Christmas day 2008, brought with it a rare wind event to the Coachella Valley, creating a ton of damage including a roof being blown off the Cambridge Inn in Palm Springs. What the west end of the Coachella Valley, Yucca Valley andhigher elevations experienced wasis commonly referred toby meteorologists as a “Mountain Wave”.For the publica”Down Slope Wind Event”. Most pilots are aware of these because they are a very real risk to aircraft and create conditions that cause aircrafts to lose lift and crash.
Down Slope Wind Events are not like the normal windy conditions. They cangenerate a lot stronger wind gusts than a typical wind event and result ina lot more damage. Thereal difference though is in how the stongwind gustsare actually created, resulting in the Down Slope Wind actually being able to produce stronger gusts than the typical wind.
Your typical strong wind is created by a pressure difference, a high pressure system and low pressure system being to close to one another. In a minute we will explain how the Down Slope Wind Event forms.
Down Slope Wind Events occur a couple times a year here in Southern California. They are more common on the leeward (east side in the Northern Hemisphere)side of the Rockies where winds have been recorded to gust to near 120 mph before.The Christmas dayevent was a little bit stronger than a typical event. The atmosphere onThursday had strong winds in the middle part of the atmosphere at about 8,000 feet and they approached the mountains at the most favorable angle for the strongest wind gusts to occur, the perfect combination.
Wind gusts inYucca Valleygusted up to49 mphduring theevening hours with gust to 37 mph atPalm Springs InternationalAirport. Stronger gusts were likely in the Valley based on the damage that was created but there were no weather instruments in those locations to record the wind speeds. In the west end of the Valley strong wind gusts continued to pumle the area well into the overnight hours, toppling trees, power poles and ripping shingles off roofs.
There is one distinct aspect of the atmosphere Christmas day that had to be present for this event to evolve. That was the presence of a “stable” layer of air at approximately 15,000-20,000 feet above the ground. We will use several analogies to simplify this, the “stable” layer acts as a brick wall. It is simply a layer of air that is not buoyant (doesn’t rise).
As winds of 35 mph at 8,000 ft.(blue arrows) come in off the Pacific Ocean perpendicular (90 degree angle) to the San Jacinto Mountainsthey hit the mountains as Figure 2 shows.This air acts like a rubber ball and bounces off the west face of the mountain and up into the higher parts of the atmosphere.
On its journey up it ran into the “stable” air (brick wall) at approximately 15,000 to 20,000 feet and is bounced back to the Earth below.At 15,000 to 20,000 feetwinds were blowing at approximately64 mph, that bouncing ball of air will get sped up if you will while at that altitude and then bringits new acquired speeddown to the Earth below. Now it won’t totally inherit all of the64 mph wind speed.A rough estimate would be to average the starting speed of the air,35 mph, with the 64 mphwind speedin the “stable” layer,and you come up with a gust possible of 50 mph. Which is approximately what was recorded in Yucca Valley.
Thenas the air or “rubber ball” as we call it,crashes into the ground it creates strong wind gusts at random times and locations west of the base of the mountains asFigure3shows in the areas in red. This isn’t the end of the story for this rubber ball though. It’s acting like a rubber ball after all!!! We all know that rubber balls can’t bounce just once. After hitting the ground the air bounces off the ground and heads back towards our “stable” air or brick wall.
It again is on a crash course with the “stable” layer of air. It hits it again!!Figure3shows this, as the air (blue arrows) rises, like all pockets of rising air that have enough moisture in them, the rising air creates a cloud. Like before though it bounces back towards the Earth and this air is sinking, so cloud do not form.
What is taking place is that our air is maintaining its momentum and has become trapped between the ground and the “stable” layer (brick wall). As a bouncing ball does, this air bounces up and down between the ground and the “stable” layer. It’s what we call a trapped wave.Figure 3shows this pocket of air or “bouncing ball” bouncing eastward from the base of the San Jacinto Mountains. The strongest wind gusts will always occur with in the first or second crash with the ground.During the evening of Christmas dayYucca Valley recorded a wind gust of 49 mph which was most likely the result of a “second” bounce of the wave hitting the ground down stream of the mountains. In Figure 4 taken during the Feburary 3, 2008 event you can even see the cloud that formed at the “highest” point of the trapped air’s “bounce”. This image is looking northwest towards the Banning Pass. On Christmas day during the evening if you would havelooked towards the base of the San Jacintos you would have seen a similar shaped cloudnear the mountainsthroughout the duration of the event.
Earlier we said that the wind hit the San Jacinto Mountainsat a 90 degree angle along with our “stable” layer being in place. Those are the two major ingredients that do not come together to often around here.
Most of the time the winds don’t hit the mountains at the correct angle-90 degrees, or a “stable” layer isn’t in place, or the winds at 15,000-20,000 feet are not as strong, or the winds at 8,000 feet are not as strong.
Other mountain waves or down slope wind events that occur in the Coachella Valley several times a year usually do not have as strong of winds at 15,000-20,000 feet and they don’t hit the San Jacinto Mountainsdirectly perpendicular to the mountain face. Both of those factors greatly reduce the resulting wind gusts.
The wind hitting the mountain square at a 90 degree angle is like a car (wind) running directly into a wall (mountain), it comes to a stop and thenbounces backwards, but the car wanted to continue traveling forward but the wall got in the way. Figure 6 shows the airdoes almost the same, but it can’t not go backwards so its forced up and over the mountain with most of its momentum (wind speed)in tack (while gaining extra speed from the”stablelayer”winds)still traveling forward.
Where as ifthe wind hits at a smaller angle, say 45 degrees, or from the northwest, thewind speed(momentum)is distributed to the north and south along the west face of the mountain, so the winds that actually make it up and over the mountain moving southeast are not as strong. Figure 7 helps visualize this effect, with windson the west side of the mountains moving at slower speeds.Making the car analogy, thecar (wind)having a glancing blow to a wall (mountain), will continue inthe general direction it was traveling but at a slightly different angle and it keeps going…..in wind terms, thewind gets stuck on the west side of the San Jacinto Mountains.
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Check out this site for more info on mountain waves: http://moe.met.fsu.edu/~rhart/mtnwave.html