延伸期预报中大气初值与海温边值的相对作用

The relative roles of initial atmospheric condition and lower boundary condition of SST for extended forecast

中文关键词:  延伸期预报  初值  边值  BCC_AGCM

英文关键词:Extended range forecast  Initial conditions  Boundary conditions  BCC_AGCM

中文摘要:

      初始条件记忆和下垫面条件是延伸期可预报性的主要来源，它们在不同时段、不同区域的相对作用存在明显不同。利用中国国家气候中心最新一代大气环流模式BCC_AGCM，设计了由4组不同大气初值与海温边值构成的组合试验，研究了（大气）初值与（海温）边值对全球不同区域延伸期预报的相对作用。结果显示，模式预测技巧在3周以内强烈依赖于初值。在相同的边值条件强迫下，不同初值在月内尺度上的预测技巧差异非常明显，且在更长的时间尺度上，初值仍然能够提供一定的预测信息。从全球来看，边值强迫对预测技巧的影响从一周左右开始。在低层的850 hPa高度场上，边值的作用在热带地区于第4-5候与初值相当，其他区域的边值影响达到与初值相当的时间滞后于热带地区；在北半球500 hPa高度场上，边值的作用在热带地区第5候前后达到与初值作用相当，其他区域这个时间则推迟至第6候前后，比对低层的影响时间滞后1-2候；对于东亚地区而言，边值的贡献在第2候就已显现，对预测技巧产生了明显的改进。在延伸期尺度上，边值强迫主要作用的区域为低纬度区域，且对500 hPa高度场的影响更为稳定，在该区域第5候以后有较为明显的改进。初值与边值对延伸期预报都具有相当重要的作用，认识初、边值条件的相对作用能够为延伸期预报的改进奠定基础。

英文摘要:

      The initial condition and lower boundary forcing are the main sources of the predictability in extended range, and they play different roles in different time scales and different regions. Using the latest version of BCC_AGCM, four experiments with different combinations of atmospheric initial condition and sea surface temperature (SST) boundary forcing are designed in this study to investigate the roles of atmospheric initial condition and SST boundary forcing in the extended prediction over different regions around globe. Results show that the skills in extended prediction are strongly dependent on initial condition at lead times less than three weeks, and different initial conditions with the same boundary conditions lead to significant differences in the prediction skills at sub-monthly time scales. However, the initial condition can still provide some useful information for the prediction exceeding monthly time scales. From a global perspective, SST boundary forcing affects the prediction skill at lead times of more than one week. It will take roughly four to five pentads for boundary conditions to have the same effect as initial conditions have on the predictability of geopotential height at 850 hPa in the tropics, and this lag time will be longer in other regions. Similar results can also be found in the geopotential height prediction at 500 hPa except that the lag time is 5 pentads in the northern hemisphere and 6 pentads in other regions, which means the above effects in the upper atmosphere lag behind that in the lower atmosphere by one to two pentads. In the region of East Asia, the contribution of boundary forcing to the prediction skill appears on the second pentad accompanied with significant improvements in forecast at this time. As for the extended forecast, impacts of the lower boundary forcing are concentrated in low-latitude regions, and are more stably shown in the forecast of geopotential height at 500 hPa. Significant improvements caused by the boundary forcing are shown in the forecast after the 5th pentad. The results indicate that both initial conditions and boundary conditions are very important in extended forecast, and understanding the relative roles of initial and boundary conditions in the extended prediction is the basis to improve forecast skills.