3-DIMENSIONAL NUMERICAL STUDY OF SHALLOW CONVECTIVE CLOUDS AND PRECIPITATION INDUCED BY LAND-SURFACE FORCING

A state-of-the-art mesoscale atmospheric model was used to investigate the three-dimensional structure and evolution of shallow convective c louds and precipitation in heterogeneous and homogeneous domains. In g eneral, the spatial distribution of clouds and precipitation is strong ly affected by the landscape structure. When the domain is homogeneous , they appear to be randomly distributed. However, when the landscape structure triggers the formation of mesoscale circulations, they conce ntrate in the originally dry part of the domain, creating a negative f eedback which tends to eliminate the landscape discontinuities, and sp atially homogenize the land water content. The land surface wetness he terogeneity of the domain and the total amount of water vapor present in the atmosphere (locally evapotranspired and/or advected) affect the precipitation regime. In general, the upward motion of mesoscale circ ulations generated by landscape heterogeneities is stronger than therm al cells induced by turbulence. Furthermore, their ability to transpor t moist, warm air to higher elevations increases the amount of water t hat can be condensed and precipitated. The evolution of shallow convec tive clouds and precipitation consists of a ''build-up phase'' during which turbulence is predominant and responsible for the moistening of the atmosphere. In heterogeneous domains, it is also responsible for t he creation of horizontal pressure gradients leading to the generation of mesoscale circulations. This phase occurs during the morning hours . From about 1200 until 1600 LST, clouds develop and most of the preci pitation is produced. This is the ''active phase.'' After 1600 LST, th e horizontal thermal and pressure gradients, which fed the energy nece ssary to create and sustain the mesoscale circulations, gradually disa ppear. This is the ''dissipation phase.'' The differences and similari ties obtained between three-dimensional and two-dimensional simulation s were also studied. These simulations indicate that, unless the lands cape presents a clear two-dimensional structure, the use of such a two -dimensional model is not appropriate to simulate this type of clouds and precipitation. Conversely, two-dimensional simulations can be conf idently used, provided that the simulated domain presents a two-dimens ional heterogeneity.