Boundary layer evolution during a cold pool episode in a mountain valley – a case study

A cold air pool is a topographically confined, stagnant layer of air that is colder than the air above. Cold air pools frequently occur in mountain valleys and basins, especially in winter season. Persistent cold air pools that last for multiple days can trap air in valleys and basins, allowing pollution to build up and reach unhealthy level. Forecasting when a persistent cold air pool would form and how long it would remain in an area has proven to be challenging. The WRF model was used to simulate a persistent cold pool episode observed during the Persistent Cold Air Pool Study (PCAPS) field campaign in Utah's Salt Lake Valley. The WRF simulations were first validated using various in-situ and remote sensing observations in the valley. The model results were then used to understand the boundary-layer structure and evolution during this cold pool episode. A series of sensitivity experiments were designed to help determine the relative contributions of the surface forcing and synoptic forcing to the formation and break of this episode.