Tuesday, December 11, 2018

A Classic Cloud-Topped Mixed Layer Followed by Snow

There's a fantastic view looking west this morning from the top of Snowbird's Hidden Peak, with a sea of stratus clouds over the Salt Lake Valley and the Great Salt Lake Basin.

Source: Snowbird
The scene is not unlike one might see from a hill above San Francisco in the summer as those clouds reflect what is known as a cloud-topped mixed layer, which dominate the weather along the coast of California in summer.  Below is the sounding from the Salt Lake City airport.  I have added a grey bar to denote the cloud layer.  From the surface to the top of the cloud deck, the temperature decreases rapidly with height and the atmosphere is well mixed.  Right at the top of the cloud layer is an inversion layer in which the temperature increases about 1.6ºC (about 3ºF) over a depth of about 60 m (200 ft).  That doesn't sound like much, but it is sufficient to keep a lid on the valley atmosphere.  Further aloft, a series of stable layers extend to nearly 600 mb, well above the crest level of the Wasatch Range. 

Sounding source: SPC
Although it makes for depressing skies, we are better off with a cloud-topped mixed layer than a strong inversion based near the valley floor.  That's because in a cloud-topped mixed layer, radiative cooling at cloud top drives turbulence and keeps the atmosphere relatively well mixed within and below the cloud layer.  Hence the term cloud-top mixed layer.

Source: Pataki et al. (2005)
As a result, our pollution is mixing through a layer that is about 850 meters (2775 feet) deep, which is right up to the base of the inversion.  That's much better than when the inversion is based very near the valley floor.

The development of this cloud topped mixed layer is one reason why PM2.5 levels have remained at moderate levels the last two days.  Basically we are seeing pollution dilution due to its development.
Source: Utah Division of Air Quality
All else being equal, a cloud-topped mixed layer with a deep cold pool and elevated inversion results in lower PM2.5 levels on the valley floor than a shallow cold pool with a near-surface inversion.  On the other hand, if the benches or perhaps places like Emigration Canyon see higher PM2.5 levels.  

Looking toward the future, the overnight model runs are showing a vigorous trough passage tomorrow (Wednesday) that should crack this inversion.  Although quick hitting, the system will generate snow down to the valley floor.  Students with finals tomorrow should consult National Weather Service Forecasts and plan on leaving early.  No excuses!  The official forecasts of 6-12" in the mountains are looking pretty good.  Pro Tip: Lift-served skiing tomorrow will be better late than early.  

Monday, December 10, 2018

Greed Is Good

Gordon Gecko should have been a powder skier
Last night I thought of Gordon Gecko as I was bemoaning the lack of snow over the past several days.  

We had a good run through last Monday.  The snowpack is much better than last year at this time.  But we need more.  Greed is good. 

We certainly had some fun, creamy snow ski touring on Saturday.  But skiing it involved pinballing off a few rocks and a bony brushwhacking adventure at the exit.  I kept thinking that another 3-5 feet would be nice.  Greed is good.

The skate skiing at Round Valley has been great so far this year, but on Sunday, things were starting to feel a little worn out.  I kept thinking that a few inches of high-density refresher would be nice.  Greed is good.

I haven't skied at the resorts in a while and have heard a mixture of reports, but surely a major dump or two before the holidays would be nice.  Greed is good.

However, there will be no hostile takeovers or union busting here.  Only the strait skinny.  Looks like 3 troughs will be moving through the west over the next week, but none of them look like a sure thing.  You might recall the storm cycle a couple of weeks ago when the NAEFS ensemble was tightly clustered around a significant storm, but last night's forecast for the next seven days shows huge spread being produced by both the GEFS and the Canadian Ensemble.  For snow at Alta, the mean is about 10 inches, but the range is 0 to 22.  How's that for uncertainty?  

We'll have to see how this all shakes out.  Let us hope the inversion cracks and that we can at least get a bit of a refresher.

Sunday, December 9, 2018

Intricacies of This Inversion, Part II

Building on the previous post (Intricacies of This Inversion), it is worth taking a look once again at some of the intricacies of the our current inversion as the evolution is quite interesting from a meteorological perspective.

First some definitions.  An inversion is a layer in the atmosphere in which temperature increases with height.  Within a valley or basin, a cold pool is the layer of cold air beneath that inversion.  Inversions vary in strength, depth, and altitude.  The current inversion has been characterized by an elevated inversion rather than one that is very near the valley floor.  In addition, the cold pool beneath the inversion has featured high relative humidity.  This has led to a cold pool that has not only filled with pollution, but has featured haze (small water droplets that form in a high humidity environment), and clouds. 

This morning those clouds largely fill the Salt Lake Valley, and the contrast between valley and mountains couldn't be larger.  I awoke this morning to a depressing scene, as bleak and grey as you will see in Salt Lake City. 

Meanwhile, the weather at Snowbird was stunning, with the view from the top of Hidden Peak showing a sea of stratocumulus over the Salt Lake Valley. 

This morning's sounding shows a multi layered structure from the valley floor to crest level.  From the valley floor to about 814 mb (about 6500 feet) the atmosphere was well mixed.  That means pollution is not confined to a very shallow layer near the valley floor, but is actually mixing through a depth of about 2000 feet.  At 814 mb is the base of a weak inversion.  This inversion sits at the top of the stratocumulus layer.  Then there is a second, much stronger inversion, based just below 700 mb (10,000 feet), which is being produced by the upper-level ridge that is building in. 

There are some interesting things happening, however, in the Salt Lake Valley on smaller scales.  When looked east this morning, I saw a bright spot along the east bench where the clouds and haze were thinner.   

A look at the surface observations showed light easterly flow at several east bench locations.

That easterly flow appeared to be disrupting the clouds and possibly contributing to the entrainment of drier and cleaner air into the valley cold pool.  Although the PurpleAir PM2.5 values are sometimes too high when the relative humidity is high, they are very useful for identifying pollution patterns over the valley and show lower PM2.5 indicies along the east bench of the northern Salt Lake Valley, especially north of Big Cottonwood Canyon.  These lower values extend eastward across downtown Salt Lake City.  This is consistent with the extension of easterly and southeasterly flow to the Salt Lake City airport in the station map above. 

Consistent with this easterly flow and associated entrainment of cleaner air, the air quality at Hawthorne Elementary has dropped significantly, all the way down to about 12 ug/m3. 
Source: Division of Air Quality
On the other hand, values remain high in other parts of the Salt Lake Valley to the south and to the west.  One disturbing aspect about this is that the DAQ uses the Hawthorne observations for their web site and is listing the air quality as good (i..e, green) on their web site, despite the fact that it is clearly not good in many parts of the Salt Lake Valley. 

Source: Utah Division of Air Quality
The Governor recently announced $100 million for air quality in his 2020 budget.  Whether or not this survives the legislature remains to be scene, but if it does, perhaps some of it can be used to put together a proper air quality monitoring and reporting system for the Salt Lake Valley.  Community (PurpleAir) and University of Utah (www.aqandu.org) networks are now providing valuable information that could be mined to improve air quality information and alerts.  The challenge is how to blend higher cost but higher fidelity instruments with lower cost but lower fidelity instruments.  In any event, we can certainly do better than a county-based air quality sensor based on one site. 

Friday, December 7, 2018

Intricacies of This Inversion

As far as dismal grey goes, the past few days take first prize in the Salt Lake Valley due to frequent mid-level clouds, occasional low clouds, plenty of haze, and smog.  I was surprised to hear that we reached PM2.5 levels yesterday that were unhealthy for sensitive groups as the air was still relatively clean on Tuesday and it usually takes four days to build up to that level.  However, the PM2.5 trace from Hawthorne (below) shows it took three, although it was a spike around noon that got us to over 35 ug/m3.
Source: Utah Division of Air Quality
A curious aspect of the current inversion is that it has been relatively elevated.  Yesterday's morning and afternoon soundings from the Salt Lake City Airport show the inversion base is at about 825 mb, which is about 1500 feet above the valley floor. 

Source: SPC

Source: SPC
Often, the inversion is right down near the valley floor, but in this instance, the smog is actually being mixed some.  It's just not able to mix through a deep enough layer to get out of the valley. 

The relatively elevated nature of the inversion and the deep layer of haze and pollution over the Salt Lake Valley is very evident in the Snowbird camera image from this morning.  While it is not uncommon for haze and pollution to be found in the Salt Lake Valley, the depth extends relatively far up the Oquirrh Mountains. 

In some respects, that is good news as it means the pollution is diluted a bit by the deeper layer of mixing.  On the other hand, it's still very depressing and the air quality is a best moderate and at worst into the unhealthy for sensitive groups category.

Looking through the weekend, it appears our goose is cooked, at least in the valley.  The models show an upper-level ridge building in with temperatures warming aloft and in the mountains.  The 3-km NAM sounding for Sunday Morning shows a still-elevated inversion with a shallow, moist mixed-layer near the valley floor.
Source: Tropical Tidbits
 The models are notoriously bad at getting the details of inversions right, but there's really no hope of mixout through the weekend.  Further, given the moisture in place in the valley and the forecast sounding above, I wonder if we will see more widespread fog or stratus (i.e., elevated low clouds) developing through the weekend. 

Finally, it is worth pointing out that blaming the pollution on the inversion, as I hear so often, is simply incorrect.  The inversion is simply a meteorological phenomenon that caps a pool of cold air in the valleys and basins of the intermountain west.  The pollution is caused by emissions due to the burning of fossil fuels and wood, as well as from agricultural and other sources.  As I like to say, we have met the enemy, and it is us.

Wednesday, December 5, 2018

Remarkable Overnight Temperature Rises

If you are into microclimates and the impact of radiation on temperature, the last two days have been incredible.

On Monday night, conditions were ideal for temperatures to fall and fall dramatically in lowland regions.  Tuesday night, things were different and clouds resulted in an increase in overnight temperatures overnight.

To illustrate this, let's take a look at observations from the Peter Sinks, a limestone sinkhole in the Bear River Range of northern Utah known for incredibly low overnight temperatures.

On Monday night, temperatures exhibited some ups and downs after sunset with some wild swings in temperature from about 1800 MST 3 Dec through about 0400 MST 4 Dec.  Those swings occurred in conjunction with gusty winds, which occasionally mixed out the cold pool that was forming in the Sinks.  However, after about 0400 MST 4 Dec, the wind died down and temperatures fell nearly continuously, reaching a minimum of about -35˚F at about 0900 MST 4 Dec when I suspect the low angle sun finally began to warm the sink, after which temperatures rose until the early afternoon. 

Source: MesoWest
Source: MesoWest
Last night, however, things were different.  The air was mainly calm all night and the temperature fell nearly continuously from about sunset through midnight (0000 MST 5 Dec), when it reached an incredible -40˚F.  

However, after midnight something remarkable happened.  The temperature increased 35˚F, reaching -5˚F by 0700 MST 5 Dec.  This happened despite it being night time and the air being calm or nearly calm.  Similar temperature traces can be found at many stations in northern Utah, although as you move southward, the temperature increase occurs earlier.  For example, at the Clover Site in the Rush Valley southwest of Salt Lake City, which is also known for very low overnight temperatures, the minimum temperature occurred at about 2000 MST 4 Dec, after which the temperature rose until this morning . 

Source: MesoWest
What gives?

Overnight several things happened.  First, cloud cover spread over the area, as can clearly be seen in the overnight satellite imagery.

Second, cloud base lowered.  At the Salt Lake city airport at midnight (0000 MST), a few clouds were reported at 20,000 feet above ground level, but by 0400 MST, the broken clouds covering 6/8 of the sky were reported at only 3700 feet above ground level.  

Finally, those clouds accompanied an elevated warm front that moved through the area and increased temperatures aloft.  Yesterday morning, the 700-mb (about 10,000 feet above sea level) temperature was -14.5˚C.

However, this morning the 700-mb temperature was -7.5˚C.  Note also that while there was an inversion yesterday morning from the surface to 850 mb, the atmosphere was relatively well mixed in that layer this morning (although the inversion based at about 800 mb and associated with the warm front still puts a lid on the valley atmosphere).  

So, overnight free-atmosphere temperatures aloft and at mountain elevations increased, but so did the cloud cover.  I suspect that the latter played a critical role in rising overnight temperatures in cold spots like the Peter Sinks.  With the air calm, it would be very difficult to mechanically mix out the -40ºC cold pool that had formed by midnight.  Instead, the cloud cover was probably critical.  

Clouds are important not because they "act like a blanket" (they don't), but because they emit infrared (longwave) radiation.  On a clear night with dry air in place, there is very little longwave radiation received by the ground (or snow) from the atmosphere, and temperatures plummet.  However, on a cloudy night, the incoming longwave radiation is greater.  How much greater depends on the temperature at cloud base, with low, warm clouds emitting more longwave radiation than high, cold clouds.  This radiation is then received by the Earth's surface or the snowpack, which cools at a slower rate than would occur under clear skies.   

Last night provided an extreme example of this in which clouds actually produced increasing temperatures.  In Peter Sinks, temperatures were -40˚F (which equates to -40˚C) before the clouds spread in.  Based on this morning's sounding above, the temperature at cloud base by this morning probably reached at least -10˚C and perhaps as high as -6˚C.  The amount of infrared radiation emitted by the clouds and also by the snowpack strongly increases with temperature.  Thus, with much warmer clouds spreading over a cold snowpack, the snowpack would begin to receive more longwave radiation than it was emitting, and would begin to warm.  That energy subsequently is transferred into the atmosphere, resulting in an increase in the near-surface air temperature.  

This process really has nothing in common with how blankets work.  Blankets keep you warm primarily by reducing the mixing of air near your body with the air farther away.  This keeps relatively warm air near your skin, reducing the transfer of heat to the atmosphere.  Clouds don't directly affect the transfer of heat from the Earth's surface to the atmosphere.  They affect the temperature by changing the radiation balance and providing more longwave radiation than clear sky.

Monday, December 3, 2018

Nice Storm Cycle Winding Down

The storm cycle that has blessed us is coming to and end and, when combined with the Thanksgiving weekend storm cycle, leaves us in pretty good snowpack shape for early December.

As of midnight last night, every Wasatch Snotel except for Parish Creek (98%) and Mill D North (94%) was above median snowpack water equivalent for the date.  

Source: NRCS
Those are also the only Snotels in the central and northern Wasatch with less than 5 inches of snowpack water equivalent.  If you are looking for the fattest snowpack, you'll find it in the upper Cottonwoods where Snowbird and Brighton are at 6.6 and 6.5 inches, respectively, and Ben Lomond Peak and Trail, which are at 8.2 and 5.3 inches, respectively.  The 5.3 inches at Ben Lomond Trail represents 241% of median and is especially impressive given that it sits just below 6000 feet. 

Although there may be some lingering snow showers today, the storm cycle is pretty much winding down.  This week represents a break in the action as the next major storm is expected to pass to our south, although it will throw some precipitation our way midweek.  

Most downscaled NAEFS members initialized at 0000 UTC (6 PM MDT) yesterday afternoon are showing less than 6 inches of snow over the next 7 days, and most of that was last night

Thus, this looke like a relatively quiet week.  Hopefully the backcountry snowpack will begin to strengthen and heal.  

Sunday, December 2, 2018

Weekend Ski & Snow Update

I got out for my first ski tour of the season yesterday and was pleased to see more snow than we saw probably at any point until late January of last year.  Conditions were good, with the snowpack dictating cautious meadow skipping.

USA bowl was apparently the "it" place for most as it looked nearly as tracked as Solitude did across the street.

The resorts were packed as traffic in the canyon was clogged in the morning and cars were parked up and down the road as far as the eye could see.  Sadly, we enter another season with little progress made to alleviate congestion and no real plan in sight.  

Shifting to more positive news, the snow continues this morning with about 4" of pixie-dust overnight on the Alta-Collins stake through 8 am.  The situation right now is quite interesting as we are basically just to the northeast of an area of strong northwesterly flow where there are a number of small-scale eddies, known as mesovorticies,.  I have identified three of these vorticies in the 1-h RAP forecast below, which is overlaid on the 1500 UTC (8 AM MST) satellite and radar pattern below.  

The circulation associated with the middle mesovortex is clearly evident in the radar image below.  

This is a situation in which the radar isn't really all that useful for inferring what is happening in the central Wasatch.  Note the the lack of echoes, or the existence of only very weak echoes, in the imagery above, especially on the Park City side.  Nevertheless, it is clearly snowing over there.  

There are a number of possible reasons for this.  One is that the storm might be quite shallow, so the radar beam is overshooting shallow snow-producing clouds.  Another is that the water equivalent rates are very low (i.e., the snow is low density) and the ice crystals are relatively small, and thus the snow is producing very little backscattering of energy back to the radar.  

The periods of light snow will continue today and tonight.  I'll be honest.  I'm not sure how much we're going to see.  The flow is weak and shifty and the snow-density is very low (probably 5% or so), so a small amount of water adds up to a lot of snow.  Further, check out the spread in water equivalent for Alta being produced by the models.  Anywhere from 0.1 to 0.9 inches from 2 AM this morning through 5 AM MST tomorrow morning (03/12Z).  

At 5% water content, that's a range of 2 to 18 inches of snow.  Since 2 AM, we've already had 3 inches at Alta Collins.  18 is unlikely, but another 4-8 seems doable, with higher totals if we end up on the high side of these model forecasts.  It should all be cold smoke.