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 streams & planetary waves

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Jet Streams

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Jets: ~ Geostrophic Balance

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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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Cyclones & Anticyclones

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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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North American Cyclone

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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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Tropopause Shear Line Instability

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Tropopause Shear Line Instability

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Tropopause Shear Line Instability

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Tropopause Shear Line Instability

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Moist Convection

Strong vertical mixing

tropopause

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Kelvin—Helmholtz Instability

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Gravity Waves

Wavelength ~10 km

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Gravity Waves

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How do we better understand cyclones & anticyclones?

Need to filter other disturbances from the equations…

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Ertel Potential Vorticity

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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.

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Height-Latitude Tropopause Profile

Hoinka (1998)