Pathways relating soil moisture conditions to future summer rainfall within a model of the land-atmosphere system

In this paper, the key pathways and mechanisms through which soil moisture conditions affect future rainfall over the U.S. Midwest are investigated using a regional climate model. A series of numerical experiments are performed to identify these pathways using the drought of 1988 and flood of 1993 as representative events. The results suggest that the soil moisture–rainfall feedback is an important mechanism for hydrologic persistence during the late spring and summer over the midwestern United States. They indicate that the feedback between soil moisture and subsequent rainfall played a significant role in enhancing the persistence of the drought of 1988 and the flood of 1993. It is found that there is a pronounced asymmetry in the sensitivity of simulated rainfall to specified initial soil moisture. The asymmetry acts to favor a stronger soil moisture–rainfall feedback during drought conditions as opposed to flood conditions. Detailed analyses of the simulations indicate that the impact of soil moisture on both the energy and water budgets is crucial in determining the strength of the soil moisture–rainfall feedback. Anomalously high soil moisture tends to 1) increase the flux of high moist static energy air into the planetary boundary layer from the surface via an increase in net surface radiation, 2) reduce the planetary boundary layer height thus increasing the moist static energy per unit mass of air, and 3) reduce the amount of entrained air of low moist static energy from above the planetary boundary layer. Each of these effects are additive and combine to increase the moist static energy per unit mass of air in the planetary boundary layer. This increase results in an increase in the frequency and magnitude of convective rainfall events and a positive feedback between soil moisture and subsequent rainfall