The influence of sensor overpass time on passive microwave derived snow water equivalent measurements

Passive microwave derived retrieval of terrestrial snow water equivalent (SWE) is strongly influenced by snowpack wetness. The presence of water in the crystal matrix increases the microwave emissivity, destroying the between channel brightness temperature gradient used to make quantitative estimates of SWE. In order to obtain the most accurate SWE imagery, overpass times from orbiting sensors such as the Special Sensor Microwave/Imager (SSM/I) can be chosen so that the diurnally coldest and driest snowpack is being monitored. The authors evaluate the ro/spl circ/le of sensor overpass time when mapping SWE by comparing two datasets of five-day averaged, boreal winter season (December, January, and February) SWE imagery for a ground validated prairie study area. The first dataset is derived from SSM/I morning overpass times, the second from afternoon overpass times. Correlation analysis and modified mean bias error calculations are used to quantify the association between the two datasets. In addition, a series of principal components analysis tests quantify the spatial and temporal association between the two time series.