Tuesday, January 15, 2013

An Ill (North) Wind Doth Blow

Strong northerly flow is ushering in much warmer air aloft as a high amplitude ridge builds over the western United States. This is not good news for northern Utah.

As Jim “Head Blogger” Steenburgh suggested a few days ago, this massive warm up aloft, which is already apparent in the morning sounding, will isolate a very cold pre-existing air-mass in the valleys of the intermountain west setting up for a persistent cold-air pool… or as Jim put it, “the mother of all inversions.”

KSLC 12 UTC sounding. Note the strong northerly flow aloft above the inversion.

The warming aloft, which often accompanies the initiation of a persistent cold-air pool, is the result of both subsidence and warm air advection. These two processes occur in concert today along an upper level front, which marks the departing edge of the cold trough that impacted Utah over the weekend. The slowly descending frontal zone will result in a continued increase of 700 hPa (or near ridge top) temperatures to around 0 C by Wednesday morning.

Potential temperature time height showing the departing cold trough, upper level front, strong subsidence and formation of the cold-air pool.

Meanwhile surface temperatures will remain quite cold. The still fresh snow that blankets the Salt Lake Valley will render the feeble mid-winter sun all but irrelevant, reflecting most of the incoming radiation back to space. With little energy available to heat the atmosphere, the convective boundary layer (CBL) growth (which helps to redistribute pollution emitted at the surface) will be suppressed. Moreover, the presence of the strong descending inversion layer from aloft will further limit the upward growth of the CBL.

So we have both a top down (e.g. warming aloft) and bottom up (e.g. wimpy CBL) contribution to a strongly stable atmospheric profile. In combination these processes will limit the vertical mixing of pollution, while the confining topography and light winds prevent lateral displacement… in other words air stagnation.

In fact, we can already see PM2.5 levels increasing within the stagnating air towards the national ambient air quality standard (NAAQS), and the event has barely begun!

PM2.5 concentration at Hawthorne Elementary School

The one piece of good news for today is that clouds associated with the upper level front prevented strong surface radiative cooling last night, (compare -2 F on Mon. vs 11 F  on Tues. as the low temperature at KSLC). As a result what little heat that is convected into the boundary layer today should have an easier time mixing pollution away from the surface.

Tonight, on the other hand, skies are expected to clear and we could grow a strong surface inversion, which will make matters much worse. In fact over the next few days we may also see fog or stratocumulus clouds develop within the cold pool. And don’t hold your breath for the inversion to break anytime soon. The GFS shows little hope in the next 7 days of significant weather capable of mixing out the inversion.

So, on second thought, do hold your breath (and try not to drive).  

-Neil Lareau

1 comment: