Using Potential Vorticity to Represent the Effect of Stratosphere-Troposphere Exchange on Vertical Ozone Distributions in the Front Range during the FRAPPE 2014 Field Campaign

Stratosphere-troposphere exchange (STE) influences tropospheric column ozone (O₃) significantly, and quantifying its contribution to surface O₃ is crucial within the context of understanding the impacts of ozone precursor emission reductions. Here, the online coupled WRF-Chem model is used to simulate O₃ during FRAPPE (Front Range Air Pollution and Photochemistry Experiment), July-August, 2014. Continetal scale (12km resolution) and regional scale (4km resolution) simulations were performed using 4 different strategies for O₃ lateral boundary and initial conditions (BC/IC). Aircraft, ozonesonde, tower and surface data from FRAPPE are used for evaluation. The case using a simple climatological profile demonstrates that model O₃ biases within the boundary layer (BL) are highly correlated with biases well above the BL introduced by the IC/BCs. When IC/BCs are specified according to the larger scale global model or the O₃ derived from data assimilation within NWS/NCEP Global Forecast System, overall statistics show little bias, but transient multi-day features in observed upper level O₃ are not well reproduced. The case with upper level IC/BCs partitioned according to potential vorticity (PV) allows O₃ to be specified according to finer scale thermodynamic properties associated with STE. Resolved features as small as 80km horizontally and 2 km vertically demonstrate effective downward transport of O₃ from intrusion events, O₃ laminae over the western U.S., and helps explain some of the transient features observed in the upper-level O₃ observations. The exchange between upper-level and BL O₃ within the context of terrain induced wave forcing within the model is also discussed.