Gravity Waves Phil Evans Paul Domm Gravity Waves

Gravity Waves Phil Evans Paul Domm

Gravity Waves § Buoyancy oscillations – Should be called buoyancy waves § Only exist in stably stratified atmosphere § Dynamical Process http: //sprg. ssl. berkeley. edu/atmos/gj_science. html

Static Stability Overview § Stably stratified atmosphere – θ increases with height – A parcel adiabatically displaced from its equilibrium height downward will become positively buoyant. – Displaced upward, parcel will become negatively buoyant. § dθ/dz > 0 § dθ/dz = 0 § dθ/dz < 0 Statically stable Statically neutral Statically Unstable

Buoyancy Oscillations § Adiabatic oscillations of a fluid parcel about its equilibrium level. § Period of oscillation – τ = 2π/N – buoyancy frequency § Average values of N = 1. 2 x 10 -1 s-1 § Period buoyancy oscillation is about 8 min

Gravity Wave Formation § Stably Stratified Atmosphere § Topographic flow e. g. Flow Over Mountains

Formation Cont’d § Downdrafts hitting the ground § Updrafts penetrating the tropopause

Transports & Balance § GW can provide moisture convergence which in turn drives the wave § Geostrophic adjustment processes § Mass adjustment (carries momentum)

Propagation § § § Achieve pressure equilibrium PGF Crests and troughs will travel outward in all directions – Example: throwing a stone into a pond.

Atmospheric preconditioning § § § http: //www. mcwar. org/articles/cafe/gw/gravwaves. html Stably stratified Frontal inversion Strong wind shear aloft

Convection § Amplitude of wave can force parcel above LCL, LFC § Can force/hinder condensation and convection § Clear air turbulence (CAT) – If air is too dry or upward forcing is not strong enough.

Physical characteristics § § Amplitude: 1 -15 mb Wavelength: 50 – 500 km Period: 1 – 4 hours Range of wave speeds: 12 – 500 km/hr

Detecting g-waves § Microbarographs – Can detect pressure fluctuations with 0. 001 mb precision. – Useful when no visible signs are present. § Visible Satellite – Only useful when g-wave forces parcel above LCL or LFC (produces condensation)

Considering g-waves in forecasting § G-waves can trigger convective events § G-waves can interact with existing dry lines § Example – Jarrell, Texas tornado outbreak

Case Study § Jarrel Supercell

http: //www. vvm. com/%7 Ecurtis/Jarrell. htm

http: //www. mcwar. org/articles/cafe/gw/gravwaves. html

earthobservatory. nasa. gov/ Newsroom/New. Images/

http: //www. gesource. ac. uk/worldguide/html/image_84. html