Application of the NASA scatterometer (NSCAT) for determining the daily frozen and nonfrozen landscape of Alaska

The seasonal transition of the land surface between frozen and nonfrozen conditions affects a number of terrestrial processes that cycle between wintertime dormant and summertime active states. The relatively short (2.14 cm) K-u-band of the space-borne NASA scatterometer (NSCAT) is sensitive to changes in dielectric properties, associated with large-scale shifts in the relative abundance and phase (frozen or thawed) of canopy and surface water. We used a temporal change detection analysis of NSCAT daily radar backscatter measurements to classify surface freeze/thaw state across a 1.4 million km(2) region of Alaska from January to June 1997. In the spring, radar backscatter measurements showed pronounced decreases (1.6-4.9 dB) relative to reference frozen state conditions, which corresponded with sustained maximum daily air temperature measurements above 0.0 degreesC and total decreases in measured snow depths from 28% to 61% of seasonal maximum values. We classified the daily frozen and nonfrozen area for the region based on the sign (+/-) of the radar backscatter temporal difference relative to frozen and nonfrozen reference conditions. These results compared favorably (e.g., r(2) = 0.881; p less than or equal to 0.0001) with frozen area estimates derived from the regional weather station network. NSCAT-derived estimates of the timing and spatial variation in regional thaw during spring were also generally consistent with seasonal increases in river discharge for five major Alaska basins. The NSCAT sensor appears to be responsive to changes in dielectric properties associated with surface freeze/thaw transitions over broad boreal and arctic landscapes. Further study involving longer time-series information, alternative radar wavelengths, and finer spatial scales is needed, however, to resolve the various components (i.e., soil, vegetation, snow) of the regional radar freeze/thaw signature for improved monitoring of the circumpolar high latitudes. (C) Elsevier Science Inc., 2001.