Evaluation of the Long-term Performance of Microwave Radiation Imager Onboard Chinese Fengyun Satellites

Abstract

Accurate brightness temperature (BT) is a top priority for retrievals of atmospheric and surface parameters. Microwave Radiation Imagers (MWRIs) on Chinese Fengyun-3 (FY-3) serial polar-orbiting satellites have been providing abundant BT data since 2008. Much work has been done to evaluate short-term MWRI observations, but the long-term performance of MWRIs remains unclear. In this study, operational MWRI BTs from 2012–19 were carefully examined by using simultaneous Advanced Microwave Scanning Radiometer 2 (AMSR2) BTs as the reference. The BT difference between MWRI/FY3B and AMSR2 during 2012–19 increased gradually over time. As compared with MWRI/FY3B BTs over land, those of MWRI/FY3D were much closer to those of AMSR2. The ascending and descending orbit difference for MWRI/FY3D is also much smaller than that for MWRI/FY3B. These results suggested the improvement of MWRI/FY3D over MWRI/FY3B. A substantial BT difference between AMSR2 and MWRI was found over water, especially at the vertical polarization channels. A similar BT difference was found over polar water based on the simultaneous conical overpassing (SCO) method. Radiative transfer model simulations suggested that the substantial BT differences at the vertical polarization channels of MWRI and AMSR2 over water were partly contributed by their difference in the incident angle; however, the underestimation of the operational MWRI BT over water remained a very important issue. Preliminary assessment of the operational and recalibrated MWRI BT demonstrated that MWRI BTs were substantially improved after the recalibration, including the obvious underestimation of the operational MWRI BT at the vertical polarization channels over water was corrected, and the time-dependent biases were reduced.

摘 要

准确的亮温(BT)是大气和地表参数反演所需重要的基础保障。中国风云三号 (FY-3) 系列极轨卫星上的微波成像仪 (MWRI) 自 2008 年以来一直在提供丰富的亮温数据。目前已有不少研究评估MWRI短期观测能力,但对MWRI长期表现仍不太清楚。在本文研究中,使用先进的微波成像仪AMSR2同步观测亮温作为参考,仔细检查评估了2012-2019 年MWRI业务化亮温数据质量。研究发现FY-3B卫星上MWRI与AMSR2 在这8年期间的 亮温差异随着时间的推移而逐渐增加。陆面上,FY-3D的MWRI 亮温相比较FY-3B上MWRI,与AMSR2 亮温更为接近;而且MWRI/FY3D 的升降轨差异也远小于 MWRI/FY3B,这表明 MWRI/FY3D 的性能优于 MWRI/FY3B。水面上的AMSR2 和 MWRI 之间存在显著亮温差异,尤其在垂直极化通道。基于圆锥扫描同步重叠 (SCO) 方法,在极地水域也出现类似的亮温差异。利用辐射传输模式模拟结果比较分析,发现水面上MWRI 和 AMSR2 在垂直极化通道明显的亮温差异一部分是由它们的入射角差异造成,另一部分是MWRI亮温业务数据明显低估造成。初步评估MWRI业务定标和再定标 亮温数据表明,再定标的MWRI 亮温数据质量得到了显著改善,包括订正了业务化MWRI垂直极化通道亮温数据在水面明显低估的问题,减少亮温随时间变化的偏差趋势。