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Mesoscale M. D. Eastin
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 StructureFronts:
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
Warm
Cold
Mesoscale M. D. Eastin
Frontal StructureExamples:
Note: Contours are of potential temperature
Cold Front Occluded Front
Warm Front Forward-tilting Cold Front
Mesoscale M. D. Eastin
Frontal StructureCross-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:
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→ Downward motion
DivDt
D +
_VortMin
VortMax
JET
VorticityDecrease
VorticityIncrease
VorticityIncrease
VorticityDecrease
JET
Descent
AscentDescent
Ascent
LeftExit
LeftExit
RightExit
RightExit
LeftEntrance
LeftEntrance
RightEntrance
RightEntrance
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
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 DrylineCommon 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 DrylineCommon 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
West-East Cross SectionsMorning (6 am LST) Late Afternoon (6 pm LST)
Extend from Tuscon, AZ to Shreveport, LA
Solid Lines are potential temperature (θ in K)
Dashed Lines are mixing ratio (w in g/kg) Moist
Moist
DryDry
CappingInversion
CappingInversion
Mesoscale M. D. Eastin
The DrylineSignificance:
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
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 positionjust prior to sunrise
Thermals mix out shallow moist layerDry line position moves east
New dryline position
T0T1
Capping Inversion
The Dryline
Mesoscale M. D. Eastin
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)
• In the late afternoon to early evening the dryline begins to move back westward…Why?
Deeper thermals continue to mix outshallow moist layer on west edge
Dryline positions
T0T1T2T3T4
Capping Inversion
The Dryline
Mesoscale M. D. Eastin
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)
Dryline surges westward
Noon 6 pm
Midnight 6 am
Schematic of Diurnal Evolution
From Parsons et al. (2000)
The Dryline
Mesoscale M. D. Eastin
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,
From Bluestein (1993)
TriplePoint
Ordinary Frontal
Convection
SevereStorms
The Dryline
Mesoscale M. D. Eastin
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
Schematic of Downward Transport
Example of a Dryline Bulge
The Dryline
Mesoscale M. D. Eastin
Numerical Simulation Examples:
Plan Viewanimation
Courtesy of Ming Xue at the University of Oklahoma
Cross Sectionanimation
The Dryline
Mesoscale M. D. Eastin
Gust FrontsBasic 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 FrontsThree – Dimensional Structure:
Mesoscale M. D. Eastin
Air Motions within a Gust Front:
• Air parcel trajectories (labeled A → G) in a mature gust front
From Droegemeier and Wilhelmson (1987)
AB
GDInitial
Locations
Gust Fronts
Mesoscale M. D. Eastin
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
Gust Fronts
Mesoscale M. D. Eastin
Sea-Breeze FrontsBasic 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
Mesoscale M. D. Eastin
Coastal FrontsBasic 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)
• 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 fluxes warming the shallow cold air
Palmer Divide
Cheyenne Ridge
Mesoscale M. D. Eastin
Topographically Induced Fronts
From Wilson et al. (1992)
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
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
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: 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.