Date of Award

Spring 5-1-2006

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geography, Geology, and Anthropology

First Advisor

Gregory D. Bierly

Second Advisor

John E. Oliver

Third Advisor

Ryan Jensen

Abstract

North Pacific atmospheric blocking is an upper tropospheric phenomenon that occurs modally in the northwestern quadrant of the North American cyclogenetic region primarily during the winter season. In this study, Colorado lee cyclones, which develop to the east of the Rocky Mountains, are investigated to see if differing strengths of North Pacific atmospheric blocking influences cyclone frequency, intensity and tracks during the study period 1965-1995. Three blocking categories, strong blocking, anti-blocking and weak-index blocking were created based on indices provided by previously published research. Additionally, an analysis of upper tropospheric circulation during strong blocking and anti-blocking winter seasons was made to determine if the mean position of 500 mb geopotential heights influence Colorado lee cyclone intensity. During the period under investigation, a total of ninety-four Colorado lee cyclones occurred during the months of December, January and February. A Chi square statistical test indicates that there is no significant increase in Colorado lee cyclone frequency during strong blocking, anti-blocking and weak-index blocking winter seasons. Mean central pressure values were analyzed with an ANOV A statistical test and the results indicate that there is no significant difference in Colorado lee cyclone intensity during the different blocking winter seasons. Mean values were derived for latitude, longitude and azimuth for cyclones between cyclogenesis and forty-eight hours following cyclogenesis in six-hour temporal periods. An ANOV A statistical test indicates that there are significant differences in Colorado lee cyclone paths, once they depart the cyclogenesis region, during the different blocking winter seasons. An inspection of mean 500 mb geopotential heights reanalysis graphics, during strong blocking and anti-blocking winter seasons, indicates that the mean position of the western ridge, coupled with above normal geopotential height rises over the Bering Strait region during strong blocking winter seasons, and below normal mean 500 mb geopotential heights over the Bering Strait region during anti-blocking winter seasons, likely contributes to the significant difference in cyclone tracks.

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