Mesoscale Airflow Over Mountains: Orographic Drag and Upstream Convective Initiation

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Mountains exert a profound influence on the weather and climate. This project will investigate two important aspects of the interaction between mountains and the atmosphere. The first will be to examine how the drag exerted by mountains on an air stream is distributed among different vertical levels in the atmosphere and how that drag feeds back on the larger scale flow. The second effort will examine the circumstances in which low-level blocking can trigger deep convective thunderstorms upstream of a mountain and how the resulting convection evolves in the presence of the mountain and a time-varying large-scale flow. Intellectual Merit: When an air stream encounters a mountain barrier, mountain waves may be set up above and downstream of the barrier and the low-level flow may be blocked and diverted around the flanks of the mountain. These disturbances are too small in scale to be resolved in global weather and climate models, but they produce an important drag on the large-scale flow that must be parameterized. This project will attempt to place such parameterizations on a firmer theoretical footing and also determine the large-scale responses to orographic drag through the analysis of high-resolution numerical simulations. Similar high-resolution simulations will be conducted to examine the conditions under which a moist air stream blocked by a mountain can undergo sufficient low-level destabilization and lifting to trigger deep convection upstream of the mountain itself. The resulting thunderstorms are inherently non-steady, and we will examine their onset and decay in time varying large-scale flows. Such upstream convective triggering is thought to be a significant factor in regulating the distribution of precipitation in the south Asian monsoon. The Principal Investigator also will examine the influence of thermally driven circulations on the generation of these convective events. Broader Impacts: The research and professional development of two graduate students will be supported. Results should help improve the parameterization of low-level gravity wave drag in large-scale models essential for global weather forecasting and much climate research. Global climate models suggest there will be shifts in winds and moisture fluxes in response to global warming, but the resolution of those models is too coarse to allow them to directly simulate changes in the convection. Therefore, understanding the role played by mountains in triggering deep convection, both in the south Asian monsoon and in other regions, will be helpful in the management of water resources as the climate warms.