AMSR-E melt patterns on the Southern Patagonia Icefield

Abstract

Passive-microwave 37 GHz vertically polarized (V) brightness temperature (Tb) 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 Tb's for 2002-08 for high-elevation (>1200ma.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 Tb threshold for melt (252 K). This Tb 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 Tb signals. Tb histograms for pixels west of the Andean divide have a normal distribution above the melt threshold. We interpret the Tb histogram as controlled by surface moisture; the shape and position with respect to Tb are retained with changes in both latitude and elevation, and the region is known to have a moist climate. Tb is not driven by seasonal temperature changes in the northwest sector of the icefield because the Tb 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.