Density Currents, Waves Density Currents, Waves and Bores; Little Known and Bores; Little Known

Hazards to AviationHazards to AviationJim JohnsonJim Johnson

National Weather ServiceNational Weather Service Dodge City, KSDodge City, KS

AIRMASS 2005AIRMASS 2005

Necessary Bore IngredientsNecessary Bore Ingredients

• 1. Stable boundary layer w/ inversion to act as a ducting mechanism.

• 2. Something to disturb the stable layer.a. Gravity Wave

b. Density Current

c. Front acting as density current

Bore CharacteristicsBore Characteristics

• 1. Undular bore or standing wave/sa. One or more successive, parallel waves.

b. Breaking waves are rare.

c. Not necessarily associated with a density or gravity current.

d. Most common aviation hazard = wind shear when low level jet present.

Density/Gravity Current Density/Gravity Current Characteristics Characteristics

• 1. Density/Gravity Currents a. Kelvin-Helmholtz instabilities.

b. Elevated mixing head (flow dependent).

c. Turbulent mixing head (flow dependent).

d. Coexisting Bore type (flow dependent)

Bore may outrun current.

e. Wind shear presence (flow dependent).

f. Seldom visible!

g. Most common aviation hazard = turbulence.

Elevated

Head

Kelvin-Helmholtz

Instabilities

Denisty/Gravity Current Flow Denisty/Gravity Current Flow DependenciesDependencies

Common High Plains EventsCommon High Plains Events

Hazard to AviationHazard to Aviation

Suggested TAF ProcedureSuggested TAF Procedure

• Since both turbulence and wind shear are possible there is no provision in the current TAF code to describe the hazard. However, inclusion of a low level wind shear remark will alert the pilot to the presence of at least part of the flight hazard even if wind shear criteria are not met.

Questions?Questions?