Parameterization of the Arctic sea ice microwave emissivity between 1.4
and 36~GHz, for large scale applications
Abstract
Modeling sea ice microwave emissivities at large scales presents
challenges, due to complex interactions between the microwave signal and
the sea ice environment. For the preparation of the Copernicus Imaging
Microwave Radiometer mission (CIMR) that focusses on the pole
monitoring, a pragmatic parameterization of the sea ice emissivity over
the Arctic is proposed, providing consistent emissivity
parameterizations between 1.4 and 36~GHz, for V and H
polarizations. Satellite-derived microwave emissivities are calculated
from AMSR2, SMAP, and SMOS observations, subtracting the atmospheric
contributions and the surface temperature modulation using ERA5
meteorological reanalysis. The resulting Arctic sea ice emissivities are
analyzed, alongside sea ice geophysical parameters Mercator model
outputs and ERA5, to identify the pertinent variables for the emissivity
parameterization. Sea ice thickness emerges as a crucial factor,
particularly at 18 and 36~GHz.
A large training database of coincident satellite-derived emissivities
and geophysical parameters is set up, to develop a Neural Network
parameterization of the emissivities based on the geophysical
parameters. This pragmatic methodology establishes a direct link between
calculated emissivities and physical sea ice properties, eliminating the
need for a priori assumptions. Promising emissivity results are
obtained, with RMSE below 0.02 for most channels, reaching 0.04 at
36~GHz. Part of the error is expected to come from
uncertainties in the input geophysical parameters, especially the sea
ice thickness and the snow depth above sea ice. The emissivity frequency
dependence is checked, and the emissivity angular variation of the
1.4~GHz is derived from the analysis of the SMOS-derived
emissivities.