Soil moisture control of large-scale precipitation efficiency over a heterogeneous land surface

Abstract:

Soil moisture heterogeneity can induce mesoscale circulations due to differential heating between dry and wet surfaces, which can, in turn, trigger precipitation. Considerable insight has been gained from previous idealized modeling studies. In this work, we conduct cloud-permitting simulations over a 100 km x 25 km idealized land surface, with the domain split equally between a wet and dry region, each with homogeneous soil moisture. Each simulation is run with fixed soil moisture for 100 days to allow the atmosphere to achieve a quasi-equilibrium state with the given land surface. It is then run for one additional day, allowing the soil moisture to freely vary. We find that soil moisture controls the resulting precipitation through two different mechanisms. The first mechanism is the mesoscale circulation caused by differential heating: all else being equal, as the dry domain gets drier, the mesoscale circulation grows stronger due to the increased heating differential, which promotes increased precipitation over the dry domain. The second mechanism, which has received less attention in previous studies, is the effect of soil moisture on the large-scale precipitation efficiency (the ratio of precipitation to the sum of moisture convergence and surface evaporation over the dry domain), particularly at low soil moisture values: all else being equal, as the dry domain soil moisture approaches zero, the large-scale precipitation efficiency rapidly decreases. The two mechanisms counteract one another, but we find that the second mechanism dominates as the dry domain soil moisture approaches zero, meaning that decreasing soil moisture in the dry domain ultimately decreases precipitation in the dry domain. This work highlights an important mechanism by which synoptic scale soil moisture may regulate soil moisture-precipitation feedbacks at smaller scales, by partially determining the large-scale precipitation efficiency.