Mechanisms of Intermountain Cold Front Evolution

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Perhaps nowhere in the world is the development and evolution of cold fronts more complex than over the Intermountain West of the United States. Upstream mountain ranges, most notably the Sierra Nevada, modify land-falling cold fronts, which in turn influence orographic precipitation processes and rates over downstream mountain ranges such as the Wasatch Mountains. In other events, intense cold fronts develop or intensify downstream of the Sierra Nevada, producing dramatic temperature falls, dust storms, and high winds that can exceed 40 m s-1 and have produced more than $15 mil in property damage. In addition to orographic effects, intense surface heating, deep convective boundary layers, and elevated moist convection are also important, yet knowledge of the influence of these processes on frontal evolution and dynamics remains elusive with considerable debate concerning their combined effects. Using the Intermountain West as a natural laboratory, the Principal Investigator will conduct a comprehensive three year research program to examine the combined effects of surface heating, boundary layer processes, elevated moist convection, and orography on frontal evolution. The key questions to be investigated are (1) how are land-falling fronts structurally and dynamically modified as they traverse a major mountain barrier like the Sierra Nevada, (2) what role do large-scale, orographic, diabatic, and boundary layer processes play in the rapid development of strong cold fronts over the Great Basin, and (3) how does the evolution of Intermountain cold fronts compare and contrast with that found in other mountainous or arid region of the world? The project will involve observational analysis and numerical modeling. High-density surface observations from the MesoWest cooperative networks and the Meteorological Assimilation Data Ingest System (MADIS), radar observations from the NEXRAD radar network, and wind profiler data from California, Nevada, Idaho, and Utah will be used to describe the observed structure and evolution of selected cases of frontal interaction with the Sierra Nevada and rapid frontogenesis over the Great Basin. Real-data simulations by the Weather Research and Forecast (WRF) model will be used to provide high-resolution dynamically consistent datasets for diagnostic analysis, while idealized simulations and sensitivity studies will be conducted to isolate orographic effects or the influence of boundary layer and diabatic processes on frontal evolution. The research will contribute to overall understanding of frontal dynamics and evolution, particularly the effect of orographic, diabatic, and boundary layer processes. Benefits of the research for society at large include improved understanding and prediction of cold fronts and their associated hazardous weather over the Intermountain West, as well as other regions of the world. These improvements should help reduce societal vulnerability to hazardous weather in Nevada, Utah, Arizona, Idaho, and Colorado, the five fastest growing states in the nation. Other broader impacts include the integration of research results into undergraduate and graduate courses at the University of Utah, the mentoring of two graduate students, and the development of on-line instructional modules for educating students, meteorologists, and other individuals interested in the atmospheric sciences.