Dynamics of Heavy Precipitation over Mesoscale Mountains

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Better prediction of flooding in mountainous terrain has been identified as a high priority research area under the interagency U.S. Weather Research Program. Under this award, the Principal Investigator (PI) will perform numerical and observational studies aimed at improving scientific understanding of mesoscale orographic rainfall and flooding. The data set to be used is that collected during the international Mesoscale Alpine Programme (MAP). The following questions will be addressed:1. What are the effects of atmospheric stability and mountain geometry on the formation and propagation of orographically induced mesoscale convective systems?2. What are the basic ingredients for producing orographic heavy rainfall or flooding?3. How do upper-tropospheric high potential vorticity (PV) air masses or troughs interact with deep warm desert air from North Africa to develop heavy orographic rainfall or flooding in the Alps?To address the first question, the PI hypothesizes that propagation of convective systems is controlled by the upstream Froude number and the vertical moisture flux is enhanced by the Alpine concave geometry in the Lago Maggiore area of Italy. The PI will then perform a series of numerical sensitivity experiments involving different atmospheric instabilities and mountain geometry. The research will analyze radar, wind profiler, and satellite data to verify numerical model results and increase understanding of the underlying dynamics.With respect to the second question, it is hypothesized that the basic ingredients for producing orographic heavy rainfall or flooding are: high precipitation efficiency, the presence of low-level jet, a steep mountain, favorable mountain geometry, strong vertical motion induced by the synoptic system, and slow movement of the orographically induced convective system. To test this hypothesis, the PI will produce synoptic and mesoscale analyses of MAP cases as well as historical Alpine and US flooding events to explore the relative importance and relationship between these ingredients.To address the third question, the PI hypothesizes that an upper-tropospheric high PV air mass or trough helps transport deep moist air from the ocean, enhances the southerly low-level jet, and induces upper-level divergence in phase with the orographically forced upward motion, which is favorable for the development of deep convection. Numerical simulation experiments and data analysis will be conducted to explore this hypothesis.Successful completion of this research will provide insights into the important factors that control orographic precipitation. This potentially could lead to improved flash flood and other precipitation forecasts in mountainous terrain.