Polarization signature in micro-wave humidity sounder window channels.

摘要

Microwave Humidity Sounders (MHS) onboard NOAA-15, -16, -17, -18, -19 and EUMETSAT MetOp-A, -B provide radiance measurements at a single polarization state at any of the five observed frequencies. Microwave Humidity Sounder (MWHS) onboard FengYun-3 (FY-3) satellite has a unique instrument design and provides dual polarization measurements at 150 GHz. In this study, the MWHS polarization signal is investigated using observed and modeled data. It is shown that the quasi-polarization brightness temperatures at 150 GHz display a scan angle dependent bias. Under calm ocean conditions, the polarization difference at 150 GHz becomes non-negligible when the scan angle varies from 10 to 45 degrees and reaches a maximum when the scan angle is about 30 degrees. Also, the polarization state is sensitive to surface parameters such as surface wind speed. Under clear-sky conditions, the differences between horizontal and vertical polarization states at 150 GHz increase with decreasing surface wind speed. Therefore, the polarization signals from the cross-track scanning microwave measurements at window channels contain useful information of surface parameters. What is more, cloud liquid water is also proved to be a non-ignorable factor reducing the difference between horizontal and vertical polarization states at 150 GHz. Also, the availability of dual polarization measurements allows a one-to-one conversion from the antenna brightness temperature to sensor brightness temperature if there exit a cross-polarization spill-over.