Thursday, December 1, 2011

Downslope Windstorm Anatomy

The event today was quite remarkable, with peak gusts at one MesoWest station exceeding 100 mph.

There are many people out there wondering, however, why the wind weakened abruptly as one moved westward away from the Wasatch Mountains.  This weakening has to do with the structure of high-amplitude mountain waves.  In such a wave, high momentum air from aloft descends rapidly and accelerates on the lee (in this case west) side of the mountain range.  Once in the lee, the flow frequently exhibits a hydraulic jump in which the flow ascends rapidly.  A rotor, or an area of reversed flow at the surface, is typically found downstream of the hydraulic jump.

Source: Whiteman (2000)
Most of us have seen a hydraulic jump before, but in water not the atmosphere.  Here's an example.

Source: GFDL/Wikipedia Commons
At the surface, the transition from strong downslope flow to return flow in the rotor can be incredibly abrupt.  Check out the MesoWest plot below, which shows strong easterly flow near the Wasatch Mountains, but three stations along I-15 with westerly flow.  In particular, there is a 55 knot sustained east wind at Farmington, but two stations just to the west in the surrounding area report west winds.  

Very impressive! 

1 comment:

  1. The 12Z (1 Dec) SLC sounding is very interesting, as it goes right up through the rotor. Looks like the center of it was near 815 mb (6000 feet MSL) where winds were calm. Perhaps the sharp stable layer near 700 mb marks the top? Also, the sounding is quite dry. Seems like the moisture field must have increased later to get those clouds in the video.