Synoptic

1
Synoptic Mesoscale Fronts
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Synoptic Mesoscale Fronts

• Fronts and Jet Streaks The Basics
• Common Structure on the Mesoscale
• Coupling with Jet Streaks
• Mesoscale Fronts
• Dry Line
• Gust Fronts
• Sea-Breeze Fronts
• Coastal Fronts
• Topographically Induced Fronts

3
Frontal Structure

• Fronts
• Pronounced sloping transition zones in the
  temperature, moisture, and wind fields
• Contain large vorticity gradients and vertical
  wind shears
• Cross front scale (10-100 km) is often an order
  of magnitude smaller than along front scale
  (100-1000 km)
• Shallow (1-5 km in depth)
• Most often observed near the surface, but also
  occur aloft near the tropopause
• Important for mesoscale weather
• Rapid local changes in weather
• Associated with clouds and precipitation
• Often provide the necessary trigger for
  initiating deep convection

Cold
Warm
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Frontal Structure
Examples
Note Contours are of potential temperature
5
Frontal Structure
Cross-Section
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Coupling with Jet Streaks

• Divergence and vertical motion patterns
  associated with upper-level Jet Streaks
• Using a simplified vorticity equation
• Vorticity Divergence
• Change
• Thus, the vorticity change experienced by
• an air parcel moving through the jet streak
• will lead to
• Vorticity decrease ? Divergence aloft
• ? Upward motion
• Vorticity increase ? Convergence aloft

Vort Max
Left Exit

Left Entrance
Vorticity Increase
Vorticity Decrease
JET
_
Vorticity Decrease
Vorticity Increase
Right Exit
Right Entrance
Vort Min
Left Exit
Left Entrance
Ascent
Descent
JET
Descent
Ascent
Right Exit
Right Entrance
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Coupling with Jet Streaks

• The orientation of a surface front and an
  upper-level jet streak can lead to either
• enhanced (deep) convection or suppressed
  (shallow) convection along the front
• Enhanced Convection ? Left exit or right
  entrance region is above the front
• ? Helps destabilize
  the potentially unstable low-level air
• ? Increases the likelihood of deep
  convection

8
Coupling with Jet Streaks

• The orientation of a surface front and an
  upper-level jet streak can lead to either
• enhanced (deep) convection or suppressed
  (shallow) convection along the front
• Suppressed Convection ? Left entrance or
  right exit region is above the front
• ? Prevents
  destabilization of the potentially unstable air
• ? Decreases the likelihood of deep
  convection

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The Dryline

• Common Characteristics and Structure
• Can be defined as a near surface convergence
  zone between moist air flowing off
• the Gulf of Mexico and dry air flowing off
  the semi-arid, high plateaus of Mexico and
• the southwest United States
• Observed from southern Great Plains to the
  Dakotas ? east of the Rockies
• Occur between April and June when a surface high
  is located to the east and
• westerly flow aloft and a weak lee-side
  surface low is located to the west
• The 55F isodrosotherm or the
• 9.0 g/kg isohume are often used
• to indicate dryline position
• Dewpoint gradient often 15F
• per 100 km or larger
• Wind shift and moisture gradient
• are not always collocated

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The Dryline

• Common Characteristics and Structure
• Large diurnal variations
• Morning ? Shallow (below 850 mb)
• ? Furthest westward extension
• ? Moist layer capped by strong nocturnal
  temperature inversion
• Evening ? Deeper (up to 750 mb)
• ? Furthest eastward extension
• ? Dry mixed-layer on west side often
  extends up to 500 mb

Morning (6 am LST)
Late Afternoon (6 pm LST)
West-East Cross Sections
Extend from Tuscon, AZ to Shreveport,
LA Solid Lines are potential temperature (?
in K) Dashed Lines are mixing ratio (w in
g/kg)
Capping Inversion
Capping Inversion
Dry
Dry
Moist
Moist
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The Dryline

• Significance
• Convection is frequently initiated along the
  dryline
• Often develops into severe thunderstorms,
  producing strong winds, hail, and tornadoes
• Over 90 of such convection
• develops within 100 km
• of the line on the moist side
• Has important implications
• for agriculture
• Occur during the peak
• of growing season
• Hot / Dry to the west
• (need to irrigate more)

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The Dryline

• Evolution and Movement
• Daytime Eastward Motion
• Moves rapidly via sudden leaps (after sunrise)
• Motion is much faster than would occur from
  advection aloneHow?
• Turbulent mixing induced
• by solar heating begins
• to erode the shallow west
• side of the dry line

Initial dryline position just prior to sunrise
Thermals mix out shallow moist layer Dry line
position moves east
Capping Inversion
T0
T1
New dryline position
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The Dryline

• Evolution and Movement
• Daytime Eastward Motion
• Moves rapidly via sudden leaps (after sunrise)
• Motion is much faster than would occur from
  advection aloneHow?
• Process continues
• throughout the day
• (T0 ? T4)
• In the late afternoon to
• early evening the dryline

Deeper thermals continue to mix out shallow moist
layer on west edge
Capping Inversion
T0
T1
T2
T3
Dryline positions
T4
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The Dryline

• Evolution and Movement
• Night time Westward Motion
• During the day, a heat
• low develops west of
• the dryline, driving low
• level air toward the line
• When the sun sets,
• radiational cooling
• weakens the westerly
• flow (dry, cloud free)
• much quicker than it
• weakens the easterly
• flow (moist, cloudy)

Schematic of Diurnal Evolution
Noon
6 pm
Midnight
6 am
From Parsons et al. (2000)
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The Dryline

• Interaction with Synoptic Fronts
• Synoptic-scale cold fronts often catch and
  interact with dry lines
• The point of intersection is called the
  triple-point
• Location of enhanced convection
• Front provides an additional source of lift
• Front now has access to moist air
• Severe thunderstorms often
• occur near the triple point
• on the warm moist side,

Triple Point
Severe Storms
Ordinary Frontal Convection
From Bluestein (1993)
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The Dryline

• Dryline Bulges
• Eastward bulges occasionally develop
• during the afternoon hours
• 80-100 km in scale
• Preferred location for convective initiation due
• enhanced convergence
• Occur when mid-tropospheric winds are strong
• Result from the deep turbulent mixing west
• of the dryline transporting strong westerly
  winds
• from aloft down toward the surface

Example of a Dryline Bulge
Schematic of Downward Transport
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The Dryline
Numerical Simulation Examples
Cross Section animation
Plan View animation
Courtesy of Ming Xue at the University of Oklahoma
18
Gust Fronts

• Basic Characteristics and Structure
• Generated within thunderstorms by either
• precipitation loading or evaporative cooling
• at mid-tropospheric levels
• Negative buoyancy brings cool air down to
• the surface, where it spreads out, creating
• outflow boundaries ? gust fronts
• Horizontal scale ? 10 to 50 km
• Vertical scale ? 1 to 2 km
• Time scale ? 1 to 6 hours
• Forward motion ? 5 to 20 m/s
• Often responsible for generating new
• convection due to the enhanced
• convergence and ascent along their
• leading edge

From Wakimoto (1982)
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Gust Fronts
Three Dimensional Structure
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Gust Fronts

• Air Motions within a Gust Front
• Air parcel trajectories (labeled A ? G) in a
  mature gust front

G
D
Initial Locations
B
A
From Droegemeier and Wilhelmson (1987)
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Gust Fronts

• Sequence of Surface Events during Mature Gust
  Front Passage
• Change in wind speed and direction
• Direction may rotate 180
• Speed initially decreases
• prior to frontal passage
• and then rapidly increases
• soon after frontal passage
• Decrease in temperature on the
• order of 2 to 5C
• Increase in pressure (1 mb)
• Initial rise is non-hydrostatic,
• a dynamic effect created by
• the collisions of two fluids
• Second rise is hydrostatic,

22
Sea-Breeze Fronts

• Basic Characteristics and Structure
• Result from differential surface heating/cooling
  along coasts on light wind days
• Day ? Heating over land (positively buoyant
  air rises)
• ? Onshore flow near surface offshore flow
  aloft
• Night ? Cooling over land (negatively buoyant
  air sinks)
• ? Offshore flow near surface onshore flow
  aloft
• Front develops where onshore flow collides with
  background synoptic flow

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Coastal Fronts

• Basic Characteristics and Structure
• Stationary boundary separating relatively
• warm moist air flowing off the ocean from
• relatively cold dry air flowing off the
  continent
• Occur in the late fall and early winter
• from New England to Texas
• Often form during cold air outbreaks
• and cold-air damming events
• Boundary between rain and freezing rain/snow
• Temperature gradients of 5-10C over 5-10 km
• Convergence zone enhanced by land-sea
• friction contrasts

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Topographically Induced Fronts

• Denver Convergence Zone
• Generated by synoptic-scale easterly flow
• converging with shallow cold air flowing
• down topography (ridges and mountains)
• Cold air originates in the nocturnal
• boundary layer at high elevations
• Air begins to flow down the slopes
• and valleys
• Converges with synoptic-scale
• easterly flow by mid-morning
• and begins to push eastward
• onto the Great Plains
• Usually dissipates by mid-afternoon
• due to solar heating and surface

Cheyenne Ridge
Palmer Divide
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Topographically Induced Fronts

• Denver Convergence Zone
• Convergence line can help initiate
• deep convection ? non-supercell
• tornadoes often form during such events
• The topography in the Denver area often
• leads to the development of a cyclonic
• circulation ? enhances convergence
• Other Topographic Fronts
• Such circulations occur near most mountain
• ranges, including the Appalachians, when
• synoptic flow is weak and toward the range

Denver Convergence Zone
From Wilson et al. (1992)
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Synoptic Mesoscale Fronts

• Summary
• Frontal Structure on the Mesoscale
• Coupling between Fronts and Jet Streaks
• Vertical motion pattern
• Impact on convection
• Dry Lines (structure, significance, evolution,
  bulges)
• Gust Fronts (basic characteristics, structure,
  air flow patterns)
• Sea-Breeze Fronts (structure, physical
  processes)
• Coastal Fronts (structure and physical
  processes)
• Topographic Fronts (structure and physical
  processes)

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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 New England coastal
frontogenesis. Quart. J. Roy. Meteor. Soc., 101,
957-978. Droegemeier, K. K., and R. B.
Wilhelmson, 1985 Three-dimensional numerical
modeling of convection produced by interacting
thunderstorm outflows. Part I Control simulation
and low level moisture variations. J. Atmos.
Sci., 42, 23812403. McCarthy, J., and S. E.
Koch, 1982 The evolution of an Oklahoma dryline.
Part I A meso- and sub-synoptic scale analysis.
J. Atmos. Sci., 39, 225-236. Nielsen, J. W.,
1989 The formation of New England coastal
fronts. Mon. Wea. Rev., 117, 13801401. Parsons,
D.B., M.A. Shapiro, and E. Miller, 2000 The
mesoscale structure of a nocturnal dryline and of
a frontal-dryline merger. Mon. Wea. Rev., 128
,11, 3824-3838. Schaefer, J. T., 1974 The
lifecycle of the dryline. J. Appl. Meteor., 13,
444-449. Schaefer, J. T., 1986 The Dry Line.
Mesoscale Meteorology and Forecasting, Ed Peter
S. Ray, American Meteorological Society,
Boston, 331-358. Wakimoto, R. M., 1982 The
life cycle of thunderstorm gust fronts as viewed
with Doppler radar and rawinsonde data. Mon.
Wea. Rev., 110, 10601082. Wilson, J. W., G. B.
Foote, N. A. Crook, J. C. Frankhauser, C. G.
Wade, J. D. Tuttle, and C. K. Mueller, 1992 The
role of boundary-layer convergence zones and
horizontal roles in the initiation of
thunderstorms A case study. Mon. Wea. Rev.,
120, 1785-1815. Wilson, J. W., and W. E.
Schreiber, 1986 Initiation of convective storms
at radar observed boundary-layer convergence
lines. Mon. Wea. Rev., 114, 25162536.