PBL mesoscale perturbations over the western United States during the summer months

The main objective of this research is to identify wind and geopotentialheight perturbations over the westem United States. The perturbations are fairly small and theorized as being generated over the plateau region in the western U.S. by low-Ievel circulations. The circulations exist on the plateau due to the temperature contrasts between the elevated terrain of the plateau and lower terrain adjacent to the plateau. Sixty-six cases included in the data set are separated into four categories; well-defined and not well-defined perturbations at 00 UTC and 12 UTC. Mean conditions of lapse rates, temperatures and geopotential heights are computed and analyzed for each category to propose possible locations of and conditions most likely to produce perturbations.

The possible effect of perturbations on moist-convection coverage is also analyzed. Surface data from various observation stations in the westem U.S. has been collected for the sixty-six cases and employed to compute an average coverage of moist-convection for the included states. Comparisons are made in the magnitude of coverage values between states and in coverage values between categories for each state.

Finally, the Rapid City Flood is examined to detect possible influence of perturbations on conditions leading to the flood event. Rainfall of up to 300 mm was observed during a four-hour period from 00 UTC to 04 UTC on 10 Jime 1972 along the eastem slopes of the Black Hills in southwestem South Dakota. Geopotential-height data is analyzed for three days previous to the flood event and the day of the flood event.