基于CMA-GEPS的延伸期预报能力评估

Assessment of Extended-Range Prediction Capability Based on CMA-GEPS

关键词:  CMA-GEPS  延伸期天气  集合预报  MJO  预报能力评估

Key words:  CMA-GEPS  extended-range weather  ensemble prediction  MJO  assessment of prediction capability

摘要:

基于 CMA-GEPS系统开展 1~35天的延伸期集合预报，并对该系统的延伸期尺度天气进行预报能力评估。结果表明：关于 500 hPa 位势高度，以距平相关系数（ACC）为表征的集合预报有效天数在北半球和南半球分别为9天和8.7天，且在北半球呈现季节循环特征，即冬（夏）季值高（低），为大气内在性质的表现；定量分析离散度-均方根误差关系表明，集合预报系统比确定性预报在延伸期尺度上可预报性更高，且北半球及南半球的潜在可预报天数分别为 18天和 16天。关于 2 m温度，CMA-GEPS在延伸期尺度上可较好地描述温度场的空间分布特征，其较大的系统偏差主要位于热力强迫显著的高原或沙漠地区。关于 MJO，CMA-GEPS 对 MJO 的有效预报技巧达到 15 天，优于一般的大气模式，说明 CMA-GEPS 有潜力进一步发展延伸期天气预报。进一步诊断分析表明：CMA-GEPS对MJO预报的强度偏弱，这与CMA-GEPS描述的热带对流系统偏弱有关；传播速度前 8天略偏快，8天之后偏慢；CMA-GEPS可较好地预报出 MJO 东传及北传运动；比较发现，CMA-GEPS对环流信号传播特征的预报优于对流信号，且描述的MJO东传优于北传特征。

Abstract:

This study assessed the extended-range prediction capability of the CMA-Global Ensemble Prediction System (CMA-GEPS). Using this system, we conducted 35-day ensemble prediction experiments and evaluated the forecast capability. Results showed that, at the geopotential height of 500hPa, the ACC skills of the extended-range ensemble prediction system were 9 days and 8.7 days in the northern and southern hemispheres, respectively. The ACC skills in the northern hemisphere exhibited a seasonal cycle with higher prediction skills in winter and lower skills in summer, reflecting the inherent atmospheric properties. Quantitative analysis of the spread-root mean square error (RMSE) skill relations revealed that the ensemble forecasting system was more predictive than deterministic prediction on the extended scale. The potential forecast days in the northern and southern hemispheres were 18 and 16 days, respectively. In terms of 2m temperature, CMA-GEPS effectively captured the spatial distribution characteristics of the temperature field on the extended period scale. Further analysis showed that prediction errors for 2m temperature mainly occurred in desert or plateau areas with significant thermal forcing effects. Additionally, CMA-GEPS demonstrated skillful forecasting of the Madden-Julian Oscillation (MJO) with a lead time of 15 days, outperforming other general circulation models. This suggested that CMA-GEPS has the potential to develop extended-range ensemble predictions further. Moreover, analyses revealed that the predicted intensity of MJO was weaker than the analyses, possibly due to a weak tropical convective system. The predicted propagation speed for the first 1-8 days was slightly faster, while the predicted propagation speed of the 9-35 days was slower compared to the analyses. CMA-GEPS effectively predicted the eastward and northward signals of MJO. Furthermore, the prediction of circulation signal propagation characteristics was better than that of convective signal, and MJO eastward propagation characteristics were better predicted than northward propagation.