敦煌绿洲夏季边界层特征的数值模拟

Numerical Simulation of Summer Boundary Layer Characteristics in Dunhuang Oasis

使用美国NCAR新版MM5V3.6非静力平衡模式,采用三重嵌套方法,模拟研究了沙漠绿洲的环流及边界层特征,并且与无绿洲试验进行了比较。结果表明:沙漠中的绿洲在夏季是一个冷源,地面感热通量和潜热通量与周围的沙漠地区存在较大差异。在绿洲区,感热通量小,潜热通量大;在沙漠区,感热通量大,潜热通量小。绿洲边界层顶低,沙漠边界层顶高。绿洲可以改变沙漠地区的大气稳定度,使得原来大气层结不稳定的沙漠地区出现较稳定的大气层结。沙漠绿洲改变了原有沙漠地区的环流结构及温、湿场的分布,使得在绿洲上空大气下沉,沙漠上空大气上升,从而产生了绿洲上空大气干冷,沙漠上空大气暖湿的边界层特征。绿洲边缘的沙漠形成的较大的湿气柱围绕着绿洲,起到了保护绿洲的作用。沙漠绿洲低空呈现辐散气流,使得绿洲上风方的沙漠地区风速减小,绿洲下风方的沙漠地区风速增大。

To study desert-oasis circulation and boundarylayer characteristics in Dunhuang Oasis summer, a set of triple nested numerical simulation experiments with and without the Dunhuang Oasis was conducted, using NCAR nonhydrostatic meso-scale model, MM5V3.6 and the NCEP reanalysed grid data on July 24, 2001. The main results show: The oasis surrounded desert is a cold source in summer. The surface sensitive heat and latent heat fluxes in the oasis are different from its surroundings. In oasis, the sensitiveheat flux is small and latent heat flux is large; but vice versa, in desert. The boundary layer height is low in oasis and high indesert. Oasis could make the unstable stratified atmosphere in desert change to stablein oasis and it changes the circulation and the distribution of temperature and humidity there. The vertical motion is downwards over oasis and upwards over desert, then the air is dry and cold over oasis; warm and moist over desert. There is the wet air colume around oasis, which playsa key role in protecting oasis. It is divergence flow in lower layer over oasis which slow down the desert wind speed in upstream direction of the oasis, but quicken the desert wind speed in downstream direction of the oasis.