Retrieving K-Band Instantaneous Microwave Land Surface Emissivity Based on Passive Microwave Brightness Temperature and Atmospheric Precipitable Water Vapor Data

An algorithm has been developed for retrieving instantaneous K-band microwave land surface emissivity using only brightness temperature and atmospheric precipitable water vapor content (PWV) data. The radiative transfer model is simplified to a new microwave emissivity retrieval model using two assumptions: 1) emissivities at 18.7 and 23.8 GHz with horizontal polarization are approximately equal, and 2) simple parameterizations exist between atmospheric transmittance and PWV and between atmospheric effective radiating temperature and PWV. The new technique does not need infrared land surface temperature as the input data, and it overcomes the limitation of previous algorithms under cloudy conditions. The estimated instantaneous emissivities are validated at single points and in the regional area. The results demonstrate that this simplified algorithm has the best root mean square error of 0.017, a bias of 0.004 in the single-point validation, and an R-2 of 0.66 and a RMSE of 0.021 in the regional validation. This simplified algorithm has the potential to obtain instantaneous microwave land surface emissivity under both cloud-free and cloudy conditions.