Yet more observations from the planet show that modelers misunderstand the water based part of the climate – on our water based planet.
Modelers thought that dry ground would decrease afternoon storms and rainfall over those frazzled parched lands (though I don’t remember many headlines predicting “More Drought means Fewer Storms” ). But observations show that storms are more likely to rain over dry soil. Why? Probably the dry soil heats up faster than moist areas thanks to the cooling effect of evaporation, and that in turn creates stronger thermals over dry land. Modelers assumed that wetter soils means more evaporation and thus more rain, but the moisture laden air is evidently coming from further away.
It’s another example of a point where climate modelers assume a positive feedback, yet the evidence suggests the feedback is negative. Once again water appears to be the dominant force with feedbacks (it does cover 70% of the surface). In a natural stable system the net feedbacks are likely to be negative. Positive feedbacks make the system less stable (and more scary and harder to predict.)
Climate change models misjudge drought: “A four-nation team led by Chris Taylor from Britain’s Centre for Ecology and Hydrology looked at images from weather satellites which track the development of storm clouds across the globe.”
“The data trawl covered six continents, looking at surface soil moisture and rainfall patterns on daily and three-hourly time steps, with a resolution of 50 to 100 kilometres, over a decade.
“It’s tempting to assume that moist soils lead to higher evaporation, which in turn stimulates more precipitation,” said Wouter Dorigo of the Vienna University of Technology, a co-author.
“This would imply that there is a positive feedback loop: moist soils lead to even more rain, whereas dry regions tend to remain dry… (But) these data show that convective precipitation is more likely over drier soils.”
Compare it to the model predictions:
Chris Taylor, meteorologist, from Britain’s Centre for Ecology and Hydrology.
Afternoon rain more likely over drier soils
Nature advance online publication 12 September 2012. doi:10.1038/nature11377
Authors: Christopher M. Taylor, Richard A. M. de Jeu, Françoise Guichard, Phil P. Harris & Wouter A. Dorigo
Land surface properties, such as vegetation cover and soil moisture, influence the partitioning of radiative energy between latent and sensible heat fluxes in daytime hours. During dry periods, soil-water deficit can limit evapotranspiration, leading to warmer and drier conditions in the lower atmosphere. Soil moisture can influence the development of convective storms through such modifications of low-level atmospheric temperature and humidity, which in turn feeds back on soil moisture. Yet there is considerable uncertainty in how soil moisture affects convective storms across the world, owing to a lack of observational evidence and uncertainty in large-scale models. Here we present a global-scale observational analysis of the coupling between soil moisture and precipitation. We show that across all six continents studied, afternoon rain falls preferentially over soils that are relatively dry compared to the surrounding area. The signal emerges most clearly in the observations over semi-arid regions, where surface fluxes are sensitive to soil moisture, and convective events are frequent. Mechanistically, our results are consistent with enhanced afternoon moist convection driven by increased sensible heat flux over drier soils, and/or mesoscale variability in soil moisture. We find no evidence in our analysis of a positive feedback—that is, a preference for rain over wetter soils—at the spatial scale (50–100 kilometres) studied. In contrast, we find that a positive feedback of soil moisture on simulated precipitation does dominate in six state-of-the-art global weather and climate models—a difference that may contribute to excessive simulated droughts in large-scale models.
Christopher M. Taylor, Richard A. M. de Jeu, Françoise Guichard, Phil P. Harris & Wouter A. Dorigo ‘Afternoon rain more likely over drier soils’ will be published in Nature on 12 September 2012. www.nature.com DOI 10.1038/nature11377
Press release: Eurekaalert
H/t to Mark in Perth