MM5三维变分系统在北京地区冷暖季背景场误差的对比分析

Contrast Analysis of Background Error of MM5 3DVAR System in Cold and Warm Seasons in Beijing

NMC方法是目前较广泛采用的一种对模式背景场误差协方差进行统计分析的一种方法。本文根据积累的2002年8月份和2003年2月份各一个月模式预报结果,采用NMC方法,计算了中尺度模式MM5V3在北京地区的冷暖季背景场误差,详细给出其气候统计特征。通过对比分析发现,背景场误差特征对于不同的模式变量、水平分辨率、垂直层各不相同,冷暖季背景场误差也有不同的特征,其差别主要表现在风场。这些特征与模式模拟区域的平均天气状况相对应,同化应该在各模式区域分别进行。MM5三维变分系统在北京地区的实际应用中,应发展根据实际季节变换背景场误差协方差矩阵的方法。

NMC method is widely used to statistically analyze background error covariances currently. In this paper, based on a month-long series of model forecast fields in August 2002 and February 2003, the background error covariance of high-resolution mesoscale model MM5V3 in cold and warm seasons, which is about to pre-operationally run in Beijing region, is computed using NMC method. The detail climatological statistical characteristics are present as well. The results show that different model variables has different background error characteristics, even the same model variable has different background error characteristics when model resolution or model vertical-level changes. The characteristics are also different in cold and warm seasons, mainly wind field. All those characteristics are corresponding to averaged weather condition in the forecast region. Consequently, 3DVAR should be done respectively for different model domains. In practical run of MM5 3DVAR system in Beijing, method used to compute and update background error covariance should be developed incorporating actual seasonal variation of weather condition.