Relationships between snow cover and atmospheric circulation are difficult to isolate because of the complex nature of their interaction. While the insulative properties of snow cover greatly influence energy exchange with the atmosphere, single cyclonic events can alter snow cover patterns radically. Passive microwave derived satellite observations of snow cover show potential to provide synoptically sensitive information because of all-weather imaging capabilities and rapid scene revisit time. In this study, we seek to identify the dominant patterns of clustering in snow water equivalent (SWE) imagery using the Getis statistic, a local indicator of spatial association. The SWE data were derived from five day averaged Special Sensor Microwave/Imager (SSM/I) brightness temperatures using the Canadian Atmospheric Environment Service dual channel algorithm. The analyzed data span one winter season (December, January, and February 1988/89) and are limited to a ground validated North American Prairie scene.

The Getis statistic (Gi*) provides a measure of the spatial dependence of each pixel to the surrounding pixels, and also indicates the relative magnitudes of the digital numbers in a specified pixel neighbourhood. Variable window size in the computation of Gi* allows for spatial association to be computed at a range of distances from the center pixel. National Meteorological Center (NMC) gridded atmospheric data (500 mb geopotential height; 700 mb temperature) were incorporated into the study to investigate the notion that the spatial orientation of the Getis statistic clusters provide information on the dominant direction of movement of synoptic systems responsible for snow deposition in the region. Results show that the snow cover regime for winter, 1988/89 is characterized by persistent snow cover composed of numerous non-contiguous spatial clusters. The direction of atmospheric airflow as expressed by the 500 mb geopotential height field corresponds strongly to the orientation of surface snow cover clusters with no time lag. The 700 mb temperature field is also a controlling influence on the snow cover clusters both by modifying cluster orientation, and reinforcing cluster magnitude.