ANALYSIS OF FEEDBACK MECHANISMS IN LAND-ATMOSPHERE INTERACTION

The initiation of a hydrologic drought may depend on large-scale or te leconnective causes; however, local positive feedbacks in the land-atm osphere system are believed to contribute to the observed persistence and intensification of droughts. In this study a basic linearization t echnique is combined with a nonlinear stochastic model of land-atmosph ere interaction to analyze and, more importantly, quantify feedback me chanisms that arise in the coupled water and energy balances at the la nd surface. The model describes land-atmosphere interaction by four co upled stochastic ordinary differential equations in soil moisture, soi l temperature, mixed-layer humidity, and mixed-layer potential tempera ture. The solution is a physically consistent joint probability distri bution. The steady and perturbation-induced parts of the model equatio ns are decomposed into the dependence of each component physical proce ss upon each model state. Because of the negative correlation between soil moisture and soil temperature, the physical mechanisms that serve to restore each state individually (largely soil moisture control of evaporation and temperature dependence of saturation specific humidity ) act as significant anomaly-reinforcing mechanisms for the other stat e.