Monday, July 26, 2010

Anatomy of a Microburst

At 6:46pm yesterday evening, a strong microburst (likely strong enough to bring down a commercial airliner, had it been at the end of a runway) occurred in northwest Wichita a few miles northeast of Mid-Continent Airport. Since these are fairly rare, I thought it might be of interest to view the microburst in several different ways:

First, here it is in the radar reflectivity data (the type you usually see on TV weathercasts). The center of the microburst is near the deepest red pixel. The time is 6:45pm.

A second view of the microburst comes from the Wichita Terminal Doppler Weather Radar (TDWR), an instrument designed specifically to detect microbursts, installed in the wake of Delta 191 and the series of downburst-related crashes in the 70's and 80's. Compare the location of "Wichita" on the two radar images and the freeway (Interstate 235, not labeled, but wraps around the west side of the city) on both maps to orient yourself.
This radar senses the wind in great detail. The radar at 6:46pm is showing a 46 knot (53 mph, deep brown pixel) wind blowing toward the north on the north side of the microburst and a winds blowing toward the south at 39 mph (light green) on the south side of the microburst. This represents 92 mph of wind shear, likely enough to bring down an aircraft!

From the ground, the microburst looked like this from my back yard about 10 miles away.
There is the abrupt transition between heavy rain and no rain at cloud base and the typical "hourglass" shape in the rain shaft. Had there been nothing in the way, I am certain I would have seen the curl signature at the "toe" of the microburst. Here is a photo of a microburst curl.
The rain is actually "falling" up.

UPDATE: 8/4/10: I was going back through my photos classifying them and I found a curl on one frame, which is posted immediately below. Compare the image above to the image below. There were tree limbs blown down by the microburst in northwest Wichita.  End update.


How do microbursts cause plane crashes? The diagram below will help explain (click to enlarge).
Using yesterday evening's microburst as an example, a plane landing toward the south (left to right on the diagram) would encounter a headwind of 53 mph, which would increase the lift on the aircraft causing it to rise above the black line indicating the normal landing path. A pilot would attempt to get back on the landing path by pointing the nose of the plane down. Just then, the plane would reach the strong sinking air causing it to lose altitude rapidly. Then, it encounters a tailwind causing the lift to decrease. The plane crashes short of the runway.

Added image to original post: Circles depict the amount of wind shear in knots. This display is in the control tower of major airports. Courtesy of: KXAS TV

Fortunately, most airports with commercial airline traffic are equipped with a Low-Level Wind Shear Alert System. Forty-four airports have TDWR. Since these two systems have been installed and since pilots began to be trained in microburst avoidance in 1987, those once-frequent disasters have been all but eliminated.

5 comments:

  1. Fascinating Mike! Thanks so much for posting. Nice job on capturing an image of the storm too.

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  2. Mike -

    Your work continues to be superb. Glad you are still watching out for our railroad.

    Rollin

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  3. Rollin,

    It is truly our pleasure. Thanks for giving WeatherData its first opportunity to work in railroading!

    Mike

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  4. GREAT work! Very timely information. We had 1 unverified report of 3-4" diameter tree branches down with that one! Keep up the great work!

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  5. Great pictures. Well done.

    I was at the site of a down burst in Upper Michigan about 30 years ago when I was a film cameraman for the local TV station. It was scary to see the damage in the trees. And it's lucky when it is trees and not buildings and people.

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