GRAPES全球三维变分同化中卫星微波温度计亮温的背景误差及在质量控制中的应用

模式变量背景误差在观测空间的投影,也即观测变量的背景误差包含了变分同化系统的重要信息,其在诊断和分析变分同化系统中资料的影响等方面具有重要作用,特别是在背景场检查质量控制中。在GRAPES全球三维变分同化(3DVar)系统中仅给定了控制变量的背景误差,并未直接给定观测变量的背景误差。为了能够对GRAPES全球3DVar进行全面的诊断和分析,改进卫星微波温度计资料的质量控制,推导出GRAPES全球3DVar同化系统控制变量随机扰动方法估计观测变量的背景误差的公式,为分析和改进GRAPES全球3DVar提供了一个有力工具,并进而估计了AMSU-A亮温的背景误差,分析了AMSU-A不同通道亮温的背景误差特征,将其应用于GRAPES全球3DVar的AMSU-A亮温的背景场检查质量控制中。结果表明,控制变量随机扰动方法估计的GRAPES全球3DVar同化系统AMSU-A亮温的背景误差正确合理。同化循环预报试验结果表明,亮温的背景误差在背景场检查中的应用显著提高了GRAPES全球3DVar同化的亮温资料的数量,显著提高了GRAPES南半球对流层中高层位势高度场的预报技巧。在GRAPES全球3DVar同化系统中推导和实现的控制变量扰动方法为诊断和分析GRAPES全球3DVar观测资料同化效果提供了有力工具。 

Estimation of background error for brightness temperature in GRAPES 3DVar and its application in radiance data background quality control

Background errors (BEs) for observable quantities are valuable for diagnostic analysis of the response of the data assimilation system to these observable quantities and play an important role in improving the variational data assimilation system. BEs are widely applied to assess impacts of observable quantities on analysis. The condition number of the Hessian matrix is determined by background errors for observable quantities, observation errors and observation count number. The background errors of observable quantities can also be used in the procedure of background check in data quality control. However, the background errors are specified in terms of those quantities that lead to a compact formulation of the background term J_b, i.e. the balanced stream function, unbalanced Exner pressure, velocity potential and specific humidity in GRAPES 3DVar system. It is not clear how the magnitudes of these background errors can be compared with various observation errors. The formulations of control variable randomization method in GRAPES are illustrated by applying the observation operators of GRAPES 3DVar system to a set of random vectors drawn from a population whose probability density function is given by an assumed background error covariance. This method is applied to estimate the background error for AMSU-A radiance data in grid-point space. The background error for radiance data is then applied in its quality control scheme. The results of forecast experiments show that more radiance data are assimilated using the new quality control scheme, which has a positive effect on geopotential height forecast in both the southern and northern Hemisphere. A statistically significant improvement in the anomaly correlation scores for geopotential height simulation in the southern Hemisphere at all forecast time is obtained. More than 0.2 day of forecast skill of geopotential height at 500 hPa is obtained in southern Hemisphere.