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: 


Popular posts from this blog

What is the Rain Shadow and What's Wind Gotta do With It?

Image
The dreaded rain shadow - something that those of us who live east of the Sierra crest are more than familiar with and hate to hear. So what is this rain shadow and why does it even exist? Lets start with the topography just to our west... The Sierra Nevada mountain range runs 400 miles from north-to-south and is approximately 70 miles wide. It includes the highest peak in the lower 48, Mt Whitney at 14,505 feet, and rises above 10,000 feet in many locations. Left: The Sierra Nevada Mountains highlighted in blue. Right: A relief map showing elevation (in meters) of the Sierra. The Sierra are a significant barrier for Pacific storms which typically move from west to east. The movement of air masses and associated weather cannot go through the mountains and is forced up and over the mountain range. As Pacific moisture is driven eastward by the wind, the air mass is lifted up and over the Sierra. Rising air will cool and once it reaches the point of saturation, condense into
Read more

NWS Wind Forecasts: AKA What's the Deal With Friction?

Image
We have had a number of comments on social media which mention much lower observed wind speeds locally than are in the forecast. Well, you are unlikely to know this (it is not exactly advertised!) but National Weather Service wind forecasts are for 10 meters, or almost 33 ft, above the ground. As we are about to explain, that makes all the difference in the wind between eye-level (~5-6 feet) and ~33 feet.  It is natural to wonder the purpose of a wind height forecast well above where you and I typically reside. The main reason for this is the complication in forecasting wind as one nears the ground due to frictional influence caused by the surface. Here is a diagram showing the concept of wind speeds change as one gets away from the ground.  As you can see above, there is a rapid speed reduction (gray dashed line) near the ground with a lesser change away from the surface. Note that the above diagram is generalized. How much reduction is realized depends on the nature of the surface wh
Read more

Forecasting "Dry Microburst" Potential From Soundings and Observations

Image
Summer afternoons in the valleys of eastern California and western Nevada usually feature dry low level relative humidity. In certain cases where low levels are very dry (single digit relative humidities) and enough instability for showers and/or thunderstorms exists, there is the heightened possibility for severe (over 60 mph) outflow winds. Now, severe winds are possible from strong thunderstorms in many cases, even without very dry surface relative humidity. However, what makes very dry surface or low level conditions special is that severe outflow winds are possible even with w eak  convection such as light showers...or even just cumulus clouds! When severe winds are produced by convection with little or no rainfall reaching the ground, it is referred to as a "dry microburst". Microbursts can be of significant danger to aviation and, if they are strong enough, life and property. Before we move onto what conditions we look for to determine the threat for dry microbursts,
Read more