1. Density Currents, Waves and Bores; Little Known Hazards to Aviation Jim Johnson National Weather Service Dodge City, KS Dodge City, KS AIRMASS 2005

2. Necessary 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

3. Bore Characteristics 1. Undular bore or standing wave/s a.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.

7. Density/Gravity Current 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.

9. Elevated Head Kelvin-Helmholtz Instabilities

11. Denisty/Gravity Current Flow Dependencies

15. Common High Plains Events

18. Hazard to Aviation

22. Suggested 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.

23. Questions?