变网格大气模式对1998年东亚夏季风异常的模拟研究

本文利用法国国家科研中心(CNRS)动力气象实验室(LMD)发展的可变网格的格点大气环流模式LMDZ4,对1998年东亚夏季降水进行了模拟,考查了变网格模式对东亚夏季降水的模拟性能.结果表明,模式在一定程度上能模拟出东亚夏季降水的极大值中心、夏季风雨带以及降水由东南向西北递减的空间分布特征.模式基本再现了1998年夏季两次雨带的进退特征,包括降水强度、雨带范围等,从而合理再现了1998年夏季江淮地区的"二度梅"现象.与观测相比,模拟的不足在于:在陡峭地形区附近存在虚假降水;江淮和华北地区以及四川盆地存在水汽输送的气旋式辐合偏差,同时高层环流辐散偏强,使得下层暖湿空气辐合上升、降水偏多;在东南地区存在反气旋式的水汽输送偏差,30°N以南地区降水偏少.对于1998年的"二度梅"现象,模拟偏差主要表现为长江中下游地区两次(特别是第二次)较强降水持续时间偏短,强降水范围偏小,而黄淮和华南地区却降水偏多.分析表明,模式对两次梅雨期降水的模拟偏差直接受环流形势模拟偏差的影响.LMDZ4区域模式版本的特点一是区域加密,二是加密区内预报场每10天向再分析资料恢复一次.敏感试验结果表明,LMDZ4加密区向强迫场的10天尺度恢复总体上有利于提高模式对华北降水的模拟能力,而对长江流域和华南降水的模拟具有不利影响.较之均匀网格模拟试验,加密试验由于在东亚的分辨率大大提高,对东亚夏季降水模拟效果更好.

Simulation study on the anomalous East Asian summer monsoon in 1998 using a variable-grid atmospheric general circulation model

In this paper, a variable-grid atmospheric general circulation model LMDZ4 developed by French National Center for Scientific Research (CNRS) Laboratoire de Météorologie Dynamique (LMD) was used to simulate the 1998 East Asian summer precipitation. The performance of the variable-grid model in the simulation of summer precipitation over East Asia is examined. The results show that the summer flood of 1998 over East Asia is generally reasonably simulated by LMDZ4. Both the maximum center and the major rain-band, including the decreasing trend of the total precipitation from the southeast to the northwest, are reproduced to a certain degree. The strength of the model is that it reasonably reproduces two march and two retreat processes of the rain band, including both rainfall intensity and rain band coverage, and thus well reproduces the double-Meiyu over the Yangtze River Basin during the summer of 1998. However, the model still shows some weaknesses. A large bias is seen in the precipitation field over steep topography. Meanwhile, there are more low-level moisture transport convergences and stronger high-level circulation divergences over the Yangtze-Huai River Basin, North China and Sichuan Basin, which lead to more precipitation over there. Whereas, anti-cyclonic moisture transport biases and associated less precipitation are seen over the region south to 30°N. As for the double-Meiyu over the Yangtze River Basin (YRB) during 1998 summer, the duration period is shorter and the coverage is smaller than the observations for the two heavy precipitation events (especially for the second one). The model simulates more precipitation over the Yellow-Huai River Basin and South China (SC). Analysis shows that the simulated circulation biases well explain the short of model in simulating precipitation. The exceptional characteristics of LMDZ4 regional version lie in local zoom and the forecasting fields in the zoomed region recovering to reanalysis data at a 10-day timescale. The results from sensitive tests show that the recovery generally improves the simulation of precipitation over North China, but has bad effect on that over YRB and SC. Compared with the uniform-grid simulation, the local zoom at East Asia greatly increases the resolution and produces better simulation for the summer precipitation over East Asia.