The synoptic climatology of the Red River Region

Abstract

This thesis develops a synoptic climatology for the Red River Region (RRR) of southern Manitoba, North Dakota, South Dakota, and Minnesota. The Sums-of-Squares technique is used to objectively classify each of the days of 1946-84 according to the type of pressure pattern observed 1) at the 500 mb level over much of North America west of the Great Lakes, 2) at the 500 mb level over a smaller area of west-central North America, and 3 ) at mean-sea level (MSL) over the same small area. Eighteen large-area 500 mb, 20 small-area 500 mb, and 29 MSL patterns are used to represent the most common synoptic situations. The characteristics of these representative patterns are described, and their average monthly frequencies used to determine natural 500 mb and MSL pressure pattern seasons. Four 500 mb and four MSL seasons are identified. Winter and summer 500 mb regimes are found to be most dissimilar, with winter dominated by many large troughs pulling northern air into the central part of North America, including the RRR, and summer dominated by zonal and ridge-associated patterns. The winter MSL regime shows more highs north of the RRR than does the summer, and fall exhibits more lows toward the north. Spring and fall are transitional months at both pressure levels. Results are discussed in relation to the seasonal migration of the polar front. The average weather characteristics associated with the synoptic types at Winnipeg, Manitoba, and Fargo, North Dakota, are calculated and compared. Temperature conditions are assessed by calculating mean departures from mean daily temperatures and precipitation characteristics are evaluated by calculating precipitation efficiencies, or amounts of total precipitation produced relative to frequency of occurrence. Temperatures in the RRR are found to be strongly correrated to wind direction, and precipitation potential related to apparent vertical motions in the atmosphere. Finally, discriminant analysis and the MSL classification is used to identify the surface pressure patterns whose frequencies in the six months immediately preceding April are significantly different in years with low and high Red River flows, in April, at Emerson, Manitoba. The weather normally produced by the patterns selected as good predictor variables is discussed in relation to the hydrometeorological conditions known to be conducive to extreme Red River flows.