西北太平洋热带气旋潜势分布和年际变率的数值模拟

The tropical cyclone genesis potential index over the western North Pacific and its interannual variability as simulated by the LASG/IAP AGCM

热带气旋潜势指数是定量表征影响热带气旋生成的大尺度环境条件指标,在不能显式模拟热带气旋的气候系统模式中,常被作为热带气旋的代用指标。基于中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室发展的AGCM GAMIL2.0模式在历史海温驱动下的积分结果,评估了该模式对热带气旋潜势气候态、季节循环和年际变率的模拟能力。并分别从影响热带气旋潜势分布的热力因子(相对湿度、热带气旋最大风速)和动力因子(垂直风切变、绝对涡度、垂直抬升速度)的角度,讨论了造成热带气旋潜势模拟误差的原因。结果表明,在西北太平洋地区,模式能够合理再现热带气旋潜势的气候态分布,但由于GAMIL2.0模拟的相对湿度偏大且向东延伸,造成了热带气旋潜势大值区较之再分析资料偏大且偏东10°。由于GAMIL2.0模拟的季风槽位置偏北偏强,导致模拟的热带气旋潜势季节循环北进偏早而南退偏晚。在年际变化方面,GAMIL2.0能合理模拟出热带气旋潜势在ENSO正负位相东西反向的变化特征,但位于20°—30°N的加强和减弱区的分界线偏西,这与模拟的垂直速度和相对湿度的模拟误差有关,进一步分析表明,这是由于模拟中ENSO事件期间的西北太平洋异常上升中心比观测偏西且偏强造成的。

The tropical cyclone genesis potential index （GPI） is a useful metric for gauging the performance of global climate models in the simulation of tropical cyclone genesis. The authors have assessed the performance of the LASG/IAP AGCM GAMIL2.0 in the simulation of GPI over the western North Pacific （WNP）. Since the value of GPI is determined by large scale environmental factors including the low-level vorticity （850 hPa）, the relative humidity at 700 hPa, the magnitude of vertical wind shear between 850 and 200 hPa, the maximum the potential intensity （MPI） and the vertical velocity, the bias of the model in the GPI simulation is discussed by analyzing these foregoing thermal and dynamical factors. The model results are compared to the ERA-40 reanalysis data. The results show that both the spatial pattern and the seasonal cycle of the GPI distribution in the WNP are reasonably simulated by the GAMIL2.0. Due to the overestimate of the relative humidity, the simulated GPI extends eastward to 170°E, which shifts eastward about 10° relative to the reanalysis. Another deficiency of the model is that the monsoon trough is about 5° north to the reanalysis; the model overestimates the GPI in the early season （May-June） and later season （Sep- Oct）, but underestimates the GPI in the mature season （July- Aug）. For the interannual variation, the response of GPI to ENSO in WNP is well simulated, including the eastward shift of genesis location during the E1 Nino phase, and westward shift during the La Nina phase. One limitation of the simulation is that the anomalous convective center shifts westward about 20° in comparison to the reanalysis, which leads to the biases of both vertical velocity and relative humidity distribution. Thus the boundary between positive and negative bias zones over 20° - 30°N shifts westward. The analysis has pro- vided a useful reference for the future improvement of the model.