Partial Removal of Stable Boundary Layers in an Urbanized Mountain Basin

An observational analysis of the partial removal of a cold-air pool (CAP) from Utah's Salt Lake Valley is presented. The analysis uses data collected during IOP 1 of the Persistent Cold-Air Pool Study, including measurements from surface meteorological stations, radiosondes, radar wind profilers, a radio acoustic sounding system (RASS), and a laser ceilometer.

The CAP erosion occurred at night during a period of enhanced regional pressure gradients, warm air advection, and strong southerly winds associated with an approaching short-wave trough. Strong winds and warm air first penetrated to the surface within the southern and western portions of the Salt Lake Valley, displacing the CAP to the north. The boundary between the cold air within the CAP and the warm air to the south formed a pseudo-warm front, which moved northwards through the valley while southerly winds increased aloft. The temperature difference across the front was as much as 8 K Km-1. As the front approached an observing site in the valley center, data from the wind profiler, RASS, and ceilometer indicate that the CAP thinned from the top down. Simultaneous radiosonde observations also showed that the CAP developed a sloping upper surface, wherein the depth of cold air increases towards regions of lower pressure. As the front passed over the observing site a burst of strong winds accompanied a rapid increase in temperature. Elsewhere in the valley, the front pulsed north and south, crossing over some measurements sites multiple times. Later, as the mid-tropospheric trough moved downstream, the front returned southward, restoring CAP conditions to all locations within the valley.

We hypothesize that turbulent erosion, advective processes, mountain waves, and density current dynamics all contribute for the displacement of the CAP, though high-resolution numerical simulations will be required to elucidate the comparative roles of these disparate mechanisms.