On the Synergy of SMAP and AMSR2 for Estimating Snow Depth on Arctic Sea Ice

The objective of this letter is to extend the commonly used gradient ratio (GR) method for Arctic sea ice snow depth (SD) estimation by combining brightness temperatures (TBs) from the Soil Moisture Active Passive (SMAP) and the Advanced Microwave Scanning Radiometer 2 (AMSR2). The L-band (1.4 GHz) channel from SMAP together with higher frequencies (i.e., 6.9, 10.7, 18.7, and 36.5 GHz) from AMSR2 was used to calculate GRs, which was then used to derive empirical SD retrieval algorithms based on five years of Operation IceBridge (OIB) SD measurements acquired on Arctic sea ice during springtime. Results show that the GR(1/19) at vertical polarization is suitable for SD estimation over both first-year ice (FYI) and multiyear ice (MYI) and could achieve the best performance with correlation $r$ and root mean square distance (RMSD) values being −0.80 and 5.95 cm, respectively. More importantly, there exists a one-to-one relationship between SD and GR(1/19) independent of sea ice types, which is an advantage of the GR(1/19) over the previous GR(7/19).