Fronts: Structure and Observations
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Fronts: Structure and Observations. Fronts – Structure and Observations. Definition and Characteristics Definition Common Characteristics Frontal Slope Frontal Types Cold Fronts Warm Fronts Occluded Fronts Coastal Fronts Upper-Level Fronts. Definition and Structure.

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Fronts structure and observations

Fronts: Structure and Observations

M. D. Eastin


Fronts structure and observations

Fronts – Structure and Observations

  • Definition and Characteristics

    • Definition

    • Common Characteristics

    • Frontal Slope

  • Frontal Types

    • Cold Fronts

    • Warm Fronts

    • Occluded Fronts

    • Coastal Fronts

    • Upper-Level Fronts

M. D. Eastin


Fronts structure and observations

Definition and Structure

  • Definition:

  • Pronounced sloping transition zone between two air masses of different density

  • Disagreements and Caveats:

    • What defines an air mass? What defines a transition zone?

  • → Are we restricted to the synoptic-scale Bergeron air mass classifications?

  • → Do baroclinic zones induced by physical geography gradients count?

  • → Do drylines with minimal temperature gradients count?

  • → Must a density gradient of certain magnitude be present?

Daytime

Cloudy

Cool

Clear-Dry

Warm

→ Do temperature

gradients that

“disappear” at

night (or during

the day) count

as fronts?

Nighttime

Cloudy

Cool

Clear-Dry

Cool

M. D. Eastin


Fronts structure and observations

Definition and Structure

  • Our Definition:

  • In this course we will use a less restrictive definition of fronts as air mass boundaries

  • without certain gradient requirements throughout the diurnal cycle, but we will omit

  • those baroclinic zones mostly locked in place by topography (e.g., drylines)

  • Significance of Fronts:

    • Forecasts must account for frontal type, frontal movement, frontal intensity,

    • the spatialdistribution of clouds and precipitation, and the precipitation type

    • Frontal zones are pre-conditioned to support severe weather

  • Common Characteristics:

    • Enhanced horizontal gradients of density (temperature and/or moisture)

    • Relative minimum in pressure (a trough)

    • Relative maximum in cyclonic vertical vorticity (distinct wind shift)

    • Strong vertical wind shear (due to thermal wind balance)

    • Large static stability within the frontal zone

    • Ascending air with clouds / precipitation (moisture availability)

    • Greatest intensity near the surface (weaken aloft)

    • Shallow (1-5 km in depth)

    • Cross-front scale (~100 km) is much smaller than along-front scale (~1000 km)

M. D. Eastin


Fronts structure and observations

Definition and Structure

Surface Pressure

Equivalent Potential Temperature (θe)

Vertical Vorticity

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • How much does a front “slope” with height?

  • Let’s derive a simple equation that can describe

  • the vertical slope of any front

  • Assumptions

    • Front is oriented east-west

    • Only consider variations in “Y-Z space”

    • Neglect variations in the X direction

    • Density is discontinuous across the front

    • Pressure must be continuous so the PGF

    • remains finite (otherwise very strong winds)

    • Equation of state (p=ρRT), thus, requires

    • temperature to be discontinuous

    • Hydrostatic Balance

    • Geostrophic Balance

    • Pressure is steady (no changes in time)

y

ρc

ρw

x

T

ρ

p

Warm

Cold

y

Front

South

North

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • The differential of pressure is:

  • (1)

  • Divide each side by Dy

  • (2)

  • Substitute in the hydrostatic equation

  • (3)

  • (4)

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • Since pressure is continuous across the front:

  • (5)

  • (6)

  • Substitution of (4) into (6) yields:

  • (7)

  • We can now solve for (Dz/Dy)

  • (8)

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • Which way can the front slope and still be “stable”?

  • The front must be able to persist for 1-2 days

  • (as fronts do in reality)

  • Thus (9)

  • And since (10)

  • Thus (11)

  • Or (12)

  • What does this imply about pressure across the front?

z

Stable

ρw

ρc

y

z

Unstable

ρc

ρw

y

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • What does this imply about pressure across the front?

  • While pressure is continuous across the front, the

  • pressure gradient is not continuous

  • Thus, the isobars must kinkto satisfy this relationship

High pressure

Or

Low pressure

High pressure

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • What can we say about the winds across the front?

  • Assume the flow is geostrophic across the front

  • and does not vary along the front:

  • (13)

  • Thus, on the warm and cold sides of the front:

  • (14)

  • Substituting (14) into (8) yields:

  • where (15)

  • Again, if and thenor (16)

  • What does this imply about the winds across the front?

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • What does this imply about the winds across the front?

  • Recall the definition of geostrophic vertical vorticity

  • Thus, cyclonic vorticity must exist across the front

  • Here are more possible examples

y

ugc

ugw

x

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • How much does a front slope with height?

  • Returning to the frontal slope equation:

  • (15)

  • Using the Equation of State, (15) can be written as:

  • Margules Equation

  • for Frontal Slope

  • If we substitute in typical values:

This is similar to observations!

Surface fronts are shallow!

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • Similar conclusions can be reached for a front

  • oriented north-south using similar assumptions

  • Margules Equation

  • Again, frontal stability requires:

  • Thus, it can be shown:

    • The pressure gradient is discontinuous and the

    • isobars must kink across the front

    • The geostrophic wind must contain cyclonic

    • vorticity across the front

y

ρc

ρw

x

T

ρ

p

Cold

Warm

x

Front

West

East

M. D. Eastin


Fronts structure and observations

Frontal Slope

  • Synoptic-scale Vertical Motion:

  • The vertical motion immediately adjacent to a given

  • frontal slope can also be estimated:

  • where: v = cross-front velocity

  • c = the speed of the front

  • Example:

  • Dz/Dy ~ 1/300

  • v ~ 5 m/s

  • c ~ 2 m/s

  • w ~ 0.01 m/s

z

ρw

w

v

c

ρc

y

This is similar to observations!

Synoptic-scale vertical motions are weak!

M. D. Eastin


Fronts structure and observations

Cold Fronts

  • Observational Aspects:

  • Cold air advances into a warmer air mass

  • Stereotypical passage includes:

  • Thunderstorms

  • Rapid (gusty) wind shift

  • Rapid temperature drop

  • Tremendous variability in weather ranging

  • from dry, cloud-free frontal passages to

  • heavy downpours with severe storms

  • Variability related to the cold front’s spatial

  • orientation relative to the warm-conveyor

  • belt ahead of the cold front

  • Katafront → Precipitation ahead of the

  • surface front

  • Anafront → Precipitation along / behind

  • the surface front

M. D. Eastin


Fronts structure and observations

Cold Fronts

  • Observational Aspects: Katafronts

  • Warm conveyor belt parallel to surface front

  • Limited lift along the surface front

  • Most lift associated with an elevated surge

  • of cold-dry air above the surface front, often

  • called a cold front aloft (CFA)

  • Occur later in the parent cyclone’s lifecycle

  • (when the cold front has a N-S orientation)

B

A

A

B

  • Warm front precipitation

  • Convection cells ahead of CFA

  • Precipitation from CFA falling

  • in warm conveyor belt

  • Shallow warm-moist zone

  • Surface front (light precipitation)

M. D. Eastin


Fronts structure and observations

Cold Fronts

  • Observational Aspects: Anafronts

  • Warm conveyor belt crosses

  • the surface front at some angle

  • Significant lift along surface front

  • Often accompanied by a southerly

  • low-level jet just ahead of the

  • surface frontal zone

  • Increased risk of winter precipitation

  • during the cold season

  • Tend to occur earlier in the parent

  • cyclone’s lifecycle (when the cold

  • front has greater E-W orientation)

M. D. Eastin


Fronts structure and observations

Cold Fronts

  • Observational Aspects: Arctic Cold Fronts

  • Second surge of cold air

  • Very shallow

  • Strong temperature gradient

  • Often lack precipitation

  • Behind primary cold front

  • Behind false warm sector

Arctic

Cold Front

Primary

Cold Front

M. D. Eastin


Fronts structure and observations

Cold Fronts

  • Observational Aspects: Back-door Cold Fronts

  • Caused by differential

  • cross-front advection

  • along a pre-existing

  • warm/stationary front

  • Surge of near-surface

  • cold air originating

  • over a cold surface

  • moves south/southeast

  • Most common along the

  • U.S. East coast

  • Don’t assume all cold

  • fronts move southeast!!!

Back-door

Cold Front

M. D. Eastin


Fronts structure and observations

Warm Fronts

  • Observational Aspects:

  • Warm air advances into a colder air mass

  • Motion is slow than cold fronts → dependent upon turbulent mixing along stable boundary

  • Warm fronts often have shallow slopes → the pressure trough is weaker

  • (makes warm fronts difficult to analyze)

  • Low clouds / stratiform precipitation common

  • Deep convection less common

FFC

M. D. Eastin


Fronts structure and observations

Warm Fronts

  • Observational Aspects: Back-door Warm Fronts

  • Warm air advances into a colder air mass

  • Importance of source region → maritime polar air is warmer than continental polar air

  • Don’t assume warm fronts always move north!!!

M. D. Eastin


Fronts structure and observations

Occluded Fronts

  • Observational Aspects:

  • When “a fast-moving cold front overtakes a slow-moving warm front from the west”

  • Cyclone become cut-off from the warm sector → baroclinic instability ends

  • Marks the mature stage of a midlatitude cyclone → dissipation ensues

  • Rising motion above the frontal zone is weak as warm air lifted over cool/cold air

  • Stratiform precipitation is the norm

M. D. Eastin


Fronts structure and observations

Occluded Fronts

Observational Aspects: Two Conceptual Models

  • Norwegian Cyclone Model

  • Initial cyclone development from a stationary front

  • Cold front advances and “overtakes” warm front

  • Cyclone near peak intensity as “occlusion” starts

  • Extension of the occluded front is southward

  • Shapiro-Keyser Cyclone Model

  • Initial cyclone development from a stationary front

  • Fast-moving cold front “fractures”

  • A “bent back” warm front (develops)

  • As cold front surge continues, warm air becomes

  • “secluded” (or occluded) from cyclone center

M. D. Eastin


Fronts structure and observations

Occluded Fronts

  • Observational Aspects: Two Occlusion Types

  • Depend on the relative temperature of

  • the pre- and post-frontal air masses

  • Cold occlusions should be much more

  • common in the eastern US → Why?

  • Warm occlusions are much more

  • common in western Europe → Why?

  • (and have been studied more)

  • Completion of your homework will provide

  • a new perspective to all this “conventional

  • wisdom” regarding occluded fronts!

M. D. Eastin


Fronts structure and observations

Coastal Fronts

  • Observational Aspects:

  • Strong temperature contrast caused

  • by warm-moist maritime air adjacent

  • to cold-dry continental air

  • Temperature differences of 5°-10°C often

  • occur over distances of 5-10 km

  • Shallow (less than 1 km deep)

  • Occur during the cold season (Nov-Mar)

  • Form along concave coastlines

  • (New England, Carolinas, Texas)

  • Cross-front structure similar to warm front

  • Pressure field often an “inverted trough”

  • Heaviest precipitation on “cold side”

  • Often the boundary between rain and

  • frozen precipitation types

  • Can serve as a primary or secondary site

  • for cyclogenesis

M. D. Eastin


Fronts structure and observations

Coastal Fronts

  • Observational Aspects: Formation

  • Cold anticyclone north or northeast

  • of frontal location → onshore flow

  • Onshore flow acquires heat / moisture

  • via strong surface fluxes from relatively

  • warm offshore waters (Gulf Stream)

  • Differential friction at coastline causes

  • distinct wind shift that favors frontal

  • formation along the coastline

  • Can be enhanced by cold-air damming

  • events along the Appalachians

  • Can be enhanced by a land breeze

M. D. Eastin


Fronts structure and observations

Coastal Fronts

Observational Aspects: Motion

Onshore migration→ anticyclonic shifts eastward

→ geostrophic wind intensifies or primarily onshore

Offshore migration→ anticyclonic shifts northward

→ geostrophic wind weakens or primarily along-shore

M. D. Eastin


Fronts structure and observations

Upper-Level Fronts

Observational Aspects:

  • Sharp thermal gradients in the upper/middle troposphere → don’t extend to the surface

  • Associated with “tropopause folds” whereby stratospheric air is drawn down into the

    troposphere → subsidence due to ageostrophic flow near jet streaks (right-exit region)

    → subsidence produces adiabatic warming (thermal front)

    → subsidence leads to vortex stretching (pocket of high PV)

Isentropes (solid)

Isotachs (dashed)

Potential Vorticity (solid)

Jet

Core

Jet

Core

Subsidence

Tropopause

Upper-level

Front

M. D. Eastin


Fronts structure and observations

Upper-Level Fronts

Observational Aspects: Significance

  • Have little to no impact on synoptic or mesoscale weather

  • Regions of strong clear air turbulence → significant hazard to aircraft

  • Regions of mixing between the troposphere and stratosphere

  • Transport→ Radioactivity downward

  • → Ozone downward

  • → CFCs upward

B

A

A

B

M. D. Eastin


Fronts structure and observations

References

Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of Weather

Systems. Oxford University Press, New York, 594 pp.

Bosart, L. F., 1985: Mid-tropospheric frontogenesis. Quart. J. Roy. Meteor. Soc., 96, 442-471.

Lackmann, G., 2011: Mid-latitude Synoptic Meteorology – Dynamics, Analysis and Forecasting, AMS, 343 pp.

Miller, J. E., 1948: On the concept of frontogenesis. J. Meteor., 5, 169-171.

Newton, C. W., 1954: Frontogenesis and frontolysis as a three-dimensional process. J. Meteor., 11, 449-461.

Petterssen, S., 1936: A contribution to the theory of frontogenesis. Geopys. Publ., 11, 1-27.

Sanders, F., 1955: An investigation of the structure and dynamics of an intense surface frontal zone. J. Meteor, 12,

542-552.

Schultz, D. M., and C. F. Mass, 1993: The occlusion process in a midlatitude cyclone over land, Mon. Wea. Rev., 121, 918-940.

Shapiro, M. A., 1980: Turbulent mixing within tropopause folds as mechanisms for the exchange of chemical constituents

between the stratosphere and troposphere. J. Atmos. Sci., 37, 995-1004.

Shapiro, M. A., 1984: Meteorological tower measurements of a surface cold front. Mon. Wea. Rev., 112, 1634-1639.

M. D. Eastin


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