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State University of New York - Albany: "Weather forecasting problems related to intermountain cyclogenesis"

Final Report

Intermountain cyclogenesis has been studied much less frequently than lee cyclogenesis. In particular, frontogenetical and precipitation processes, as a function of the prevailing synoptic scale flow regime in a region of complex terrain, have not been investigated in sufficient depth to generate operationally-useful guidance to forecasters. The focus of this project was on cool season cyclogenesis events over the intermountain region of the western United States. The primary emphasis was on documenting the complete life cycle of individual weather systems that move in from the Pacific Ocean and work their way eastward across the Continental Divide.

An analysis was performed of the Nevada cyclogenesis event of March 10-12, 1991 in which the models performed poorly in predicting the locations of both the 500 mb and surface features during the event. In order to better understand the physical processes, various diagnostic tools were used to examine the structure of the vertical velocity field associated with cyclone development. The primary diagnostic tool was the calculation of quasi-geostrophic (QG) Q vector convergence and divergence (QCD) at all of the mandatory levels from objectively analyzed fields. This analysis found that two separate but well-defined areas of upward QCD forcing existed. The QCD forcing at the mandatory levels was then integrated using relaxation techniques to yield an actual QG omega. This was done to diagnose the QG vertical velocity during the event and to study the relatively common forecast problem in which the implied direction of the vertical motion from QCD forcing varies with height through the troposphere. The result was a three-dimensional picture of the QG vertical velocity.

Finally, a kinematic omega was computed from the same mandatory level data set (with and without topography), and NGM initial omega and QG forcing fields were also analyzed. The result is a more thorough understanding of the physical processes which contributed to the cyclogenesis, as well as an enhanced understanding of the operational application of QCD diagnostics.