Monday, January 14, 2019

Contrasts between Persistent and Diurnal Cold Pools

Great examples of persistent and diurnal cold pools exist presently over northern Utah.

The persistent cold pool exists over the Great Salt Lake Basin, including the Salt Lake Valley, where widespread fog and stratus are present, as illustrated by yesterday afternoons modis imagery.  It is sometimes difficult to discern fog and stratus from snow in visible imagery, but the smooth nature of the coverage and the inability to see the Great Salt Lake are dead giveaways. 

Modis Imagery 13 January 2019
One can also use multiple satellite channels to detect fog and stratus, as is done in the image below for 0915 UTC (0215 MST) this morning.  Note the area of white covering the Great Salt Lake Basin, as well as the Uinta Basin to the east. 

Source: NWS Aviation Weather Center
The sounding from the Salt Lake City International Airport shows a relatively deep cold pool extending from the surface to 814 mb (6250 ft).  The temperature then increases 6ºC in a sharp inversion layer (indicated by orange).  Above that level, the atmosphere is quite dry throughout the remainder of the troposphere (green). 

Sounding source: SPC
The low clouds over the Salt Lake Valley sit right at and below the base of the inversion, resulting in a sea of stratus covering the Salt Lake Valley, as can be seen from the Snowbird Prism Cam.

Source: Snowbird
Below the stratus, a dreary, dark scene exists, with cloud base somewhat poorly defined.  The photo below was taken looking south from the upper Avenues. 

As we have discussed previously, although it looks dark and dreary, the reality is that a deep cold pool of this type, capped by stratus, results in mixing through a deeper layer than when the inversion is based right at the valley floor.  As a result, while there is still pollution, the concentration levels along the valley floor are not as bad as in periods when the inversion is lower.  Note, for example, how PM2.5 concentrations over the past couple of days have been lower than they were on the 10th and the 11th. 

Source: Utah Division of Air Quality
In contrast, a diurnal cold pool exists this morning in the Snyderville Basin near Park City.  There, with clear skies overnight, temperatures plummeted and at 15:16 UTC (0816 MST) were -8ºF at Silver Junction and Kimball Junction.  It was also 2ºF at the Park City Golf Course.   In contrast, it was in the mid 20s mid mountain at Park City Mountain Resort. 

Although the persistent cold pool present over the Salt Lake Valley doesn't burn off during the day, the cold pool in the Snyderville Basin is diurnal.  It forms at night and burns off during the day. 

Compare, for example, the temperature traces (red line) at the Salt Lake City airport and Silver Creek Junction (note scale change).  There is practically no diurnal (daily) temperature cycle at the Salt Lake City International airport.  Instead, there is a gradual decline in temperature during the period.  This reflects the presence of low clouds, which reduce incoming solar radiation received at the ground as well as the overnight cooling.  We remain capped by an elevated inversion all day long. 

In contrast, there is a huge diurnal cycle at Silver Creek.  Temperatures last night and the previous night fell to below zero, but yesterday afternoon, the high was 30ºF.  Here, a shallow cold pool forms under clear skies each night, and then burns off during the day. 

Residents of the Summit Park area observe some remarkable temperature variations on days like this.  Note how it is -8ºF at Kimball Junction, -6ºF along Pinebrook Boulevard near I-80, 9˚F near the top of Pinebrook Road, and 21˚F across I-80 at the Parley's Summit SNOTEL.

My skate ski today, which will be my final ski of the season in Utah, will need to wait until afternoon when temperatures are more tolerable...


  1. Have noticed something a little unique with this inversion. There is a large area of snow cover over Wyoming and eastern Idaho which (coupled with clear skies in those areas) has been generating an area of very cold surface temps. A lot of this cold air has been overflowing into northern Utah from the northeast, which I think has strengthened the inversion at SLC in terms of temperature structure, but at the same time has apparently pushed out most of the pollution with improving air quality from north to south over the past few days. There has been enough moisture remaining to keep the stratus deck mostly intact, but the dew point and temperature have been dropping near the surface with this colder and drier air filtering in from the northeast.

    1. I certainly agree too about the stratus and deeper mixed layer as described, but thought this could possibly be an additional factor improving air quality in this case. Dave