The phrase cold pool is used for several meteorological phenomenon, including topographically confined layers of cold air that are confined by a warmer layer aloft.
There are two types of cold pools: diurnal and persistent. Diurnal cold pools form during the evening or night and decay with surface heating the following day. Persistent cold pools occur mainly in the winter and last through at least one daily heating cycle or more. Colloquially, persistent cold pools in northern Utah are known as inversions.
There is a remarkable transition in the characteristics of cold pools in northern Utah from February to March. During this period, days are getting longer and the angle of the sun relative to the horizon at a given time of day is increasing, resulting in a dramatic increase in the amount of solar energy intercepted by the Earth's surface and available to heat the lower atmosphere.
Through about the end of February, especially when there is snow on the ground, there's insufficient solar energy to destroy cold pools that reside in many northern Utah valleys and basins. As a result, cold pools tend to be persistent. They can hang around for days and, where there are local emissions, lead to worsening air quality.
But in early March, we cross a threshold and cold pools typically become diurnal. They form at night, and decay during the day.
In the Salt Lake area, some of the most impressive diurnal cold pools from in the Rush Valley south of Tooele and the Snyderville Basin near Park City. Last night provided a nice example of the latter. Below are observations from the Park City area at about 7:15 AM this morning, which is near the time of minimum temperatures. At mountain locations, especially near ridgelines, temperatures were predominantly in the 20s. For example, at the Mount Baldy observing site at 9347 ft elevation at Deer Valley ski area, it was 27˚F.
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| Source: https://www.wrh.noaa.gov/map/?obs=true&wfo=slc |
However, at lower elevation in the Snyderville Basin, such as along I-80, US-189/40, and SR-224, in and around the area labeled "Parley's Park" on the map above, temperatures ranged from 12-15˚F. Such a cold pool forms frequently on clear nights over the Snyderville basin in part because it is relatively enclosed, which favors light winds that limit mixing with the warmer air aloft.
This time of year, however, the cold pool will erode quickly during the morning. Below is a time series of temperature, dewpoint, and relative humidity at Silver Creek Junction and the I-80 to US-189/40 interchange. Focus in particular on the period from 1-4 March, which also featured clear skis. There is a warming trend during this period with maximum and minimum temperatures increasing, but also a very large diurnal variation in temperature of about 45˚F between. Temperatures tend to rise rapidly in the morning and then stabilize near the maximum for a few hours in the afternoon and then fall quickly near and following sunrise, before falling more gradually in the late night and early morning hours.
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| Source: https://www.wrh.noaa.gov/map/?obs=true&wfo=slc |
On the 4th, the diurnal variation was much smaller. This was due to the clouds that moved in from Wednesday night to Thursday morning, resulting in less cooling. Diurnal cold pools can be very sensitive to the presence of clouds.
Excellent post on cold pools with great examples. Thank you. Pretty much every town/city in the West has had some kind of winter cold pools (Grand Junction, Steamboat Springs, Silverton, Front Range of Colorado, Tucson). Mainly because these locations are located in the valleys and surrounded by mountains to some degree. In Tucson, there can be cold pools and drainage within cold pools like near the usually dry rivers which can be 5 to 10 degrees colder than on the valley floor in the winter mornings .
ReplyDeleteRuss Scott