Turbulence

John Bravender, Aviation Program Manager

Ray Tanabe, Warning Coordination Meteorologist

National Weather Service – Honolulu, HI

john.bravender@noaa.gov(808) 973-5282

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.

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