Orographic Precipitation and Airmass Transformation

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An improved understanding of, and the ability to model, orographic precipitation and air mass drying would contribute to solving a number of earth science problems involving: flash flood prediction, water resources, mountain glaciers, ice core interpretation, erosion, rain-shadow deserts and inter-ocean water transport. Most current models either oversimplify or fail to resolve key moist orographic flow processes. In this research, the Principal Investigator will test existing mesoscale models and develop new theories combining airflow dynamics and microphysics.The highest priority is to compare existing model runs with data from the Mesoscale Alpine Programme (MAP). This is the only detailed data set using new technologies that allows moist air flow over a major mountain range to be diagnosed. Four MAP cases have been identified with differing characteristics. For each case, output from three numerical models will be compared with conventional and aircraft data. If the agreement is satisfactory, detailed interpretation of the output will follow. Several theoretical questions will be addressed in the analysis: the relative role of moist ascent versus flow splitting and blocking; triggered convection; the accuracy of parcel trajectories; the effect of model resolution; the testing of the "threshold" theory of precipitation efficiency; sensitivity to microphysical algorithms; and the conservation of equivalent potential temperature in mixing flows. The second phase of the research will include data and publications from three contemporary medium scale experiments managed by other groups. The results from these projects will be compared to put the MAP results into a broader context and allow cautious generalizations to be developed.In the final phase of the project, applications of relevant orographic precipitation models to other geoscience disciplines (see items above) will be undertaken, broadening the impact of the work. A scholarly survey of the field will be attempted. Considerable influence on educational programs should be seen from this research activity. Undergraduate and graduate students and post-docs will be involved in the field work and data analysis.