slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Fronts: Structure and Observations PowerPoint Presentation
Download Presentation
Fronts: Structure and Observations

Loading in 2 Seconds...

play fullscreen
1 / 31

Fronts: Structure and Observations - PowerPoint PPT Presentation


  • 137 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Fronts: Structure and Observations' - malha


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide2

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

slide3

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

slide4

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

slide5

Definition and Structure

Surface Pressure

Equivalent Potential Temperature (θe)

Vertical Vorticity

M. D. Eastin

slide6

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

slide7

Frontal Slope

  • The differential of pressure is:
  • (1)
  • Divide each side by Dy
  • (2)
  • Substitute in the hydrostatic equation
  • (3)
  • (4)

M. D. Eastin

slide8

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

slide9

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

slide10

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

slide11

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 then or (16)
  • What does this imply about the winds across the front?

M. D. Eastin

slide12

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

slide13

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

slide14

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

slide15

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

slide16

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

slide17

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

slide18

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

slide19

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

slide20

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

slide21

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

slide22

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

slide23

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

slide24

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

slide25

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

slide26

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

slide27

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

slide28

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

slide29

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

slide30

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

slide31

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