Thursday, December 14, 2017

Three Surprises about Yesterday's Weather

Richard Feynman.  Photo: Tamiko Thiel
"The only way to have real success in science is to describe the evidence very carefully without regard to the way you feel it should be. You must try to explain what's good about it and what's bad about it equally."
- Richard Feynman
The above quote is one that every forecaster should keep in mind.  A forecast is a scientific hypothesis, and improving as a forecaster requires careful evaluation of your hypotheses without cherry picking evidence or being overly kind (or harsh) on yourself.

As a scientist and as a forecaster, I get especially excited about surprises.  Things that happen that I can't explain or didn't anticipate.  In other words, they lie outside the bounds of my hypotheses.    These provide great learning opportunities and in some cases fodder for future scientific research.

Yesterday's weather provided three surprises.  The first was the haze evident in the Wasatch Mountains above the valley pollution, as discussed in depth in the previous two posts.

The second was the snow shower activity in the late afternoon and overnight.  Yes, it wasn't much, adding up to 0.13" of water equivalent and an inch of snow at Alta-Collins, but it was more than I expected.

The third was the stirring and partial mixout of pollution in the Salt Lake Valley.  Although it is faint, I can actually see Lone Peak from my office right now and observations from Hawthorne Elementary show a significant decline in PM2.5 concentrations overnight.  Great news, although air quality remains moderate.
Source: DAQ
The question then becomes why were these surprises.  In the case of the smoke from California, it was a case of inadequate situational awareness.  The smoke had clearly been blowing offshore and northward along the Pacific Coast and the thought of it actually moving inland never really crossed my mind until I was skinning yesterday morning.

The other two surprises, the snow and the pollution mixout, are related to the intensity of the short-wave trough that was dropping down the back (eastern) side of the ridge.  These are both related to the intensity of the trough, which in forecasts brushed by Utah, kept us in northerly flow at 700 mb, and dropped crest-level temperatures to only -3ºC.


Instead, the trough was further west and stronger.  It brought in more humidity, stronger flow, and dropped 700-mb temperatures to about -6ºC overnight.


This led to the somewhat more productive snowshowers, but also the cooler temperatures aloft and stronger flow helped provide some ventilation for the valley.

Could this have been anticipated?  Perhaps as a low probability possibility.  For example, I went through several SREF forecasts and could find a member or two that put out more than .10" of precipitation at Alta.  An example is below.

We don't currently mine the ensembles for guidance related to pollution mixout (my bias is snow), but perhaps if we consulted the full range of model solutions, we could find a few with a sharper trough.  The European center model, for example, had a bit of a sharper trough than the GFS and might have provided a bit better guidance.

Source: Weatherbell
Looking over all the data, chances are I wouldn't have expected the mountain precipitation and pollution mixout as a high probability outcome.  They would have been low probability outcomes, but in the range of possibilities nonetheless.  All of this illustrates the importance of assessing the full range of possibilities and not zeroing in on one forecast possibility.  After all, we live in a chaotic world.

All of that being said, I'm glad we got an angry inch of snow and a pollution stirring.

Wednesday, December 13, 2017

Incredible PM2.5 Observations from KSL Chopper

Following up on the previous post, our intrepid MesoWest Team has partnered with KSL-TV to collect PM2.5 observations from their helicopter, Chopper 5 (much thanks to KSL for enabling this incredible resource).  The data today is simply incredible.

Below shows one period during which the chopper was flying across the Salt Lake Valley and over portions of the Wasatch Front.  The chopper position varies, as well as its altitude, and it is clear that sometimes it it flying in clean air (blues) other times "dirty" air with much higher PM2.5 values (red).

The scattergram below illustrates all the PM2.5 measurements during that period based on altitude. Below about 2100 meters, the helicopter frequently flew in the valley pollution.  PM2.5 values were generally above 35 ug/m3.  There are, however, a few places at those elevations where the air was fairly clean.  This is not surprising given the tendency for there to be waves and other features on the inversion. 

Between about 2100 and 2600 meters, the air is mostly clean.  All observations were below about 30 ug/m3 and the vast majority were < 5 ug/m3.  

However, go higher up and there are some horrible PM2.5 values again, some as high (or higher) as in the polluted air in the valley.  This is what I suspect is the smoke from the California fires.  This smoke isn't uniform.  There are some pockets of clean air, but the PM2.5 levels are quite high in some locations.  

This is simply incredible data.  I've never seen anything like it.  Wintertime pollution trapped in the valley.  Smoke from extreme wildfires at upper levels.  Remarkable.  Thanks to the MesoWest team and KSL for making this happen.  Note that this data is freely available in real time at http://utahaq.chpc.utah.edu/

Smoke from California May Have Arrived in the Mountains

Many people over the past few days have wondered if some of our pollution is from the California fires and up until today, the answer was no.

If you live in the lowlands along the Wasatch Front, the answer to that question is probably still no.  We are in a soup of locally produced pollution that is quite isolated from changes aloft.

However, in the mountains, the answer might be yes.

I went up to Alta this morning for a quick tour and workout, and was quite surprised to see some dense haze in the canyon.  This haze was especially apparent looking down canyon from the upper Albion lot.


After our climb, the view toward Heber showed some dense haze that appeared to be thicker aloft than right near the surface in the valley.


What was quite strange was how spotty and variable the haze was.  At times, the air looked quite clear in one area, but hazy in another.  One thing that was clear is that there didn't seem to be any spread of pollution from the valley to high elevations, as suggested by the photo below.


Thus, I don't think the mountain haze is from the valley pollution.  Providing further evidence of this, the morning sounding at KSLC showed an incredibly strong inversion, with the temperature within the inversion increasing more than 10ºC.  My experience with such strong inversions is that the air is typically quite clean once you are above them.


I wonder if instead some smoke from the California fires has finally crested the ridge and dropped down into northern Utah. Over the past several days, that smoke has been pushed offshore and at times northward along the Pacific Coast.  Monday's Modis image, for example, shows the smoke offshore, along the coast, and wrapping northward off the Northwest Coast.

Modis Image 11 December 2017.  Source: NASA
Yesterday, however, that smoke had spread inland across much of Washington and Oregon, and possibly across northern Idaho.

Modis Image 11 December 2017.  Source: NASA
 Thus, it is possible that the northwest flow has finally transported that smoke into our area.  As I write this, we haven't gotten a Modis overpass yet, but one will be coming soon and it should be interesting to see what it shows.

Note that this does not mean that the smoke is contributing to our problems in the valley.  The smoke is likely elevated and the inversion that prevents the pollution from entering the valley, also limits the spread of smoke aloft into the cold pool.  The smoke instead simply adds to the misery by reducing the clarity in the mountains.

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.