Soil Moisture–Atmosphere Interaction in a Moist Semigeostrophic Model of Baroclinic Instability

Abstract

Attention is focused on the dynamical coupling between the soil moisture and atmospheric processes, such as in the presence of a growing moist baroclinic wave. A simplified scheme, representing the mass and energy exchanges between the soil and the atmospheric boundary layer, is incorporated into the inviscid semigeostrophic equations for the evolution of a meridionally independent baroclinic disturbance in the moist atmosphere. The model is formulated in terms of coupled, nonlinear, ordinary differential equations, allowing for simple and extensive numerical simulation. Using such an approach, the question is addressed to what extent and through which mechanisms a heterogeneous soil moisture distribution affects the evolution of the baroclinic waves and in particular the estimate of the potential precipitation. The interaction between land and atmosphere is found to have both a local effect and a large-scale effect. The local effect consists of modifying the vertical lapse rates of dry and equivalent potential temperatures in the troposphere. The large-scale effect is realized through the global dynamics of the baroclinic wave and in particular through the rates of mass and energy advection and the strength of the ageostrophic frontal circulation, which depend strongly themselves on the temperature lapse rates. Both mechanisms are found to be substantially dependent on both the local moisture content of the soil and on the large-scale moisture differences, highlighting their feedback on precipitation.