The Uinta Basin Snow Shadow: Impact of Snow-Depth Variation on Winter Ozone Formation

After heavy snowfall in the rural Uinta Basin, Utah, elevated surface ozone can occur when a persistent, reinforcing pool of cold air traps emissions from local oil and gas industry operations. Insolation permits photolysis and unhealthy buildup of surface ozone. Wintertime ozone is rare in the U.S. Intermountain West—more so a summertime issue in urban areas—and historically predicted poorly in numerical models. It follows a better understanding of prevailing snow patterns is important for constructing predictive models of ozone concentration. The Basin’s location leeward of the Wasatch Mountains provides conditions for a precipitation shadow, where rain or snow accumulation is suppressed by sinking air close to the lee. Mechanically, in westerly flow during winter, descending air parcels may accelerate, warm, and dry, hence clearing clouds that limit snowfall. Complicating spatial variations, wind-driven forcing may erode snow depth through drifting, melting, sublimation, etc. Sparse in situ observations and poor coverage of low-elevation radar tilts blocked by surrounding terrain compound the difficulty of snow-depth prediction, and due to tight connection with ozone formation, complicating prediction of snow sufficient to initiate the cold pool reinforcing feedback loop. Expectedly, ozone concentration observations track with snow coverage; more challenging was diagnosing a shadow effect and impact on ozone due to data sparsity and noise. The uncertainty but importance in linking snowfall variation to serious impacts of ozone levels on economy and public health motivates efforts to improve the region’s snow-depth measurements for purposes of nowcasting and machine-learning training.