Tuesday, December 12, 2017

Inversion Tidbits and Long-Range Prospects

Yesterday's satellite imagery summarized the ridge-dominated weather of western North America quite well with extensive fog found in the major basins, many of the valleys of British Columbia and the Northwest United States, and the Great Salt Lake Basin.  At the same time, smoke from the California Fires covered much of the offshore eastern Pacific Ocean.  If you look carefully, it appears that some of this smoke has been carried northward to the Queen Charlotte Islands.

Composite MODIS image from NASA.
Within the Salt Lake Valley, the pollution went into overdrive yesterday, with PM2.5 levels skyrocketing in the morning to unhealthy levels.  Unlike previous nights, when PM2.5 dropped considerably, levels declined only modestly overnight and remain unhealthy for sensitive groups. 

PM2.5 concentrations at Hawthorne Elementary.  Source: DAQ
Looking for a brightside?  The frosty trees make for a beautiful Christmassy scene.  

We are so desperate for weather that I feel the need to mention that there is actually a weak short-wave trough dropping down the back (eastern) side of the ridge and passing through our area Wednesday night.  

Yup, that's your weather for the week.  It will bring somewhat cooler temperatures to the mountains, perhaps helping with the snowmaking efforts and might stir the upper part of the inversion a bit.  Emphasis on might.  Low elevations will likely remained mired in pollution. 

I am a bit more optimistic that the trough on Saturday is strong enough to give us at least a partial mix out.  It's still soon to say if it will scour it all out.  Sometimes, the coldest, most polluted air at the lowest elevations can be quite stingy. 

Snowfall totals for the mountains presently look paltry.  About half the members in our downscaled NAEFS ensemble generate 2 inches or less.  A few members go for more.  A game changer is unlikely. 

The word "pattern change" is being thrown around a lot, but I bet you'll have a hard time finding anyone who can tell you what that means.  I have yet to see any indication from any ensembles that we are going to shift from the high-amplitude pattern that has dominated for weeks and in which there are very deep ridges and troughs at upper levels, to a more progressive pattern with stronger westerly flow.  Instead, there may be some shifts in the position of the ridges and troughs.  For example, some of the GEFS 10-day forecast members below have a ridge upstream of the west coast of North America, rather than near its present location along the west coast or just inland. 

Source: Penn State E-wall
Those shifts could be important if they lead to a slowly evolving but wet pattern for Utah.  However, looking at the GEFS solutions above, some might bring us some snow, others keep us dry.  Why waste time talking about this range of possibilities?  Like thermonuclear war, the best option is not to play.  

Thus, hope we get something from the trough on Saturday and at minimum hope it cracks the inversion.  It's the only slim hope we have for mountain snow over the next week.  After that, your guess is as good as mine. 

Monday, December 11, 2017

Diurnal Intricacies of the Inversion

PM2.5 concentrations during our current inversion event have shown remarkable variations from day to night.

Below is a time series of PM2.5 measured at our mountain meteorology lab at the University of Utah showing a clear long-term upward trend, but also a tendency for PM2.5 concentrations to spike just before noon, remain elevated until mid to late afternoon, and then decline.

Source: MesoWest
What are the causes of this diurnal behavior.  There are several possible contributors.

First, there is the possibility that photochemistry - chemical reactions occurring in the presence of sunlight, are contributing.  Comparison of the above plot with the incoming solar radiation below shows some relationship, with the PM2.5 exhibiting a bit of a lag relative to the solar radiation.

Source: MesoWest

Another possibility is that temperature is playing a role since it also affects the PM2.5 chemistry.  Again, there is some correlation.  

Source: MesoWest
Finally, there is the transport possibility as the winds are also changing diurnally, with a good correlation between wind direction and PM2.5 concentrations.  

Source: MesoWest
Another perspective is provided by the someone hacked-up graphs below, based on data collected at the University of Utah by our MesoWest team over the 24-hour period ending this morning at 10 AM (the hacking reflects my splicing of their multiple graphs together).  The top figure is derived using a laser that is shot vertically through the pollution.  The color fill is backscatter, a measure of how much of the laser light is reflected back to the ground, with higher values roughly correlated with greater PM2.5 concentrations (brown-white being the dirtiest air).

The plot begins on the left at 10 AM on Sunday when the local flow just shifted to predominantly westerly (some variability from SW-NW).  Surface PM2.5 concentrations during this period are quite high and, in addition, the pollution is quite deep.  At just after 1700 MST (5 PM), the flow shifts abruptly to ENE, which reflects the onset of down valley flow from Red Butte Canyon.  This marks the beginning of a gradual decline of surface PM2.5 concentrations, as well as a decrease in PM2.5 concentrations aloft.

Source: MesoWest
There is a brief lull in the wind that occurs just before 2300 MST (11 PM MDT), with the flow becoming somewhat erratic.  Without the inflow of cleaner air from Red Butte Canyon during this period, the surface PM2.5 values climb, although things don't change too much aloft.  Finally, after midnight, the ENE flow returns and PM2.5 values drop again, although there are a few spikes during the night that may correlate with declines in wind speed (I haven't bothered to check yet...so take this comment for what it is worth).

At the end of the time period, the PM2.5 values climb again, abruptly, when the flow shifts to westerly.

All of this illustrates some of the intricacies of these inversion events.  Pollution concentrations vary in the vertical (yes, there is clean air up there), although if you look carefully at the plot above, you can see that it's not as simple as polluted air near the ground and non-polluted air aloft.  There are layers.  In addition, pollutant concentrations vary horizontally and at the University of Utah one can clearly see the migration of pollutant-laden air onto campus when the wind shifts to westerly in the morning.

What role photochemistry and temperature play in all of this is unclear to me.  I suspect it plays a secondary role compared to meteorological factors, but I am not an atmospheric chemist and over the years I've learned that when all you have is a hammer, everything looks like a nail.  In other words, as a meteorologist, I might be guilty of placing too much weight on meteorological factors.

One thing to keep in mind is that not all inversions look or behave like this and even this one might behave differently in the days to come.  As a scientist, I think what we see over the next few days will be "interesting."  As a citizen, I wish the damn thing would just blow away.

Saturday, December 9, 2017

Scenes from the Inversion

Classic inversion conditions are now apparent over northern Utah.  Here are a few photos.

First, the view of the valley smoke that befuddled me, and is discussed in the previous post.  It sure looked like clouds this morning, but as soon as I descended down into it, I smelled campfire and knew I was in error.  Turns out it was smoke from a major arson fire near 500 South and 200 East. 

In contrast to the cesspool in Salt Lake, morning at Alta was splendid and whiter than expected.

A bit later.  A good example that no matter where you are in the Salt Lake Valley today, you're only a few hundred meters (vertically) from clean, pristine air. 

Valley cold pools can be found just about everywhere over northern Utah right now.  If there's emissions, there's pollution.  The smog below is in the Heber Valley. 

Just about anyone who has skied in Little Cottonwood knows this view.  Unfortunately, it is common in the wintertime.

And, this afternoon back in the Avenues.  Ick. 

On the positive side, air quality remains in the moderate category so far. 
Source: DAQ
On the negative side, we have at least 5 days left of this, and air quality is going to worsen.

Overnight Stratus Formation?

I woke up this morning and was surprised to see a shallow layer of stratus already beginning to form over the northeast portion of the Salt Lake Valley.

I thought it might be smoke in the early morning light, but after watching it for a little while, I think it's cloud.  It's a bit unusual for the first stratus in an inversion to form in this part of the valley.  Typically it happens at lower elevations and near the airport, but perhaps the distribution of cloud cover is having an effect.

As one might surmise from the photo, the atmosphere is now quite stable over the Salt Lake Valley.  The morning sounding shows temperatures increasing from -4.7ºC at the surface to 3.4ºC at 763 mb (8200 feet).

Source: RAL
We're stuck with this now for several days.

You're going to hear a lot of talk about a possible trough passage toward next weekend, but the strength, structure, timing, and implications for northern Utah vary quite a bit across the various GEFS ensemble members.  

Source: GEFS
Keep the faith.

Friday, December 8, 2017

Heartbreak Ridge Tightening the Inversion Noose

Noontime smog yesterday, looking southwest from the Natural History Museum of Utah, University of Utah
Given that our last storm was winding down on Monday, I'll call today Day 4 of Heartbreak Ridge.

So far, the pollution buildup has been modest.  Because the center of the ridge has been along the Pacific Coast, we've been on the downstream side, temperatures aloft have been cool, and the inversion relatively weak and elevated.  This has enabled some vertical mixing of pollutants through a decent portion of the valley atmosphere.  As a result, the increase in pollution has been gradual and we've been fluctuating between good and moderate air quality.  

Source: Utah Division of Air Quality
However, Heartbreak Ridge is sliding eastward and the inversion is strengthening, as can be seen in the soundings from yesterday afternoon (top panel below) and this morning (bottom panel below).  

Source: University of Wyoming
Note in particular the warming in the layer between about 800 and 700 mb (6500–10000 feet), which equates to a strengthening of the "lid" over the valley atmosphere.  This morning, temperatures near the base of that layer increase about 5ºC through a depth of around 50 mb (1500 feet). 

The NAM sounding loop below (note: this is a skew-t diagram, not directly comparable to the diagrams above) shows further warming aloft over next two days, with temperatures aloft warming an additional 5ºC.  
Thus, the inversion will be strengthening and lowering through the weekend.  It appears we will be in the grips of the inversion at least through the next work week, unless a system stronger than presently advertised slides down the back side of the ridge and gives it a stir. 

Model Products Information

We have been having some problems with the server that hosts weather.utah.edu and it has been down intermittently the past two days.  Behind the scenes (and unrelated to the outages), I've been updating some of our products.  Options for the GFS now include global and regional plots from the 0.25 degree latitude-longitude grid (we've been using the old 0.5 degree grids), higher frequency (every 3-h to 240 hours), more regional sectors (e.g.,  Intermountain, Northwest, Southwest), and time-height section options that match the time period of the NAM for comparison.  Some little used plots are gone, such as the Indian Ocean sector.  

Thursday, December 7, 2017

Posts Written But Not Published

Source: https://sarkaimak.wordpress.com/2013/04/15/unpublished/
I noticed yesterday that sometime in October, the Wasatch Weather Weenies reached 2500 posts.  Unbelievable.  I've written about 99.99% of those and that is an obscene amount of work!

Our top-10 most viewed posts are popular for a mixture of reasons including Google-search happenstance, popular posts for a moment in time, and content that is timeless.

10. Blast from the Past: Ski Magazine February 1978.  Powder and Alta nostalgia.  Always a winner.

9. Powder Explosion. Photo collage from our snow adventures on the Tug Hill Plateau.  Nice to see the eastern US makes an appearance.

8. El Niño Likely for the 2015-16 Winter.  Seasonal outlooks and posts on El Niño/La Niña get huge readership, despite the fact that I almost always conclude that such information has little value for skiers in Utah.

7. Disastrous Heartbreak Ridge to Develop.  Wow.  This one is only a few days old and it has skyrocketed into the top 10.  If it bleeds, it ledes, and Heartbreak Ridge bleeds.

6. Pound for Pound the Snowiest Place in Utah.  I love Ben Lomond Peak and the north Ogden Valley.  Good to see you do too.

5. The "Official 2017/18 Ski Season Outlook.  Another seasonal outlook, although this one was written with tongue firmly in cheek.  The outlook, republished below, looks to be verifying well in California and Nevada, as well as Hawaii, but the "Better than Colorado" for Utah could be in jeopardy if this ridge hangs around.

4. West To Be Tickled by Fabio.  A good example of gaming the system with a frequently googled name.  In this case, Fabio was a former eastern Pacific hurricane with remnants spreading into the western US.

3. Tour de France Weather.  Weather always affects the tour, and this post continues to get a lot of traffic.

2. Outlook for the 2013–2014 Ski Season.  Another seasonal outlook.  This one was popular, because the outlook we issued was basic and honest.  WE HAVE NO IDEA!

1. Let's Rock.  Sort of a shame that this short post is #1, simply because there are so many people Googling Let's Rock.  There's no other reason to go here.

OK, so that's the top 10 based on page views, but there's another top-10 list (technically a top 7 list) that is more interesting, and that is the list of posts never published.  There aren't many of these because I am stubborn as hell.  Typically when I write a post I get an idea, I think it will take 5 or 10 minutes, I start to write it up, and I realize I'm in over my head.  I polish the turd quickly and hit the "publish" button and move on, hoping for the best.

However, every now I realize that there's no polishing the turd.  This typically occurs with politically controversial topics related to climate change, or more philosophical posts about science and weather forecasting.

With that being said, here are the seven posts never published out of 2500+:

7. Science Is Never Settled

6. Air Quality Irony

5. What Tree Rings Tell Us about Utah Climate

4. Obama's Carbon Reduction Plans

3. Climate Change: "Action is Urgently Needed"

2. Do Climate Scientists Really Ignore Natural Climate Forcings?

1. The Future of the Weather Forecaster

These posts were written with good intentions, but haven't yet made it to blog-worthiness.  However,  there's no such thing as good writing, only good rewriting.  Perhaps they will appear in the future.