Basic Anatomy of a Downslope Wind Event (credit: Sam Lillo and Tropical Tidbits)
DISCUSSION: In any given moment, clouds obscure the usually easy-to-spot mountain ranges in the distance. Suddenly, the clouds part and give way to sunshine. Or say for potential hikers on the trail, they are rushed with a strong flow of air that is much warmer than its surroundings and at times could pose danger simply because of the intensity at which wind the wind is blowing, especially when not known and when caught under the tree line. The phenomenon known as a downslope wind event is defined as a channel of wind that is directed down a mountain slope and . Usually driven by subsidence, or when air parcels in the atmosphere have the tendency to descend or sink towards the surface, downslope flows are responsible for the overall drying and warming of the atmosphere since this air warms adiabatically as it approaches the surface, and thus clearing of most if not all cloud cover if clouds are in the vicinity. Most of these downslope wind events are also referred to as a “föhn” or “chinook” winds and are usually tied to larger dynamical forces that are larger than the slope itself.
In recent literature, there are a couple of proposed mechanisms for the development and sustenance of impactful downslope wind events: An example of this type of event is when flow over the crest of the mountain transitions from subcritical (stable and laminar) to supercritical (turbulent and unstable). This is very similar to what is seen during a hydraulic jump, where potential energy is converted to kinetic energy as the waves ascend then descend through the crest. As an alternative hypothesis, vertically propagating waves that have a larger amplitude undergo a partial reflection especially on the leeward side of the mountain.
Forecasting these downslope wind events become a challenge and return back to understanding the three aforementioned mechanisms that could be driving their development. However, several key ingredients stand out that give forecasters clues they need. For instance, long ridges with gentle (steep) slopes on the windward (leeward) sides of the mountain can alter the effective terrain shape because of upstream blocking. Downslope wind events are also likely to occur in environments where the humidity is lower since it is hypothesized that higher moisture content dampens the gravity wave amplitude. Of course, one of the most notable points of detection would be a sudden drop in barometric pressure matched by an equally rapid rise at the conclusion of the event. It does raise the question, “what are the most critical components that make up the main cause for the high winds?”
It goes without saying that, while these windstorms are spectacular in nature and not all windstorms are of severe levels, they are still capable of producing significant damage. In particular, one case that stands out is the October 1997 blowdown event across the Mount Zirkel Wilderness Area in northwest Colorado. In total, roughly 13,000 total acres of trees were blown down while the maximum winds occurred for roughly ~30 minutes. Closer to the Foothills of Colorado, Boulder and Fort Collins have been subjected to damaging windstorms with winds exceeding 100+ mph on more than three dozen combined occurrences. This is a relatively new field of study for most researchers, but the overall benefits of understanding these windstorms will help in forecasting them in the future.
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© 2019 Meteorologist Brian Matilla
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