南极中山站夏季下降风数值模拟个例研究

Numerical simulation of summer katabatic wind at Zhongshan Station,Antarctica:A case study

南极内陆地面辐射冷却产生的近表层冷空气,沿高原斜坡向下流动而形成下降风,其分布形态决定了南极大陆近表层风场的主要特征。我国南极中山站全年均受下降风的强烈影响。夏季晴天时,中山站的下降风一般在傍晚开始出现,风速在午夜达到极值,在次日中午之前逐渐减弱,风速有显著的日循环特征。本文选取南极中山站2010年1月的夏季下降风个例,使用常规地面气象观测资料和Polar WRF极地大气数值模式进行了分析研究。结果表明：中山站夏季夜间晴天出现偏东向的下降风时,近地面风速变化趋势与地面气温呈负相关,相关系数为-0.91。数值模拟发现,中山站下降风在距地面高度约100～150m之间时风速最大,约为15～21m/s。在下降风发生时,近地层大气存在逆温现象。下降风较强时,近地层逆温也较强,逆温层厚度约为200～300m,逆温强度约为4～6℃。在地面摩擦的作用下,中山站近地面下降风风向为东南,随着高度的增加,风向逆时针偏转,最终趋于与地形等高线平行。没有太阳直接辐射时,南极大陆地区存在持续的逆温层,逆温层的出现加强了下降风气流,随着逆温的增强,大风区逐渐西移,且面积不断增加。在夏季太阳辐射造成的逆温消失的短暂时间内,逆温时产生的下降风尚不能完全消失,由此形成了较稳定的风向空间分布特征。

Katabatic winds are airflows that occur above a cold sloped surface.They are driven by gravity that causes colder and denser air masses to move downhill.Katabatic winds play a crucial role in the surface wind regime of Antarctica.Chinese Zhongshan Station,Antarctica,is strongly influenced by the katabatic wind all the year round.In sunny summer days,the katabatic wind at Zhongshan Station usually begins in the evening.The wind reaches its maximal speed at mid-night and then decreases before the next noon,with a significant diurnal cycle characteristics.The katabatic wind case in austral summer,January 2010,at Antarctic Zhongshan Station is analyzed with employing of conventional meteorological observations and the Polar WRF model.The results indicate that when katabatic wind emerges at nights of clear sky,the wind speed near the ground and surface air temperature trends are negatively correlated to-0.91.Simulations from Polar WRF show that the height of the maximum speed of the katabatic wind,15-21m/s,is between 100 and 150mfrom the ground.Near surface air temperature inversion always accompany with the katabatic wind.With the thickness of 200 to 300m,the inversion has its strength of about 4-6℃.The near-ground katabatic wind at Zhongshan Station always blows from the southeast due to the ground friction.The wind direction deflects counterclockwise and eventually becomes parallel with the terrain contours while the height increasing.When there is no direct solar radiation,continuous inversion presents at Antarctica continent and enhances the katabatic air flow.With the enhancement of the inversion,windy area gradually shifts westward and the acreage increases.In the short time when inversion disappears due to solar radiation in summer times,the katabatic wind would not completely disappear,thus forming a relatively stable spatial distribution of wind direction.