Observations of MMW backscatter from snow near grazing incidence

In the winter of 1998 and 1999 the University of Michigan made numerous polarimetric backscatter measurements of snow at 35 and 95 GHz at depression angles between 2/spl deg/ and 14/spl deg/. This is the first known measurement of the backscatter from snow at these frequencies and angles that are accompanied by extensive ground truth including snow depth, air temperature, snow wetness and snow wet density. Measurement techniques for both the radar data and ground truth are described. Comparisons between the observations and ground truth are presented and correlated with expectations for backscatter from snow derived from previous measurements of snow at higher angles of depression. For example, dry snow at 95 GHz exhibits sufficient extinction that the material underlying the snow is not observed with as little as 4 cm of snow cover. Comparison of diurnal measurements of the backscatter to simultaneous measurements of snow wetness 2 cm down into the snow pack show very strong hysteresis, indicating the sensitivity of the W-band radar to moisture is confined to a region very close to the top of the snow pack.