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Variational Quality Control of Non-Gaussian Innovations in the GRAPES m3DVAR System: Mass Field Evaluation of Assimilation Experiments

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Abstract

The existence of outliers can seriously influence the analysis of variational data assimilation. Quality control allows us to effectively eliminate or absorb these outliers to produce better analysis fields. In particular, variational quality control (VarQC) can process gray zone outliers and is thus broadly used in variational data assimilation systems. In this study, governing equations are derived for two VarQC algorithms that utilize different contaminated Gaussian distributions (CGDs): Gaussian plus flat distribution and Huber norm distribution. As such, these VarQC algorithms can handle outliers that have non-Gaussian innovations. Then, these VarQC algorithms are implemented in the Global/Regional Assimilation and PrEdiction System (GRAPES) model-level three-dimensional variational data assimilation (m3DVAR) system. Tests using artificial observations indicate that the VarQC method using the Huber distribution has stronger robustness for including outliers to improve posterior analysis than the VarQC method using the Gaussian plus flat distribution. Furthermore, real observation experiments show that the distribution of observation analysis weights conform well with theory, indicating that the application of VarQC is effective in the GRAPES m3DVAR system. Subsequent case study and long-period data assimilation experiments show that the spatial distribution and amplitude of the observation analysis weights are related to the analysis increments of the mass field (geopotential height and temperature). Compared to the control experiment, VarQC experiments have noticeably better posterior mass fields. Finally, the VarQC method using the Huber distribution is superior to the VarQC method using the Gaussian plus flat distribution, especially at the middle and lower levels.

摘 要

离群值的存在会严重影响变分资料同化分析的准确性. 质量控制能够使同化系统有效地剔除或吸收离群值观测, 从而获得更佳的分析场, 尤其是变分质量控制 (VarQC) 可以有效处理灰色区域的离群值. 因此, VarQC在变分资料同化系统中得到了广泛的应用. 本文利用 “高斯分布+均匀分布” 和Huber norm分布的污染**态分布模型分别推导了两种VarQC算法. 这两种VarQC算法可以质量控制具有非高斯新息向量分布的离群值观测. 随后, 两种VarQC算法被应用于 GRAPES模式层的三维变分 (m3DVAR) 资料同化系统. 理想观测同化试验的检验表明, Huber分布的VarQC方法比“高斯分布+均匀分布”的VarQC方法具有更**的稳健性. 实际观测同化试验结果表明, 两种VarQC试验的观测分析权重分布与各自理论的权重曲线变化一致, 证明了基于GRAPES m3DVAR系统建立的VarQC方案应用的**确性. 随后的个例和长期同化试验分析表明, 观测分析权重大小的变化幅度和分布与质量场 (位势高度和温度) 的分析增量变化具有一致的响应关系. 与控制试验相比, VarQC试验具有更准确的后验分析质量场, 证明了Huber分布的VarQC算法优于“高斯分布+均匀分布”的VarQC算法, 尤其在对流层中下层.

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Acknowledgments

The authors thank Yinghui LU for his helpful advice and grammar correction. Jie HE is supported by the China Scholarship Council. This work is primarily sponsored by the National Key R&D Program of China (Grant No. 2018YFC1506702 and Grant No. 2017YFC1502000). We acknowledge the High Performance Computing Center of Nan**g University of Information Science & Technology (NUIST) for their support of this work. The datasets in this paper are archived and accessible on the supercomputer of NUIST.

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Correspondence to Xulin Ma.

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Article Highlights

• The VarQC method using the Huber distribution (Huber-VarQC) is implemented in the GRAPES m3DVAR system.

• Huber-VarQC reveals strong robustness against outliers.

• Huber-VarQC is superior to the VarQC method using the Gaussian plus flat distribution (Flat-VarQC) in the improvement of the mass field in the GRAPES m3DVAR system.

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He, J., Ma, X., Ge, X. et al. Variational Quality Control of Non-Gaussian Innovations in the GRAPES m3DVAR System: Mass Field Evaluation of Assimilation Experiments. Adv. Atmos. Sci. 38, 1510–1524 (2021). https://doi.org/10.1007/s00376-021-0336-3

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  • DOI: https://doi.org/10.1007/s00376-021-0336-3

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