Friday, December 24, 2010

Salt Lake stratus, valley circulations, and mountain surface hoar

If you were in the mountains today, you may have observed a nice example of the diurnal "breathing" of the canyons of the Wasatch Mountains.  Little Cottonwood Canyon, for example, was cloud free at about 9:30 am.

Looking west down Little Cottonwood Canyon
9:30 AM 24 Dec 2010
This afternoon, however, the stratus had pushed back up the canyon to an elevation of about 7000 feet. 

Looking northwest toward Little Cottonwood Canyon
1:30 PM 24 Dec 2010
The cloud evolution is consistent with the diurnal circulations that occur in Little Cottonwood Canyon.  Down valley drainage flows at night erode and advect the stratus out of the canyon at night, whereas up-valley flows during the day transport it back into the canyon.  Those of you on the east bench around Olympus Cove were treated this morning to a brief period of clear skies as the morning outflow from Parleys, Emigration, and Big Cottonwood eroded away that stratus overnight.  When those flows stopped, the stratus returned.   

Also of note is the surface hoar we found at elevations below 9000 feet in White Pine Canyon.

Curiously, there was little surface hoar above 9000 feet.  At issue is if this is a reflection of the large-scale temperature inversion that existed overnight.  The sounding from the Salt Lake airport this morning shows a pronounced inversion and decrease in specific humidity with height right at 700 mb (about 10,000 feet).  

Even below this level, there is a fairly strong vertical gradient in specific humidity, so perhaps the potential for vapor deposition from the atmosphere to the snowpack decreased above about 9000 feet and this led to less surface hoar development?  

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