Influence of Sensor Overpass Time on Passive Microwave-Derived Snow Cover Parameters

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. 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 the diurnally coldest and driest snowpack is being monitored. In this study we seek to evaluate the role of sensor overpass time when mapping SWE by comparing two data sets of 5-day averaged, winter season (December, January, and February) SWE imagery for a ground-validated prairie study area. The first data set 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 data sets. In addition, a series of principal components analysis (PCA) tests have been utilized to quantify the spatial and temporal association between the two time series. Results indicate a strong agreement in SWE retrievals between the two data sets, with little systematic bias that can be attributed to sensor overpass time. Differences in snow-covered areas are more marked, with the morning overpass data consistently estimating greater snow extent. The PCA indicates that the results of time series analysis can be influenced by the choice of sensor overpass time, although the dominant characteristics of the seasonal snow cover evolution are captured by both data sets. Surface temperature data are utilized to illustrate the dependence of algorithm performance on the physical state of the snowpack.