A statistical analysis of extreme chinooks in Montana as they relate to large scale circulation changes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w6634595m

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  • A statistical analysis of extreme chinooks in Montana as they relate to large scale atmospheric circulation changes is the subject of this thesis. A mean chinook situation was identified which indicated that a critical pattern of the surface flow always contained a Polar or Arctic airmass which overspread the prairie areas of Eastern Montana to the Rocky Mountain front just prior to the onset of the chinook. On Chinook-Day this surface flow changes to directions from the south-southeast to south-southwest. This surface pattern was accompanied by a strong 500 MB flow from the northwest on Chinook- Day. The first phase of the statistical analysis used daily changes in the maximum and minimum temperatures to identify 36 dates of intense chinooks. Selected examples from these 36 chinooks were then analyzed to identify three special cases that show a variation in the penetration of the chinook eastward from the Rocky Mountain front in Montana. This study identified as extreme chinooks those events in which the change in maximum daily temperatures were equal to or greater than two standard deviations from the mean daily temperature change for the day of the winter period, November through March, on which the chinook occurred. The mean temperature changes were calculated for 28 stations with reliable data for the period of time from 1895 to 1976. Thirty six intense (extreme) chinooks were identified. The synoptics of each were combined by the superposed epoch technique to produce a "mean chinook". Synoptic data for Chinook-Day and a Five Day Lead period (Day -5 to Day -1) were interpreted from the mean sea-level and 500 mb height charts of the specific dates involved. These data were collected by the use of a 5° diamond grid covering the area extending from 10° to 70° North Latitude and 165° East to 85° West Longitude. Analysis of the 36 intense chinook events revealed a three part Montana longitudinal zonation termed Mountain, Midland and Prairie. The chinook of November 17, 1959, which occurred at all 28 stations in the Prairie, Midland, and Mountain Zones was classified as an Extreme chinook. The chinook of March 25, 1964, common to all but three stations in the Mountain and Midland Zones was termed a Moderate chinook. The chinook of January 11, 1949, a Weak chinook, represented those events common to only the Mountain Zone. The Extreme chinook was characterized by a high index circulation and its characteristic zonal flow. The zonal flow at the surface was from the south-southeast, while at 500 mb the flow was from he northwest. The Weak chinook was illustrative of a low index circulation. The meridional flow of this chinook, both at the surface and 500 mb, restricted the chinook to the immediate leeward side of the Rocky Mountains in Montana. The Moderate chinook was characterized by a moderated high index circulation. The zonal flow returned to the direction characteristic of the Extreme chinook. The stratification technique and the statistical analysis used in this study were valid since the Extreme and Moderate chinooks resemble the synoptics of the mean of the 36 intense chinooks. The Weak chinook, with its very small frequency of occurrences, indicated the exception rather than the rule for extreme chinooks in Montana.
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  • description.provenance : Approved for entry into archive by Deborah Campbell(deborah.campbell@oregonstate.edu) on 2013-10-01T20:49:04Z (GMT) No. of bitstreams: 1 GrantRolandLester1980.pdf: 3405375 bytes, checksum: 186d9f4856f07bf30e6b0c30babd96b6 (MD5)
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