Transcript Mesoscale Fronts

Synoptic & Mesoscale Fronts
Mesoscale
M. D. Eastin
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
Mesoscale
M. D. Eastin
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
Mesoscale
M. D. Eastin
Frontal Structure
Examples:
Cold Front
Occluded Front
Warm Front
Forward-tilting Cold Front
Note: Contours are of potential temperature
Mesoscale
M. D. Eastin
Frontal Structure
Cross-Section:
Mesoscale
M. D. Eastin
Coupling with Jet Streaks
Divergence and vertical motion patterns associated with upper-level Jet Streaks
• Using a simplified vorticity equation:
D
 Div 
Dt
Vort
Max
Left
Entrance
Vorticity
Increase
+
Left
Exit
Vorticity
Decrease
JET
Vorticity
Change
Divergence
• Thus, the vorticity change experienced by
an air parcel moving through the jet streak
will lead to:
Vorticity decrease → Divergence aloft
→ Upward motion
Vorticity
Decrease
Right
Entrance
_
Vorticity
Increase
Right
Exit
Vort
Min
Left
Entrance
Left
Exit
Descent
Ascent
JET
Vorticity increase → Convergence aloft
→ Downward motion
Ascent
Right
Entrance
Mesoscale
Descent
Right
Exit
M. D. Eastin
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
Mesoscale
M. D. Eastin
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
Mesoscale
M. D. Eastin
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 55°F isodrosotherm or the
9.0 g/kg isohume are often used
to indicate dryline position
• Dewpoint gradient often 15°F
per 100 km or larger
• Wind shift and moisture gradient
are not always collocated
Note: Drylines also occur in
India, China, and
west Africa
Mesoscale
M. D. Eastin
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)
West-East Cross Sections
Extend from Tuscon, AZ
to Shreveport, LA
Solid Lines are potential
temperature (θ in K)
Dashed Lines are mixing
ratio (w in g/kg)
Mesoscale
Late Afternoon (6 pm LST)
Capping
Inversion
Capping
Inversion
Dry
Dry
Moist
Moist
M. D. Eastin
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)
• Warm / Humid east
Mesoscale
M. D. Eastin
The Dryline
Evolution and Movement:
Daytime – Eastward Motion:
 Moves rapidly via sudden “leaps” (after sunrise)
 Motion is much faster than would occur from advection alone…How?
• 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
Mesoscale
M. D. Eastin
The Dryline
Evolution and Movement:
Daytime – Eastward Motion:
 Moves rapidly via sudden “leaps” (after sunrise)
 Motion is much faster than would occur from advection alone…How?
• Process continues
throughout the day
(T0 → T4)
Deeper thermals continue to mix out
shallow moist layer on west edge
Capping Inversion
T0
Dryline
positions
T1 T2
T3 T4
• In the late afternoon to
early evening the dryline
begins to move back
westward…Why?
Mesoscale
M. D. Eastin
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
Schematic of Diurnal Evolution
Noon
6 pm
 When the sun sets,
radiational cooling
weakens the westerly
flow (dry, cloud free)
much quicker than it
weakens the easterly
flow (moist, cloudy)
Midnight
6 am
 Dryline surges westward
From Parsons et al. (2000)
Mesoscale
M. D. Eastin
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
Triple
Point
occur near the triple point
on the warm moist side,
Ordinary
Frontal
Convection
Severe
Storms
From Bluestein (1993)
Mesoscale
M. D. Eastin
The Dryline
Dryline Bulges:
• Eastward “bulges” occasionally develop
during the afternoon hours
Example of a Dryline Bulge
• 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
Schematic of Downward Transport
Mesoscale
M. D. Eastin
The Dryline
Numerical Simulation Examples:
Plan View
animation
Cross Section
animation
Courtesy of Ming Xue at the University of Oklahoma
Mesoscale
M. D. Eastin
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
• Under special conditions can help maintain
intense long-lived squall lines…more on
this in the future
From Wakimoto (1982)
Mesoscale
M. D. Eastin
Gust Fronts
Three – Dimensional Structure:
Mesoscale
M. D. Eastin
Gust Fronts
Air Motions within a Gust Front:
• Air parcel trajectories (labeled A → G) in a mature gust front
Initial
Locations
G
D
B
A
From Droegemeier and Wilhelmson (1987)
Mesoscale
M. D. Eastin
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 5°C
• Increase in pressure (~1 mb)
• Initial rise is non-hydrostatic,
a dynamic effect created by
the collisions of two fluids
• Second rise is hydrostatic,
the thermodynamic effect
from the cold air
• Onset of light precipitation
Mesoscale
M. D. Eastin
Sea-Breeze Fronts
Basic Characteristics and Structure:
 Result from differential surface heating/cooling along coasts on “light wind” days
→
→
Night →
→
Day
Heating over land (positively buoyant air rises)
Onshore flow near surface – offshore flow aloft
Cooling over land (negatively buoyant air sinks)
Offshore flow near surface – onshore flow aloft
• Front develops where onshore flow collides with “background” synoptic flow
Mesoscale
M. D. Eastin
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°-10°C over 5-10 km
• Convergence zone enhanced by land-sea
friction contrasts
Mesoscale
M. D. Eastin
Topographically Induced Fronts
Denver Convergence Zone:
 Generated by synoptic-scale easterly flow
converging with shallow cold air flowing
down topography (ridges and mountains)
Cheyenne Ridge
• 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
Palmer Divide
• Usually dissipates by mid-afternoon
due to solar heating and surface
fluxes warming the shallow cold air
Mesoscale
M. D. Eastin
Topographically Induced Fronts
Denver Convergence Zone
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
From Wilson et al. (1992)
Mesoscale
M. D. Eastin
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)
Mesoscale
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: 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, 2381–2403.
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, 1380–1401.
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, 1060–1082.
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, 2516–2536.
Mesoscale
M. D. Eastin