Thursday, December 1, 2011

It Has Started

The winds shifted to easterly near the Wasatch Front between 1 and 3 am and we now have a full-blown downslope wind event underway.

We've already had a peak gust to 74 miles per hour in Centerville and 69 miles per hour at the University of Utah.

As is often the case in downslope windstorm events, the strong winds are confined to very near the Wasatch Mountains.  Note how light the winds are at the KSLC airport and that there is a northwest wind at Hill Field.  This is evidence of a hydraulic jump and rotor.  The hydraulic jump is where the flow transitions rapidly from horizontal to vertical and there is a pronounced wave with a wave downstream.  Within that wave, the flow can actually reverse, forming a rotor, which sometimes contains sub-rotors, embedded vorticies with strong turbulence.

Hydraulic jump and rotor (Doyle and Durran 2007)
Sometimes there is return flow, which is flow directed in the opposite direction, at the surface beneath the rotor.  That appears to be the case in the image above at Hill Field.  A beautiful wave cloud can sometimes form on the top of the rotor.  Perhaps we'll see one when the sun comes up.  Until then, back to bed.

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