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Assimilation of GNSS reflectometry delay-Doppler maps with a two-dimensional variational analysis of global ocean surface winds
Quarterly Journal of the Royal Meteorological Society ( IF 8.9 ) Pub Date : 2021-04-06 , DOI: 10.1002/qj.4034
Feixiong Huang 1 , James L. Garrison 1 , S. Mark Leidner 2 , Giuseppe Grieco 3 , Ad Stoffelen 3 , Bachir Annane 4 , Ross N. Hoffman 5
Affiliation  

Direct remote-sensing observations (e.g., radar backscatter, radiometer brightness temperature, or radio occultation bending angle) are often more effective for use in data assimilation (DA) than the corresponding geophysical retrievals (e.g., ocean surface winds, soil moisture, or atmospheric water vapor). In the particular case of Global Navigation Satellite System Reflectometry (GNSS-R), the lower-level delay-Doppler map (DDM) observable shows a complicated relationship with the ocean surface wind field. Prior studies have demonstrated DA using GNSS-R wind retrievals inferred from DDMs. The complexity of the DDM dependence on winds, however, suggests that the alternative approach of ingesting DDM observables directly into DA systems, without performing a wind retrieval, may be beneficial. We demonstrate assimilation of DDM observables from the NASA Cyclone Global Navigation Satellite System (CYGNSS) mission into global ocean surface wind analyses using a two-dimensional variational analysis method. Bias correction and quality-control methods are described. Several models for the required observation-error covariance matrix are developed and evaluated, with the conclusion that a diagonal matrix performs as well as a fully populated matrix empirically tuned to a large ensemble of CYGNSS observation data. The 10-m surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational forecast are used as the background (i.e., prior in the variational analysis). Results are compared with independent scatterometer (the advanced scatterometer (ASCAT), the oceansat-2 Scatterometer (OSCAT)) winds. For one month (June 2017) of data, the root-mean-square difference (RMSD) was reduced from 1.17 to 1.07 m·s−1 and bias from −0.14 to −0.08 m·s−1 for the wind speed at the specular point. Within a 150-km wide swath along the specular point track, the RMSD was reduced from 1.20 to 1.13 m·s−1. These RMSD and bias statistics are smaller than other CYGNSS wind products available at this time.

中文翻译:

同化 GNSS 反射计延迟多普勒图与全球海面风的二维变分分析

直接遥感观测(例如,雷达反向散射、辐射计亮度温度或无线电掩星弯曲角)在数据同化 (DA) 中的使用通常比相应的地球物理反演(例如,海面风、土壤湿度或大气水蒸气)。在全球导航卫星系统反射计 (GNSS-R) 的特殊情况下,可观测的低层延迟多普勒图 (DDM) 显示出与海面风场的复杂关系。先前的研究已经证明了使用从 DDM 推断的 GNSS-R 风反演的 DA。然而,DDM 对风的依赖的复杂性表明,将 DDM 可观测值直接摄取到 DA 系统中而不执行风检索的替代方法可能是有益的。我们展示了使用二维变分分析方法将来自 NASA 旋风全球导航卫星系统 (CYGNSS) 任务的 DDM 观测值同化到全球海面风分析中。描述了偏差校正和质量控制方法。开发和评估了所需观测误差协方差矩阵的几个模型,得出的结论是,对角矩阵的性能与根据经验调整到大量 CYGNSS 观测数据的完全填充矩阵一样好。来自欧洲中期天气预报中心 (ECMWF) 业务预报的 10 米地表风被用作背景(即,在变分分析之前)。结果与独立散射计(高级散射计 (ASCAT)、oceansat-2 散射计 (OSCAT))风进行比较。-1和偏差从 -0.14 到 -0.08 m·s -1用于镜面反射点的风速。在沿镜面反射点轨迹的 150 公里宽的条带内,RMSD 从 1.20 减少到 1.13 m·s -1。这些 RMSD 和偏差统计数据比目前可用的其他 CYGNSS 风产品小。
更新日期:2021-06-03
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