Observations and Simulations of a Lake Breeze with High Ozone Concentrations in the Salt Lake Valley

Urban emissions from the Salt Lake Valley can be transported toward the Great Salt Lake by nocturnal down-valley flows. Pollutants over the lake are concentrated in a shallow boundary layer and may be transported back toward the city in the daytime by lake breezes. During the late afternoon of 18 June 2015, ozone concentrations in the densely populated Salt Lake Valley rapidly increased by ~20 ppb after the passage of a strong lake breeze front. Ozone observations from an enhanced network were available from state air quality measurement sites, additional fixed locations, and mobile platforms including a news helicopter. The southward progression of the well-defined lake breeze front through the Salt Lake Valley was observed by wind, temperature, and moisture observations available at automated weather stations as well as radial velocity scans from a nearby Terminal Doppler Weather Radar. Strong flow opposing the lake breeze increased convergent frontogenesis and delayed the onset of its passage through the Salt Lake Valley.

The ability of numerical models to simulate the development and progression of lake breeze in the Salt Lake Valley is investigated. Lake breezes on both 17 and 18 June 2015 were simulated using the Weather Research and Forecast model at 1-km horizontal resolution over northern Utah. The model was initialized at 0000 UTC 14 June 2015 using hourly analyses at 3-km resolution from the High Resolution Rapid Refresh model. The underlying surface state was improved by specifying the areal extent and surface temperature of the lake observed during June 2015. An urban canopy parameterization was added as well to better simulate urban effects on wind and heat fluxes. These modifications improved the model simulation particularly for the more typical lake breeze event on 17 June. However, on 18 June weaker than observed opposing southerly flow allowed the development of the simulated lake breeze front to occur too early and its subsequent speed up the valley was too fast. The results of this simulation are compared with the operational HRRR analyses and forecasts available on that day. This study highlights how the timing and intensity of subtle mesoscale features can affect a lake breezes and ozone concentrations within an urban region. Improvements to operational numerical weather models would add skill to air quality forecasts.