Abstract
Abstract. The turbulent flux parameterization schemes in the surface layer are crucial for air pollution modeling. There have been some deficiencies in the prediction of air pollutants by atmosphere chemical models, which is closely related to the uncertainties of the momentum and sensible heat fluxes calculated in the surface layer. The differences between two surface layer schemes (Li and MM5 schemes) were discussed, and the performances of two schemes were mainly evaluated based on the observed momentum and sensible heat fluxes during a heavy haze episode in Jing-Jin-Ji in eastern China. The results showed that the aerodynamic roughness length z0m and the thermal roughness length z0h played major roles in the flux calculation. Compared with the Li scheme, ignoring the difference between z0m and z0h in the MM5 scheme induced a great error in the calculation of the sensible heat flux (e.g., the error was 54 % at Gucheng station). Besides the roughness length, the algorithm for the surface turbulent flux as well as the roughness sublayer also resulted in certain errors in the MM5 scheme. In addition, magnitudes of z0m and z0h have significant influence on the two schemes. The large z0m and z0m∕z0h in megacities with a rough surface (e.g., Beijing) resulted in much larger differences of momentum and sensible heat fluxes between Li and MM5, compared with the small z0m and z0m∕z0h in suburban areas with a smooth surface (e.g., Gucheng). The Li scheme could better characterize the evolution of atmospheric stratification than the MM5 scheme in general, especially for the transition stage from unstable to stable atmospheric stratification, corresponding to the PM2.5 accumulation. The biases of momentum and sensible heat fluxes from Li were lower, about 38 % and 43 %, respectively, than those from MM5 during this stage. This study indicates the superiority of the Li scheme in describing regional atmospheric stratification and an improved possibility of severe haze prediction in Jing-Jin-Ji in eastern China by coupling it into atmosphere chemical models.
Highlights
Adequate air quality modeling relies on the accurate simulation of meteorological conditions, especially in the planetary boundary layer (PBL) (Hu et al, 2010; Cheng et al, 2012; Xie et al, 2012)
The definitions and influences of the roughness sublayer (RSL) on the calculation of the turbulent flux are discussed in detail
It is necessary to consider the RSL effect in the calculation of the turbulent flux, especially for rough terrain such as forest or large cities. z0m is defined as the height at which the extrapolated wind speed following the similarity theory vanishes
Summary
Adequate air quality modeling relies on the accurate simulation of meteorological conditions, especially in the planetary boundary layer (PBL) (Hu et al, 2010; Cheng et al, 2012; Xie et al, 2012). The relationship between the flux and the atmospheric profile in the atmospheric surface layer is a critical factor for air pollution diffusion, especially under stable stratification conditions (Li et al, 2017). Another study (Vautard et al, 2012) of mesoscale meteorological models pointed out there was a systematic overestimation of near-surface wind speed in the stable boundary layer which should contribute to the underestimation of surface concentrations of primary pollution. The observed momentum and sensible heat flux data covering one complete haze process at Gucheng station were used to evaluate the two schemes, focusing on the transition stage from unstable to stable atmospheric stratification, corresponding to the PM2.5 accumulation. This study may provide the prerequisite for coupling the Li scheme into atmosphere chemical models in the future
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