MICROWAVE CHARACTERISTICS OF SNOW COVERED SURFACES IN SOUTHERN GREENLAND AND THEIR IMPACT ON GSMAP SNOWFALL RETRIEVAL

Abstract

 In the global satellite precipitation product GSMaP (Global Satellite Mapping of Precipitation), Greenland is being masked due to the difficulty in snowfall estimation over the region. In southern Greenland, the GSMaP’s radiometer retrieval algorithm for the Global Precipitation Measurement (GPM) Microwave Imager (GMI) tends to estimate significantly larger snowfall rates compared to other global precipitation products, which suggests an overestimation bias of the GSMaP snowfall. This study investigates the causes of the bias focusing on the microwave characteristics of the snow-covered surfaces in southern Greenland.

 Normally, snow-cover land surfaces are accompanied by a scattering signature of microwaves, where the brightness temperature (TB) at the high-frequency channel is lower than that of the low-frequency (i.e., TB19 – TB89 > 0). However, in southern Greenland, the scattering signature of snow-cover suface is absent (i.e., TB19 – TB89 < 0). The geographical pattern of such scatter-free snow-cover surfaces in southern Greenland shows a close similarity to that of the GSMaP snowfall overestimation, impling the unique microwave characteristics are related to the GSMaP’s snowfall overestimation. Because the lookup tables used for GSMaP snowfall estimation are developed based on global observations, it is geared toward general relationships between microwave TBs and surface snowfall. The result of this study suggests that a separate lookup tabel is reauired for the snowfall estimation over snow-cover scatter-free surfaces.