Tuesday, December 19, 2023

Inversion Intricacies

Last night, the cold pool beneath the inversion got shallower and the upper Avenues emerged from the smog.  I took the photo below from near 13th Avenue on my way into work.  It shows the clear skies at upper bench level, but also the thin lens of smog enveloping downtown and the lower valley.  

Upper air soundings collected at the airport yesterday morning and this morning show very subtle but important changes.  Yesterday morning, the inversion base was elevated and near 860 mb (about 400 to 500 feet above the valley floor.  Beneath the inversion was the valley cold pool that contained the fog and low-level stratus clouds.  The flow was weak in and above this layer and didn't reach 20 knots until about 775 mb (about 3300 feet above the valley floor).  

Source: SPC

This morning, however, the inversion is based right on the valley floor and temperatures below about 800 mb (about 2500 feet above the valley floor) have warmed, whereas they have cooled aloft.  20 knot winds are now evident at around 800 mb.  

Source: SPC

So, we have scoured out some of the cold air at the lowest levels (resulting in the low-level warming), leaving the fog and pollution to being confined to near the valley floor.  

A look at the PM2.5 concentrations from PurpleAir shows we are still in the unhealthy for sensitive group levels across much of the valley floor, but along the east bench, there is a rim where concentrations are either good or moderate.  

Source: PurpleAir

This is an example of what meteorologists call top-down cold pool erosion.  The flow aloft is increasing and beginning to scour out the coldest air.  It's a tough job though.  It takes a lot of energy to scour out the remarkably cold and dense air that is currently at the lowest elevations.  It would be easier to scour that air out if we had a bonafide trough with cold air moving in at upper levels, but that's not on the docket for the next few days.

The models, due to lack of resolution, an inability to properly include the presence of such shallow features, and the challenges of incorporating the complicated nature of near-surface physical processes, are not particularly good at dealing with these top-down mixout events.  I'm not exactly sure how things will play out today, but my best guess is that a shallow layer of cold air and pollution will remain resident over the Great Salt Lake and the central and northern Salt Lake Valley.  It might retreat for a short time, only to return, including to the University of Utah and some bench areas, as the flow becomes upvalley and upslope with surface heating during the day.  

I'm also not sure about how things will play out over the next few days, but at the moment, I don't think we will fully crack the inversion and get a total mixout everywhere.  I think we will see a persistent cold air pool at low levels near the lake and in many low elevation areas in the Salt Lake Valley and along the Wasatch Front.  The southern Salt Lake Valley and upper bench areas may luck out at times with clearer air.  

Hopefully I'm wrong and we crack this thing.  

1 comment:

  1. Appreciate your inversion posts and knowledge sharing more than anything!