Detection of snow melt and freezing in Himalaya using OSCAT data

Abstract

A study of the snow cover melt and freeze using Ku band Oceansat scatterometer (OSCAT) HH polarised backscatter coefficient ( $\sigma ^{\mathrm {0}}_{\text {HH}}$ ) for 2011 and 2012 is reported for the Himalayas, which contain the world’s largest reserve of ice and snow outside polar regions. The analysis shows spatial and temporal inter-annual variations in the onset of melt/freeze across four regions (Upper Himalaya, Western Himalaya, Central Himalaya, and Eastern Himalaya), nine elevation bands and four aspect zones. A threshold based on temperature– $\sigma ^{\mathrm {0}}_{\text {HH}}$ relation and average $\sigma ^{\mathrm {0}}_{\mathrm {HH }}$ for the months January–March was used for melt/freeze detection. When the three consecutive images (6 days) satisfied the threshold, the day of first image was selected as melt onset/freeze day. The average melt onset dates were found to be March 11 ± 11 days for Eastern Himalaya, April 3 ± 18 days for Central Himalaya, April 16 ± 27 days for Western Himalaya, and May 12 ± 18 days for Upper Himalaya. Similarly average freeze onset dates were found to be August 23 ± 27 days for Eastern Himalaya, September 08 ± 24 days for Central Himalaya, August 27 ± 11 days for Western Himalaya, and September 13 ± 11 days for Upper Himalaya. All the zones experienced the melt onset earlier by around 20 days in 2011 at elevation above 5000 m. All the zones experienced freeze earlier in 2012, with onset being 18, 19, 11, and 21 days earlier in Eastern, Central, Western, and Upper Himalaya, respectively.