Further Developments in Estimating Cloud Liquid Water over Land Using Microwave and Infrared Satellite Measurements

Abstract

Refinements and improvements of an earlier technique to retrieve the cloud liquid water path (LWP) of nonprecipitating clouds over land surfaces using Special Sensor Microwave/Imager (SSM/I) 85.5-GHz measurements are presented. These techniques require estimates of the microwave surface emissivity, which are derived in clear-sky regions from SSM/I measurements and window infrared measurements from the Visible and Infrared Spin Scan Radiometer on GOES-7. A comparison of forward model calculations with SSM/I measurements in clear regions demonstrates that over a 7-day period the surface emissivities are stable. To overcome limitations in the single-channel retrieval method under certain situations, a new method is developed that uses a normalized polarization difference (NPD) of the brightness temperatures. This method has the advantages of providing estimates of the LWP for low clouds and being extremely insensitive to the surface skin temperature. Radiative transfer simulations also show that the polarization difference at 37 GHz may be useful for retrievals in high water vapor environments and for large cloud LWP. An intercomparison of the different retrieval methods over Platteville, Colorado, reveals large discrepancies for certain cases, but the NPD method is found to agree best with coincident ground-based microwave radiometer measurements of cloud LWP. This success is primarily due to the larger than average surface polarization differences near the Platteville site. While the NPD method shows promise in distinguishing between low, moderate, and high values of cloud LWP, a comprehensive validation effort is required to further evaluate its accuracy and limitations.