On detection of the thermophysical state of landfast first-year sea ice using in-situ microwave emission during spring melt

In this study we examine the critical linkages between thermophysical properties and microwave emissions of landfast snow-covered first-year sea ice during spring melt. For this we analyzed the temporal evolution of radiation fluxes, electro-thermophysical properties and microwave emissions, and perform model simulations to evaluate the observations. The results show five major microwave signature events: brine-rich, blowing snow, melt onset, the onset of funicular regime, and freezing. A brine-rich snow basal layer can considerably increase the snow wetness in the upper and mid layers, resulting in a significant increase in complex permittivity that in turn increases in polarization difference (δp) at 19 and 37 GHz. A dense (∼ 0.40 g cm^{- 3}) wind-packed snow surface layer, during a blowing snow event, was found to increase the permittivity (i.e., surface reflectivity) that in turn increases δp in microwave emissions. Melt onset caused by sustained warming (above - 5 °C) corresponded to increased δp of ∼ 9 K at 19 GHz. The most dramatic increase in δp (up to 17 K at 19 GHz) coincided with the occurrence of a rainstorm. During a freezing, melt-freeze events enlarged snow grains and led to formation of ice lenses and layers within the snow, thereby significantly decreasing microwave emissions. We found that these five factors state above were critical to the melt indicators (i.e., ΔT_B(H) (T_B(19H) - T_B(37H)) and XPGR ([T_B(19H) - T_B(37V)]/[T_B(19H) + T_B(37V)])) commonly used in the satellite melt detection algorithms. The results suggests that the absolute value of T_B(19H) (brightness temperature of horizontal polarization at 19 GHz) would be a good indicator along with ΔT_B(H) (or XPGR) to delineate the melt onset from ambiguous factors (i.e., a brine-rich slush layer or wind-packed layer), and that the funicular stage of snow melt on sea ice could be unambiguously detected by either ΔT_B(H) or XPGR.