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.  


  1. Do I ski tomorrow or Thursday, if I can only get up to Solitude one day?

    1. We typically don't make decisions for you without significant financial support...

    2. I would ski Snowbird regardless of the day. Solitude isn't very good.

    3. Way less crowded though, so you're more likely to find untracked snow.

  2. Att 11am today the inversion was all the way up to the new wildlife crossing at the top of parleys.

  3. Which factors decide how high above the valley floor the inversion layer develops? Why is this layer almost touching the valley bottom in some situations but very often it is higher above ground? Is the elevated inversion layer caused by heating of air in the valley by cars, buildings and radiation that manages to penetrate the thin status?

    1. This is complicated as it is dependent on many factors including the strength of the sinking motion aloft beneath the ridge, whether or not fog or low clouds develop, the transfer of moisture and heat from the ground, snow, and lake in to the atmosphere, whether or not it precipitates, and the length of the inversion.