黄土高原复杂地形上边界层低空急流对近地层湍流的影响

Impact of low-level jets on turbulent in nocturnal boundary layer over complex terrain of the Loess Plateau

利用中尺度气象数值模式(Weather Research and Forecasting Model,WRF)模拟风场,结合兰州大学半干旱气候与环境观测站(Semi-Arid Climate and Environment Observatory of Lanzhou University,SACOL)湍流观测资料,分析了黄土高原复杂地形上稳定边界层低空急流对近地层湍流活动的影响.黄土高原复杂地形上稳定边界层低空急流的形成与地形作用引发的局地环流有关.低空急流对近地层湍流活动有强烈影响,剪切作用使小尺度湍涡活动加剧,湍动能增大,同时非平稳运动被压制.低空急流发生时,观测数据有87.3%是弱稳定情形(梯度理查森数小于0.25);而无低空急流时,对应时段的观测表明65.4%属于强稳定层结(梯度理查森数大于0.3),非平稳运动造成湍流功率谱在低频端迅速增大.与无低空急流和弱低空急流情形相比,强低空急流发生时,近地层湍动能增大1倍,湍动能在垂直方向上的传递增大1个量级,且方向向下,约为-3×10-3 m3·s-3,湍流在上层产生并向下传递.

This paper examines the impact of low-level jets（LLJs）on the turbulent in nocturnal boundary layer, using the data collected by eddy covariance （EC）and boundary layer meteorological measurement systems at the Semi-Arid Climate and Environment Observatory of Lanzhou University（SACOL）. The Weather Research and Forecasting model is used to understand flow structure characteristics in the stable boundary layer over the complex terrain of the Loess Plateau. Over the complex terrain of the Loess Plateau,the formation and maintaining of LLJs may be related to the local circulation caused by topography in a stable boundary layer. The presence of LLJs results in strong turbulence and weak stability with gradient Richardson number （Ri）less than 0.25 by the strong shear. On the other hand, synchronous observations show that 65.4% are strong stable stratification of Ri〉0. 3 ,in the absence of jet activity. The contribution from non-stationary motions results in sharp increasing in the low frequency end of power spectra for horizontal wind speed. Compared with the no/weak LLJs situations, the turbulent kinetic energy doubles when strong LLJs occur. Vertical transportation of turbulent kinetic energy has an order of magnitude increase in value,and the turbulent is transported downward from aloft in the boundary layer,about -3 × 10^-3 m^3 · s^-3. This means that the pollutants may be transported downward and accumulated at the bottom of boundary layer.