Afternoon rain more likely over drier soils

Abstract

(2012), Afternoon rain more likely over drier soils, Nature, 489(7416), 423-426 (10.1038/nature11377) Land surface properties influence the partition of radiative energy between latent and sensible heat fluxes during daytime. A climatically important process during dry periods is the impact of soil water deficit on limiting evapotranspiration, leading to increased surface heating of the lower atmosphere 1-2 . Soil moisture can influence the development of convective storms through modification of low level atmospheric temperature and humidity 1,3 , which in turn feeds back on soil moisture. There is considerable uncertainty in how soil moisture affects convective storms across the world, due to both a lack of observational evidence and uncertainty in large-scale models 4 . Here we present the first global-scale observational analysis of the coupling between soil moisture and precipitation. This shows that across all six continents studied, afternoon rain falls preferentially over soils which 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. We find no evidence in our analysis of a positive feedback i.e. preference for rain over wetter soils, at the 50-100 km scale studied. In contrast, our analysis of six state-of-the-art global weather and climate models demonstrates the dominance of a positive feedback of soil moisture on simulated precipitation. These results suggest a fundamental weakness in the sensitivity of convection to the land surface in large-scale models, which may contribute to excessive simulated droughts.Soil moisture influences precipitation on a range of time and space scales 5 . In drought-affected continental regions, weak evapo-transpiration leads to reduced atmospheric moisture content over a period of days, potentially suppressing subsequent precipitation 6 . When soil moisture anomalies are extensive, surface-induced perturbations to the atmospheric heat budget may modify synoptic scale circulations 2 , in turn affecting moisture advection from the oceans 7 . On smaller scales, the development of convective clouds and precipitation can be influenced by local surface fluxes over the course of the day 1,3 . Theoretical considerations [8][9] suggest that, in an undisturbed atmosphere, the likelihood and sign of a surface feedback will be determined by the atmospheric profiles of temperature and humidity. Thus one might expect regional variations in the strength and sign of convective sensitivity to soil moisture [10][11] . Mesoscale soil moisture variability can also influence the feedback via the development of daytime circulations 12 , which provide additional convergence to trigger convection [13][14] .There have been a number of studies examining the impact of the land surface on observed rainfall in different regions of the world. Analyses in Illinois 15 and West Africa 16 have indicated positive correlations between ...

Seasonal and Diurnal Variations in Rainfall Characteristics over the Tropical Asian Monsoon Region Using TRMM-PR Data

“…A satellite observational study of soil moisture and precipitation in some regions showed that a drier surface induces stronger precipitation particularly over the relatively drier land-surface conditions (Taylor et al 2012). However, additional studies are needed to determine if their results are in accordance with those in the tropical Asian monsoon regions where the land-surface conditions are relatively wet.…”

Effects of explicit convection on global land‐atmosphere coupling in the superparameterized CAM

“…Our results here indicate that the convective schemes used in GCMs might also be part of the reason. Berg et al [2013] also attributed the difference of TFS results and those in other studies showing negative feedbacks [e.g., Carleton et al, 2008;Wang et al, 2009;Taylor et al, 2011Taylor et al, , 2012 to the coupling indices framework and the spatial scale difference. The positive feedback as shown through TFS in our coarse spatial resolution simulations ($2.88 here) is not necessarily contradictory with those studies that emphasize scales of coupling in the 10-100 km range accounting for surface heterogeneity, which are not included in our framework.…”

“…Counterintuitive effects of explicit entrainment can also introduce exotic behaviors [Hohenegger et al, 2009]. Thus, both positive and negative soil moisture feedback on precipitation can occur as shown in many studies [Hohenegger et al, 2009;Findell et al, 2011;Taylor et al, 2011Taylor et al, , 2012Gentine et al, 2013c;Guillod et al, 2015].…”

“…Despite the large diversity of coupling signals in different global circulation models (GCMs), they tend to agree on several regions of strong coupling signals, initially identified in the Global Land-Atmosphere Coupling Experiment (GLACE) [Koster et al, 2004[Koster et al, , 2006, which has inspired a series of global land-atmosphere coupling studies in the past decade. These studies range in spatial and temporal scales and use a combination of models, reanalysis or satellite products [e.g., Dirmeyer, 2006Dirmeyer, , 2011Dirmeyer et al 2012;Findell et al, 2011;Taylor et al, 2012], with associated trade-offs. For instance, observational-based analyses are limited by an inability to explore the relationship of causality and short temporal and sparse spatial coverage, while reanalysis and models are impaired by their PBL and cloud parameterization schemes [Seneviratne et al, 2010;Ferguson et al, 2012;Guillod et al, 2015], though this can be mitigated to some degree by assimilating rainfall data [Findell et al, 2011].…”

A simplified model of precipitation enhancement over a heterogeneous surface

“…Several observational studies (e.g., Taylor et al, 2012) showed that in the Sahel region, thunderstorms occur preferably over regions drier than their surroundings. In other areas of the world the synoptic forcing is usually so strong that a robust relationship of causality between soil moisture and precipitation cannot be found (Tuttle and Salvucci, 2017).…”