Modulation of Airflow Within Complex Terrain by Moist Processes

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This project seeks to better understand and quantify the physical processes involved in the newly proposed concept of wet drainage flow in major river valleys under sustained rainfall. The suggested mechanism behind this phenomenon is air subsiding as a result of cooling from melting and evaporation of precipitation particles. The subsiding air concentrates in river valleys, which act as air drainage. The resulting down-valley flow can develop underneath an opposite-directed flow of moist air that is lifted onto the topographic barrier in which precipitation is formed.The analyses to be carried out during this project are focused on (a) in-depth evaluation of ground-based and airborne Doppler radar and thermodynamic data collected in major river valleys on the south side of the European Alps during the Mesoscale Alpine Program (MAP), and (b) numerical simulation of the wet drainage flow phenomenon and numerical experimentation to delineate the conditions under which this occurs and evaluate a potential feedback to orographic precipitation formation mechanisms.This research is focused on the effect of moist processes on the flow of air within valleys, complementing substantial work performed on airflow in valleys under dry (clear sky) conditions. The proposed analyses will help to better define the new concept of wet drainage flow and assess the relevance of this flow phenomenon to dynamic and microphysical processes ranging from the local (valley) scale to the topographic barrier and larger scales. The research is tightly knitted to MAP, which is focused on exploring the mesoscale effects of complex topography on precipitation formation and flooding. MAP is conceived as an integrated program of basic research with direct applications in the realm of numerical mesoscale weather prediction. Orographic precipitation mechanisms in the Alpine region, as well as many other major mountain ranges around the world, are an important element of flooding, landslide and debris flow hazards.