown to be much more questionable as the lines wiggle about much more over shorter timescales and sometimes go in completely opposing directions.  This shows uncertainty increasing over shorter timescales.

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less accurate predictions over a short time scale

On the shorter timescale of decades or less, model uncertainty increases as climate variability over short time scales is naturally large from one year to the next i.e. one year can be colder or warmer than the previous due to internal variability of the climate system (i.e.chaos).  Models don’t handle this small scale short term climate chaos very well!  One model run predicts cooling in a particular decade, while another predicts warming for the same decade.  It turns out that predicting climate change over smaller timescales is more unpredictable than predicting changes over broad sweeps of time! This is because of the internal chaotic nature of the climate system and model uncertainty being greater at this higher resolution. For example, models will handle these short term variables differently: How will ocean heat uptake respond?  What will happen to ocean currents and regional climates? How will snow cover respond?  Will volcanoes erupt? How will cloud cover and type change? … and many more besides.

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climate model variables

Despite these transient climatic uncertainties over short time scales the overall direction of change towards a new climatic equilibrium in the long term is confidently predicted by all the models. So… longer timescales are better handled by models than short.

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Scenario uncertainties increase over time in long range climate prediction models

The exception to this reduction in uncertainty over time is scenario uncertainty.  Most long range climate model charts include a wide range of predictions.  This wide range is not due to model uncertainty or uncertainties over internal climate variability. Scenario uncertainty is the uncertainty over predicting future emissions of greenhouse gases.  In other words, uncertainty over our own human behaviour in the future, which is historically difficult to predict!  This is the reason why the IPCC charts show several “RCP trends”.  Representative concentration pathways (or emissions scenarios) show a range of human response to the climate crisis… ranging from drastic curbs to emissions and lower growth, through business as usual, to increased emission pathways presumably due to high growth with no curbs to greenhouse gas emissions. These all yield obviously different outcomes.  The uncertainty over human response, known as the scenario uncertainty, increases over time.  Despite scenario uncertainty, each RCP is an accurately modelled temperature change based on changes in radiative forcing, the wide range of results is due to our own unpredictable behaviour more than climate chaos or uncertainties in the models.

2. Geographic coverage

On a long term global scale, climate system uncertainties are reduced while uncertainties increase over how climate change will occur over small geographic regions.  Predicted global mean annual temperature change is therefore more certain than, for example, regional European or UK temperature change in 100 years.  Future climate change on regional scales is more uncertain than on a global scale due to small scale variabilities.

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So… prediction of climate change over short time scales and regional scales is more uncertain than predicting longer range changes on a global scale.  Models struggle to resolve climate change at small time scales and small geographic scales. Overall however, longer term climate outcomes are more certain than short term small scale weather action.  Even with the immense complexity of the climate system, computer models can provide confident predictions of future climate within a range of scenarios.  The IPCC quote levels of confidence and certainty for their climate predictions and, even for the end of the century, they claim high confidence in their predictions ranging from “likely” to “more likely than not” for various scenarios.

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IPCC AR5 report on confidence in computer model temperature predictions

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the range of climate predictions will hit the probable climatological outcome

In conclusion, maybe this analogy might help: Perhaps climate prediction is a bit like a football match: complex uncertainties about precisely where the ball will go during the game are perhaps completely impossible to forecast beyond the first few seconds of the game.  Any detailed minute-by-minute action on the field thereafter becomes increasingly uncertain over time as countless variables come into play, including some chaos.  For example, individual player performance on the day, how the players and teams interact, the chaotic nature of the ball, the nature of the pitch… these are internal variables that make it almost impossible to model accurate step by step action of a game perhaps beyond the first few kicks after the whistle blows.  This is why small scale high resolution weather forecasts are still limited to less than a week ahead.  Nevertheless, despite such short term uncertainty the overall score and outcome of the game is still possible to predict with confidence, particularly within a certain range. The same can be said for long range climate models.

This post has only scratched the surface of climate and weather uncertainty. Please let me know of any errors you spot in this post. Further reading available here:

further reading

http://www.easterbrook.ca/steve/2010/07/tracking-down-the-uncertainties-in-weather-and-climate-prediction/

https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch1s1-6.html

http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf

https://www.skepticalscience.com/climate-models-intermediate.htm

http://www.skepticalscience.com/chaos-theory-global-warming-can-climate-be-predicted-intermediate.htm

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HIGH pressure dominates but is it all calm?

High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather.  However, HIGH pressure is not as unexciting as all that.  Anticyclones can sometimes be surprisingly windy especially round the edges.  We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.

So… buckle up for the ride and let’s get super-geostrophic!  Wind blows from HIGH pressure to LOW pressure.  The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure.  Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere.  Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.

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The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!

The pressure difference between high and low pressure determines the speed of wind.  Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post.  Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially.  Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind.  In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind.  So why is the wind blowing so strong when there is no LOW for miles?

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Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems.  The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe.  These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface.  The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.

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Wind is a result of pressure differences across the planet surface.  Wind wants to blow from high to low pressure.  This is called the pressure gradient force.  Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path.  The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.

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Assuming a constant isobar spacing.  Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind.  In the chart above the wind is shown as a black arrow.  In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force.  Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure.  As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars.  This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.

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Here is the HIGH pressure situation.  This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure.  As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars.  This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing.  These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.

Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them.  Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.

useful reference

http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/grad.rxml

http://www.metoffice.gov.uk/learning/wind/what-causes-wind

http://education-portal.com/academy/lesson/factors-that-affect-wind-pressure-gradient-forces-coriolis-effect-friction.html

Update #2 25/12/14: update: cold weather arriving after this LOW, heavy rain overnight Fri-Sat; snow marginal for SE early Sat am, more likely for Midlands and EA, cold weather arrives in lee of this system.  MetOffice warnings updated:

Update #1 25/12/14 latest MetOffice chart lifts pressure and pushes track further south, with low moving SE across our area.  This reduces wind speed, still brings in colder air flow though with risk of snow increased for back northern edge of the system with NE winds. For SE possible sleet/snow on Downs early Sat am. Evaporative cooling could yield more snow for SE if rain sufficiently heavy (drags down cold uppers). Gale risk gone but replaced by some heavy rain, marginal snow risk and retaining the cold easterlies in the aftermath on Saturday with pressure building to dry bright frosty conditions.

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After a pleasantly cool bright and dry Christmas Day, an interesting depression due on Friday and through Saturday is likely to usher in a period of colder weather for the UK and SE in particular. The situation is a little uncertain still but the run of warm mild gloomy temperatures lately this December, already pushed aside gently by a weak cold front passing south through the country today, are likely to be pushed further down into some “proper”cold after the storm passes through by Sunday. This storm, forms in the Atlantic along the polar front and quickly races east towards the UK on Boxing Day Friday.  Storms tend not to deepen much if they move fast, which this one does at first: crossing half the Atlantic in a matter of 24 hours. The storm is mixing some airmasses with contrasting temperatures: cold polar air in the north is about to get up close and personal to mild warm Tropical air from the south west.  They are due to meet in the LOW pressure over the UK soon, so expect some interesting weather!  You can spot the impact of the storm on the upper air temperature chart below but also see the steeper drop to colder conditions thereafter.

GEFS shows cooler days ahead

GEFS shows cooler days ahead

The ECM charts below show upper air temperatures at around 1500m. These “850hPa” charts are commonly used as guides to airmasses because air at 1500m (850hPa pressure level) is not affected by changes day and night or surface characteristics, it is therefore a good guide to true airmass characteristics.  Note the really cold airmass to the north meeting comparatively warm air to the south and SW in this LOW.

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For the South of England the LOW will initially push warmer tropical air ahead with rain arriving for us in the SE on a warm front sometime Friday pm (top diag above Sat 00hrs).  The warm sector is likely to be windy with gusty SW winds and a considerable accumulation of rain, 10-20mm overnight into Saturday.  The warm sector tropical air mass (upper air +5C) could have temperatures near double figures whilst the polar air bearing down from the north is a much more frigid airmass (upper air -6C).  The contrast between these two airmasses could make the frontal rain particularly heavy while the cold front contrast could even have an odd rumble of thunder as cold air undercuts the warm and forces it aloft.  The skew-t diagrams below show the contrast in these two airmasses.

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The LOW centre crosses the North of England and into the North Sea overnight into Saturday when, due to it’s location under the left exit of the jetstream, it is forecast to deepen to possibly around 980mb. quite low especially for a depression located so near the shore.  Deepening occurs as the jetstream aloft encourages air to rise off the surface because air is diverging aloft.  So air is rising off the surface quicker than it can be replaced by air arriving: hence falling surface pressure. This commonly occurs when lows interact with jetstreams on their left hand side, near the exit of a jetstreak.

The classic frontal depression with cold and warm fronts separated by a warm sector only lasts for a matter of hours before the cold front, pushing forward more dynamically than the warm, catches up the warm front and pushes the remaining warm air into the upper atmosphere.  This is an occlusion and signals the end of the development stage of a depression.  The central pressure usually starts to rise after occlusion has occurred.

Whilst the situation is still uncertain, it is likely that Friday afternoon and Saturday will be windy and increasingly cold as the winds veer clockwise from the SW through to North and finally NE and E.  It is the latter NE and E winds that will bring the colder air to the UK and the SE especially.  Continental Europe is currently very cold so any air flowing from this direction will be chilly.  Cold crisp continental air will stay with us for a while as high pressure builds to the west and pushes north over Scotland while the LOW moves over Europe.  This setup allows easterly winds to flow over the UK.  Dry cold is expected as the pressure is likely to rise and stay high.  Expect some frosty nights. The duration of the HIGH varies between models but certainly should keep things cold and crisp through to the New Year.

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cold new year

Further ahead a split in the polar vortex and stratospheric warming are dominating weather chat and these are set to possibly bring colder conditions through January.  On the other hand, Phase 3-4 of the MJO (Madden Julian Oscillation) is usually associated with a positive North Atlantic Oscillation which brings milder westerlies to the UK.  So, it’s interesting times ahead, stay tuned and Happy Christmas!

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The MetOffice charts above show the formation and life-cycle of a December 2014 “weather bomb”, involving the process more properly called rapid cyclogenesis. There are reasons why meteorologists dislike the term “weather bomb” but perhaps the most obvious is that the sensationalist short-hand use of the term “bomb” detracts from the complex processes and variable scale and location of impacts.  The term “bomb” tends to hype stories in the press that can cause over-reaction and unnecessary concern. On the other hand it gets people reading about the weather, which is a good thing (like this post, ahem!).

Nevertheless, a “weather bomb”, a term borrowed from the US and New Zealand, is short-hand for a potentially extreme event.  Bomb depressions are deep low pressure systems that form by the process of Rapid Cyclogenesis (RaCy for short).  RaCy is the rapid formation of a deep depression when the central pressure falls more than 24mb in 24 hours.  Such RaCy depressions are usually of marine origin. About 12 such RaCy bomb depressions hit the UK in the exceptionally stormy winter last year 2013.  Although by no means the most powerful, the first and most famous RaCy depression of last winter was the St Jude storm that hit Southern England with moderate force in October 2013.  Pictures below are from that event and can be compared to the enormous scale of the more recent Atlantic bomb depression of December 2014.

The “bomb” depression that struck this December 2014 seemed to catch media attention, despite the impressive weather impacts being almost wholly restricted to the less populated NW, especially Scotland, where people are entirely used to coping with such lively weather.

http://www.stornowaygazette.co.uk/news/local-headlines/weather-bomb-cuts-off-power-to-18k-homes-1-3630321

December 2014 rapid cyclogenesis: the weather story

The December 2014 “weather bomb” was a depression (low pressure system) which formed rapidly far out west in the Atlantic between SE Greenland and Iceland.  The formation was associated with a fast moving jetstream and the surface convergence of sub-tropical air from the south west meeting a frigid NW polar airstream from Canada and more local air direct from the Greenland ice cap.  The big temperature differences between these air masses accelerated uplift and the lowering of central pressure.

impressive but not the day after tomorrow

impressive but not the day after tomorrow

Descending dry stratospheric air is another defining feature of RaCy systems.  Cold dry air from aloft turbo-charges the depression as it is injected into the depression.  The cold air aloft increases lapse rates in the surface airmass and causes air to rise more purposefully creating a dramatic fall in central pressure.  Descending cold dry stratospheric air can be spotted on the water vapour satellite images as a dark dry slot ingressing into the depression circulation over time and following hard on the heels of the cold front as it is blasted across the Atlantic.  The water vapour images below show the rapid development of the system during Tuesday 8 December.  In later images it is possible to see the speckly cumulonimbus clouds emerging in the unstable cold sector following the cold front. Such instability was caused by the descending dry air.

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Rather than going through the rather measured development stages of a Norwegian Model cyclone, a RaCy depression usually follows a life cycle more like the Shapiro-Keyser model below (though at the time of writing I am not certain as to whether the December 2014 RaCy depression formally fitted all aspects of this model).  Several key characteristics of the December 8 cyclone fit the S-K model fit and this is the usual model associated with RaCy depressions.

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The Shapiro-Keyser depression life-cycle model often features a cold front that is blasted rapidly ahead.   so rapidly that it “fractures” from the wrapping warm front further north. This is known as a T-bone fracture and experts can identify the moment of fracture using satellite photos. Additionally, cf course, upper air moves faster than the surface wind that suffers frictional drag even across relatively smooth ocean.

satellite features of emerging RaCY depression

satellite features of emerging RaCY depression

This meant that the cold front moved so rapidly that it split vertically into a fast moving upper front and a slower moving surface cold front. The cold front literally had its head ripped off!  The frigid upper cold air travelled over a shallow moist zone of warmer sub-tropical air and it is this that increased lapse rates and caused immense instability in the polar air stream that eventually arrived in Scotland.  Instability can be seen on the visible satellite pics as speckly masses of cumulonimbus clouds shown best in the satpic above.  In the charts and sat pics below note the wind speed associated with this polar air and the tropical air preceding it in the warm sector.

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In the S-K model the cold front is sometimes weakened during the formation process while the warm front remains active, wrapping itself in knots around the central “eye” of the storm.  The 850mb chart below shows temperatures of this cold upper air at 1500m above Scotland. The bomb depression this December seems to have matched this because, while the cold front was relatively weak (narrow squall line) the exceptionally unstable polar air behind it was arguably the defining characteristic of this system, bringing persistent convective storms and an outstanding 5000 lightning strikes and thunder-snow blizzards across higher ground in Scotland during the advection of this exceptionally cold and unstable air for an Atlantic NW airstream.

In the S-K model depression life-cycle the warm sub-tropical air is eventually left “sequestered” as a warm pool trapped in the middle of the mature depression which is called a “warm seclusion”.  The usual process of occlusion is bypassed as the centre of the low fills with warm air.  Meanwhile, the rapidly overshooting upper cold front causes S-K cyclones to often elongate in appearance on surface pressure charts, a feature associated with the rapid forward acceleration of the cold front in relation to the tightly wrapped, almost stationary, wrapped warm front. It is this tightly wrapped warm front (sometimes shown as occluded on weather charts) that shows another defining feature of S-K depressions.

As our initial bomb LOW pressure moved due east and filled and decayed offshore near Norway, a wave depression further south on the Polar Front also “bombed-out” to the SW of the UK and swept across Southern England on Thursday-Friday 11-12 Dec.

This was a separate small scale system but technically another rapid cyclogenesis as central pressure fell more than 24mb in 24 hours, but only just.  This illustrates the varying scale of bomb cyclones: some cover vast areas, some a small.  The 11-12 Dec RaCy depression was much smaller in size and intensity, max wind speeds were much more restricted and the whole system several magnitudes smaller in scale than the “mother” cyclone further north. Charts below show the evolution of this storm.

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Finally, the North Atlantic Oscialltion is a measure used to describe and forecast the mean pressure pattern over the Atlantic. A positive NAO indicates “normal” conditions with low pressure over iceland and high over the Azores. This is associated with a zonal west to east flowing jetstream and fast moving cyclones moving rapidly west to east bringing generally mild conditions to the UK in winter. Note the recent positive pattern matching the westerly flow and active zonal jetstream causing the RaCy depressions.  When the NAO turns negative the jetstream is often more wiggly and flows between latitudes in a more meridional flow potentially bringing cold air from the north when pressure patterns are more slow moving and even “blocked”.  A negative pattern is often associated with cold winter weather for the UK. The NAO is not a driver of weather, merely an indicator of pressure patterns.

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For a bit of fun we invented our own local Wight-Wash Oscillation (WWO) which is a measure of pressure across the south of England between The Wash and the Isle of Wight.  This would give an approximately similar local version of the NAO but just for fun!  We noted a WWO difference in pressure of 10mb during St Jude and only 9mb during the recent bomb wave depression.  The WWO particularly suits the passage of wave depressions across the Midlands which tend to yield the highest wind speeds for the SE.  It would also work in negative conditions which would give cold easterly winds in winter. Note this measure is just for fun!

Positive NAO remains likely on the run-up to Christmas 2014 so chances of a White Christmas is much reduced. Remember that a White Christmas for us in SE England is the rare exception to the rule.  On a brighter note, the earliest sunset has just passed and we can at least look forward to later sunsets from now on!

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A dedicated team of students from Reigate Grammar School weather station successfully launched a high altitude balloon (HAB) and retrieved the payload on Tuesday 25 November 2014.   After months of meticulous planning and with close cooperation and assistance from the scientists at the Met Office, Met Research Unit, Cardington Boundary Layer Research Facility and in collaboration with Chris Hillcox Near Space Photography, our students launched the 800g balloon and then used GPS trackers to chase and locate the payload as it fell to earth after an extraordinary 3.5 hour flight collecting some of the best amateur high altitude balloon photos of near space.  The students and staff were delighted by the success of their mission which took careful planning and preparation to reduce the risk of failure, which always remained a possibility.  The balloon took 2 hours to ascend 100,000 feet (30km) into the stratosphere, through temperatures as low as -70C in the tropopause, before the balloon burst at pressures 100x lower than at the surface.  The descent took over an hour.  Videos of the story can be found below.  See below also for “Why did we do this?”

Planning

Students spent weeks prior to the launch designing and building a lightweight box (150g) that would carry the payload safely to an incredible altitude up to 30km into the atmosphere.  The payload consisted of several HD cameras, spot GPS trackers and a GSM locator with meteorology sensors which were weighed to ensure the balloon would be able to lift them to the target burst height at 30km (100,000 feet) with 3.5m cu of helium.  All equipment was thoroughly tested (several times).  GPS trackers were tested to ensure phones would pick up the track reliably.  A laptop was enabled to operate on a mobile connection.  GoPro cameras were tested and checked, as were HTC cameras.  A school logo was laser cut onto a small plastic arm that would sit in front of one of the cameras during the ascent.  CAA approval was applied for and received.  Scientists at the Met Office research unit were also closely consulted regarding gas type (99.5% helium, not party grade!) and the practicalities of the launch location such as trees and masts.  Risk-benefit assessments based on MetOffice launch assessments were completed and approved. Various risks needed to be considered beyond allergies to latex and the dangers of helium inhalation,  particularly regarding student involvement in retrieval that might require travel across land that lacked nearby public access.  We checked and adjusted school insurance to ensure full coverage for the flight.  However, in the end, it was the good will and enthusiasm shown by the individuals involved with the student launch that drew the threads together and allowed us to proceed with this unique educational challenge.  Acknowledgements are listed below.

Predicting the right weather 

To avoid the balloon payload shooting off across the North Sea we had to choose a day when the jetstream and higher altitude winds in the stratosphere were relatively slack.  Winds in different directions with height would also be useful to ensure the balloon did not travel too far for retrieval purposes.   A single date could therefore not be fixed in advance due to changing autumnal weather conditions.  Parents of students were sent an open letter with details regarding the potential for this expedition and ways we would contact them to ensure students were available as any possible launch date approached.  The weather at all altitudes was studied carefully and a potential launch date emerged for Tuesday 25 November as a weak high pressure pushed the jetstream and high altitude winds further north.  A suitably safe landing site was also critical to mission success and the HABHUB website landing predictor was regularly consulted in the run up to the launch.  Conditions improved throughout the period until Tuesday looked the most ideal.  We were GO FOR LAUNCH!

 The launch

The launch day started early as the team had to travel well beyond the flight paths of Gatwick and Heathrow.  Our launch site was in Bedfordshire with a predicted landing site NE of Cambridge.

Amanda Kerr-Munslow, a Met Office Boundary Layer Research Scientist, gave the group a very interesting tour of the site, including lasers that measured cloud height.  Dave Bamber the Met office facilities manager then helped students inflate the balloon.  The payload box had last minute modifications completed and was then prepared and carried to the launch bed.  The balloon was carefully transferred, inflated, to attach to the payload at the launch site.  Cables checked, final preparation and then the countdown!  The pictures below show some screenshots captured by the movie cameras on the ascent to 100,000 feet.

short film: (FULL FILM scroll below)

(please refresh the page if video is from another playlist)

Payload retrieval

The chase and retrieval were very exciting!  Despite predictive forecasts showing a broadly good landing area (open farmland, flat, few forests, rural, few pylons etc) it was not certain that it would avoid landing in problem locations like hedges or trees, pylons, main roads, rivers, ditches, inaccessible fields remote from a road or someone’s roof!  It was an exciting moment for the team when the tracks stopped at Wicken Fen in Cambridgeshire and the maps appeared to show a landing site in an open field which appeared to be accessible from the road.  Reflective vests on and trackers in hand we rushed off to search and found the payload with the shredded remains of the burst parachute precisely on target.   A wonderfully memorable moment after an exciting and highly rewarding adventurous day for everyone involved.

Why did we do this?

It has been a long-term aim of the RGS weather station to launch a HAB.  Reasons are myriad but the experience of this has been a high quality challenging learning and teamwork experience for students and staff. Practical skills, meteorology and an understanding of the atmosphere were all critical for students to grasp to increase the chances of a successful launch and, importantly, retrieval.  The students more than rose to the challenge!  At RGS the sky is not the limit!

A driving force behind this enterprise has been to raise climate and weather awareness amongst students and more widely.  The images and story brought back beautifully illustrate the fragile atmosphere in which we all live and upon which the human race depends.  From our balloon view the atmosphere was shown as a thin blue veil of gases held precariously against the surface with the blackness of space directly above and not far away.  It is of course widely accepted that humans are forcing climate change which might potentially threaten our existence because we have altered the workings of the delicate atmospheric envelope.  The site and sound of planes cruising far below our balloon demonstrated how we might dominate a tiny hospitable part of the planet but that we reside perilously close to utterly hostile environments.  In this respect it is no surprise that we have probably damaged this delicate life-protecting atmosphere with our activities.  We would like anyone using our pictures to ensure that climate change is explicitly mentioned and that our balloon launch post is linked to any material used.

What next?

This project has proved that the stratosphere is closer than ever and increasingly accessible. School exploration of the stratosphere is now within the budget of schools or groups of schools that can collaborate (rather timely considering recent political announcements).  Currently it costs around £200 to launch a payload to 30km, to launch a further 30km above that would cost £200 million (Antares rocket cost)!  The exploration and science that schools can do at 30km is breathtaking! Floating balloons are also possible for longer experiments.  The IPCC consider that our climate is on the cusp of irretrievable change that will have great impacts in the next 100 years.  It is therefore of the utmost importance that schools engage their students with weather and climate and that exam syllabuses include meteorology and climate across a variety of disciplines.

costing the earth: different missions

This video of the flight has been edited from 6 hours of footage from 3 cameras pointing in different directions (GoPro Hero3, 2 HTCs) over the course of the 3.5 hour flight.  It also includes screenshots and photos showing the story of how students put their payload into near space.

(please refresh the page if video is from another playlist)

11 video questions for classes: (ask “why” / “how” for extension)

  1. Why is the flight path not straight?
  2. At what height do clouds stop?
  3. It rains ice particles from blue sky at some points in the flight.  Why might this happen?
  4. Does all rain that falls out of a cloud reach the ground?
  5. Are any clouds man-made?
  6. Listen to the wind and noise picked up by on-board camera mics during the flight: how does it change?
  7. How does the colour of the sky change with altitude?
  8. How “near space” actually is 100,000 feet (30km)?
  9. Which was the coldest part of the ascent?
  10. As the balloon burst there was a puff of “smoke”, what was this?
  11. Shreds of burst balloon were seemingly left hanging above the descending payload.  What do you think happened to these?

RGS launch team:

  • Piers Rex-Murray
  • Tom Tatham
  • Jasmine Hull
  • Louis Chambers
  • Edgar Povey
  • George Beglan
  • Harry Persand
  • Chris Meredith
  • Fraser Cadman
  • Matt Taylor

RGS staff:

  • Simon Collins
  • Peter Klein
  • Vanessa Ramsden

Acknowledgements and sincere thanks to the following:

Huge sincere thanks to the following for helping us to get our HAB project off the ground.

  • Near Space Photography : Chris Hillcox Chris was incredibly helpful in giving students ownership of their launch in all stages whilst offering any amount of technical know-how we needed to increase chances of success. Chris gave his time extremely generously throughout the enterprise and his expertise was pivotal in the success of the mission.  Many thanks Chris!
  • Royal Meteorology Society RMetS : especially Sylvia Knight who encouraged us to pursue the project and put us in touch with Amanda. Thank you Sylvia!
  • Met Office, Met Research Unit, Cardington : Boundary Layer Research FacilityDavid Bamber and Amanda Kerr-Munslow were on the launch site at Cardington.  Amanda gave us a thorough tour of the scientific research unit and assisted with all practical aspects of getting it organised at Cardington.  David was a hero for staying up all night on an IOP and then staying on to manage students in inflating the balloon.  Many thanks Amanda and David.
  • RGS DT department: thanks to DT department for their workshop space and the free use of their tools and being so patient when we left polystyrene all over the place.  Thanks to Martin especially for helping us with the lazer logo cutting.

Interested in a HAB launch or weather stations for schools?  Contact RGSweather!

short video version

news:

http://www.surreymirror.co.uk/Reigate-school-sends-weather-balloon-near-space/story-25116859-detail/story.html?ito=email_newsletter_surreymirror

The “Spanish Plume” is a special weather set-up which can produce thunderstorms in the UK, as well as France and Benelux counties and beyond.

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A Spanish Plume is when hot dry air from the Spanish Plateau moves north towards the UK.  This occurs when LOW pressure exists in the Bay of Biscay and a HIGH pressure builds further east over Europe or N Europe / Scandinavia. An “omega block” is one such pattern but several different types of Spanish Plume are recognised.

As the warm dry Spanish air moves north from Iberia it picks up moisture from the Bay of Biscay, increasing the humidity of the lower layers but retaining dry air aloft.  The combination of these two airmasses, moist surface flow and dry upper flow, creates instability and potential thunderstorms. The unstable airmass can further lift over any relatively cooler air residing over N France and the UK (isentropic lifting). The lifting of the air column allows thunderstorms to grow even further.

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Jetstream and upper flow from the South

Thunderstorms are directed north on the upper winds and jetstream from the south where, given an approaching cold front, lapse rates can increase further and the intensity of storms can grow over France, Benelux and the UK and further into Europe.  The Spanish Plume is a complex weather pattern which needs some special ingredients to prime the atmosphere. Like all thunderstorms, Spanish Plume storms are based on convection in an unstable atmosphere which occurs when thermals of air rise uninhibited to a great height creating tall cumulonimbus clouds.

Unfortunately, thunderstorm formation is difficult to forecast, often being altered by local factors below the resolution of many forecast models.  The localised nature of storms also means that some places may see a tumultuous thunderstorm of epic proportions while other places not far away will see very little impressive action or nothing at all. Quite frequently plumes will skirt to the east of the UK and miss us entirely, storms staying on the continent, as initial forecasts of a “direct hit” are corrected east.  Cool sea surface temperatures in the English Channel can also deflate imported storms from France. So some understanding of the plume might help figure out how much we can expect from any convective action forecast with a Spanish Plume event!

For a wonderful time lapse of the plume passing over the UK watch this…

saturdays weather

Read below to find out the ingredients for a Spanish Plume but, if you are in a rush, then read the BBC weather summary here (it’s a bit shorter!) 🙂

Setting the stage:

The large scale “synoptic” pattern required to prime the atmosphere for a Spanish Plume is for a cut-off LOW pressure to locate around the Atlantic coast of Spain, ideally moving into the Bay of Biscay. The LOW in the Atlantic therefore sits to the SW of the UK.  An upper level trough nudges this LOW east across a hot Iberian plateau and France picking up humidity across the Bay of Biscay.  Strong solar heating over Spain creates a thermal heat LOW circulation and local thunderstorms here.  The stage is set for a Spanish Plume!

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Spanish Plume set-up

Warm plume followed by cold front chaser!

How does this “plume” head towards the UK and create thunderstorms here? Prior to the arrival of the upper warm Spanish plume in the UK, a humid surface SE flow from the continent feeds heat and moisture into the UK at low levels, steepening lapse rates. SE winds in the UK in summer frequently herald thunderstorms due to the low level advection (movement) of heat and moisture from the continent, both critical to feed thunderstorm formation.

Meanwhile, the upper SW flow (jetstream) ahead of the upper level trough pumps warm air from the Spanish Plateau northwards. This plume rides a conveyor belt of air rising towards and over a warm front as it travels north in a warm humid sector. Now an approaching cold front from the west is needed to really kick off storms by destabilizing the atmosphere!

The LOW pressure system has cool moist Atlantic air wrapping behind the cold front (seen on satellite pics below as the white line of cloud extending from the mother low to the NW).  A warm surface conveyor is driven NE in the developing warm sector ahead of the cold front, guided by upper level winds and the jetstream.

The rising warm plume has cool Atlantic air moving in aloft ahead of the cold front.  This cool air aloft can override the warm plume, increasing lapse rates and destabilizing the atmosphere further.  Sometimes this scenario can cause huge thunderstorms called meso-scale convective systems (MCS) or sometimes supercells.

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Any solar heating will further rapidly destabilize the atmosphere creating surface based storms where temperatures rise. Storms are often squeezed into a narrowing band or chain ahead of the cold front.  The cold front itself often weakens as air tends to sink after the passage of the thunderstorms.  The cold front can pass over unnoticed behind the thunderstorm event, its main job is to advect cold air over the rising warm plume to increase lapse rates.

HIGH pressure over the Mediterranean can also assist the process by advecting warm / hot air out of North Africa, over Spain, into France and finally reach Britain across the English Channel.  The more heat the better.

Sounding unstable 

By itself, a warm moist wind wafting up from Spain will not necessarily create the biggest bangs.  Other, ingredients are required to cook-up a good thunderstorm, with large hail, cg (cloud to ground) lightning and thunder and even funnel clouds or a tornado. Thunderstorms need an unstable atmosphere to trigger the air to rise rapidly UP. Such triggers can be spotted on a skew-t or sounding chart.  Cooler dry air sinking from aloft ahead of the upper trough can destabilize the atmosphere by increasing lapse rates and CAPE (convective available potential energy). This occurs because dry air at mid levels (4000m-7000m; 400-600hpa) can cause evaporative cooling of the atmosphere.  Such cooling aloft will mean that rising parcels remain unstable to a greater height and rise freely.  The sounding below shows all the ingredients that caused a supercell storm (note dry slot at 500mb). (this is for Texas, but same applies!)

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thunderstorm sounding

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unstable sounding for Glasgow airport July 1 2015

Another important factor in thunderstorm formation especially for the UK is TIMING.  The arrival time and combination of unstable air masses, fronts and moisture is what makes or breaks storms. Any element that is premature or delayed can be the death of expected storm formation. The best example of this is whether unstable air arrives during a sunny warm day or at night, when surface based heating is absent.  In the case of the Spanish Plume event 7 June, the unstable air arrived at night over a comparatively stable boundary layer surface air mass.  The only storms that resulted were elevated thunderstorms embedded in the unstable upper air ahead of the cold front (which oddly brought warmer conditions later in the day as cloud cleared and sun came out but too late to coincide with the impressive instability of the plume).

A cap in the sounding can also enhance convection.  A temperature inversion can hold heated air near the surface building convective energy which is held to the ground until the cap is broken by daytime heating… if it is sunny enough, cloud cover will spoil the event.  Busting the cap can create explosive and dangerous thunderstorms, rarely seen in the UK.

Here is a summary of some other ingredients as it applies to the Spanish Plume like the one on 7 June 2014 over the UK:

Heat! the SE of England is due to get warm or even hot on Saturday with Tmax temps well over 25c in sunny spots.  This will warm the air at the surface and, like a hot air balloon, these air “parcels” will want to rise (called lift). Warm air is also being moved into the country from the south by a process called advection.  Charts showing the heat energy providing the potential for air to rise are called CAPE: convective available potential energy.  In the UK we are pleased with CAPE values of 200-500j/Kg for some thunderstorms.  On Saturday we might expect values of up to 3000j/Kg. The other measure used to assess thunderstorm potential is the LIFTED INDEX (LI) this measures the difference in temperature between a parcel of air lifted to 5000m and the temperature of the air around it.  Negative figures show the air is buoyant and ready to rise.  Values of -8 or -9 are unusually low and show a very unstable airmass with the potential for plenty of lift!

In a Spanish Plume event the upper air contains enough energy and moisture to produce elevated thunderstorms even in the absence of surface heating: this means moderate thunderstorms can occur at night and with extensive cloud cover. The morning moderate storms experienced in the SE on Saturday 7 June were all elevated thunderstorms because extensive cloud cover throughout the morning meant an absence of surface based heating to kick off more purposeful convective activity.  Here’s an excellent blog explaining elevated thunderstorms compared to surface thunderstorms that would occur due to surface heating.

Moisture! local SE winds are frequent precursors for thunderstorms in England. This is because, in summer, SE winds are often humid with a relatively high water content, advecting (moving) lots of moisture into the SE: high dew points illustrate this with some reaching 20c on Saturday… a muggy humid day. This moisture will be required, of course, to form clouds.  To form really big clouds you need a lot of water in the atmosphere.  Once water vapour starts to condense it releases latent heat and this heat gives additional lift to convection and feeds thunderstorm formation.  Interestingly and perhaps counter to what might be expected, a dry plume of air mid-way up through the atmosphere is also an important ingredient to the production of big thunderstorms.  Drier air at mid-levels aloft gives the atmosphere added instability for the production of thunderstorms.  The reason for this is that dry air cools more rapidly with height than moist air (because rising moist air releases latent heat when clouds inevitably form and this additional heat reduces the rate of cooling of saturated air with height).  The rate of cooling with height is called the “lapse rate” and the lapse rates or temperature gradient on Saturday is steep. So moisture is a critical ingredient to storm formation because it controls instability and cloud formation.

The skew-t charts show temperature change with altitude.  They are called Skew-t because the temperature lines are skewed off the vertical slightly.  Whilst they are initially odd to look at, focus on the red and blue and dashed lines: if the red and blue lines are close together it means the air is saturated (cloudy).  If they are far apart then the air is dry. If the red line skews to the right then this is known as an inversion where temperatures can increase or stay the same with height.  Such an inversion will prevent thermals rising and the formation of clouds from surface convection, a critical ingredient for big thunderstorms.

more analysis of Skew-t

more analysis of Skew-t

The other ingredient the skew-t chart shows above is plenty of wind shear with height, in this case speed shear.  Wind shear is the change of wind speed and/or direction with height.  Increasing wind speed with height has the effect of dragging air off the ground like a hoover. On Saturday there is a jetstream moving overhead during the day and this will cause divergence aloft and encourage more air to lift off the surface. Strong directional wind shear, when winds turn at angles through the atmosphere and when different winds directions meet around fronts, is also a trigger for tornadoes.  The UK quite regularly has tornadic conditions in lively convective thunderstorm weather but the ingredients for tornadoes are a even more fickle!  Our tornadoes are considerably less powerful than those in the USA but can be of great interest and still cause damage by uprooting trees, damaging rooves and chimneys and even tipping cars.

Lift! like a hot air balloon, a warm bubble of air will only rise if it is warmer than the air around it.  On Saturday morning a CAP will exist in the atmosphere that will prevent air lifting far off the surface, thus preventing extensive vertical lift. The cap is known as a temperature inversion and the cap on Saturday is pretty solid, so little convection is expected early on, except possibly in unstable upper atmospheric layers which might cause elevated thunderstorms.   During the day, if there are suitable breaks in cloud cover, the sun will heat the surface and this will start to break down the cap.  A strong cap has the effect of building energy and heat below and, if the surface heats up sufficiently then the cap can be broken suddenly.  This usually happens in the afternoon when a sudden explosive thunderstorm could be produced.  In the mid-west of the USA tornado chasers call this process “busting the cap” and it produces the possibility of extreme weather.  On charts the cap is shown as convective inhibition that acts against convection but, at the same time, can be an ingredient for extreme weather.  Another mechanism that is capable of lifting air rapidly is the proximity of fronts which can mechanically lift the air as different air masses converge.  On Saturday, a slow moving cold front is forecast to be located along a north/south axis through the Midlands and S England, moving gradually east… this could be the focus of most activity if the cap remains solid further east.

Convergence of winds! winds converging at the surface are another local or regional factor that enhances convection. Converging winds at the surface tend to slow down and pile up, like lorries slowing down uphill and causing congestion.  When surface winds pile into each other (converge) they can only go one way… UP!  Convergence often occurs at the coast where winds coming from the sea slow down due to friction over the land.  This is one reason why the south coast often produces thunderstorms or enhances them as they cross the Channel (so long as the Channel sea surface temperature is warm enough – another story!). The other map below shows areas where there is a sufficient combination of wind shear, heating and energy to possibly start to rotate a thunderstorm: rotating thunderstorms are called supercells and are capable of producing tornadoes.  These are the “big daddy” of thunderstorm cells and would be an awesome site for anyone able to catch a photo before or after the inevitable heavy rain, hail, lightning, darkening skies and thunder!

So the action of winds converging at the surface and diverging aloft or doing the reverse, is important to thunderstorm activity. The charts below (website ) are a cross-section through the atmosphere from S-N and W-E across the UK before and during the plume.  The streamlines showing wind vectors illustrate the turbulent nature of the air during the plume.  There are elements of both lift and subsiding air at different elevations showing the complexity of the plume event.

Even with so many ingredients primed for thundery weather here in Reigate on Saturday it still remains a risk potential rather than a certainty.  Local factors might inhibit convection, too much cloud in the morning could reduce surface heating and the cap may not be broken, so no thunderstorms! It’s a matter of waiting to see the conditions nearer the time to make a final forecast and often it remains uncertain until the very day.  Nevertheless, there is a possibility of explosive convective thunderstorm action on Saturday, mostly in the Midlands and the East, if the CAP is BUST!  If this occurs then instability of the atmosphere will rapidly build extremely tall cumulonimbus clouds up to 10,000 metres tall.  The UKMO issued an uncharacteristically early warning of heavy rain for Saturday on the basis of this Spanish Plume event. Others will doubtless follow, but it is safe to ignore the daft scaremongering of the Daily Express but do keep a watchful eye on any dark clouds!

Although an accurate forecast is beyond the scope of this post and best left to the professionals, one broad indicator of possible storm pattern over the course of the day is set by convergence of winds and the likely position of the front as it progresses west to N/east.  The first plume of instability arrives in N France later on Friday and some might arrive S England Friday pm with low risk thunderstorm activity imported across the Channel from France (as it happened this occured only in the SW).  The charts for Saturday below show how the main location of biggest storm activity could move broadly from west to NE during Saturday and clear off into the N Sea overnight. (as it happened the front moved much quicker and clear S England and most parts of Central England by lunchtime leaving a bright afternoon with bubbly Cu).

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The animation here also shows the total rainfall expected to accumulate during the 48 hour plume event.

Whatever happens, do watch out for altocumulus castellanus in clear skies or asperatus undulatus (pic above) or shelf clouds and cumulonimbus developing ahead of storms as the plume arrives on Saturday and then watch for any cumulonimbus clouds exploding if the cap is bust later on!  Please send in your photos of any interesting weather phenomenon to RGSweather : on twitter and facebook and email.

Comments and any additional information always welcome!

sources: many thanks to these sites:

Soil moisture: possibly the most under-rated meteorological measurement!  Rarely do weathermen get animated about the extent of wet sod across the country.  Nevertheless, soil moisture, usually measured in centimetres of water in the top two metres of soil or as % saturation (see maps below), has been found to control continental scale weather patterns, summer maximum temperatures and even heat waves and the extent of droughts.  So we ignore soil moisture at our peril, especially as soil moisture also controls vegetation growth and death and the ability of farmers to grow food.

How wet the soil obviously relates to how much it has rained recently.  During winter, in mid-latitudes, soils usually become increasingly saturated with a surplus of water building up as inputs of precipitation exceed evaporation which is reduced in the cooler months and shorter days.  During the summer, soils tend to become increasingly depleted of their moisture content as evaporation (output) exceeds precipitation (input).  This input and output of moisture forms an annual balance known as a soil moisture budget and is shown in the graph below.

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High rainfall during the winter builds up a lot of water in the soil.  In the spring time a high soil water content “uses up” more energy from the sun in the process of evaporation.  The more energy “used up” in evaporation, the more energy is lost from the system to produce sensible warming at the surface. Hidden energy, or latent heat, is required to change liquid water into water vapour.  So “latent cooling” reduces the amount of energy available to warm the atmosphere as long wave radiation.  So local temperatures can be depressed over areas of wet soil especially during a spring when wetter-than-usual soils might take a long time to dry out.  It might also be expected that, after a wet winter, there could be a cooler period until such time that the soil dries out locally and more energy becomes available to produce a sensible heat flux at the surface.

In fact, the effects of soil moisture go far beyond these micro-climatic changes and can have impacts that are continental in scale.  In a 2007 study (see below) it was found that 25% extra soil moisture could reduce continental Europe-wide temperatures by up to 2c from average summer maximums. Likewise, a 25% reduction in soil moisture could raise temperatures across continental Europe by 2c in the study period.  It was also discovered that higher winter and spring soil moisture could raise summer precipitation levels and change continent-wide pressure patterns.

The study from 2007 used reanalysis of computer weather models to investigate the impact of soil moisture on the European 2003 heat wave, the warmest for 500 years.  This heat wave killed over 20,000 people and caused crop damage.  The study found that by re-running computer models just with different soil moisture values, the maximum temperatures and heat wave intensity varied greatly.  Dry soils during the spring increased summer heat wave intensities while wet soils reduced the maximum temperatures.  The difference was significant, and in some localized regions the intensity of heat anomalies varied by 40% simply due to different soil moisture content at the outset of the model runs.  The largest differences were mainly located over central Europe.  It seems that differences in soil moisture have most impact across central continental Europe and progressively less impact on summer temps with increased distance further north.

Not only did soil moisture control temperatures, it also had a control over continental pressure patterns.  Dry soils built pressure through the middle troposphere, while wet soils could lower pressure.  This has numerous positive feedbacks: wetter soils reduce pressure which increases cloud formation and summer rainfall that enhance the wet soils.  Dry soils build pressure, reducing cloud formation, reducing summer rainfall, further drying out soils.

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So, in summary:

  • If the soil-moisture deficit is high, the dry soils raise the sensible heat flux, producing a deeper, warmer, drier low-level atmosphere: raising temperatures and enhancing surface heating and drying.  Increased drought risk.
  • If soil moisture is high, the latent heat flux by evaporation and transpiration dominates, enhancing cloud formation and a tendency for cooling.  lower temperatures and enhancing rainfall and further wetting of the soil.  Increased wet summer.

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The full article is here: Fischer_heat_waves_2007 (1)