AMSR-E melt patterns on the Southern Patagonia Icefield

Passive-microwave 37 GHz vertically polarized (V) brightness temperature (T b) measurements from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) are used to monitor the extent and timing of snowmelt on the Southern Patagonia Icefield (SPI) in Chile and Argentina. Twice daily T b’s for 2002–08 for high-elevation (>1200 m a.s.l.) pixels exhibit a bimodal histogram, typical of snow-covered regions in Yukon, Alaskan icefields and the Greenland ice sheet. The low count between the two populations represents the T b threshold for melt (252 K). This T b value with the ±18 K diurnal amplitude variation threshold quantifies onset and duration of the spring melt–refreeze period and is used to identify melt regimes and seasonal T b signals. T b histograms for pixels west of the Andean divide have a normal distribution above the melt threshold. We interpret the T b histogram as controlled by surface moisture; the shape and position with respect to T b are retained with changes in both latitude and elevation, and the region is known to have a moist climate. T b is not driven by seasonal temperature changes in the northwest sector of the icefield because the T b threshold is exceeded 75% of the time. For all pixels, the spring melt–refreeze period has shortened by a mean of 10 days a−1 and a mean of 16 days a−1 for pixels with bimodal distributions between 2002 and 2008.