A numerical study of moist stratified flows over isolated topography

Abstract

In order to improve our understanding of orographic precipitation processes, a study of a 3-D flow over and around an isolated, Gaussian shaped mountain, with circular horizontal section, has been tackled. The effect of moisture is analysed systematically, by estimating the role of different parameters in the case of a simple stratified flow. The presence of humidity in the atmosphere can effectively change the flow regime with respect to the dry case, with the same wind and temperature upstream profiles. The energy exchange due to the release of latent heat tends, in general, to increase “flow over” an obstacle in detriment of “flow around”. This result can be interpreted, to a first approximation, as a change in the local value of the Brunt-Vaisala frequency. However, as regards the interpretation of results, questions arise due to the irregular distribution of saturated portions, particularly downstream, and due to the fact that the volume of saturated air is a function itself of the flow regime near the obstacle. Some non-linear effects, such as the formation of upstream blocking and lee vortices, are confined to higher mountains in comparison with the dry case. The critical adimensional height for wave breaking is also moved by condensation to higher mountains, since moisture tends to decrease the wave amplitude. An increase of the adimensional height up to a certain value increases the volume of saturated air and the amount of precipitation; however, the opposite effect occurs for very high mountains. In general, the drag is strongly reduced in comparison with the dry case.