ON THE PHYSICS OF DROUGHTS .2. ANALYSIS AND SIMULATION OF THE INTERACTION OF ATMOSPHERIC AND HYDROLOGIC PROCESSES DURING DROUGHTS

In this study a simple general circulation model (GCM) is developed to investigate the interaction of atmospheric and hydrologic processes during droughts. Only the vertically and latitudinally averaged mean temperature and mean water vapor content over the strip of Earth between 30 and 50-degrees-N latitudes are considered as atmospheric state variables; only two hydrologic state variables, the water storage (through the water balance equation) and ground temperature, are considered for describing the behavior of the hydrologic system. The ocean temperature is specified as a function of season only. The coupling between the atmosphere and the Earth's surface occurs through exchanges of thermal energy (radiation, sensible and latent heat fluxes) and of water (evapotranspiration and rain), which are parameterized functions of the four state variables of the atmospheric-hydrologic interactive system. Using this GCM, the mechanism leading to drought conditions is investigated. It is shown to be a non-linear positive feedback mechanism in the sense that it feeds on itself. Imposing a temperature wave over a geographical region (western USA in this study) for a short period of the order of few months induces geophysical conditions in that region which tend to reduce the hydrologic water storage for a period of time of the order of years (called 'drought growth period'). These conditions consist initially of increased cloudiness over the region, which reduces the net radiative flux of energy available at the surface. This in turn reduces the evapotranspiration which is needed to achieve condensation conditions in the atmosphere. Consequently, a reduction in rainfall occurs. After the end of the drought growth period, the hydrologic system recovers the water storage mainly owing to decreased evapotranspiration (related to reduced water storage) while rainfall resumes its normal levels. The time duration necessary to return to the climatological average conditions of water storage (called 'drought recovery period') is proportional to the peak drought severity during the growth period.