Estimation of Atmospheric Microwave Radiation Parameters Over the Arctic Sea Ice From the AMSR2 Data

A new approach is developed to estimate atmospheric microwave radiation parameters (atmospheric radiation
$T_{a}$
and its optical thickness
$\tau $
) over the Arctic sea ice (SI) at the frequencies of the Advanced Microwave Scanning Radiometer 2 (AMSR2). The approach is based on the results of numerical modeling of the brightness temperature (BT) of vertically and horizontally polarized microwave radiation of the Arctic SI–atmosphere system under nonscattering conditions. The main point of the approach is the retrieval of
$\tau $
at 89 GHz (
$\tau _{89}$
) from BT polarization difference (PD) at 89 GHz under the assumption of constant SI emission PD at 89 GHz.
$T_{a}$
and
$\tau $
at the other AMSR2 frequencies and
$T_{a}$
at 89 GHz are estimated using
$\tau _{89}$
and atmospheric water vapor column (WVC), based on the results of numerical modeling and neural networks (NNs)-based inversion algorithms. Since the retrievals of WVC can be done over SI from the AMSR2 data with the previously developed algorithm, the suggested approach allows
$T_{a}$
and
$\tau $
retrieval at the AMSR2 channel frequencies using only AMSR2 measurements without any additional data. The method is restricted by the use of the “absorption only” form of the radiative transfer equation (RTE) and is not applicable in the months from April to October when cloud and precipitation scattering start to play an important role in microwave radiation transfer. The method allows estimating the SI effective emissivities from BT measurements by means of the atmospheric correction without any reanalysis or operational analysis data. It may be used both in SI surface property studies and in the determination of the tie points in the SI concentration (SIC) retrieval algorithms. The uncertainties of
$T_{a}$
and
$\tau $
estimation greatly depend on the error of the assumption of constant SI emission PD at 89 GHz.