New dual‐frequency microwave technique for retrieving liquid water path over land

Abstract

We present and demonstrate a new methodology for retrieving liquid water path over land using satellite‐based microwave observations. As input, the technique exploits Advanced Microwave Scanning Radiometer for EOS (AMSR‐E) brightness temperature polarization‐difference signals at 37 and 89 GHz. Regression analysis performed on model simulations indicates that over variable atmospheric and surface conditions these polarization‐difference signals can be simply parameterized in terms of the surface emissivity polarization‐difference (Δε), surface temperature, liquid water path (LWP), and precipitable water vapor (PWV). By exploiting the weak frequency dependence of Δε, a simple expression is obtained which enables fast and direct (noniterative) retrievals of LWP. The new methodology is demonstrated and validated using several months of AMSR‐E observations over (1) the Southern Great Plains (SGP) of the United States and (2) an area near Montreal, Canada, instrumented during the Alliance Icing Research Study II (AIRS II) field campaign. Comparisons are also made with MODIS LWP retrieval results for one scene over the SGP region. Retrieval results in clear‐sky conditions indicate an uncertainty on the order of 0.06 mm, in agreement with theoretical estimates. In cloudy conditions, results using the new method are systematically smaller than results for both ground‐based microwave radiometers and MODIS but are well correlated.