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

Abstract

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 X 10(-3) m(3) . s(-3). This means that the pollutants may be transported downward and accumulated at the bottom of boundary layer.