Advanced SynopticM. D. Eastin Fronts: Structure and Observations

Advanced Synoptic M. D. Eastin

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

Fronts – Structure and Observations


Definition and StructureDefinition:

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?

Cloudy

Clear-Dry

Cool

Cool

NighttimeDaytime

Cloudy

Clear-Dry

Cool

Warm

→ Do temperature gradients that “disappear” at night (or during the day) count as fronts?

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 spatial distribution 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)


Definition and Structure


Definition and StructureSurface Pressure Equivalent Potential Temperature (θe)

Vertical Vorticity


Frontal SlopeHow 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)

Front

T ρ

y

ColdWarm

NorthSouth

p

ρc

ρw

y

x


Frontal Slope• The differential of pressure is:

(1)

• Divide each side by Dy

(2)

• Substitute in the hydrostatic equation

(3)

(4)

Dzz

pDyy

pDp

Dy

Dz

z

p

y

p

Dy

Dp

gz

p

Dy

Dzg

y

p

Dy

Dp


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)

Dy

Dzg

y

p

Dy

Dzg

y

pc

c

w

w

cw pp

cwDy

Dp

Dy

Dp

wc

wc

g

yp

yp

Dy

Dz


Frontal SlopeWhich 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?

y

z Stable

Unstable

y

z

0Dy

Dz

wc

0

wcy

p

y

p

wc

wc

g

yp

yp

Dy

Dz

wcy

p

y

p

ρw

ρc

ρw

ρc


Frontal SlopeWhat 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 kink to satisfy this relationship

wcy

p

y

p

Low pressure

High pressure

High pressure

cy

p

wy

p

Or


Frontal SlopeWhat 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 then or (16)

What does this imply about the winds across the front?

y

p

fug

0

1

ccgc y

p

fu

0

1

ww

gw y

p

fu

0

1

wc

gcgw

g

uuf

Dy

Dz

)(0

2

)( cw

0Dy

Dzwc 0 gcgw uu gcgw uu

0gv


Frontal SlopeWhat 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

ugc

ugw

y

x

gcgw uu

y

u

x

v ggg

y

ugg


Frontal SlopeHow 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:

wc

gcgw

g

uuf

Dy

Dz

)(0

cw

gcgw

TTg

uufT

Dy

Dz

)(0

300

1

1010

10103002

114

Kms

mssK

Dy

Dz This is similar to observations!

Surface fronts are shallow!


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

wc

gcgw

TTg

vvfT

Dx

Dz

)(0

ρc ρw

y

x

Front

T ρ

x

WarmCold

EastWest

p

0Dx

Dz


Frontal SlopeSynoptic-scale Vertical Motion:

• The vertical motion immediately adjacent to a given frontal slope can also be estimated:

where: v = cross-front velocityc = the speed of the front

Example:

Dz/Dy ~ 1/300 v ~ 5 m/s c ~ 2 m/s

w ~ 0.01 m/s

Dy

Dzcvw )(

v

z

yρc

ρw

c

wDx

Dzcvw )(

This is similar to observations!

Synoptic-scale vertical motions are weak!


Cold FrontsObservational 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


Cold FrontsObservational 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)

1. Warm front precipitation2. Convection cells ahead of CFA3. Precipitation from CFA falling in warm conveyor belt4. Shallow warm-moist zone5. Surface front (light precipitation)

A

B

BA


Cold FrontsObservational 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)


Cold FrontsObservational 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

PrimaryCold Front

ArcticCold Front


Cold FrontsObservational 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-doorCold Front


Warm FrontsObservational 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


Warm FrontsObservational 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!!!


Occluded FrontsObservational 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


Occluded FrontsObservational 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


Occluded FrontsObservational 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 providea new perspective to all this “conventionalwisdom” regarding occluded fronts!


Coastal FrontsObservational 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


Coastal FrontsObservational 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


Coastal FrontsObservational 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


Upper-Level FrontsObservational 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)

JetCore

JetCore

Potential Vorticity (solid)

Subsidence

Upper-levelFront

Tropopause


Upper-Level FrontsObservational 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

A

A

B

B


ReferencesBluestein, 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 constituentsbetween 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.

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Advanced SynopticM. D. Eastin Fronts: Structure and Observations