A Simplified Approach to Retrieve the K-Band Microwave Surface Emissivity Under Clear Skies

Microwave land surface emissivity (MLSE) at the K-band plays a key role in driving geophysical parameters, such as land surface temperature (LST). However, satellite-based MLSE currently is hard to be quickly retrieved in clear skies since the time cost is high in removing atmospheric contributions. In this letter, one clear-sky atmospheric profile dataset, including a wide range of precipitable water vapor (PWV) values, was constructed using the thermodynamic initial guess retrieval database for analyzing numerical relationships between PWV and atmospheric parameters. Then, a simplified algorithm was developed for accurately retrieving instantaneous K-band MLSEs (18.7 and 23.8 GHz) under clear skies, which can significantly save the time of atmospheric correction. The sensitivity analysis shows that PWV is a key factor affecting MLSE estimation at 23.8 GHz, and the brightness temperature (BT) uncertainty has a greater impact on MLSE estimation than LST. In addition, with LST derived from the moderate-resolution imaging spectroradiometer, BT from the Advanced Microwave Scanning Radiometer Earth Observing System (AMSR-E), and the ERA5 reanalysis PWV in 2008, the proposed algorithm was, respectively, applied in Europe and the United States for presenting its applicability at a station scale and regional scale. The actual sounding profile and global AMSR-E MLSE product were used as validation datasets. Results indicate that the simplified approach has a good performance with root-mean-square errors (RMSEs) less than 0.02 in the site and regional validations, whereas there are some apparent overestimations in estimating clear-sky MLSEs, especially for 23.8 GHz. We believe that the proposed approach is promising for retrieving other parameters.