Advanced Synoptic M. D. Eastin
Fronts: Structure and Observations
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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)
Advanced Synoptic M. D. Eastin
Definition and Structure
Advanced Synoptic M. D. Eastin
Definition and StructureSurface Pressure Equivalent Potential Temperature (θe)
Vertical Vorticity
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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!
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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!
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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)
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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!!!
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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!
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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
Advanced Synoptic M. D. Eastin
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.