Tropospheric Moisture Sounding Using Microwave Imaging Channels: Application to GCOM-W1/AMSR2

Traditionally, space-based microwave imagers have been used exclusively for providing total atmospheric water vapor along with hydrometeor and surface parameters, using a combination of window channels. These channels operate away from the strong water vapor absorption lines used for atmospheric moisture sounding. Nevertheless, they are sensitive to the vertical distribution of moisture in the atmosphere. This is due to the fact that the water vapor absorption continuum varies monotonically, and the wing effect of the strong absorption lines has a nonconstant effect, across the microwave spectrum. Therefore, the transmittances of window channels are different, allowing a varying level of penetration into the atmospheric column. To demonstrate the capability of imagers to operate as sounders, we introduce penetration depth, which defines the vertical sensitivity of window channels to moisture. In addition, we evaluate the degree to which data from spaceborne microwave imagers, specifically the Global Change Observation Mission-Water 1 (GCOM-W1) Advanced Microwave Scanning Radiometer-2 (AMSR2) sensor, can be used to perform sounding of tropospheric moisture. The capability is assessed using numerical weather prediction (NWP) analysis as the reference. In simulations using a regression approach, the performance of AMSR2 moisture sounding matches that of Suomi-NPP ATMS up to 700 hPa, with standard deviations of 10%-20%. AMSR2 outperforms other pure moisture sounders (Microwave Humidity Sounder and Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie) up to 500 hPa, before degrading rapidly. The performances using real observations and a physical algorithm are consistent with those found in simulations. These results highlight new applications for microwave imagers like GCOM-W1 AMSR2, including in Nowcasting and NWP, which are heavily reliant on lower tropospheric moisture information.