青藏高原大地形对南海冷涌事件影响的数值模拟

Numerical simulations of Tibetan Plateau effects on cold surge events over South China Sea

利用大气环流模式NCAR CAM3,通过比较敏感性试验与控制试验的结果,讨论青藏高原大地形高度对南海北部冷涌事件及环流的可能影响。结果表明,地形的绕流作用对阿留申低压和东亚大槽的产生有着重要作用,当大地形不存在时,两者消失。地形高度变化对大陆冷高压的强度有影响,随着地形高度的降低大陆冷高压强度减弱。即大地形对中高纬行星尺度平均槽脊具有重要作用,地形高度降低会导致中高纬环流经向度减小,不利于冷空气南下,从而影响冷涌事件的强度和发生频率。当地形高度减半时,由于大陆东部地势相对平坦,南海北部北风强度增加,使得南海北部冷涌的出现频率略有增加。当无地形存在时,由于无青藏高原大地形的存在,使得东亚中高纬环流的经向度显著减小,冷空气南下活动明显减弱;同时没有大地形激发冷锋后的Kelvin波向南运动,导致南海北部北风强度减弱,南海北部冷涌的出现频率也显著减少。

Cold surge is one of main features of East Asian winter monsoon,and can lead to high influence weather,for example,lowtemperature and chilling injury. The cold surge over the northern South China Sea( SCS) is closely associated with the outbreaks of cold air influencing most part of China,and results from the interaction between the mid-high latitudes circulation and the lower latitude circulation. Southward movement of cold surge shows two kinds of gravity wave,which are influenced by dynamical effects of large-scale topography significantly. The high-frequency inertia-gravity and Kelvin waves are correspondence to the front of cold surge events and the maximum northerly center of cold surge,respectively. In this paper,based on the atmospheric circulation model NCAR CAM3,the influences of altitude changes of the Tibetan Plateau on the cold surge over the northern South China Sea and circulations have been investigated by comparing the results between the sensitivity experiments and the control experiment. The results showthat the flowaround topography is closely associated with the formation of the Aleutian lowand East Asian trough,and when the large scale topography does not exist,the Aleutian lowand East Asian trough disappear. The changes of topography altitude does not influence the formation of cold high over the continent,but it can influence its intensity,showing that the intensity of continental cold high decreases with the decrease of topography altitude of Tibetan Plateau. The large scale topography plays an important role on the planetary mean trough and ridge over mid-high latitudes. The decrease of topography altitude leads to the decrease of meridionality of circulation over mid-high latitudes,which is not conducive to cold air southward and further influences the intensity and frequency of cold surge events. When the topography altitude is halved,due to the eastern part of the continent is relatively flat,the intensity of the northerly over the northern SCS increases,leading to the increase of occurrence frequency of cold surge over the northern SCS. When there are not topography,the meridionality of circulation over mid-high latitudes in East Asia decreases significantly,and the southward movement of cold air decreases obviously. Meanwhile,the southward movement of Kelvin wave triggered by the Tibetan Plateau disappears,inducing the decrease of northerly over the northern SCS and the significant decrease of occurrence frequency of cold surge over the northern SCS.