Basin Cold Pools of the Rush Valley

As mentioned yesterday, we are in a pattern that lends itself to the development of strong cold pool, especially in topographic basins.  The Peter Sinks are the king of all Utah cold spots, but the Rush Valley is a close second and, more importantly, is well instrumented.

The Rush Valley is poorly named as it is really a closed hydrologic basin located south of Tooele and East of Provo.

Map Background Source: ACME Mapper

It is surrounded by a jumbled mess mountain ranges including the Stansbury and Onaqui Mountains to the west, the Sheeprock and Tintic Mountains to the south, the Oquirrh Mountains and Thorpe Hills to the east, and South mountain to the north.  Much of the valley floor is below 5100 feet, but the low point is at Rush Lake, just south of the Stockton Gap, at about 4960 feet.  The topography surrounding the Rush Valley has two major weaknesses.  One at what I call Stockton Gap (~5120 ft), the other at Five Mile Pass (~5300 ft).  Between the West and East Tintic Mountains, Boulter Summit, sits at about 6000 feet.

On a clear night like last night, the Rush Valley gets cold.  Really cold.  This morning, the minimum temperature at stations near the floor of the valley ranged from 29–34F, with the coldest temperature at the Pony Express Marker.

Minimum 24-h temperatures reported to MesoWest.  Winds
plotted are the most recent observation within the hour
ending at 1427 UTC 21 Sep 2011.

The time series from the Pony Express Marker shows a diurnal temperature range of about 50F.  As is often the case in basins, the temperature fell most rapidly during the first part of the night, then fell more slowly, reaching a minimum near sunrise.

Contrast this with Ophir Station (5561 ft) on the east bench or Vernon Hill (5761 ft), which sticks up in the southern part of the Rush Valley, where the minimum temperatures were only 53 F and 58F, respectively.   At Vernon Hill, temperatures remained between 58 and 63 F for much of the night.

One of the "cool" (pun intended) things about cold pools is that they tend to be most intense below the lowest gap, and this is supported by the evidence above.  In the case of the Rush Valley, that's Stockton Gap, where all sorts of wild things happen.  At the Stockton Bar MesoWest site, the maximum temperature yesterday was about 75 F.  There was a modest drop in temperature around sunset, followed by relatively steady temperatures until about 0430 UTC.  At this time, the temperature dropped from 65 to 58 F.

This temperature drop coincided with a wind shift from northerly to southerly, which likely occurred when the cold pool in the Rush Valley reached sufficient depth to pour through Stockton Gap.

Following this wind shift, the wind remained from the south for the remainder of the night, although you can see some warming and cooling cycles that are somewhat regular.  These could be due to sloshing of the cold pool (sometimes referred to as a seiche).

Curiously, the temperature at Five Mile Pass, which is about 180 feet higher than Stockton Gap, only dropped to 44F.  The shallow, intense cold pool near the valley floor does not extend as high as Five Mile Pass.  Whether or not this is due simply to nocturnal radiative and turbulent processes, which perhaps are not sufficient to deepen the cold pool that much, or due to a loss of mass through Stockton Gap, remains for further investigation.