Archive for August 13th, 2012

Using Weather Radar to detect Thunderstorm Winds

Published on Monday, 13 August 2012 19:12

David Reimer

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Yesterday evening, residents in the northwestern portion of the D/FW metroplex were dealing with severe weather, mainly in the form of severe wind gusts. Many of you are already aware of the specific areas that received significant damage from straight-line winds. I’ll leave that to the media outlets to cover, as they do it so well. What I want to explain to you is how the National Weather Service and weather geeks alike are able to detect and estimate the speed of strong winds inside a thunderstorm.

While weather radar has existed since the 1950s, it was not until the early 1990s that weather radar was commonly used to detect more than just rain within a thunderstorm. The WSR-88D (Weather Service Radar – 1988 Doppler) is the latest system of radars deployed across the United States. These radars show not only how heavy the rain is in a thunderstorm, but which direction and speed the wind is moving inside the thunderstorm. In most cases, broadcast meteorologists display radar in what’s called Reflectivity Mode on the air which only shows rainfall.  Because of that, many individuals don’t even realize that we can detect the winds inside a thunderstorm as well!

A radar set in Velocity Mode is used to detect and display the wind speed and direction inside a thunderstorm. This is particularly useful in diagnosing circulations, or “rotation” within a thunderstorm. I’ll discuss in a separate blog the process used to detect a possible tornado by using velocity data, but for now I want to concentrate on how we can use this data to warn for straight-line winds and microbursts/downbursts.

This image is showing Base Velocity data from 6:59 PM on Sunday, August 12. This specific product is useful in analyzing straight-line winds and microbursts. I realize this graphic is a tad messy compared to my usual array of products, but it does show the data quite nicely. Using the scale on the bottom portion of the image, we’re able to correlate the wind speeds on the radar to actual values. At this point, the radar was detecting winds over 60 MPH all the way from Lake Worth over to I-820 near the I-35W interchange. At this point, I was sitting on I-820 just east of FM-1220/Azle Road measuring winds from 55 MPH up to 62 MPH via the weather station that I have attached to the top of my vehicle.  Another storm chaser on 35W measured a 67 MPH wind gust. The highest wind measured in this particular zone was 76 MPH. Luckily, radar detected these winds five to ten minutes before they even reached the ground.  Radar measures at different levels in the atmosphere.  These levels are called “Tilts”  so Tilt 1 would be closest to the ground, while Tilt 4 would be roughly the top of the thunderhead.  National Weather Service meteorologists detected this downburst event in its beginning stages further up inside the thunderstorm.  They can tell by looking at the different Velocity Mode radar tilts, up and down inside the thunderstorm, that the wind speeds were increasing in a downward pattern.   This is how we are able to detect microbursts with our summer thunderstorms and how the National Weather Service can adjust the wind speed in their severe weather warnings based on this radar data. By the time this signature was evident on radar, the National Weather Service in Fort Worth had inhanced their wording in the Warning for Tarrant County and helped increase awareness about the seriousness of this storm.

If you have any questions about anything we’ve talked about or are wondering about something weather related, shoot us a note!