WRAL WeatherCenter Blog

Rough Air on Approach...

Posted July 30, 2007

Radar reflectivity 27 July 07

You may have seen the story on our Friday evening or Saturday morning news, or here on WRAL.com (http://www.wral.com/news/local/story/1644020/), about the American Airlines flight from London that encountered a strong burst of turbulence or wind shear on approach to RDU. Several flight attendants were apparently treated for minor injuries and the passengers received a pretty significant scare, one noting that it felt like the plane suddenly dropped 50 to 75 feet. Although this is a difficult number to estimate, the feeleing of having the bottom drop out is by no means a good one, especially when you're already at a fairly low altitude.

Although we don't have information on the exact flight path and height above ground of the aircraft when it encountered the turbulence, we do know it occurred Friday afternoon when there were several thunderstorm cells in the vicinity. Greg Fishel did a little hunting through Doppler radar imagery Friday evening, and retrieved the images above. The first is a reflectivity image (basically how much radio energy is being scattered back to the radar, with the greatest amaounts indicating locations of heavy precipitation and/or hail) that shows a moderately intense thunderstorm cell over southeastern Franklin County between Wake Forest and New Hope.  The second and third images are Doppler Velocity displays, which show wind components going away from the radar in shades of red and those approaching the radar in shades of green. In these images, we see a divergence signature just northwest of New Hope (winds toward the radar as high as 46 mph, with winds away from the radar as high as 20 mph just beyond the strong inbound winds, meaning air in the middle is rapidly spreading outward). Since we are sweeping the storm at a rather low altitude (around 1500 feet) in these images, the divergence may indicate a strong downbust is underway, with the air impinging the ground being forced to spread rapidly outward. These outflows can be rather shallow, and there can be significant turbulence or strong vertical wind shear along the upper edge of the outflow.

This is the kind of system that aircraft typically attempt to route around in order to avoid being caught in strong updrafts, intense downdrafts and the intense turbulent shear at the boundary between those. However, even outside the storm there can be outflows that can affect flight performance and that can create a lot of discomfort for passengers on occasion. While there's no way to be certain exactly what the situation was with the incoming flight on Friday, it seems like a situation where the descending aircraft could have settle into a turbulent shear zone at the top of a strong outflow - this could create a lot of bouncing and shaking, for example - and then crossed into the pool of outlfow air below. with a sudden change in ambient wind direction and speed. If this change in direction is such that the plane is suddenly subjected to a strong tailwind, or equivalently a sudden decrease in headwind, then it's effective airspeed (and the lift associated with that airspeed) will rapidly decrease. If this occurs fast enough, the plane can drop in a hurry, until the pilot can increase power settings enough to offset the lost airspeed.

In any of its forms, strong or severe turbulence can be a scary experience, especially for passengers who don't have any sense of being in control of the aircraft. I've been in a couple of those situations, once at Washington National when very strong wind shear near the surface had our plane going up, down and rolling hard left and right, to the point that it was unlandable and the pilot did a go-around after descending to about 1-200 feet off the ground. Luggage bins were popping open and one person had to be treated for heart trouble, and I definitelty had some white knuckles myself that day. My other big turbulence encounter was on a smaller aircraft at flight altitude one evening in the vicinity of strong shear aloft, this time in clear air turbulence. Looking out the window a bit earlier, I had seen Kelvin-Helmoholtz wave clouds (last image above) not far away, and apparently we flew into and through a breaking wave just a little later. The plane was jostled around pretty hard, then whooshed upward, followed by a very hard and sustained descent (the flight attendant flew up off the floor) then just as suddenly we were back in fairly calm air and the rest of the trip was uneventful.

If you have a turbulence encounter you'd like to share below, feel free.

On a much lighter note (lighter than air, in fact!) I wanted to pass along this link that Chris Thompson sent today - it's a time lapse of hot air balloons that is a real different way of seeing them - be sure to keep an eye on the poor eagle toward the end of the video... see the video at

http://www.liveleak.com/view?i=c85_1185663859

7 Comments

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  • Mike Moss Jul 31, 2007

    JDNCSU - thanks for the feedback from someone who was there. Interesting to hear that it was apparently a mainly shear event without much turbulence. There can be places or periods where the flow along the interface between the two layers of air with different wind speed and directions is almost laminar, with little in the way of strong mixing. Sounds like you may have descended into the next layer down in a smooth region like that, so that the main effect was a rapid loss of lift. It would be neat to see a trace from the flight recorder showing stuff like radar altimeter versus airspeed versus ground speed versus estimated flight level winds, etc.

  • Mike Moss Jul 31, 2007

    astokes - sorry I somehow forgot to include the balloon video link when I saved the blog yesterday. I remembered last evening and added it then - hope it works for you!

  • JDNCSU Jul 30, 2007

    I was actually on the flight mentioned...hard to remember exact time, but everyone had already stowed their laptops, etc., and all of the service items had been cleared, so there was not much to fly around (except the crew unfortunately). I don't think the captain had made the "flight crew prepare for landing" announcement yet so we still had a way to go, seems like we had about 20-25 mins until landing, and we were approaching from the NE. Unlike Mr. Moss' experience, there was almost no turbulence (this was a 777), skies were clear, and the bottom just dropped out and ended with a hard bump. I would think the 50-75 ft estimate was pretty accurate. I wasn't nervous because it was over so fast. I've been flying over 20 years and this wsa the first time something of this magnitude occurred. Definitely will serve as a reminder to always keep my seatbelt fastened.

  • curiousgeorgia Jul 30, 2007

    Thought about another event even longer ago! It was when Mount Taal outside of Manila erupted. It was back in the sixties, we had just stopped over for a couple of days on our way back to the States. The 'plane (a DC 8) had taken off from the main airport and was gaining altitude when we were hit by some kind of weird turbulence that shook the whole plane from side to side instead of up and down. All the fusilage creaked most terribly, but we went on to complete the flight safely! What do you think happened? Would it have been an effect from the eruption? We were not in ash clouds at that time. I never experienced anything like it again. (my Dad was with WHO so we did lots of long flights around the world)

  • curiousgeorgia Jul 30, 2007

    This was quite a number of years ago--my parents and I were taking a flight from Germany to the States and out over the Atlantic we hit something the pilot told us later was clear air turbulence. Since my parents and I had been on many looong flights over the years we had the habit of buckling ourselves in even when the light was not on so we could doze if we fancied it. When the event occurred the jet seemed to drop like a stone and everything went into free fall. Drinks were floating around, so were people! Fortunately the flight was pretty empty so there weren't a lot of people to get bumped around. We were lucky because we stayed put!

  • go2beachgirl Jul 30, 2007

    Mike - Am I missing the Hot Air balloon link??? It's possible - its been a whopper of a Monday! Thanks!

  • Travised Jul 30, 2007

    I was watching a family members business flight come in, and weather in the area on another screen at the same time. The flight had a good 75 minutes to go on it, so I put in in the corner of my screen and kept NOAA radio on in the room. As it came into the region I noticed a front in the Bravo airspace outta the south, and the weather had come closer from the west. Not being able to listen in to the metro air bands I kept watching those two windows, the one tracking the flight and the weather radar. At first it looked like they were going to refuel in the metro, but they just were reassigned airspace by that field and headed out to the home field. Made it back about 5 minutes before the main downpour hit.

    It was funny. I called into the office, and the family member. Told them they should be landing about now. He called in 5 minutes said they had just landed. She told him she knew!!