Tuesday, November 13, 2012

New paper finds models have it wrong again & predict excessive droughts

A new paper published in Nature finds that "across all six continents studied, afternoon rain falls preferentially over soils that are relatively dry compared to the surrounding area."  The finding is yet another example of negative feedbacks in the climate system that help to stabilize climate. However, climate models are programmed to assume the opposite: that dry areas will become drier and wet areas wetter due to a fictitious positive-feedback mechanism. The authors find that this erroneous positive-feedback assumption results in excessive predictions of drought in computer models, stating,
 "We find no evidence in our analysis of a positive feedback—that is, a preference for rain over wetter soils—at the spatial scale (50–100kilometres) 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."
Related posts on the abject failure of climate computer models

Afternoon rain more likely over drier soils



Nature
 
489,
 
423–426
 
(20 September 2012)
 
doi:10.1038/nature11377
Received
 
Accepted
 
Published online
 
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 atmosphere12. Soil moisture can influence the development of convective storms through such modifications of low-level atmospheric temperature and humidity13, 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 models4. 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 (50–100kilometres) 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.

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