Effects of rain on CFOSAT scatterometer measurements

• Published in: Remote sensing of environment 2022-06, Vol.274, p.113015, Article 113015
• Main Authors: Zhao, Xiaokang, Lin, Wenming, Portabella, Marcos, Wang, Zhixiong, He, Yijun
• Format: Article
• Language: English
• Subjects: Backscatter; Backscattering; C band; CFOSAT; Global precipitation; Heavy rainfall; Incidence angle; Inversion; Numerical prediction; Numerical weather forecasting; Oceanography; Pencil beams; Radar; Radar backscatter; Rain; Rain effects; Rainfall; Rainfall simulators; Satellite observation; Scatterometer; Scatterometers; Superhigh frequencies; Surface wind; Weather forecasting; Wind; Wind data; Wind quality; Wind speed
• ISSN: 0034-4257; 1879-0704
• DOI: 10.1016/j.rse.2022.113015

The Ku-band scatterometer onboard China France Oceanography Satellite (CFOSAT) observes the sea surface with two conically scanning fan beams. Compared to the prior Ku-band pencil beam scatterometers, this innovative observing mechanism provides more independent backscatter measurements at varying incidence and azimuth angles, as such it brings challenges for the sea surface wind inversion, particularly under rainy conditions. In this paper, the rain effects on the CFOSAT SCATterometer (CSCAT) are investigated using the collocated numerical weather prediction (NWP) wind data and the Global Precipitation Measurement (GPM) microwave imager (GMI) rain data. Similar to the prior Ku-band or C-band scatterometers, the sensitivity of CSCAT radar backscatter to rain substantially varies with wind speed, radar polarization and incidence angle. However, due to the complex observation geometries, rain effects on the CSCAT retrieved winds is more complex than that of prior scatterometers, which may lead to a remarkable underestimation of CSCAT wind speed at high winds and heavy rain conditions. A simple simulation method is used to clarify the relation between the retrieved wind speed and the dependency of radar rain effects on the incidence angle. It is found that the backscatter measurements at low incidence angles, which are generally underestimated at high winds and heavy rainy conditions, have a larger influence on the wind inversion minimization, leading to much lower retrieved wind speeds than those of ECMWF and the pencil beam scatterometer (e.g., Haiyang-2B scattometer). Under low and moderate rain conditions though, a more compensated effect between low and high incidence angle measurements is found, leading to generally unbiased CSCAT high winds, in contrast to the generally underestimated pencil-beam scatterometer winds. •High winds under heavy rain are underestimated by the CFOSAT scatterometer (CSCAT).•The dependency of rain effect on the incidence angle is explored for CSCAT.•Rain effect on the CSCAT σ0is more complex than that of prior scatteormeters.•A simulation method is used to investigate the rain impact on the wind retrieval.