| A basic understanding of the characteristics of turbulence in the boundary layer and the fluxes between the earth’s surface and atmosphere in the semiarid region of the Loess Plateau is of significance for understanding of the land-atmosphere interaction processes and their climate feedback mechanism, improving the accuracy of numerical weather and air pollution forecast and other aspects. However, the complex terrain limits the understanding on the turbulence of this region.To gain an insight into the characteristics of turbulence in the boundary layer over the complex terrain of the Loess Plateau, data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) are analyzed. Vertical advection and soil heat storage are main sources of imbalance and contribute nearly80%of the energy residuum; the composite of the minor surface energy balance terms, including air heat storage, vertical movement of moisture in soil and photosynthesis, jointly account for10%of the residuum. The flux attenuation due to spatial separation of sensors and sampling frequency could not be ignored, which alerts us to the possibility that CO2flux may also be underestimated. On the other hand, the losses of turbulent energy fluxes influence CO2flux derived from open-path analyser via application of the WPL algorithm. In the semiarid region of the Loess Plateau, during the daytime, the strong density effect caused by sensible heat results in CO2flux observation precision strongly dependent on the sensible heat flux. While in the nighttime, the behavior of CO2concentration spectrum is consistent with that of water vapor concentration.Non-stationary motions are the important cause of turbulence, resulting in fluxes being scattered, deviating from that estimated by the mean airflow, accompanied by great fluctuations in horizontal velocity but much smaller fluctuations in vertical velocity, and the turbulence is intermittent. Turbulence in a stable boundary layer can be divided into two different forms of movement,"local turbulence" shear generated with small-scale, and "nonstationary motions" with scales just greater than those of the local turbulence, and the scale analysis shows that the critical scale between the two is2-4min. Under weakly satble condition, the standard deviation of u, v, and w normalized by friction velocity u is2.54,2.21and1.35, respectively. The generation of turbulence by non-stationary motions is a dominant factor in turbulence when winds are weak. A method has been proposed to identify and efficiently isolate nonstationary motions from turbulence series, and then the characteristics of non-stationary motions are examined. Nonstationary motions occur more often with decreasing wind speed, and last from a few to20minutes with the nonstationary velocity around1.0m s-1. Unlike site over homogeneous and flat underlying surface where turbulence nonstationarity is a function of stability, over a complex terrain site such as SACOL, the wind speed is a much more significant factor rather than the stability parameter to determine the occurrence frequency of nonstationary motions. Then turbulence is categorized into three regimes based on the behaviors of nonstationary motions and local turbulence. Regime1considers stationary turbulence with a wind speed greater than3.0m s-1, and the Monin-Obukhov similarity theory (MOST) can be used to calculate the turbulence momentum flux. Regime2examines intermittent turbulence where the MOST is competent to evaluate the local turbulence momentum flux, but not nonstationary motions. Regime3involves wind speed that is less than the threshold value, where nonstationary motions are dominant, local turbulence is independent of the mean flow, and where the MOST may well be invalid.The presence of low-level jets (LLJs) results in strong turbulence and weak stability with gradient Richardson number (Ri) less than0.25by the strong shear. The turbulence is more active and stationary, transported downward from aloft in the boundary layer, about-3×10-3m3s-3. With the absent of jet activity, synchronous observations show that65.4%are strong stable stratification of Ri＞0.3, and non-stationary motions are more frequent. |
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