On the dynamical coupling of large-scale spatial patterns of rainfall and soil moisture

Abstract

It is well-recognized, although not really well understood, the impact that land-atmosphere interaction has on the large variability in the spatial structure of continental climate. This paper focuses on a specific dynamical coupling between the soil moisture and atmospheric processes, such as in the presence of a growing moist baroclinic wave. Starting from the inviscid semi-geostrophic equations for the evolution of a meridionally independent baroclinic disturbance in the moist atmosphere, a discretized fluid-dynamical model is formulated. A quite general scheme for the representation of the mass and energy exchanges between the soil and the atmospheric boundary layer is also included, and analytical solutions to the non-linear instability problem are derived. The analysis of these solutions show how a heterogeneous soil moisture modifies the dynamics of the baroclinic wave up to the formation of atmospheric fronts, and how a rough estimate of the associated rainfall potential is affected as well. In particular, when the baroclinic wave develops in such a way that the warm precipitating anomaly develops on top of wet soil, with dry soil on the cold anomaly side, both the occurrence of the frontal collapse and the rainfall potential are favored. A positive, spatially structure, feedback of soil moisture on its own precipitation forcing is then identified.