Abstract
We examine the influence of a passing weather system on a persistent cold-air pool (CAP) during the Persistent Cold-Air Pool Study in the Salt Lake Valley, Utah, USA. The CAP experiences a sequence of along-valley displacements that temporarily and partially remove the cold air in response to increasing along-valley winds aloft. The displacements are due to the formation of a mountain wave over the upstream topography as well as adjustments to the regional horizontal pressure gradient and wind-stress divergence acting on the CAP. These processes appear to help establish a balance wherein the depth of the CAP increases to the north. When that balance is disrupted, the CAP tilt collapses, which sends a gravity current of cold air travelling upstream and thereby restores CAP conditions throughout the valley. Intra-valley mixing of momentum, heat, and pollution within the CAP by Kelvin–Helmholtz waves and seiching is also examined.
Highlights
The disruption of persistent cold-air pools (CAPs) arising from passing weather systems is examined
Based on all the PCAPS data available, we show that the Intensive Observing Period 1 (IOP-1) CAP progressed through four stages in its life cycle: formation, disturbance, persistence, and break-up (Fig. 2)
In this paper we have documented the complex evolution of a persistent cold-air pool (CAP) that was disturbed by a passing shortwave trough
Summary
The disruption of persistent cold-air pools (CAPs) arising from passing weather systems is examined. Lee and Pielke (1989) examine the interaction of a mountain wave with a lee-side CAP in idealized simulations, and show that the CAP is displaced by the mountain wave unless there is an adverse pressure gradient generating an opposing surface flow toward the mountain They found that turbulent erosion was a minimal factor in CAP evolution despite strong shear. We use data collected in the Salt Lake Valley of northern Utah during the Persistent Cold-Air Pool Study (PCAPS, Lareau et al 2013) to examine the passage of a shortwave trough during a multi-day CAP This trough–CAP interaction produced a variety of waves, displacement and fronts that disrupted the otherwise quiescent CAP. We analyze the observed changes in CAP structure and develop a simple conceptual model for the trough–CAP interaction based on the observations and an idealized numerical simulation using a large-eddy simulation (LES) version of the Weather Research and Forecasting (WRF-LES) model (Skamarock et al 2008)
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