Melt in Antarctica derived from Soil Moisture and Ocean Salinity (SMOS) observations at L band

Melt occurrence in Antarctica is derived from Lbandobservations from the Soil Moisture and Ocean Salinity(SMOS) satellite between the austral summer 2010-2011and 2017-2018. The detection algorithm is adapted froma threshold method previously developed for 19 GHz passivemicrowave measurements from the special sensor microwaveimager (SSM/I) and special sensor microwave imagersounder (SSMIS). The comparison of daily melt occurrenceretrieved from 1.4 and 19 GHz observations showsan overall close agreement, but a lag of few days is usuallyobserved by SMOS at the beginning of the melt season.To understand the difference, a theoretical analysis is performedusing a microwave emission radiative transfer model.It shows that the sensitivity of 1.4 GHz signal to liquid wateris significantly weaker than at 19 GHz if the water is onlypresent in the uppermost tens of centimetres of the snowpack.Conversely, 1.4 GHz measurements are sensitive to waterwhen spread over at least 1m and when present in depthsup to hundreds of metres. This is explained by the largepenetration depth in dry snow and by the long wavelength(21 cm). We conclude that SMOS and higher-frequency radiometersprovide interesting complementary information onmelt occurrence and on the location of the water in the snowpack.