What is a Downslope Wind? (Caution: Knowledge dropping ahead)

Some of you may have read the forecast discussion the past couple of days and noticed the reference to "downslope winds" or "downslope enhancement". What does it MEAN? Well we are going to explain that to you today in the simplest terms that we can...so here we go!

First of all, wind is typically driven by pressure differences, which are influenced by thermal differences. The greater the difference, the stronger the wind. (Video here) This type of wind is fairly typical, and we would call it a "gradient driven wind". Downslope conditions are characterized by winds encountering a barrier and attempting to get past that barrier. Let's take a look at this by relating it to something we are more familiar with, like water running in a creek!





Let's pretend in a perfect world there is a straight, slow moving river with a smooth riverbed. In this ideal scenario (without friction, physics, etc.), the water flows down the river quietly and calmly with no ripples or eddies within the water flow or on the surface. Depending on your adventure preference via kayak, this would be pretty easy-going and relatively boring. Now let's take a look at what reality would be in a river. Toss in some rocks, riverbed irregularities, and swift-moving water. The water would also move at different speeds at any level within the river, which would change things up. Notice the flow now! Let the kayak adventure begin. 

Mathematically, air and water movement can be calculated with very similar equations, which makes this comparison fairly true to life. So similar to water, when the wind encounters a barrier, propagating waves (eddies/turbulence) result downstream of the barrier. The stability of the atmosphere determines whether the waves will be pushed downward towards the ground (see image on the right) or become trapped midair (trapped lee waves on the left below).



The breaking region of a downslope wind can be thought of as similar to a hydraulic jump region that many are familiar in seeing in rapids, spillways, and even your own kitchen sink at home. This is because the quickly moving fluid, or atmosphere, crashes into a region of relatively lower speed. Just think about being in a big wind event in the Sierra -- winds are almost always stronger at the upper elevations than they are in the valleys below. See the images below for hydraulic jump examples in fluids, with the picture on the right similar to the jump region in a downslope wind event.


Meteorologically, how do we get the setup that we need for downslope conditions to occur? Here are some downslope tidbits:
  • Requires a strong jet perpendicular to the Sierra
  • A ridgetop inversion is needed to develop the wave breaking region, see examples of this in actual upper air soundings from downslope wind events below.
  • Typically occur 3-6 times a year with the core months late October through early May
  • Strongest winds will be immediately in the lee of the Sierra
  • Wind speeds 60 to 90 mph have been recorded in the Reno area during these strong events with gusts of 100 to 150 mph observed in the Sierra. 
 Typical downslope windstorm soundings with the ridgetop inversion level highlighted in the yellow circle:



Downslope winds have been known to cause quite a bit of damage in the lee of the Sierra. Impacts include, but not limited to: power outages, semi trucks blowing over, blowing dust, fence damage, multiple airline cancellations/delays, and roof damage. Even with all of the potential destruction, it is hard to argue that the lenticular clouds often resulting from these winds (plus some moisture) aren't spectacular! 


Photo taken from NWS Reno office, March 15, 2015


Time lapse of a long duration lenticular cloud on November 7, 2009. This was a trapped wave event with strong winds at ridge top level, but only moderate winds at the surface.


Need more information? Here are some resources for our weather geeks: 


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