Afternoon rain more likely over drier soils

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Abstract

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 atmosphere1,2. Soil moisture can influence the development of convective storms through such modifications of low-level atmospheric temperature and humidity1,3, 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 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 (50–100 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.

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Figure 1: Preference for afternoon precipitation over soil moisture anomalies.
Figure 2: Sensitivities of pre-rain-event soil moisture to mean soil moisture and time of day.
Figure 3: Simulated preference for afternoon precipitation over soil moisture anomalies.

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Acknowledgements

This research was partly funded by the European Union (FP6) WATCH Integrated Project (contract 036946), the UK National Centre for Earth Observation and the European Space Agency STSE Water Cycle Multi-mission Observation Strategy (WACMOS) project (ESRIN/contract number 22086/08/I-EC. We thank A. Beljaars, S. Seneviratne and D. Parker for discussions on this topic. We also thank the CMORPH, TRMM, PERSIANN and GPROF teams for the provision of their precipitation data, the World Climate Research Programme’s Working Group on Coupled Modelling, and the centres who provided modelling data in Figure 3.

Author information

C.M.T. and R.A.M.d.J. conceived the study, C.M.T. performed the analysis and wrote the paper, R.A.M.d.J. and W.A.D. provided expertise on soil moisture data sets, F.G. interpreted the convective responses in models and observations, and P.P.H. devised statistical tests. All authors discussed the results and edited the manuscript.

Correspondence to Christopher M. Taylor.

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Supplementary information

Supplementary Information

This file contains Supplementary Text, Supplementary Tables 1-4, Supplementary Figures 1-12 and additional references. (PDF 1509 kb)

Supplementary Data

This zipped file contains a zipped file containing monthly mask files for ASCAT and AMSR-E soil moisture datasets and a text file which contains information on the content and format of the mask files. (ZIP 1763 kb)

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Taylor, C., de Jeu, R., Guichard, F. et al. Afternoon rain more likely over drier soils. Nature 489, 423–426 (2012) doi:10.1038/nature11377

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