Ambient Wind Effects on the Initiation and Development of Cumulus Clouds over Mountains

Abstract

The effects of ambient winds on the initiation and development of cumulus clouds over mountains are investigated. A case with the atmosphere initially at rest is compared with a case in which a wind increasing linearly with height is imposed from the ridge line upward to the top of the grid. Preliminary conclusions are that cloud initiation is stimulated in the ambient wind case. In addition, the cloud forms over the down-wind slope rather than directly over the peak and develops in situ. However, the cloud in the linear wind-shear case eventually dissipates, raising questions about the effects of the side and top boundaries on the cloud. Three more cases are integrated to test the influence of the boundary conditions. The results prompt the conclusions that the cloud will be initiated earlier because of the wind effects but will not develop as rapidly as the case with no initial wind, unless the cloud top comes within a few grid intervals of the top. A more realistic wind is then modeled to test its effect on the initiation and development of clouds. In two cases a mountain wave is simulated, which allows motion down to the plain on either side of the mountain ridge. The initiation and development of clouds in these two cases are completely different from those in the other cases. The clouds form 1 km downwind of the ridge, develop rapidly with respect to liquid water content, and move with the wind out of the grid. These mountain-wave cases reveal a unique generating process for the clouds. Essentially, the mountain dams the airflow upstream. Heating of the upwind slope creates a wave in the airflow upwind. Small perturbations in potential temperature and water vapor form at the upwind boundary, propagate into the grid and become superimposed on this wave. They amplify with time and are advected up the mountain slope and into that part of the model's atmosphere which has been moistened by a large circulation cell downwind. Here the warm, moist bubbles initiate clouds. This is one method of generating cloud streets or of creating pulsations in stationary clouds formed in the downwind circulation cell.