大气环流模式CAM4对亚洲气候的高分辨率模拟

A high resolution simulation on Asian climate by Community Atmosphere Model 4

中小尺度地形不仅在区域尺度上影响降水和环流形势分布,还会影响季风、西风急流等大尺度的天气系统。新的全球模式分辨率得到明显提高,本研究通过对比分析CAM4高分辨率配置下的模拟结果与高分辨率APHRO降水和ERA-Interim再分析资料,评估高分辨率CAM4对亚洲地区气候系统的模拟性能。结果表明,该模式不仅对亚洲季风-干旱气候系统的大尺度分布特征有良好的模拟能力,还能很好地模拟出季风区和内陆干旱半干旱区年均和季节平均降水的高、低值中心以及水平风场、地表温度和海平面气压的区域差异性特征,尤其是山脉地形附近的降水极值和风场变化。此外,CAM4与气象资料间在不同空间尺度上的空间相关性均很好,其中地表温度的相关系数在0.9以上,降水在亚洲地区超过0.7。

Background, aim, and scope Asian climate is characterized by the Indian monsoon, the East Asian monsoon and inland aridity, which is complex and sensitive to global change. The formation and variation of the climate system are correlated with lots of external forcings, such as solar radiation, landsea distribution and ice sheet. The Tibetan Plateau is also demonstrated to have a vital effect on the Asian monsoons and inland aridity. Besides the large scale plateau, the meso- and small-scale mountains also show great influence. These mountains not only affect the regional precipitation and circulation, but also influence large scale weather systems such as monsoons and westerly jet flow. The climate model CAM4 is with much higher resolution, it can reflect a more realistic topography and resolute those meso- and small-scale mountains to evaluate the climate effect of them. Materials and methods The precipitation data is a daily database from Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of Water Resources of Japan. Its spatial resolution is 0.25°×0.25° from 1951 to 2007. Other meteorological variables such as sea level pressure, horizontal wind and temperature is from ERA-Interim reanalysis data of European Center for Medium-Range Weather Forecasts（ECMWF）. The reanalysis data is from 1979 to present with a resolution of 0.75°×0.75°. Besides the meteorological data, the climate model we used to simulate is the CAM4. The CAM4 is the sixth generation of the NCAR atmospheric general climate model. The horizontal resolution is 0.47°×0.63°（latitude×longitude） in the simulation. Results With the apparently increased resolution of global climate model, the modeling result conducted by CAM4 with high resolution is compared with APHRO precipitation and ERA Interim reanalysis data to evaluate whether and in what extent the simulating capability is improved.（1） The CAM4 with high resolution can well simulates the large scale Asian monsoon — aridity climate pattern.（2） More importantly, the Asian monsoon precipitation are well presented, including the Indian monsoon rain centers over the west coast of the Indian subcontinent, the southern Himalaya mountains, Bangladesh and the west coast of Indochina peninsula, and the East Asian monsoon rain gradient decreases from southeast to northwest. The CAM4 also shows more precipitation in winter and less precipitation in summer over arid inland of the Central Asia, which is different compared to inland China, where the most raining season is summer.（3） The atmosphere circulation simulated in high-resoluted CAM4 is more realistic, such as the 850 h Pa wind difference over the Indian ocean, the Indian subcontinent, the Bay of Bengal and the Indochina peninsula of the Indian monsoon region, the wind direction turning from southwesterly to southeasterly over east China of the East Asian monsoon region, and the summer bypassing flows over the south Tianshan mountain in Xinjiang.（4） Besides, the surface temperature and sea level pressure patterns of the Asian monsoon and interior aridity are also well simulated, including the large scale climate systems such as the Siberian high, the Aleutian low and the western Pacific high, and the seasonal variations of the meso-scale pressure and temperature centers and gradients.（5） The spatial correlations show that the correlation between CAM4 and meteorological observation are significant on global, North Hemisphere and continent scale. The surface temperature has a very high correlation between high-resoluted CAM4 model and meteorological data. The correlation coefficient of surface temperature is above 0.9. The correlation coefficient of sea level pressure, 850 h Pa zonal wind and meridional wind is above 0.9, 0.9 and 0.8 on global scale, respectively. The correlation coefficient of precipitation over Asia is no less than 0.7. Discussion The results suggest that the precipitation center is always located over upwind regions no matter over monsoon or inland arid areas while the downwind regions always get minimum rainfall. The character is accordance with the relationship between the convective activity and mountains. Besides, the wind field also changes near mountainous regions and obviously different from other regions. The related temperature distribution can be directly influenced by temperature advection change caused by wind variation. As a result, the atmosphere circulation and vertical structure are changed, which can also influence large scale climate systems. Lots of studies show that more realistic precipitation and wind field can be simulated with higher topography resolution in climate models. Therefore, high-resoluted climate models need to be used to study the detailed monsoon and arid climate and inner difference over Asia and study the relationship between precipitation, atmosphere circulation and mountains. Conclusions The results show that the high-resoluted CAM4 model can improve the simulation skill over Asian, where the climate system is characterized by complex monsoons and aridity. With the improved resolution, the differences between the Indian monsoon and the East Asian monsoon, and between the arid Central Asia and inland China are obviously presented. Besides, it suggests a great influence of meso- and small-scale mountains on convection centers and precipitation distribution. Recommendations and perspectives The high-resoluted CAM4 model is very helpful for detailed study of the complex Asian climate and to deep our understanding of the influence of the mountain topogaraphy. The sensitivity experiments related to meso-scale mountain need to be designed to further finger out its climate effect.