Structure and Evolution of Intermountain Cyclones

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Weather systems and their accompanying precipitation are often profoundly affected by complex terrain. Knowledge of the influence of orography is somewhat limited, partially because observations in these regions are difficult to obtain. The Principal Investigator will investigate the processes contributing to the development and evolution of intermountain cyclones and associated winter storms. Intermountain cyclones typically develop or intensify to the lee of the Sierra Nevada, a region of frequent cyclogenesis, and can produce damaging winds and snowfall over Nevada, Utah, and adjoining states. They are also precursors for Colorado cyclogenesis, and ultimately affect the evolution of fronts and precipitation to the lee of the Rockies.The objectives of the research are to determine (1) how synoptic scale, orographic, and diabatic processes influence the development and evolution of intermountain cyclones, (2) how major mountain barriers like the Sierra Nevada modify frontal structure and dynamics, and (3) how moisture transport and downstream orographic precipitation are impacted by air mass transformation over upstream mountain barriers.The project will involve observational analysis and numerical modeling. High-density surface observations from the MesoWest cooperative networks, radar observations from the WSR 88D radar network, and wind profiler data from California, Nevada, Idaho, and Utah will be used to describe the observed mesoscale structure and evolution of selected intermountain cyclones. Idealized and real-data simulations by the MM5 and Weather Research and Forecast (WRF) models will be used to examine large-scale, orographic, and diabatic contributions to cyclogenesis, determine how the Sierra Nevada and Great Basin ranges alter the structure and dynamics of fronts, and quantify water vapor fluxes, sources, and sinks during airflow over major topographic barriers.The project represents the first comprehensive study of intermountain cyclone evolution to employ high-density surface observations and high-resolution numerical simulations. The relevance of previously developed orographically modified cyclogenesis theories will be evaluated in a region that is markedly different from the lee of the Alps and Rockies, where most prior studies have been focused. Also for the first time, the impact of the Sierra Nevada on downstream frontal evolution and moisture transport will be described. Broader impacts include improved weather and climate prediction over the intermountain west, integration of research results into undergraduate and graduate education, and the mentoring of graduate students.