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.
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.
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.
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.
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.
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.
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.
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.
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!