Simulating the Evening Transition in the Pbl and Evaluating Its Contribution to the Next Day Prediction Ozone Peak Concentration



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The transition from the convective boundary layer during the daytime to the stable stratified boundary layer during nighttime after sunset plays an important role in the transport and dispersion of atmospheric pollutants. However, our knowledge regarding this transition and its feedback on the structure of the subsequent nocturnal boundary layer is still restricted. This also prevents forecast models from accurate prediction of the onset and development of the nighttime boundary layer, which determines the redistribution of pollutants within the nocturnal surface layer and the residual layer aloft. Turbulence in the stable nocturnal boundary layer is generally weak and typically characterized by intermittent bursts of activity. It often exists in isolated layers generated primarily from localized shear instabilities. As a result, turbulence is rarely in equilibrium with the conditions of the underlying surface. Given the layered structure of the nocturnal boundary layer, the spatial and temporal characteristics of turbulent activity can have a significant effect on local air quality at hourly to diurnal scales. In a case study using Large-Eddy Simulation was found a weak, but noticeable nighttime turbulent kinetic energy produced by wind shear in Houston’s planetary boundary layer (PBL). This may likely be related to observations made at the UH Moody Tower on at least two Vertical Mixing Experiment days, when peaks of carbon monoxide occurred in the evening, while the variability in wind conditions was very little. This supports a hypothesis of intermittent turbulence acting in the concentration of pollutants measured at the UH Moody Tower air quality station The overall goal of this study was to improve the current understanding of dynamical processes of the diurnal cycle of the PBL that impacts the vertical distribution of pollutants, with particular attention to the representation of the late afternoon in meteorological and air quality models. This goal was assessed throughout the sensitivity analysis of a more realistically representation of diffusion process during the decay of the convective boundary layer, typical of the evening transition, in a widely-used weather and air quality forecast model (WRF/Chem). Improvements in O3 precursors were possible without a significant increase in complexity or computer time.



Ozone, PBL, Air quality, WRF/Chem, LES, SCM, Eddy diffussivity