Q: How to predict or observe wind shear lines?

[markseker]

PROBLEM: Surface wind speed affects safety and certainly comfort and can dramatically change as a function of altitude (and many other factors). It would sure be cool to be able to predict (via published forecasts) or at least observe (via direct visual clues on a clear day) where lines of changing wind shear are likely to occur.

 

CASE IN POINT: I witnessed a dramatic and persistent wind shear yesterday on Mt Hood at 7000'. My partner and i hiked directly uphill from Timberline Lodge 6000' to 9000', turned and snoboarded back down. 4.5 hour round trip starting at 10am. after an hour of steady 20 knot easterly wind, nobody was smiling amid this spindrifty 22 degree air. skiers, lift riders and climbers all wanted a change. the climbers got it, right at slicox hut, above which was CALM DEAD AIR to at least 9000' (and much higher, it appeared). This shear line remained all day, as we shredded down thru it on our descent to the lot. On our ascent, after nearly turning around and heading down, we were glad we didn't and the payoff was a glorious single-layer clothes all-sunny day.

 

QUESTION: Does anyone know a way to predict this? Especially, how to predict it from below, inside the windy layer. Once above it, we could see the wind affects and dissappearing clouds as we looked down. But it's UP we wanted to know most about.

[mattp]

Mark-

Yesterday we were looking at Mount Rainier and speculating that the same thing might have happened there. In these situations where there is a high-pressure cell east of the mountains and air is draining over the range to toward the west, I bet it is pretty common for the high volcano's to be above the disturbance. I bet, too, the aviation weather reports cover this.

[Anna]

Wind shear is definitly something I do not want to experience on landing or taking off when the airplane is traveling slightly above minimum stall speed!

 

There are some ways to predict it and yes, there are aviation charts that can predict them to some degree.

 

The most common (and most dangerous in my opinion) wind shear is generally associated with a frontal system, especially one which includes a thunderstorm. Downdrafts, microbursts, there is just alot of crazy shit goin on in and around thunderstorms...scary!

 

Wind shear can be encountered when flying through a temperature inversion layer or "valley wind shear" too. This involves adiabaitic (sp?) heating of air as it moves over the mountain and then down towards the valley floor where cooler air resides (at least in this example)The shallow air mass in the valley causes the warmer air flowing down the mountain slope to flow over the top of it, resulting in a temperature inversion then our lovely horizontal wind shear. Kind of hard to explain unless I can draw it for you.....

 

Another way that mountains can create wind shear is by turbulence. As the wind blows up one side of a mountain and reaches the top, it can begin to mix turbulently. This turbulence on the lee side of the mountain is a form of wind shear. this is probably what you experienced.

 

Prediction is hard but surface analysis charts along with a winds aloft printouts will aid in figuring it out. Although you are talking about climbing, some airplanes have computer systems that will scream out "wind shear, wind shear!" before and eminent wind shear is about to occur.....

 

Hope this helps, takin a meteorology course right now and it is very interesting!

 

 

[iain]

I would make use of the weather balloon soundings. Below is the skew-T plot of the sounding data from Salem, Oregon at 0400 this morning. On the far right are wind bars which tell you direction and magnitude according to pressure change (altitude). consider 700 mb to be about 10,000 ft and work from there.

[iain]

another interesting thing to watch for on the skew-T is when the green line and red line come together. The green line is dew point, while the red is temperature. When the two meet, that is your current cloud ceiling (so you can predict if you will break out of a cloud deck and have a nice cloud sea if you push on).

[ski_photomatt]

From my limited understanding, wind shear depends strongly on the local weather and topography. The reason you experienced such a sharp transition Sunday was due to the fact that a strong high pressure was moving on shore as the storm was moving east (this caused the strong easterly (from east) winds in the Cascades Sunday). Wind was blowing over the mountains, taking the path of least resistance and I'd imagine moving around Hood like a river flows around a huge boulder sticking out of the water. Was the transition from strong wind to calm about the elevation or a little above that of the local peaks (excepting Hood of course)? I'd imagine this scenario is fairly common when a strong high follows a strong low or vice versa. Another common example is the inversions at the passes (Snoqualmie especially) during clear winter spells where easterly winds bring cold air from Eastern Washington westward setting up the low valley fog. Climbing up a few hundred feet usually sends you above the fog and wind. It's highly topographic dependent.

 

The MM5 is a regional mesoscale weather model run at UW twice daily and issues forecast soundings (like the weather balloon sounding iain posted). The main MM5 page is http://www.atmos.washington.edu/mm5rt. You want the 12km GFS run or go directly to http://www.atmos.washington.edu/mm5rt/gfsinit.html and click on Soundings.

[klenke]

Photomatt is mostly correct. In the last weekend, there was a low pressure trough off the the coast. To the east of the mountains there was cold air at higher pressure than that of the trough out to sea. The result is a pressure gradient from east to west. The gradient set up the direction of air flow. The local effects (i.e., winds) were due to topography. And in this point, photomatt is correct.

[ski_photomatt]

My bad, you're absolutely right klenke. I should have investivated a little more before typing away. My explanation of an offshore high is so wrong it's mind boggling, I shudder to think I could have let it slip The low off the coast is the same one now raining in Southern California for what it's worth.

 

[markseker]

thanks for the reply, skiphotomatt. most helpful.

[catbirdseat]

Tuesday on Rainier we had what I'd term wind shear. It was less windy at the summit that it was at Panorama Point. The wind was blowing from the NE. I think it was sinking air accelerating as it went over the ridge.

[Anna]

Is that you Paul Klenke? HOW ARE YOU? Remember me, Anna the heli-rappeller, and the hike to Vinegar Hill in the Malheur NF last summer? Got the photos, I think I talked to you after I got them though and said thanks. How was the rest of your summer travels????

[klenke]

Yes, Anna, that is me: the one and only Colonel Klenk.

 

And I'd like to point out that the plane in your avatar icon is experiencing some serious wind shear!

 

I'll send you a PM...