March 02, 2010

Why was my flight so bumpy? Part II



Well, as long as I am terrorizing all of you with tales of flight turbulence, I might as well be complete!

In my last blog I talked about the number-one source of turbulence in the skies, shear-induced turbulence. When such turbulence occurs outside of clouds it is often called clear air turbulence or CAT.

But there is another source of aircraft bumps that is often felt around here...mountain wave turbulence. Ever notice that when flying across the Rockies that sometimes the plane is rocking over and immediately to the east of the mountain crest? Or have you experienced lots of bumps flying into Denver? Mountain Wave Turbulence could be the cause!

As shown in the figure, when air moves over mountains, waves can form. Quite simply, when air is pushed up it often oscillates up and down. These oscillations can spread downwind or can propagate vertically. As these waves move up into less dense air then can amplify causing increased shear (and turbulence) or they can break (producing strong turbulence). One sign of such mountain waves are lenticular clouds (see picture at the top). Mountain wave turbulence can sometimes become severe, but generally it is light to moderate in intensity. But every once in a while you hear about jets crossing the Rockies that are jolted by the strong stuff, with people and belongings being thrown to the ceiling.

Interestingly, the Cascades generally don't produce strong mountain wave turbulence because they are too narrow, but there are exceptions to the rule. During strong easterly flow and Enumclaw-style windstorms (see my book for details), moderate to severe turbulence can occur over and to the immediate west of the Cascades.

Another important source of turbulence...but one not as frequent here in the Northwest... is convection....big cumulus and thunderstorms. With strong up and downward motion and large wind shears, such convection can produce moderate to severe turbulence. But pilots know this and generally keep a safe distance, something made easier by weather radar on most big planes.
Remember the NW Weather Workshop!!
http://www.atmos.washington.edu/pnww/

9 comments:

  1. hey Cliff,

    Will those presentations at the workshop be posted or be found on the internet for those who couldn't make it?

    Sorry I over observe the weather conditions, but tonight is truly bizarre. It is dumping buckets outside. Crazy SW wind, which is downsloping, but mid levels are SE. We don't even get thunderstorms this bad.

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  2. that happens, i think, to a lesser degree close to the ground and raptors take advantage of it. if you go out to samish island and look overhead you often see hawks and eagles hanging in one place in the sky surfing the standing wave that forms over the hump of the island sitting out in the middle of the vast tidal flats.

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  3. Isn't there a similar (to the hook-shaped clouds shown previously) cloud formation called nasi-something_or_other?

    I saw these clouds once many years ago over the breadth of the Olympics, and have been searching (in vain) for them ever since.

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  4. I photographed a series of hooked clouds similar to Cliffs photo. It was about a year ago and I was at Fry's in Renton looking west. They had formed under a uniform cloud deck.. I would guess they were at roughly 5,000'. The cloud deck was roughly 6,000' - 8,000' as observed on the ride home, obscured most of Mount Rainier.

    Flew HNL to LAX on Saturday, 5 hours of bumps, shakes, and jiggles, the flight from LAX-SEA was much better.

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  5. Not to change the subject but have you touched on this Cliff?

    I know many of us lament the snow for outdoor fun but these numbers are....yikes.

    http://www.wcc.nrcs.usda.gov/gis/images/wa_swepctnormal_update.png

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  6. Having flown between Seattle and Tri-Cities many times over the last 30 years in smaller commuter planes flown by Horizon, there have been lots of bumps coming across the Cascades on the west side. The flights usually come across just south of Enumclaw. This usually occurs when it is fairly windy.

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  7. Cliff...Can it be true??? Big dump of snow in the mountains next week???

    What's the scoop?


    Skier Jay

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  8. Hi Cliff,

    I see the NWS is talking about possible lowland snow Monday night. I realize it's a long shot any time and particularly in March but what do you think?

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