Complexities of Valley Cold Pools

As wonderful as the snow we have had since yesterday is (14" at Alta Collins as of 8 AM), what was really fascinating about this storm is how it affected the valley cold pool and associated pollution that has been resident over the Salt Lake Valley the past few days.  

The valley cold pool is the layer of cold air that resides in the Salt Lake Valley during what are known colloquially as inversions.  I prefer the term cold pool because not all inversion events feature a true inversion in which temperature increases with height (the atmosphere can still be quite stable when the temperature decreases slowly with height but is not inverted).  In addition, in some instances the inversion is elevated, so it is not really correct to call the cold air beneath it the inversion.  

OK, enough semantics.  Valley cold pools can be deep or shallow.  They can mix out quickly or slowly.  Then can be eroded by surface heating (which warms the cold pool) or from the top down by the wind. 

Observations from Hawthorne Elementary School at 1700 South 700 East over the past several days show the buildup of pollution over the Salt Lake Valley during the last work week, with PM2.5 levels peaking at unhealthy levels on February 3rd (Friday) and 4th (Saturday).  

Source: Utah Division of Air Quality

However, on February 5th, something odd happens.  First, air quality improves dramatically from about 3 AM to 9 AM when PM2.5 declines from 52 ug/m3 to 11 ug/m3.  Then, it jumps up again, in just one hour, to 49 ug/m3.  Talk about snatching defeat from the jaws of victory!  Then there is a gradual reduction through this morning, although some pollution remains at this location.  We still have not fully mixed out!

So what is going on?  With the approach of the storm on Sunday, the valley cold pool scoured out in much of the valley except the northern end and near/over the Great Salt Lake and environs where elevations are lowest and the coldest of the cold air was resident.  Lake temperatures are also quite low now, near 0˚C, so on a relatively warm, cloudy night, the air over the lake is locally cold.  As a result, we mixed out a good portion of the valley, but as seen in the photo below, taken toward the southwest and Oquirrh Mountains from the Avenues at 7:47 AM, a thin lens of cold air and pollution remained over the northwest valley (and likely over the Great Salt Lake and environs).  

I thought perhaps Mother Nature would be able to fully scour out that cold pool, but she had other ideas.  Instead the cold pool pushed southward and eastward that morning extending across a good portion of the Salt Lake Valley by 9 AM.  Hence the jump in PM2.5 concentrations at Hawthorne Elementary.  

Looking to the SSW from the upper Aves

A little while later, it started to snow.  Snow can remove some pollution from the atmosphere, but the temperature of the air aloft and the strength of the flow, which was rather weak, were not sufficient to crack and mix out the cold pool.  By 4:37 PM, the clouds were starting to move out, but the cold pool and associated pollution remained over a good portion of the Salt Lake Valley.  

Here are a two more photos showing the incredibly shallow nature of the cold pool and pollution as the sun set.

Even this morning, the cold pool and pollution refuse to give up.  Pollution levels for much of the east bench are mostly in the good category, but there are still a few pockets in the moderate category remaining, especially in the lowest elevations and west valley.

Source: purpleair.com

Better computer models and approaches are needed to forecast such complexities than we have today.  Simulating shallow cold pools is a major challenge and requires not only high resolution, but also very good information on the land surface conditions, snow cover, etc.