Gravity WavesGravity Waves
Phil EvansPhil Evans
Paul DommPaul Domm
Gravity Waves Gravity Waves
Buoyancy oscillations Buoyancy oscillations – Should be called buoyancy waves Should be called buoyancy waves
Only exist in stably stratified atmosphere Only exist in stably stratified atmosphere Dynamical ProcessDynamical Process
http://sprg.ssl.berkeley.edu/atmos/gj_science.html
Static Stability OverviewStatic Stability Overview
Stably stratified atmosphere Stably stratified atmosphere – θθ increasesincreases with height with height – A parcel adiabatically displaced from its equilibrium A parcel adiabatically displaced from its equilibrium
height downward will become height downward will become positivelypositively buoyant. buoyant.– Displaced upward, parcel will become Displaced upward, parcel will become negativelynegatively
buoyant.buoyant.
ddθθ/dz > 0/dz > 0 Statically stableStatically stable ddθθ/dz = 0/dz = 0 Statically neutralStatically neutral ddθθ/dz < 0/dz < 0 Statically UnstableStatically Unstable
Buoyancy Oscillations Buoyancy Oscillations
Adiabatic oscillations of a fluid parcel about Adiabatic oscillations of a fluid parcel about its equilibrium level. its equilibrium level.
Period of oscillation Period of oscillation – ττ = 2 = 2ππ/N/N– N – buoyancy frequency N – buoyancy frequency
Average values of N = 1.2 x 10Average values of N = 1.2 x 10-1 -1 ss-1-1 Period buoyancy oscillation is about 8 minPeriod buoyancy oscillation is about 8 min
Gravity Wave FormationGravity Wave Formation
Stably Stratified AtmosphereStably Stratified Atmosphere Topographic flow e.g. Flow Over MountainsTopographic flow e.g. Flow Over Mountains
Formation Cont’dFormation Cont’d
Downdrafts hitting the ground Downdrafts hitting the ground Updrafts penetrating the tropopause Updrafts penetrating the tropopause
Transports & BalanceTransports & Balance
GW can provide moisture convergence GW can provide moisture convergence which in turn drives the wavewhich in turn drives the wave
Geostrophic adjustment processesGeostrophic adjustment processes Mass adjustment (carries momentum)Mass adjustment (carries momentum)
Propagation Propagation
Achieve pressure equilibrium Achieve pressure equilibrium PGFPGF Crests and troughs will travel outward in all Crests and troughs will travel outward in all
directionsdirections– Example: throwing a stone into a pond. Example: throwing a stone into a pond.
Atmospheric preconditioningAtmospheric preconditioning
Stably stratified Stably stratified Frontal inversion Frontal inversion Strong wind shear aloft Strong wind shear aloft
http://www.mcwar.org/articles/cafe/gw/gravwaves.html
Convection Convection
Amplitude of wave can force parcel above Amplitude of wave can force parcel above LCL, LFCLCL, LFC
Can force/hinder condensation and Can force/hinder condensation and convection convection
Clear air turbulence (CAT)Clear air turbulence (CAT)– If air is too dry or upward forcing is not strong If air is too dry or upward forcing is not strong
enough. enough.
Physical characteristics Physical characteristics
Amplitude: 1-15 mbAmplitude: 1-15 mb Wavelength: 50 – 500 km Wavelength: 50 – 500 km Period: 1 – 4 hours Period: 1 – 4 hours Range of wave speeds: 12 – 500 km/hr Range of wave speeds: 12 – 500 km/hr
Detecting g-waves Detecting g-waves
Microbarographs Microbarographs – Can detect pressure fluctuations with 0.001mb Can detect pressure fluctuations with 0.001mb
precision.precision.– Useful when no visible signs are present. Useful when no visible signs are present.
Visible Satellite Visible Satellite – Only useful when g-wave forces parcel above Only useful when g-wave forces parcel above
LCL or LFC (produces condensation) LCL or LFC (produces condensation)
Considering g-waves in forecasting Considering g-waves in forecasting
G-waves can trigger convective events G-waves can trigger convective events G-waves can interact with existing dry lines G-waves can interact with existing dry lines
Example Example – Jarrell, Texas tornado outbreak Jarrell, Texas tornado outbreak
Case StudyCase Study
Jarrel SupercellJarrel Supercell
http://www.vvm.com/%7Ecurtis/Jarrell/Jarrell.htm
http://www.mcwar.org/articles/cafe/gw/gravwaves.html
earthobservatory.nasa.gov/ Newsroom/NewImages/
http://www.gesource.ac.uk/worldguide/html/image_84.html