Numerical Simulations Of The Extreme Atmospheric Boundary Layer Height Over Northwest China And Their Impacts On Chemical Tracer Transportations

Atmospheric boundary layer is a layer that is strongly influenced by earth surface at timescale of one hour or less. Interactions between the layer and the surface are complex thus some mechanisms are still beyond our knowledge. Atmospheric convection and turbulence are main features that differ the atmospheric boundary layer from free atmosphere and essentially determine the atmospheric boundary layer height. The mixed height influences the vertical and long-distance transportations of mass and energy, and air pollutants diffusion. The curial question is:what elements mostly influence on the atmospheric boundary layer and its mixing layer height. For instance, the depth of the observed convective boundary layer can exceed 3000 m or even up to 4000 m of height at Dunhuang station in Northwest China. In order to characterize the atmospheric boundary layer conditions and to understand the mechanisms that cause the extreme boundary layer height, an advanced research version of community weather research and forecasting model (WRF) is employed to simulate observed extreme boundary layer heights in late May and early July of 2000. The ability of the WRF model in simulating the atmospheric boundary layer height over arid areas is evaluated to compare with another station named Zhangye. It is found that the WRF model is able to capture characteristics of the observed atmospheric boundary layer height. Several key parameters that contribute to the extremely high boundary layer height are identified by sensitivity experiments. Results demonstrate the influence of soil moisture, surface albedo and wind field on the simulation of the extremely high boundary layer height. It shows the negative correlation between atmospheric boundary layer heights and perturbation of soil moisture and surface albedo. Large wind speeds also give advantage background for the heights development. In addition, the choice of planetary boundary layer scheme, land surface models and forecast lead times also plays a role in accurate numerical simulation of the atmospheric boundary layer height. The best choices are provided for the control simulations in the previous sensitive experiments. Finally, a new version of community weather research and forecasting/Atmospheric Chemistry model (WRF/Chem) is applied to study the effect of atmospheric boundary layer height on the chemistry trace transportation and concentration. The results demonstrate the changed atmospheric boundary layer heights have greatly affected on transportation and concentration of the chemistry tracer. The high atmospheric boundary layer is good for the tracer transportation and thus the results show negative correction between the atmospheric boundary layer heights and the tracer concentrations.