Transcript Lecture #14: Fronts

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
Advanced Synoptic
M. D. Eastin
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?
→ Do temperature
gradients that
“disappear” at
night (or during
the day) count
as fronts?
Advanced Synoptic
Daytime
Cloudy
Cool
Nighttime
Cloudy Cool
Clear-Dry
Clear-Dry
Warm
Cool
M. D. Eastin
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 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
Surface Pressure
Equivalent Potential Temperature (θe)
Vertical Vorticity
Advanced Synoptic
M. D. Eastin
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
y
ρc
• 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
ρw
x
ρ p
T
Warm
Cold
• Hydrostatic Balance
• Geostrophic Balance
• Pressure is steady (no changes in time)
y
South
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Front
North
M. D. Eastin
Frontal Slope
• The differential of pressure is:
Dp 
p
p
Dy 
Dz
y
z
(1)
• Divide each side by Dy
Dp p p Dz


Dy
y z Dy
(2)
• Substitute in the hydrostatic equation
p
  g
z
(3)
Dp p
Dz

 g
Dy
y
Dy
(4)
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M. D. Eastin
Frontal Slope
• Since pressure is continuous across the front:
pw  pc
 Dp 
 Dp 

  

Dy
Dy

w 
c
(5)
(6)
• Substitution of (4) into (6) yields:
 p 
Dz  p 
Dz
 

   w g
   c g
Dy  y  c
Dy
 y  w
(7)
• We can now solve for (Dz/Dy)
 p 
 p  




 

Dz
 y c  y  w 

Dy
g  c   w 
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(8)
M. D. Eastin
Frontal Slope
Which way can the front slope and still be “stable”?
 p 
 p  
 
 


y

y
Dz

c 
w 

Dy
g  c   w 
z
• The front must be able to persist for 1-2 days
(as fronts do in reality)
Dz
0
Dy
(9)
• And since
c   w
(10)
• Thus
 p 
 p 

  
  0

y

y

c 
w
(11)
• Or
 p 
 p 

  


y

y

c 
w
(12)
• Thus
Stable
ρw
ρc
y
z
Unstable
ρc
ρw
y
 What does this imply about pressure across the front?
Advanced Synoptic
M. D. Eastin
Frontal Slope
What does this imply about pressure across the front?
 p 
 p 

  


y

y

c 
w
 While pressure is continuous across the front, the
pressure gradient is not continuous
 Thus, the isobars must kink to satisfy this relationship
High pressure
 p 



y

c
Low pressure
Or
 p 



y

w
High pressure
Advanced Synoptic
M. D. Eastin
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:
ug  
1 p
 f 0 y
vg  0
(13)
• Thus, on the warm and cold sides of the front:
u gw
 p 



 w f 0  y  w
1
u gc
 p 


 y  c
(14)
(w  c )
2
(15)
1

c f0
• Substituting (14) into (8) yields:
 f 0 (u gw  u gc )
Dz

Dy
g  c   w 
• Again, if
Dz
 0 and
Dy
where
 
 c   w then ugw  ugc  0 or u gw  u gc (16)
 What does this imply about the winds across the front?
Advanced Synoptic
M. D. Eastin
Frontal Slope
What does this imply about the winds across the front?
u gw  u gc
y
ugc
• Recall the definition of geostrophic vertical vorticity
g 
vg
x

u g
y
g  
u g
y
ugw
x
• Thus, cyclonic vorticity must exist across the front
• Here are more possible examples
Advanced Synoptic
M. D. Eastin
Frontal Slope
How much does a front slope with height?
• Returning to the frontal slope equation:
 f 0 (u gw  u gc )
Dz

Dy
g  c   w 
(15)
• Using the Equation of State, (15) can be written as:
Dz T f 0 (u gw  u gc )

Dy
g Tw  Tc 
Margules Equation
for Frontal Slope
• If we substitute in typical values:
Dz 300K  104 s 1  10ms1
1


Dy
10ms2  10K
300
Advanced Synoptic
This is similar to observations!
Surface fronts are shallow!
M. D. Eastin
Frontal Slope
• Similar conclusions can be reached for a front
oriented north-south using similar assumptions
Dz T f 0 (v gw  v gc )

Dx
g Tc  Tw 
y
Margules Equation
ρc
• Again, frontal stability requires:
ρw
Dz
0
Dx
x
• Thus, it can be shown:
 The pressure gradient is discontinuous and the
isobars must kink across the front
ρ p
T
Cold
Warm
 The geostrophic wind must contain cyclonic
vorticity across the front
x
West
Advanced Synoptic
Front
East
M. D. Eastin
Frontal Slope
Synoptic-scale Vertical Motion:
• The vertical motion immediately adjacent to a given
frontal slope can also be estimated:
z
w  (v  c)
where:
Dz
Dy
w  (v  c)
Dz
Dx
v = cross-front velocity
c = the speed of the front
ρw
w
v
c
ρc
y
Example:
Dz/Dy ~ 1/300
v ~ 5 m/s
c ~ 2 m/s
w ~ 0.01 m/s
This is similar to observations!
Synoptic-scale vertical motions are weak!
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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)
Advanced Synoptic
M. D. Eastin
Cold Fronts
Observational Aspects: Katafronts
B
 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)
A
• Occur later in the parent cyclone’s lifecycle
(when the cold front has a N-S orientation)
A
B
1. Warm front precipitation
2. Convection cells ahead of CFA
3. Precipitation from CFA falling
in warm conveyor belt
4. Shallow warm-moist zone
5. Surface front (light precipitation)
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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!!!
Advanced Synoptic
Back-door
Cold Front
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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!!!
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
Occluded Fronts
Observational Aspects: Two Conceptual Models
Norwegian Cyclone Model
Shapiro-Keyser 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
• 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 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)
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
B
M. D. Eastin
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.
Advanced Synoptic
M. D. Eastin