Tuesday, January 4, 2011

Conditions around an "air stagnation advisory"

I've had a busy few days as I returned to Seattle and classes have now resumed at the Univeristy of Washington.  This is the first chance I've had to do a new post.  Since it has now been several days, I'm going to forgo the upper air analysis of the storm on New Year's Eve (sad, I know...) and press onward...

A lot of people are also posting on the forecast cold air outbreak across the central and eastern US late this week and into next week.  This is definitely a big story, and I look forward to getting into it in the days to come.  For a preview of this event at the earliest stages of its prediction, I refer you to Patrick Marsh's blog and Chris Whitehead's blog, both of which look into this cold air outbreak.

However, Seattle and much of the west coast looks to be spared this frigid air.  In fact, this morning was rather hazy in the Seattle area, but according to the weather service that's not where the big problem area was...
Fig 1 -- Watches, warnings and advisories map for eastern Washington from the NWS WFO Spokane.
Much of eastern Washington is under an air stagnation advisory throughout the morning.  Such advisories are issued when limited air movement and subsident conditions create and environment that traps dust and pollutants within the boundary layer.  This can severely deteriorate air quality.  Based on this morning's webcams out of Spokane, it looks like that's just what is happening:
Fig 2 -- Downtown Spokane webcam at 10 AM PST, Jan 4th, 2011.  From the Spokane Air Quality website.
It certainly appears that there's lots of red dust or other material in the lower atmosphere.  Definitely something to be concerned about.

So what kinds of environmental conditions promote this "air stagnation" that traps pollution?  Several things play a roll.  First, the topography of the region has a huge impact on whether air gets trapped or not.  Eastern Washington is particularly vulnerable to these kinds of events because it sits inside a topographic "bowl" with higher terrain all around.
Fig 3 -- Topographical map of Washington state.  Greens represent lower terrain while browns represent higher terrain.
Note how eastern Washington has mountain ranges or uplands surrounding the entire area--including the Cascades to the west and the Rockies (which are more in Idaho and not on this map) to the east.  That entire area is known as the Columbia River Basin as the Columbia River flows along the western and northern borders of that "bowl".

An area of low terrain like that is excellent for trapping air and pollutants because it becomes much harder to evacuate the air due to horizontal advection.  At low levels, air will be stopped by higher terrain no matter which horizontal direction it heads.  If the lapse rates are very stable (like we see when there is an inversion aloft or widespread subsidence), the air won't be able to rise up and over the higher terrain to get out either.  Therefore, absent any strong synoptic-scale forcing, the air in the low levels simply can't get out of the basin.

So do we see those kinds of conditions here today?  Here's this morning's 12Z sounding out of Spokane:
Fig 4 -- KOTX sounding out of Spokane, WA at 12Z, Jan. 4th, 2011.  From the HOOT website.
Notice that we do have an inversion--several, in fact.  From the surface up to almost 600 mb the lapse rates are largely nearly isothermal (with a few embedded layers that area not).  In fact, the 700 mb temperature is the same as the surface temperature!  This represents a very stable atmosphere--one that is going to resist any kind of upward motion. Furthermore, note the winds in the low levels of the atmosphere--10 knots or less up to 850mb.  Such weak winds are not going to do much to move that air and pollution out.  Furthermore, weak winds are present within the entire Columbia Basin:
Fig 5 -- Surface observations from eastern Washington at 17Z, Jan. 4th, 2011.  From the UW Regional Observations website.
Very, very light winds across the entire region this morning. Cold temperatures too--in the upper teens and low 20s throughout the region. That air is going nowhere.

Let's briefly look at the larger scale to see what's promoting this pattern.  Here's the analysis of 300mb winds as 12Z this morning from the UW 12-km WRF:

Fig 6 -- UW 12-km WRF 300mb wind (shaded), temperature and geopotential height analysis for 12Z, Jan. 4th, 2011.  From the UW WRF page.
I'm not the biggest fan of the color scheme on these maps as I would have made the pinks and purple the higher wind speeds instead of the lower wind speeds.  But, anyhow, on the map above the light blues and even greens indicate a strong 300mb jet coming out of the north.  We can see in the height field that there's a ridge with an axis oriented from the southwest to northeast across Vancouver Island and into central British Columbia.  We would infer that this jet is probably anticyclonically curved if it's located along the height contours around the ridge.  That would put eastern Washington square underneath the exit region of an anticyclonically curved jet streak.  This implies convergence aloft and downward motion--subsidence.  No wonder we're seeing such light winds and no air leaving the Columbia Basin--if the net vertical motion is downward, that air is just going to continue to be held in place.

Fortunately this pattern looks to be somewhat progressive and the ridge will soon be replaced by an approaching trough over the northeastern Pacific in the next few days.  This should bring some wet weather to the western side of the Cascades and snow to the Columbia Basin.

1 comment:

  1. Good subject to write about! I had to mention this for my radio forecast I did for KPDT this afternoon. Most people probably didn't have a clue what I meant by an "air stagnation advisory" haha