Interannual variations of snowmelt and refreeze timing on southeast-Alaskan icefields, U.S.A

Abstract

Twice-daily satellite observations from the Special Sensor Microwave Imager (SSM/I) indicate melt onset and refreeze on southeast-Alaskan icefields. Melt and refreeze are based on 37 GHz vertically polarized brightness temperatures (Tb) and diurnal-amplitude variations (DAV). Two types of melt regime have different summer characteristics. Onset is characterized by increasing average daily Tb and a switch from low- to high-amplitude DAV. Melt timing, calibrated using Juncau Icefield temperatures, correlates well with nearby stream hydrographs. Some pixels maintain high Tb throughout the melt season and return to low-amplitude DAV after melt onset. Refreeze on these pixels is identified by decrease in Tb and accompanying high-amplitude DAV. Other pixels maintain high DAV throughout the summer, indicating nocturnal refreeze. Fall refreeze is determined by the end of high-amplitude DAV. Interannual variability in melt timing and ablation-season length is high. Melt onset and refreeze timing show a regional tendency toward earlier glacier-melt onset and longer ablation seasons from 1988-98.