Synoptic influence on winter temperature and precipitation in western Montana.

摘要

In this dissertation, synoptic climatology is used to investigate the spatial variability of winter temperature and precipitation over western Montana. Statistical description of winter precipitation in the region shows significant variability in both spatial and temporal characteristics. Using rotated principal component analysis (RCPA) four distinct regions, that are physically reasonable, reveal the effect of the dominant airmasses and topography over the region. These regions are the northwestern part which show mainly the influence of northwesterly flow from the Pacific and of the topography of the rocky mountains; the mostly dry eastern part influenced mainly by the rain-shadow zone of the Rockies, the area between the rain-shadow zone and the moist western area where the precipitation pattern can be explained by storms which occur from the edge of fronts that move from Alberta down into the high plains; and the southern part region which has very high elevations but and for the most part characterized by the effect of the mountain ranges in northern Wyoming on the southerly flow from the gulf.;From a 4 x 5 array of self organizing maps of daily winter 700mb geopotential height, patterns to winter precipitation show a strong influence of the westerly airmasses as main contributor of precipitation. The spatial analysis of precipitation for each of the synoptic types also shows the effect of topography. This makes the northwestern part of the region the major source of snowpack for the region. Using the results from the circulation patterns, the influence of synoptic types on winter temperature and precipitation are discussed by analyzing the variability of lapse rate and orographic rate with synoptic types. The results show that lapse rates and orographic rates vary significantly with synoptic types over western Montana.;Estimates of temperature and precipitation using MTCLIM shows that synoptic types have significant influence on our ability to predict these elements in mountain regions. More precise values of lapse rates and orographic rates would predict temperature and precipitation better than long term averages of lapse rates and generalized isohyetal values.