BCC模式对北半球阻塞高压的模拟偏差评估及原因

基于NCEP/NCAR、日本气象厅的JRA55以及欧洲中期预报中心（ECMWF）最新发布的ERA5三套逐日再分析资料数据，考察国家气候中心中等分辨率（约110 km）的气候系统模式BCC-CSM2-MR和单独大气模式BCC-AGCM3-MR对北半球中高纬度阻塞高压（阻高）的模拟能力。再分析数据分析结果表明:“北大西洋—欧洲地区”以及“北太平洋中部地区”分别为北半球阻高发生的最高频及次高频区域；冬春季为阻高高发季节，夏秋季阻高频率减少至冬春季的一半左右；ERA5再分析资料中各个季节的阻高频率均高于另两套资料结果，尤其在北太平洋地区。模拟评估结果显示，单独大气模式BCC-AGCM3-MR对北半球中高纬度阻高发生频率、空间分布和季节变化特征均有较好的模拟能力，其主要偏差表现为冬春欧亚大陆特别是乌拉尔山地区阻高频率偏高，而北大西洋地区阻高频率偏低；春季北太平洋阻高频率偏低。这与模式北半球高纬度地区500 hPa位势高度场气候态偏差有关。BCC-CSM2-MR耦合模式的阻高模拟偏差总体与大气模式类似。但耦合模式中冬季欧亚大陆特别是乌拉尔山地区阻高频率减小、北太平洋春季阻高频率增大，模拟偏差减小。同时，耦合模式能够再现夏季北太平洋东西阻高频率双峰值特征。因此，海气耦合过程有助于改善对欧亚及北太平洋地区阻高频率模拟。阻高频率年际变率受到气候系统内部变率不确定性的较大影响，这也是制约阻高预测水平的重要因素。

Reproductions of Northern Hemisphere Blocking in BCC Models and Possible Reasons for the Biases

In this study, the performances of the median-resolution (about 110 km) climate system model (BCC-CSM2-MR) and atmospheric model (BCC-AGCM3-MR) developed at the National Climate Center are evaluated using three reanalysis datasets (ERA5, JRA55, and NCEP/NCAR). The results show that a high blocking tendency can be distinguished over the “North Atlantic–Western Europe” and “Central North Pacific”. The winter and spring blocking frequency is almost twice of that in summer and autumn. The blocking frequency in the ERA5 dataset is higher than that in the JRA55 and NCEP/NCAR datasets, especially over the North Pacific. Model evaluations show that the atmospheric model reproduces the main features of the Northern Hemisphere blocking frequency, spatial structures, and seasonal variations very well. The main biases of the model are overestimation over Europe–Asia in winter and spring, especially over the Ural Mountains, and underestimations over the North Atlantic. These biases are attributed to the climatological biases in geopotential heights at 500 hPa. The overall performances of the BCC-CSM2-MR model are similar to those of the BCC-AGCM3-MR model. However, the winter and spring blocking over Europe–Asia and especially over the Ural Mountains, is improved. Spring blocking and the double-peak blocking structure in summer over the North Pacific are also better reproduced in the coupled model. Therefore, air–sea coupling may help to improve the reproduction of blocking frequency over Europe–Asia and the North Pacific. The internal variability of the climate system has a great impact on the variation of the blocking frequency, which also affects the capability of the model to predict blocking highs.