Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

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Understanding Weather and Climate 3rd
EditionEdward Aguado and James E. Burt

• Anthony J. Vega

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Part 4. Disturbances

• Chapter 10
• Mid-latitude Cyclones

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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

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Cyclogenesis
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• 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

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A mature mid-latitude cyclone
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A mature mid-latitude cyclone, lifting
processes, and cloud cover
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Two examples of mid-latitude cyclones
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• 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
  systems life cycle

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• 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

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• 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 Earths surface
• Air which rotates in the direction of Earths
  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

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Relative vorticity
Vorticity around a Rossby wave
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Changing vorticity along a Rossby wave
Convergence and divergence along a Rossby wave
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Values of absolute vorticity on a hypothetical
500 mb map
Changes in vorticity through a Rossby wave
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• 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

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Temperature variations in the lower
atmosphere lead to variation in upper-level
pressure
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• 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
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• 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

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• 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

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• 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

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• 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

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• 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
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• 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

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Typical winter mid-latitude cyclone paths
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• 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

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• 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

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End of Chapter 10 Understanding Weather and
Climate 3rd EditionEdward Aguado and James E.
Burt