A Nonmonotonic Precipitation Response to Changes in Soil Moisture in the Presence of Vegetation

摘要

In many parts of the world, humans rely on afternoon rainfall for their water supply. However, it is not fully understood how land surface properties influence afternoon precipitation. In fact, disagreement remains regarding the relative prevalence of "wet-soil advantage" regimes, in which wet soils receive more precipitation than do dry soils, and "dry-soil advantage" regimes, in which the opposite occurs. Recent studies have proposed that the permanent wilting point (PWP) soil moisture threshold influences the location and organization of convective clouds. Motivated by this work, we investigate how changes in soil moisture relative to the PWP affect the timing and amount of surface rainfall, as well as how this response depends on the presence or absence of vegetation. This investigation is carried out by conducting several series of high-resolution, idealized numerical experiments using a fully coupled, interactive soil-vegetation-atmosphere modeling system. From these experiments, a new soil moisture-precipitation relationship emerges: in the presence of vegetation, simulations with moderately dry soils, whose initial liquid water content slightly exceeds the PWP, generate significantly less surface precipitation than do those with the driest or wettest soils. This result suggests that simulated wet-soil advantage and dry-soil advantage regimes may not necessarily be mutually exclusive, insofar as extremely wet and extremely dry soils can both exhibit an advantage over moderately dry soils. This nonmonotonic soil moisture-precipitation relationship is found to result from the PWP's modulation of transpiration of water vapor by plants. In the absence of vegetation, a wet-soil advantage occurs instead in these idealized simulations. Significance StatementThis modeling study suggests a new type of rainfall response to soil moisture in which intermediate-moisture soils receive less rainfall than do the driest or wettest soils. Previous studies have suggested that afternoon rainfall, which impacts populations across the globe, consistently increases or decreases with increasing soil moisture; our results suggest that this relationship can instead be nonmonotonic under certain conditions. This nonmonotonic response only occurs in the presence of vegetation, suggesting that plants play a key role in determining the soil moisture dependence of afternoon precipitation. In examining the mechanisms behind these trends, we shed light on interactions between soil, vegetation, the boundary layer, and clouds that coarse-resolution models may fail to capture.