Monday, December 19, 2011

It's Getting Deeper

Source: Snowbird
Fog and stratus has pushed up Little Cottonwood and is flirting right now with the base of Snowbird.

This is evidence of a special type of cold-pool (a.k.a. inversion) event that features what is known as a cloud-topped mixed layer.  In many of our cold-pool events, the inversion sits pretty much right on the valley floor, but in long-lasting events that develop clouds, the inversion moves upward with time.  This occurs because the radiational cooling occurs at the cloud top rather than on the valley floor.  The cloud-top cooling drives weak downdrafts and turbulence beneath the inversion.  Small amounts of air from the free atmosphere above are entrained into the cold pool beneath the inversion as this occurs, and slowly but surely the inversion climbs upward.

Schematic of the cloud-topped mixed layer in
the Salt Lake Valley (Pataki et al. 2005).
Adapted from Houze (1993).
This can be seen if you compare the morning soundings from yesterday today.  The shift is subtle, but the inversion is higher this morning.

Note also that below the temperature decreases fairly rapidly with height from the surface (-4 Cº) to the base of the inversion (-9ºC) where the top of the clouds are.  This layer in which the temperature is decreasing rapidly with height is the so-called cloud-topped mixed layer.  The clouds sit at the base of the inversion, cool, and drive weak downdrafts and mixing beneath.  Because of this mixed layer, the pollution concentrations are actually lower than they would be if the inversion were sitting on the valley floor (yes, things could be worse).  

Snowshowers in the Salt Lake Valley occur at night when the cloud topped cooling and turbulence are strongest.  What goes down, must go up, so when you have some weak downdrafts, you also have some weak updrafts elsewhere that help to fuel the snowshowers.  That's why it's snowing in Salt Lake but clear at Alta.

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