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.