atm s 542 synoptic meteorology overview gregory j. hakim university of washington, seattle, usa...
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ATM S 542 Synoptic Meteorology Overview
Gregory J. Hakim
University of Washington, Seattle, USA
www.atmos.washington.edu/~hakim
• Vertical structure of the atmosphere.
• Atmospheric phenomena by horizontal scale.
• Potential vorticity.
• Tropopause.
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Overview
Asymptotic methods• expand dependent variables in a power series.• small parameters needed.• co-operative dialog between math & physics is helpful.
Role of asymptotics in atmosphere/ocean dynamics.• simplified equations for solution & understanding.
Goals• survey atmosphere/ocean structure & phenomena.• bias toward extratropics & atmosphere.• motivate asymptotic methods for these problems.
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Jets, stirring, organized structures; waves, vortices, convection.
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Atmospheric Structure
•Atmosphere: very thin gas layer. • depth <<< radius earth.
•Troposphere: “weather layer”• ~ 10 km deep.• ~ 80% mass of atmosphere.• ~ all H2O vapor.
• Tropopause: jet streams.• wave guide.• unstable: cyclones.
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Atmospheric Energy Spectrum
Energy increases with horizontal length scale.
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Atmospheric Phenomena by Scale
Planetary waves
L ~ 10 000 kmH/L ~ .001Ro ~ 0.01
Cyclones
L ~ 1000 kmH/L ~ .01Ro ~ 0.1
Gravity waves
L ~ 10--100 kmH/L ~ .1-1Ro ~ 1
Convection
L ~ 10 kmH/L ~ 1Ro > 1
Long, slow Short, fast
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Jet stream vorticity
Waves & particles
Waves:Information flows through the medium by radiation.
Particles (vortices):Information flows through the medium by material transport.
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Cyclone Structure
• Center has lowest pressure.
• ~geostrophic winds.
• Warm air moves poleward.• and upward.
• warm front.
• Cold air moves equatorward.• and downward.
• cold front.
• Clouds & precipitation.•~ “comma” shape.
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•Intense vortex •Cold air: shallow cellular convection•Warm air: stratiform cloud
•Sharp frontal boundaries
Zoom in on cold front…
Pacific Extratropical
Cyclone
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Scale collapse at cold front: “rope cloud”---narrow line convection.
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Cyclone—Anticyclone Track Density
Primary tracks coincide with time-mean jet stream locations.
Hoskins & Hodges (2002)
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Jet-Level 2.5—6 d Variance
Maximum variance in storm tracks.
Hoskins & Hodges (2002)
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Baroclinic Waves & Packets
Cyclones & anticyclones often compose waves within larger wave packets.
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Baroclinic Waves
Wavelength ~4000 kmPhase speed ~ 15 m/s.Period ~ 3 d.Due to baroclinic instability.Organize into packets.
Lim & Wallace (1991)
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Wave Packet Phase & Group Speed
phase speed
group speed
Chang & Yu (1999)
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Wave Packet Tracks
Hakim (2003)
Jet-stream wave guides.Storm-track recycling.
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Mesoscale Phenomena & Smaller Scales
Fronts & frontal waves.
Gravity waves.
Convection.
Shear instabilities.
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Frontal Waves
Wakimoto & Bosart (2000)Paldor et al. (1994)
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How do we better understand cyclones & anticyclones?
Need to filter other disturbances from the equations…
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Vertical Profile of PV
Tropopause
• Well-defined as PV jump.
• Dynamics focus here.
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Tropopause Topography (pressure)
source: Hoinka (1998)
• High pressure over poles.
• Low pressure over tropics.
• Strong gradient in mid-latitudes.
• Stronger gradient in winter.