Thursday, February 11, 2016

A "Silver Lining" to This Inversion Episode

For the most part, the news during prolonged inversion events is bad, but I did want to share one "silver lining" with regards to the current event, which is the cloud cover and what it is doing to increase mixing in the Salt Lake Valley, especially overnight.

The air is still polluted, but these clouds are actually a help rather than a hindrance
If the solution to pollution is dilution, then the development of cloud cover over the Salt Lake Valley is an improvement over the cloud-free inversion that we had prior to yesterday.  Let me show you why.

Below is a loop of upper-air soundings collected each morning at the Salt Lake City airport over the past week.  These soundings are plotted as a "skew-t" so a line of constant temperature slopes upward to the right.  Note the two-stage evolution of the inversion event.  First, the upper levels warm dramatically while the morning surface temperatures remain relatively steady.  This occurs in the first 5 frames of the loop (note how the red line shifts rightward from about 800 to 600 mb).  Second, a mixed layer forms very near the surface in the last 2 frames, as indicated by temperatures cooling with height to an level just above 850 mb.  

Morning upper-air soundings from the Salt Lake City airport from 5–12 Feb 2016
Here are a couple of still images to further highlight this point.  On the morning of 8 February, the inversion was based right at the surface.  In such a situation, there's very little vertical mixing of the airmass.  Pollution is trapped right at or very near the valley floor.  

Morning upper-air sounding from the Salt Lake City airport on 8 Feb 2016
This morning (11 Feb), however, the inversion base is elevated and sits at 836 mb (5680 ft), almost 1500 feet above the valley floor.  Below the inversion, there is a shallow mixed layer that is allowing for some vertical mixing of pollutants.  

Morning upper-air sounding from the Salt Lake City airport on 11 Feb 2016
The growth of such a mixed layer is very common following the formation of low clouds during inversions in the Salt Lake Valley.  Radiative cooling at cloud top drives turbulence that mixes the layer in and below the clouds.  The end result is what meteorologists call a cloud-topped mixed layer.  If you are in the Salt Lake Valley this morning, you are not in the inversion, you are beneath it, in the cloud topped mixed layer.

What does this mean for pollution?  Well, it means that at night there's more mixing through a deeper layer, which reduces PM2.5 concentrations.  The presence of the mixed layer also slows the long-term rate of rise (NOTE: Concentrations are still high, so this is no reason to roast marshmallows over a fire tonight).  For example, PM2.5 levels at Hawthorne last night were not as high as they were the previous night.  In addition, there is a flattening of the long term trend.  

 As discussed in the previous post, PM2.5 concentrations at University of Utah sensors are running higher than those aw Hawrhorne, but they also suggest a flattening out over the past 24 hours or so. 

None of this means the air quality is good.  We are still in unhealthy territory.  What it does mean is that the rate of increase along the valley floor will probably slow.  Those of you on the upper benches may see more sustained high PM2.5 levels in this scenario as the base of the inversion lifts and you are more continuously enveloped in the gunk.  Perhaps this isn't a silver lining for you.  

Science wonks can see Pataki et al. (2005) for more info on these cloud-topped mixed layers and their influence on mixing and transport in the Salt Lake Valley.

Wednesday, February 10, 2016

Above My Pay Grade

Some of you have commented about the large difference between the DAQ sampler at Hawthorne and the observations collected along the Trax line as well as at Neil Armstrong Academy and the University of Utah.

For the most part, these sensors have generated pretty similar results for the first few days of the inversion.  PM2.5 at Hawthorne has risen fairly steadily during the period (with some ups and downs) with a range from midnight on the 9th to midnight on the 10th between about 38 and 54 ug/m3.  PM2.5 at Neil Armstrong Academy (sorry for change of scale) was generally close to Hawthorne through midnight on the 10th, with the range from midnight on the 9th to midnight on the 10th of about 39 to 65 ug/m3.

Source: MesoWest
Source: MesoWest
The divergence begins shortly after midnight on the 10th (last night), with NAA going to much higher values.  Similar, other samplers operated by the University of Utah at the University of Utah and along Trax are also reporting high values compared to Hawthorne.

It is above my pay grade to explain these differences.  I know little about the measurement of PM2.5, other than it is very difficult, or the instruments used, other than the fact that the DAQ and University of Utah samplers are different.  The discrepancy appears to have developed during a period when fog was in the area.  I don't think this is a coincidence, but I lack knowledge of the sensor characteristics and atmospheric chemistry to provide a reasonable hypothesis why.  I'd rather say I don't know than speculate.  

As such, in future posts during this event, I will be referencing both samplers and mentioning the uncertainty at play.  

Note, however, that the lower Hawthorne sampler was in the unhealthy category for several hours today and the 24-h average is now very close to the unhealthy threshold.  Thus, even though there are uncertainty in the measurements, this remains a serious event.

Can Mother Nature Crack This Terrible Inversion?

The situation in the Salt Lake Valley continues to deteriorate and is becoming quite serious.

Here on campus, we are currently fogged in.  Although depressing, it doesn't look as ugly as the pollution does under clear skies, but don't be fooled.

PM2.5 concentrations this morning are sky high.  The University of Utah's Trax-mounted PM2.5 sampler is measuring concentrations this morning in the 85-115 ug/m3 range (maroon filled circles below).

I did a quick inspection of the data and found a maximum concentration of 111 ug/m3, which is well into the unhealthy category.  

The terrible situation we are in is one of our own doing.  Persistent cold pools (a.k.a., inversions) are naturally occurring phenomenon in the western United States during the winter.  One shouldn't equate inversions with air pollution.  Inversions happen all over the west.  Pollution happens in those areas that experience inversions and where emissions are concentrated.  

At this point, we're up to our eyeballs in alligators.  We have absolutely terrible air quality and things are not going to improve until Mother Nature cracks this inversion.  What will that take?
The situation we have right now is one where dense, cold air is pooled in the Salt Lake Valley (and other basins of northern Utah).  From March to October, there is typically enough energy provided by the sun each day to warm that airmass and allow it to mix with the air aloft, limiting pollution concentrations.  This time of year, however, we don't have enough solar energy to mix out the cold, dense airmass.  We're basically mired in an oil and water situation.  I did a quick calculation and found that the air near the valley floor is about 25% more dense than that at crest level.  That's a big number.  

Thus, there are only two ways to get rid of this inversion.  One is to bring in even denser, colder air at upper levels, allowing what meteorologists call buoyancy driven turbulence to scour out the valley.  The other is to increase the winds near the top of the cold pool, allowing what meteorologist call mechanically driven turbulence to scour out the valley.  

One can do another quick calculation and find that for a valley temperature of 0ºC one needs to bring in an airmass with a crest-level temperature of about -17ºC to remove the cold pool with no help from the wind.  You can get a way with a higher crest-level temperature if the valley is a bit warmer (such as might be found in the afternoon), or if clouds are present (the release of heat in clouds helps invigorate turbulence).  So, perhaps we could do some damage if we got down to say -12ºC or so. 

Of course if you add wind, you can sometimes get a bit more bang for the buck.  It's very hard, however, to determine how wind will influence a cold pool unless it is very strong.  

Which brings us to our sole glimmer of hope, the trough that is forecast to brush by Utah on Saturday evening.  This will drop our crest-level (700-mb, 10,000 ft) temperatures to about -6ºC to -8ºC.

That by itself is not enough to crack the inversion, but there is also an increase in flow aloft and at low levels.  

We really don't have the tools today to determine if this will yield a full mix out (I doubt it, but can't rule it out), partial mix out at all elevations, a scouring out of the pollution from the top down, leaving a lens of pollution near the valley floor, or no change.  This is in my view a critical area for research as it is forecasting of these weaker trough passages and their influence on inversion and air pollution strength that is the hardest part of forecasting these events. 

As things stand now, all we can do is hope for the best, but I'm concerned that even if we mix out some, we're still going to see this event persist into early next week.  

Tuesday, February 9, 2016

Strengthening Inversion, Worsening Air Quality

A comparison of soundings from yesterday morning and this morning shows further strengthening of the inversion over the past 24 hours.  Yesterday morning, the surface temperature was 27ºF (-2.8ºC), with temperatures aloft maxing out at 1.4ºC just below 700 mb (10,000 ft).

Source: SPC
This morning, the surface temperature is once again 27ºF (-2.8ºC), but temperatures aloft have increased at all levels up to about 650 mb, with a maximum of 3.6ºC at 773 mb (8000 ft).

Source: SPC
Above the valley floor, the freezing level isn't hit until just over 12,000 feet.  You'll find some below freezing air in cold mountain spots this morning, but with the exception of shady north-facing spots, temperatures throughout the Wasatch Range will be above freezing today.

Air quality in the Salt Lake Valley continues to deteriorate.  Observations from DAQ sensors and from Trax-mounted sensors operated by the University of Utah are generally between 35 and 60 ug/m3, which is in the unhealthy for sensitive groups category.

Source: MesoWest
Observations from Hawthorne Elementary along 700 East reached 55.2 ug/m3 overnight, just shy of the dreaded red "unhealthy" threshold.  They have relaxed some overnight, but remain in the unhealthy for sensitive groups category.

Although there are short term ups and downs in PM2.5 concentrations, with values teaching their highest values late in the day through midnight followed by a decline, the long-term trend is upward.  That upward trend is actually not due to the inversion getting stronger (once the valley is capped off, further strengthening doesn't matter).  Instead, we're in a situation where emissions continue to accumulate in the valley.  Further worsening of the air quality will occur through at least the end of the week.

The models are flirting with a trough passage this weekend.  I say flirting because much depends on what model and ensemble member you look at.  Some have a brush by, others a more direct hit.  At this point, it's too soon to say if it will crack this thing, give us at least a partial mixout, or leave the air pollution fully intact.  The Euro provides a brush-by to the north, but perhaps enough to stir us up a bit.

Source: Penn State e-wall
That being said, there's no guarantee we'll get much relief yet and we'll have to see how the forecasts evolve in the coming days.  Steel yourself for a long ordeal, reducing driving, and hope for relief this weekend.

Monday, February 8, 2016

In the Grips of a Monster Ridge

After a run of active weather and regular powder refills from about mid December through early February, the wheels have come off our winter.

We are now fully in the grips of a monster upper-level ridge that will keep the mountains dry and the valley inverted.  On the positive side, that means warm, sunny days in the mountains.  On the negative side, we are staring directly down the barrel of what will be a very poor air quality episode.

Meteorologists use a diagram known as a Skew-T to examine upper-air soundings collected by weather balloons.  In these charts, lines of constant temperature are "skewed" (blue dotted lines below).  Yesterday morning's sounding showed a shallow inversion at low levels.  The surface temperature was 26ºF -3.3ºC and the temperature at the top of the inversion near 800 mb was about 0ºC.  Above this level, at 700-mb near the crest of the Wasatch Range, the temperature was about -4ºC.

Source: SPC
The big change overnight was warming at crest level as the ridge moved further eastward.  Temperatures at 700-mb are now around 0ºC.  A close look at the sounding reveals a temperature increase from -2.9ºC at the surface to 1.4ºC just below 700 mb.

Source: SPC
Those are the gory details.  Bob Dylan would simply say that you don't need a weatherman to know that we're in an inversion.

PM2.5 levels prior to midnight at the Hawthorn air quality site climbed well into the unhealthy for sensitive groups category.  They decreased some overnight, but popped up in the past hour to the unhealthy for sensitive groups threshold.  24-hour averages (blue dots) are now above EPA Clean-Air Act standards.

Source: Utah Division of Air Quality
Temperatures aloft will continue to rise through late Tuesday when the ridge axis is centered over the Intermountain west and the ridge is at maximum intensity.  At that time, forecast free-atmosphere 700-mb temperatures (near 10,000 feet) are near 4ºC.  

Such temperatures are very near the outer edge of anything previously observed in February over Salt Lake City.  The graph below shows the record maximum (red line), median (black line), and record minimum (blue line) 700-mb temperatures observed in soundings collected either in Ogden or Salt Lake City over the past few decades.  In February, the all-time record is 5.8ºC on Feb 1.  For the periods we are heading into (Feb 8–12), the highest previously observed is 4.4ºC.  

Source: SPC
Bottom line: This is going to be an outlier inversion event in terms of meteorological strength and duration, a situation further exacerbated by the extensive snow cover on the valley floor.  From an air-quality perspective, it will easily be the worst event of this season, with the situation worsening through the week. 

Sunday, February 7, 2016

This Inversion Is Going to Be Ugly

Smog in the Salt Lake Valley this afternoon as viewed from the Mt. Olympus Wilderness.  Antelope Island sticking up in the distance.
It's time to get real about the air quality this week.  The system that passed by yesterday, and the strong crest-level winds today, have done essentially nothing to stir up the air in the Salt Lake Valley.  We are now facing a situation where the air quality is already poor as a remarkably strong upper-level ridge builds over northern Utah.  With snow on the ground, this is essentially a worst-case scenario for air quality in February and it needs to be treated as such.

How bad is the air qualty?  Well if you wanted to know this afternoon and had to rely on the Utah Division of Air Qualty, the answer is unclear.  Here's what I got when I tried to go to their web site this afternoon. 

This is simply unacceptable, because the air quality is indeed poor this afternoon.  We know because the University of Utah operates a PM2.5 sampler at the Neil Armstrong Academy in West Valley City and it shows PM2.5 concentrations near or above 35 ug/m3.  That puts it into the unhealthy for sensitive groups category.

The graphs above show that the PM2.5 climbed quite dramatically today, more than we typically see.  I'm not sure if that might be due to photochemistry or perhaps an especially dirty airmass over the Great Salt Lake (there was a bump when the flow switched to west).

The situation is only going to worsen from here.  We will be in the grips of the upper-level ridge through at least Friday.  After that, we'll have to see.  There are some weaker systems being advertised as perhaps influencing things next weekend, but it's too far out to say how things will evolve.

The bottom line is that this is a very serious situation in which we will see poor air quality for several days.