Water Vapor Retrieval Over Cloud Cover Area on Land Using AMSR-E and MODIS

This study mainly discusses atmospheric water vapor retrieval over cloud cover area on land with the help of a newly developed surface emissivity parameter estimation method in microwave bands. In the retrieval method, the atmospheric water vapor sensitivity parameter—ratio of brightness temperature polarization difference at frequencies 18.7 and 23.8 GHz (
${{\Deltab}} {\bf Tb}_{\bf 18.7}/{{\Deltab}} {\bf Tb}_{\bf 23.8}$
)—is used to retrieve water vapor, and the surface emissivity parameter—ratio of surface emissivity polarization difference at frequencies 18.7 and 23.8 GHz (
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {\varepsilonb}_{\bf 23.8}$
) that corresponds to
${{\Deltab}} {\bf Tb}_{\bf18.7}/{{\Deltab}} {\bf Tb}_{\bf 23.8}$
—is a key parameter that affects the final precision of retrieved atmosphere water vapor. In order to estimate
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {{\varepsilonb}_{\bf 23.8}}$
in cloudy condition, we first estimated the value of
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {{\varepsilonb}_{\bf 23.8}}$
in clear condition using Advanced Microwave Scanning Radiometer for EOS (AMSR-E) brightness temperature and related MODIS atmospheric products. At the same time, it was found that gradient information derived separately from
${{\Deltab}} {\bf Tb}_{\bf 18.7}/{{\Deltab}} {\bf Tb}_{\bf 23.8}$
and
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {{\varepsilonb}_{\bf 23.8}}$
has very good correlation with each other. Based on this good correlation, the
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {{\varepsilonb}_{\bf 23.8}}$
in cloudy condition was estimated using corresponding
${{\Deltab}} {\bf Tb}_{\bf 18.7}/{{\Deltab}} {\bf Tb}_{\bf 23.8}$
and adjacent 8 days
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {{\varepsilonb}_{\bf 23.8}}$
in clear condition. With the estimated
${{\Deltab}} {{\varepsilonb}_{\bf 18.7}}/{{\Deltab}} {{\varepsilonb}_{\bf 23.8}}$
, we retrieved atmospheric column water vapor using lookup table method in cloudy condition over land. As a validation source data, the SuomiNet GPS-retrieved precipitable water (PW) vapor is used to validate the retrieved water vapor in this study. According to validation, the correlation coefficient of the two is 0.94 and the root-mean-square-error (RMSE) is 4.85 mm. It is a great improvement in water vapor retrieval using microwave in cloud cover area on land.