The Approaching Mother of All Inversions

I'm either a pessimist or a realistic optimist about the forecast for this coming week depending on your perspective.  It looks like a very strong inversion will set in and that it could persist for some time.  From a meteorological perspective, it has the potential to be even stronger than the last.  From an air quality perspective, much will depend on emissions, chemistry, and other factors.

This is a long post, but it is important because inversions have such a huge impact on the lives and health of northern Utahn's.  It has two parts.  The first discusses inversion nomenclature.  The second the forecast for the approaching event.

a. Inversion nomenclature

As discussed recently in a Salt Lake Trib article by Judy Fays, there is quite a bit confusion about the terms used to describe inversions.

A cold pool is a topographically confined layer of air that is colder than the air above it.  An inversion is a layer of air in which the temperature decreases increases with height.  As depicted below, it is possible for the inversion to sit on top of a cold pool and not extend all the way to the valley floor.  The red line shows the profile of temperature with height.  Note how it increases in the inversion layer.

In Utah, the term inversion is commonly used to describe any situation in which one can see pollution in the valley.  This is meteorologically incorrect as some so-called inversions do not feature a layer in which the temperature increases with height.  In the atmosphere, the density of air depends on both temperature and pressure and, as a result, situations in which the temperature decreases only a little with height are still quite stable.  These situations can feature elevated pollution levels and are commonly called inversions even if an inversion is not present.

When an inversion is present, it's height relative to the valley floor can be important because there can sometimes be mixing within the airmass below the inversion.  When the inversion is at or near the valley floor, all our urban emissions are trapped in a very shallow layer.  When it is higher, there is mixing through a deeper layer.  All else being equal, the pollution on the valley floor will be worse when the inversion is near the valley floor than when it is elevated.  The benches, however, can sometimes be above the inversion and in the clean (or cleaner) air if the inversion is located at or near the valley floor.

Haze is a visibility obstruction caused by dry or wet particles suspended in the sky.  Haze can be natural or have a strong human-caused component.    

Smog was originally used to describe smoky (i.e., polluted) fogs, but is now used to describe a wide variety of pollution related visibility obstructions related to particulate matter and photochemical smogs.

I've never much liked the term haze to describe the poor visibility during our so-called inversion events.  There are some inversion events in which natural haze would form, but most of the crappy air is due to pollution.  Therefore, I believe the term smog should be used.  Smog also has the advantage that it is inclusive of fog, which also forms during some inversion events.  There are some disadvantages as well, but this is my blog and I do what I want.

b. The Forecast

I'm quite concerned about the long-range forecast and the potential for unhealthy particulate matter levels in the Salt Lake Valley for several reasons.  First, we have an incredibly cold airmass and deep layer of snow over the valley.  As the forecast stands now, there's no hope that the snow will melt and very little hope that we'll erode out the cold airmass over the next couple of days.

Second, we are transitioning from a deep large-scale upper-level trough to a persistent, high-amplitude upper-level ridge over the next several days.

Forecast temperatures on the top of Mt. Baldy (11,000 ft) climb from below -10ºF today (the actual temperature on Mt. Baldy is currently -17ºF) to near freezing by Tuesday night.

That is more than a 40ºF increase, while the cold air is likely to remain entrenched in the valley.  Further, this could be a long-lived event based on the long range forecasts.  The forecast above covers 7-days, but many ensemble members keep us stagnant for longer.

The bottom line is that meteorologically this has the potential to be the Mother of All Inversions.  OK, that's hyperbole, but it will be a strong and persistent event.  What happens with pollution will depend on emissions and chemistry.  As Vince Lombardi said, the best defense is a good offense.  Today is the day to get the word out and run errands.  Tomorrow is the day to implement red burn conditions, encourage people to cut down on travel, etc.  Our air quality defense system for this event needs to be proactive.