PM2.5 Evolution - Need Help from an Atmospheric Chemist

In Salt Lake County, we are now a few days into red burn conditions, or the more specific scientific classification of "gross."

The Utah Division of Air Quality provides quasi-real-time PM2.5 data from Salt Lake City on their web site.  It is very interesting to examine the evolution over the past 5-6 days.

In particular, there is a nearly monotonic increase in PM-2.5 concentrations from New Years Day through January 4th.  Beginning on the 5th, however, a large diurnal cycle appears, with lower concentrations at night, and higher concentrations during the day.  The NO concentrations go up dramatically on these two days, with a diurnal cycle that is out of phase with the PM2.5.  Note also that the diurnal cycle increased on the 5th as the mid level clouds became less wide spread.  So we have a situation where at night and in the early morning we have a stronger surface based inversion, higher NO, but lower PM2.5.

I need help from someone with a background in atmospheric chemistry and/or PM observations to explain this.  Any volunteers?

One thing to keep in mind with a trace like this is the dangers of extrapolating short term trends.  The low PM2.5 concentrations overnight and in the morning, for example, are not an indication of improving air quality.  This afternoon we have the highest PM2.5 concentrations of the event (at least at the observing site).

Utah's First Mountain Meteorologist

We have a saying in science that there's "nothing new under the sun."  Truly original ideas are rare and we all stand on the shoulders of giants when it comes to advancing knowledge and understanding.

Today all Utah mountain meteorologists, especially those who align their weather passions with backcountry skiing adventures, stand on the shoulders of S. D. Green.  Green was a forecaster with the U.S. Weather Bureau (the predecessor to the National Weather Service) who was also an avid backcountry skier.  He toured in the Wasatch Mountains long before lifts were installed at Brighton or Alta.

It was Green who recognized the unique snow climate of the Cottonwood Canyons.  Based on his backcountry ski adventures, he stated in 1935 that "skiers will eventually find that the Brighton Basin, or the heads of the [Cottonwood] canyons within a short radius of this winter paradise, offer the best skiing to be found in the Wasatch Mountains."

Several of Green's photographs are part of the ski archives collection at the Marriott Library, all taken in the 1930s.  Have a look.  The dude was an adventurer, and he wasn't doing it on Dynafit tech bindings and rocker skis.

Special thanks to Alexis Kelner for featuring S. D. Green and the quote above in his book Skiing in Utah: A History.

Uinta Basin Cold Pool

Even after 15 years in Utah, I'm amazed at just how dramatically the temperature varies across the state when cold pools form in valleys and basins.  Case in point for today is the Uinta Basin of northeast Utah.

Source: NASA

The Uinta Basin is quite deep, with the Uinta Mountains to the south and Book Cliffs and Tavaputs plateau to the south, and higher topography to both the west near Strawberry Reservoir and to the east in Colorado.  Technically, the Uinta Basin is not fully enclosed as the Green River does cut through it roughly from north to south, but it is highly isolated topographically and a great place for persistent cold pools in the winter.  

At 1800 UTC (1100 MST) this morning, temperatures in the lowest portions of the Uinta Basin, including the towns of Duschesne, Roosevelt, and Vernal, were near 0F.  This is more than 20F colder than found in the Salt Lake Valley or at higher elevations in the surrounding highlands.

The intensity of the low-level inversion is well illustrated in the surface map above by the 0F observation at Starvation Reservoir immediately east of Duchesne (elevation 5720 feet) , and 20F observation at the Blacktail RAWS site immediately to the north (elevation 7311 feet).  

Temperatures at Starvation have been falling to well below zero at night, whereas they have remained in the positive numbers at Blacktail.  For you winter campers out there, this is why it can sometimes pay to camp high rather than low!

On the plus side, although it is cold, it is clear and sunny at Starvation today, in contrast to the haze and clouds that persist over the Salt Lake Valley.  Good ice fishing weather.  

Little Cottonwood Weather and Optics Potpourri

Today I was like a kid in a candy shop checking out the clouds and optical displays in Little Cottonwood Canyon.  I could write an entire book on what I observed, but will provide the CliffsNotes version for this post.

1. Context

A special sounding collected in West Jordan for the Persistent Cold-Air Pool Study (PCAPS) provides much needed context for the photos and analysis that follows.  The sounding, launched at 1715 UTC (1015 MST), shows two distinct inversions.  The lower inversion was between about 800 and 775 mb and caps a surface-based layer that is nearly well mixed.  The upper inversion was located just above 700 mb, or around 10,000 feet above sea level.  It also shows that the winds throughout the troposphere were very light (10 knots or less below 400 mb).  In other words, there was weak large-scale flow and thermally driven circulations associated with daytime heating were likely to dominate circulations in the mountains.

Courtesy NCAR and PCAPS observing teams

The sounding also shows that the atmosphere was saturated or nearly saturated at the base of each of the inversions.  Thin clouds were observed at times at each of these levels, as illustrated by the cloud-cam loop from Olympus Cove between 1800 and 1900 UTC (1100 and 1200 MST).  The rapid retreat of the altocumulus deck near the end of this loop, however, suggests that this cloud layer was quite sensitive to any perturbation in vertical velocity.

2. Little Cottonwood Cloud Evolution and Linkages to Thermally Driven Flows

A layer of altostratus located at the base of the higher inversion blanketed Little Cottonwood Canyon this morning, but largely dissipated by mid morning.  Clouds persisted, however, along the south-facing wall of the canyon, hanging on by a thread by about noon.

The persistence of this cloud was likely due to the development of a cross-valley circulation as the south-facing slopes of Little Cottonwood Canyon were heated by the sun, whereas the north-facing slopes were largely shaded.  As a result, there was probably a weak katabatic drainage flow over the north-facing slopes (e.g., Snowbird and Alta), which were the first to become cloud free due to the sinking motion, but upslope flow over the south-facing slopes, which enabled the altostratus to persist.  

Later in the day, however, the altostratus reformed.  I suspect this was due to the development and deepening of thermally driven up-canyon flow.  Note the thickening stratus deck in these sequential mid-afternoon panoramas taken toward the northeast from near the summit of the Sugarloaf chair.

3. Optical Displays

An added bonus was that the altostratus produced quite a bit of diamond dust.  Diamond dust is composed of tiny ice crystals and it can produce some spectacular optical displays.  We observed this beautiful sun pillar from the Sugarloaf chair 

and a partial halo (a.k.a. icebow) with just a hint of a sun dog.

4. Descent into the Valley Gunk

We departed the Alta parking lot at about 2230 UTC (1530 MST).  Skies were beginning to clear, but clouds persisted on the south-facing aspects.

At around 7000 feet, we encountered the lower inversion.  Beneath the inversion, the atmosphere was filled with haze and smog, but there was also a thin layer of stratus in places. The temperature in our highly scientific car thermometer was 25F above the haze, but reached a minimum of 19F in the lower canyon, before rebounding to 26F once in the Salt Lake Valley.  

Near the canyon mouth, we observed these crepuscular rays.  Crepuscular rays are usually produced when sunlight streams through cloud gaps, but in this case, they were created by the serrated, tree dotted south ridge above Little Cottonwood Canyon.  The apparent convergence of the sun's rays is an optical illusion, much like the apparent convergence of train tracks in the distance.

Finally, there's sunset from the Avenues.  Truly another world down here compared to Alta.  

Political Doom for Mayors

The surest way to guarantee that you will not be re-elected as mayor is for your municipality to botch snow and ice removal during a winter storm.

New York City Mayor Michael Bloomberg is learning this the hard way as he is taking serious heat for lousy snow removal after last week's snowstorm.  Paul Krugman of the New York Times called the storm "Bloomberg's Katrina."  I don't like that analog given how horrible Katrina was, but Krugman argues that the New York City administration failed to take weather warnings seriously.  That's a serious mistake these days as nor'easter forecasts are generally quite good (far better than snow forecasts in Utah) and the public is not going to have much sympathy when they saw the storm coming on the nightly news.

Ultimately, perhaps an analysis will show the snow-removal problems were beyond Bloomberg's control, but New Yorkers will probably hold him responsible anyway.  Even the relatively docile citizens of Seattle skewered Mayor Greg Nickels over the city's poor response to a major snowstorm.

I've always been amazed at snow removal efforts here in Utah.  It's easy to complain about one thing or another, but in Salt Lake, life goes on during major storms remarkably well.  Nevertheless, Salt Lake City Mayor Ralph Becker and Salt Lake County Mayor Peter Carroon need to keep it that way if they are aiming for re-election.  History shows that one poor showing in snow removal can doom a political career.

A Remarkable Snow Year, So Far

2010 has drawn to a close, but we are only 3 months into the 2011 water year (which starts 1 October) and not even half way through the 2010-2011 ski season.  However, since it is 1 January, it seems like a good time to check out the state of the snowpack.

For the Sierra Nevada and Intermountain West, it has been a remarkable ski season thus far.  SNOTEL observations show the snowpack in these regions to be at least 125% of average, with many basins over 200% of average.  Skiers and water managers rejoice!

Zooming into the Colorado River Basin and Great Basin, we see lots of cool colors, indicative of deep snowpacks, and that just warms my heart.  Many SNOTEL sites, especially in Utah, are reporting a snowpack SWE of 175% of average or greater.  Even on the Mogollon Rim near Flagstaff the snowpack is quite healthy.  The only sites with less than 90% of average snowpack are in the Gila National Forest.

Wasatch skiers know this has been a great snow year thus far, but how great?  I took a quick look this morning at the historical Snowbird SNOTEL data available from the Colorado Basin River Forecast Center website.  Thanks to our most recent storm cycle, it does appear we have the healthiest 31 December snowpack (based on SWE) at Snowbird since the 1990 water year (when data availability begins).  Below you can compare this year (green) to two other early season snow years of note.  The first is water year 2005 (red), which started with an incredible bang in late October, and the second is water year 1995 (cyan), which was a huge year for snow in November (if I recall correctly, Alta had more than 200" in November of that water year).

Uber-competitive folks out there might notice that we will fall behind 2005 if we don't get a major storm in the next week or two.  2005 was a water year characterized by a few very large storm cycles, and during that water year there was a really big one in early January (note how the SWE line is oriented vertically).  In contrast, 1995 saw lots of regularly spaced modest-sized storms, so the SWE line has a more regular sawtooth structure.  I suspect the 1995 water year was a better ski season than 2005, even if the total snowfall wasn't as large.  Once you have a healthy snowpack, you want frequent storms of modest size for a great ski season.

Of course, we are only looking back to 1990.  There are other big years before that date, including a couple monsters in the early 1980s.   Nevertheless, skiers should consider themselves fortunate, at least with the ski season so far.  Let's hope it continues.