Gravity Waves Phil Evans Paul Domm

Gravity Waves  Buoyancy oscillations –Should be called buoyancy waves  Only exist in stably stratified atmosphere  Dynamical Process

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 > 0Statically stable  dθ/dz = 0Statically neutral  dθ/dz < 0Statically Unstable

Buoyancy Oscillations  Adiabatic oscillations of a fluid parcel about its equilibrium level.  Period of oscillation –τ = 2π/N –N – buoyancy frequency  Average values of N = 1.2 x 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  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 0.001mb 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

earthobservatory.nasa.gov/ Newsroom/NewImages/