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.
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.
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.
The full article is here: Fischer_heat_waves_2007 (1)