Friday, November 22, 2013

Wind Chasers

Just as the forecast models from a few days ago suggested, a downslope windstorm occurred along the Wasatch Front between Salt Lake City and Ogden since Thursday afternoon. The National Weather Service extended the high wind warning until Saturday afternoon. 
Thursday night on the Bountiful bench, a citizen weather station reported gusts over 60 mph! Below is a time series plot of the winds for 9:00 pm Thursday to 9:00 am Friday morning. The orange dots indicate wind direction. For most of the time shown below, the wind is from the east and turns south, southeast in the early morning. The solid red lines indicates wind speed, usually averaged over 10 minutes. The dashed green line indicates wind gusts, which are those relatively short lived bursts of air that almost knock you over if it catches you off guard.
Other stations in Centerville and Farmington recorded winds speeds up to 80 mph! These winds are strong enough to cause quite a big of damage. Driving through Bountiful Friday morning we saw tipped over trash cans and fallen tree limbs, but nothing too big had fallen. However, the news had plenty of pictures showing much more damage. Thursday night, the Utah Department of Transportation closed the freeways to large semi trucks to prevent any tip overs or accidents.

This Friday Morning I participated in a some weather balloon launches. Students and staff at the University of Utah launched two weather balloons from the Bountiful bench to measure the wind and temperature profile of the atmosphere during this wind event. We set up our weather observation site on the mountain side just under the Bountiful "B".  It was sure windy up there. My friend Alex measured the wind gust at about 45 mph.

The instrument attached to a weather balloon is called a radiosonde. Below shows me holding one of the radiosondes before attaching it to the balloon. It looks like a small styrofoam cup with a wire sticking out because it that is just what it is. Inside is a small circuit board with components on it to make measurements. On the cup is a little note that reads, "If found, please call and return to the University of Utah" and a phone number and address are given.
Before attaching the radiosonde to the balloon you have to establish a GPS and radio connection with a receiver. We connect the receiver to a computer in the truck so we can receive and record the measurements the radiosonde makes when it rides through the atmosphere on a balloon. It takes a few minutes for the radiosonde to make a GPS connection, but when I heard the chirping sound, I was done walking aimlessly around the parking lot holding the. It was time to tie it to a balloon.

The hard part about filling weather balloons in the wind is, well, filling them in the wind. They want to flop and fly all over the place before we're ready to let them go. Using a big, blue tarp, we cleverly held the balloon in place as we filled the balloon with helium. Fill the balloon with helium, tie a good knot, and tie the radiosonde to the bottom by a string.

Quick countdown...5....4...3...2...1...and let her go!

Then you watch it float away until you can't see it anymore. If everything goes well, like it did for both these balloon launches, you can watch the data come into the computer. 
By radio, the radiosonde sends back measurements of pressure, temperature, and humidity. A GPS tracks its position, thus measuring wind speed. When all the data comes in, it looks like this...
(An explanation of would be rather long, so I won't go into it here. Just know that the solid black squiggly line on the right represents temperature, and the black squiggly on the left represents dew point temperature. If you so desire, more info on how to read a skew-T chart can be found here. Or stay tuned for a future blog post on skew-T/Log-P charts. For info on how I made this chart, refer to my article here.)

This video shows our adventures on the Bountiful Bench.

Well this downslope windstorm was exciting, but we don't tend to see them in Spanish Fork (location of my personal weather station). Why do areas north of Salt Lake have extreme wind events and not Utah. This questions hasn't been extensively studied, but the orientation of mountains is likely to have a most of the answers.

Downslope windstorms form when we have strong easterly winds blowing aloft. Easterly winds are can be associated with a strong cut-off low centered to the south and west of Utah, as this case shown in the weather analysis below. Winds blow around a low pressure counter-clockwise (shown by the yellow wind barbs).

These winds force cold, stable air over the mountain barrier which then plummets down the lee side of the mountain. This puts the Wasatch Front in a rain shadow, depressing clouds and precipitation.

But what is the difference between north of Salt Lake City and south of Salt Lake City? Consider the mountains upstream.
Upstream flow from Spanish Fork is intercepted by the Uintah Mountain range. In contrast, upstream of Bountiful and Farmington is much less rigid Wyoming. So, Spanish Fork residents, we may have the Uintah Mountains to thank for protecting us from these strong downslope windstorm events.

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