MesoscaleM. 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


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


Frontal StructureExamples:

Note: Contours are of potential temperature

Cold Front Occluded Front

Warm Front Forward-tilting Cold Front


Frontal StructureCross-Section:


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



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



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


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


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


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


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



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



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


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


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


Numerical Simulation Examples:

Plan Viewanimation

Courtesy of Ming Xue at the University of Oklahoma

Cross Sectionanimation

The Dryline


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)


Gust FrontsThree – Dimensional Structure:


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


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


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


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


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


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



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)


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.

Documents

MesoscaleM. D. Eastin Synoptic & Mesoscale Fronts