Development of a nonlocal convective mixing scheme with varying upward mixing rates for use in air quality and chemical transport models

Abstract

Background, aim, and scope: Asymmetrical convective non-local scheme (CON) with varying upward mixing rates is developed for simulation of vertical turbulent mixing in the convective boundary layer in air quality and chemical transport models. Materials and methods: The upward mixing rate form the surface layer is parameterized using the sensible heat flux and the friction and convective velocities. Upward mixing rates varying with height are scaled with an amount of turbulent kinetic energy in layer, while the downward mixing rates are derived from mass conservation. Results: This scheme provides a less rapid mass transport out of surface layer into other layers than other asymmetrical convective mixing schemes. Discussion: In this paper, we studied the performance of a nonlocal convective mixing scheme with varying upward mixing in the atmospheric boundary layer and its impact on the concentration of pollutants calculated with chemical and air-quality models. This scheme was additionally compared versus a local eddy-diffusivity scheme (KSC). Simulated concentrations of NO2 and the nitrate wet deposition by the CON scheme are closer to the observations when compared to those obtained from using the KSC scheme. Conclusions: Concentrations calculated with the CON scheme are in general higher and closer to the observations than those obtained by the KSC scheme (of the order of 15-20%). Nitrate wet deposition calculated with the CON scheme are in general higher and closer to the observations than those obtained by the KSC scheme. Recommendations and perspectives: To examine the performance of the scheme, simulated and measured concentrations of a pollutant (NO2) and nitrate wet deposition was compared for the year 2002. The comparison was made for the whole domain used in simulations performed by the chemical European Monitoring and Evaluation Programme Unified model (version UNI-ACID, rv2.0) where schemes were incorporated. © 2008 Springer-Verlag.