Turbulence John Bravender, Aviation Program Manager Ray Tanabe, Warning Coordination Meteorologist National Weather Service – Honolulu, HI (808)

Movie: very high resolution Clark model run on supercomputer. - Jet stream in blue - fast moving air parcels in yellow hours of computing time! Song: Clear Air Turbulence Artist: Ian Gillan Band Year: 1977

What is Turbulence? Air movement that normally cannot be seen and where air currents vary greatly over a short distance Can occur when the sky appears to be clear and can happen unexpectedly

What causes Turbulence? Convective currents Obstructions to wind flow Wind shear Wake turbulence behind an aircraft

Convective Currents Common cause of turbulence, especially at low altitudes Localized vertical motions both ascending and descending Most active on warm afternoons when the winds are light Surface heats up, air starts to rise

Convective Currents When cold air moves over a warm surface, it becomes unstable in the lower levels Convective currents can extend several thousands of feet above the surface Non-uniformity of terrain can cause considerable variability of convective currents over short distances

Convective Currents Cumulus clouds as “signposts” Taller the clouds…stronger the rising motion…stronger the turbulence Turbulence greatest in or just beneath the clouds Cloud top usually marks the upper limit of the convective current, thus turbulence Most severe case is thunderstorms…severe to extreme turbulence associated with violent rising motion.

Rain Runway Gust Front Downburst

Obstructions to Wind Flow Buildings, trees, rough terrain, mountains all disrupt air flow from being smooth into a complex of eddies and turbulent air flow. Mechanical Turbulence The degree of turbulence depends on the wind speed and shape of the obstruction The stronger the wind and rougher the terrain, the stronger the turbulence

Mountain Waves Most applicable form of low level turbulence around Hawaiian Islands If the air crossing the mountains is unstable, then the turbulence favors the windward side of the mountains If the air crossing the mountains is stable, then the turbulence favors the leeward side of the mountains (mountain waves)

Mountain Waves Lenticular clouds and lee wave clouds are a sign of mountain waves. Turbulence most commonly experienced below the “crests” of mountain waves, or where rising motion is occurring.

Low Level Wind Shear Overnight cooling creates a temperature inversion a few hundred feet above the ground Sea breezes and trade winds –e.g., Kona Fast moving cold fronts Shearlines

Clear Air Turbulence Implies turbulence in an area devoid of clouds High level turbulence…jet stream cirrus CAT develops in the turbulent energy exchange between contrasting air masses in terms of temperature, wind speed, and direction CAT most prevalent in winter, when temperature contrasts are greater between warm and cold air masses.

Preferred CAT Locations In an upper trough on the cold or (poleward) side of the jet stream Along the jet stream to the north and northeast side of a rapidly deepening surface low

Other CAT mechanisms In the absence of a jet stream, CAT can be experienced in directional wind shears Sharp ridges or troughs aloft Mountain waves Large differences in wind speed with height. Thunderstorms

Satellite Clues to CAT Transverse cirrus cloud bands along the jet stream Low level wave clouds Warming (darkening) in the water vapor imagery Elongated thunderstorm anvil tops Well defined cloud edges associated with deformation zone flow patterns.

Where to go for CAT guidance b/aviation/turb/tifcsts.htmlhttp:// b/aviation/turb/tifcsts.html