Journal Article

Afternoon rain more likely over drier soils

Guichard, Françoise, Dorigo, Wouter A., Taylor, Christopher M., Harris, Phil P., de Jeu, Richard A. M.

2012

Overview

Analysis of observations on six continents reveals a global preference for afternoon rain to fall on locally drier soils—contrary to the predictions of large-scale climate models, and suggesting that such models may exaggerate the occurrence of droughts. Rain in the afternoon Soil moisture is known to influence precipitation across a range of scales in time and space, and most models suggest that wetter soils promote higher atmospheric moisture content and favour the local development of storms. But this analysis of global precipitation data from a combination of weather satellites shows that — especially in semi-arid regions — afternoon precipitation is more likely over dry soil than over wet soil. The findings suggest that current climate models may be missing fundamental processes regulating convection and land–atmosphere interactions. 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 1 , 2 . Soil moisture can influence the development of convective storms through such modifications of low-level atmospheric temperature and humidity 1 , 3 , 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 4 . 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/106 / Arid zones / Atmosphere - chemistry / Atmospheric temperature / Austria / Climate / Climate models / Computer simulation / Desert Climate / Desiccation / Driers / Drought / Droughts / Drying / Earth, ocean, space / Ecosystem / Evapotranspiration / Exact sciences and technology / External geophysics / Feedback / Fluxes / Geography / Global weather / Hot Temperature / Humanities and Social Sciences / Humidity / letter / Meteorology / Models, Theoretical / Moisture content / multidisciplinary / Observations / Precipitation / Precipitation (Meteorology) / Rain / Scale models / Science / Science (multidisciplinary) / Semiarid lands / Sensible heat / Soil - chemistry / Soil moisture / Soil water / Soils / Time Factors / Uncertainty / Vegetation cover / Water - analysis / Water deficit / Water in the atmosphere (humidity, clouds, evaporation, precipitation)