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