Understanding Weather

and Climate

3rd Edition

Edward Aguado and James E. Burt

Anthony J. Vega

Part 4. Disturbances

Chapter 10

Mid-latitude Cyclones

Introduction Bjerknes, the founder of the Bergen school of

meteorology, developed polar front theory to describe

interactions between unlike air masses and related aspects

of the mid-latitude cyclone

The Life Cycle of a Mid-latitude Cyclone

Cyclogenesis typically begins along the polar front but may initiate

elsewhere, such as in the lee of mountains

• Minor perturbations occur along the boundary separating colder polar

easterlies from warmer westerlies

• A low pressure area forms and due to the counterclockwise flow

(N.H.) colder air migrates equatorward behind a developing cold front

• Warmer air moves poleward along a developing warm front (east of

the system)

• Clouds and precipitation occur in association with converging winds

of the low pressure center and along the developing fronts

Cyclogenesis

Mature Cyclones

• Well-developed fronts circulating about a deep low pressure center

characterize a mature mid-latitude cyclone

• Chance of precipitation increases toward the storm center

– Heavy precipitation stems from cumulus development in association

with the cold front

– Lighter precipitation is associated with stratus clouds of the warm front

• Highly unstable conditions are associated with the warm sector ahead

of the cold front

– Area may produce heavy precipitation and severe thunderstorms

associated with prefrontal waves (squall lines)

• The system is capable of creating snow, sleet, freezing rain, and/or

hail

• Isobars close the low and are typically kinked in relation to the fronts

due to steep temperature gradients

• Winds, spiraling counterclockwise toward the low, change

accordingly as the system, and its associated fronts, moves over

particular regions

A mature mid-latitude cyclone

A mature mid-latitude cyclone,

lifting processes, and cloud cover

Two examples of

mid-latitude cyclones

Occlusion

• When the cold front joins the warm front, closing off the warm sector,

surface temperature differences are minimized

• The system is in occlusion, the end of the system’s life cycle

Evolution and Movement of Cyclones

• A hypothetical mid-latitude cyclone may develop as a weak

disturbance east of Japan and travel eastward, guided by upper air

trajectories

• The system may bring rain to western North America and snow to

high elevations

• On the lee side of the Rocky Mountains, the system may undergo

strengthening, causing blizzard conditions in the central and

northeastern U.S.

• Occlusion typically occurs in the western North Atlantic

• For particular locations, weather conditions may progress from clear

to cloudy with cloud cover thickening and lowering

• Eventually, light precipitation may begin with warm front

advancement

• Winds, originally easterly, shift to southeasterly, then southwesterly

with warm front advancement

• Heavy clouds and precipitation advance with cold front approach

• Temperature and humidity plummet with cold front passage

Processes of the Middle and Upper Troposphere

Carl Rossby mathematically expressed relationships between mid-

latitude cyclones and the upper air during WWII

Rossby Waves and Vorticity

• The rotation of air, or vorticity, may be viewed as either being

absolute, the overall rotation, or relative to the Earth’s surface

• Air which rotates in the direction of Earth’s rotation is said to exhibit

positive vorticity

• Air which spins oppositely exhibits negative vorticity

• In relation to the upper air, maximum and minimum vorticity occurs

in relation to troughs and ridges, respectively

• Vorticity changes in the upper atmosphere lead to surface pressure

changes

– Decreasing vorticity in the zone between a trough and ridge leads to

upper air convergence and sinking motions through the atmosphere,

which supports surface high pressure areas

– Increasing vorticity in the zone between a ridge and trough leads to

upper air divergence and rising motions through the atmosphere, which

supports surface low pressure areas

Relative vorticity

Vorticity around a Rossby

wave

Changing vorticity along a Rossby wave

Convergence and divergence

along a Rossby wave

Values of absolute vorticity

on a hypothetical 500 mb map

Changes in vorticity through

a Rossby wave

The Effect of Fronts on Upper-Level Patterns

• Interactions between the upper air and surface and vice versa helps

establish and develop mid-latitude cyclones

• Thermal differences across cold and warm fronts lead to upper

atmospheric pressure differences due to density differences which

equate to air temperature as expressed through the hydrostatic

equation

Cold Fronts and the Formation of Upper-Level Troughs

• Upper air troughs develop behind surface cold fronts with the vertical

pressure differences proportional to horizontal temperature and

pressure differences

• This is due to density considerations associated with the cold air

• Such interactions also relate to warm fronts and the upper atmosphere

Temperature variations in the lower atmosphere

lead to variation in upper-level pressure

• Interaction of Surface and Upper-Level Patterns

– The upper atmosphere and the surface are inherently connected and linked

– Divergence and convergence relate to surface pressure differences in cyclones and anticyclones, respectively

– Surface temperatures influence vertical pressures and upper atmospheric winds

– Upper level flow patterns explain why mid-latitude cyclones exist in addition to aspects of their life cycles

– An example is the typical position of mid-latitude cyclones downwind of trough axes in the area of decreasing vorticity and upper-level divergence

Relationships between

a mid-latitude cyclone

and a trough and ridge

• An Example of a Mid-Latitude Cyclone

– April 15 - A mid-latitude cyclone is centered over the upper midwestern

U.S.

– Heavy rains, high winds, and overcast skies dominate the regions near the

central low pressure center

– Recording of wind trajectories at stations throughout the central U.S.

depict the counterclockwise rotation of the system

– The 500 mb chart shows the storm upstream of the trough axis in the

region of decreasing vorticity and upper-level divergence

– April 16 - The northeasterly movement of the storm system is seen

through a comparison of weather maps over a 24-hour period

– Occlusion occurs as the low moves over the northern Great Lakes

– In the upper air, the trough has increased in amplitude and strength and

become oriented northwest to southeast

– Isobars have closed about the low, initiating a cutoff low

– April 17 - Continual movement towards the northeast is apparent,

although system movement has lessened

– The occlusion is now sweeping northeastward of the low, bringing

snowfall to regions to the east

– In the upper air, continued deepening is occurring in association with the

more robust cutoff low

– April 18 -The system has moved over the northwestern Atlantic Ocean,

but evidence persists on the continent in the form of widespread

precipitation

– The upper atmosphere also shows evidence of the system, with an

elongated trough pattern

• Flow Patterns and Large-Scale Weather

– Zonal height patterns obstruct development of surface pressure systems

as vorticity remains constant

– Zonal conditions are indicative of rather benign atmospheric conditions

at the surface, although small scale disturbances may occur

– Meridional conditions actively support surface cyclone development as

vorticity changes appreciably between troughs and ridges

– Large-scale flow conditions in the upper atmosphere may persevere for

long periods, locking in particular weather situations to affected regions

Zonal flow pattern Meridional flow pattern

The Steering of Mid-Latitude Cyclones

• The movement of surface systems can be predicted by the 500 mb

pattern

• The surface systems move in about the same direction as the 500 mb

flow, at about 1/2 the speed

• Must predict changes in the 500 mb flow patterns in order to correctly

predict surface system movement

• Upper-level winds are about twice as strong in winter than summer

• During winter, net radiation decreases rapidly with increasing

latitude, which creates a stronger latitudinal thermal gradient

• This results in stronger pressure gradients (and winds), resulting in

stronger and more rapidly moving surface cyclones

• Winter mid-latitude cyclones may be grouped by common paths

across North America

– Alberta Clippers are associated with zonal flow and usually produce

light precipitation

– Colorado Lows are usually stronger storms which produce more

precipitation

– East Coast storms typically have strong uplift and high water vapor

content

Typical winter mid-latitude cyclone paths

Migration of Surface Cyclones Relative to Rossby Waves

• Upper-air divergence must be present for a mid-latitude cyclone to

form

• If divergence aloft exceeds surface convergence, the surface low will

deepen and a cyclone forms

• If convergence at the surface exceeds divergence aloft, the low “fills”

• Surface cyclones are pushed along the upper air wind trajectory

• They generally move in the same direction as the 700 mb winds and

at about 1/2 the speed

• The surface low generally moves southwest to northeast relative to

the Rossby wave configuration

• As such, it moves away from the region of maximum divergence

aloft, eventually dissipating as it approaches the upper-level ridge

The Modern View - Mid-latitude Cyclones and Conveyor Belts

• The conveyor belt cyclone model helps describe conditions associated

with mid-latitude cyclones through the entire profile of the

atmosphere

• The warm conveyor belt originates in the lower atmosphere of the

warm sector

– Air flowing toward the storm center is displaced aloft until it overrides

the warm front where it turns to the right (N.H.), becoming part of the

westerly flow aloft

• The cold conveyor belt lies north of the warm front

– It streams westward near the surface toward the surface low, where it

ascends and turns clockwise (N.H.) to become part of the westerly upper

air flow

• The dry conveyor belt originates in the upper troposphere as part of

the normal westerly flow

– Air sinks into the trough only to rise over the region of the surface low

before continuing along its eastward path

– Integral to maintaining separate cloud bands which give the system its

characteristic comma shape

End of Chapter 10

Understanding Weather and

Climate

3rd Edition Edward Aguado and James E. Burt