Abstract
In operational data assimilation systems, observation-error covariance matrices are commonly assumed to be diagonal. However, inter-channel and spatial observation-error correlations are inevitable for satellite radiances. The observation errors of the Microwave Temperature Sounder (MWTS) and Microwave Humidity Sounder (MWHS) onboard the FengYun-3A (FY-3A) and FY-3B satellites are empirically assigned and considered to be uncorrelated when they are assimilated into the WRF model’s Community Variational Data Assimilation System (WRFDA). To assimilate MWTS and MWHS measurements optimally, a good characterization of their observation errors is necessary. In this study, background and analysis residuals were used to diagnose the correlated observation-error characteristics of the MWTS and MWHS. It was found that the error standard deviations of the MWTS and MWHS were less than the values used in the WRFDA. MWTS had small inter-channel errors, while MWHS had significant inter-channel errors. The horizontal correlation length scales of MWTS and MWHS were about 120 and 60 km, respectively. A comparison between the diagnosis for instruments onboard the two satellites showed that the observation-error characteristics of the MWTS or MWHS were different when they were onboard different satellites. In addition, it was found that the error statistics were dependent on latitude and scan positions. The forecast experiments showed that using a modified thinning scheme based on diagnosed statistics can improve forecast accuracy.
摘要
在业务同化系统中, 观测误差协方差矩阵常被假定为对角矩阵. 但是, 卫星辐射率资料的观测误差通常存在着通道间和空间相关. 当前的同化系统中, FY-3A/B MWHS和MWHS的观测误差均根据经验假定并且被认为是不相关的, 为了提高MWTS和MWHS的同化效果, 有必要更精确地确定其观测误差协方差矩阵. 本文基于WRFDA同化系统, 使用观测与背景以及分析之间的偏差诊断了MWTS和MWHS观测误差的相关特征. 结果表明, MWTS和MWHS的误差标准差小于WRFDA中的默认值. MWTS的通道间误差较小, 而MWHS具有明显的通道间误差. MWTS和MWHS观测误差的水平相关尺度分别为120和60公里. 在不同的扫描位置和纬度位置, MWTS和MWHS观测误差具有不同的特征. 研究还发现, 对于搭载在不同卫星上的同一仪器, 其观测误差也存在着差异. 预报试验表明, 使用基于诊断结果的稀疏化方案能够提高卫星同化效果, 进而改善预报.
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Acknowledgements
This work was funded by the National Basic Research (973) Program of China (Grant No. 2015CB452802) and the National Natural Science Foundation of China (Grant Nos. 41230421, 41605075, and 41675058). We thank the CMDC for providing the FY-3A/B MWTS and MWHS data and the NCAR/UCAR Research Data Archive for providing the FNL data.
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Wang, T., Fei, J., Cheng, X. et al. Estimating the Correlated Observation-Error Characteristics of the Chinese FengYun Microwave Temperature Sounder and Microwave Humidity Sounder. Adv. Atmos. Sci. 35, 1428–1441 (2018). https://doi.org/10.1007/s00376-018-8014-9
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DOI: https://doi.org/10.1007/s00376-018-8014-9
Key words
- data assimilation
- correlated observation errors
- MWTS (Microwave Temperature Sounder)
- MWHS (Microwave Humidity Sounder)