Download G Air masses, fronts and cyclones

Air masses, fronts and cyclones
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Air masses, fronts and cyclones
Aims
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To explain the seasonal changes in weather patterns in the midlatitudes
To outline the sequence of stages in the life cycle of a migrating midlatitude wave
cyclone and their related weather patterns
Objectives
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To discuss the basis of air mass classification and to differentiate among the various
major air mass types.
To list two criteria that an air mass source region must meet.
To identify the various air masses that regularly form over or invade North America
and discuss the typical weather conditions associated with these air masses.
To describe the processes that contribute to air mass modification, identifying those
conditions that determine the degree of air mass modification.
To list at least two criteria involved with the analysis of a front on a surface weather
map.
To identify the conditions needed for frontogenesis and frontolysis.
To compare and contrast warm fronts, cold fronts, stationary and occluded fronts in
terms of their structure and associated weather.
To describe the weather sequences (i.e., changes in wind direction, pressure tendency, cloud type and coverage, precipitation and temperature) when the warm or
cold side of a wave cyclone passes an observer.
To locate the position and forecast the movement of an extratropical storm by interpreting the sequence of wind direction, pressure change, temperature change and
cloud/precipitation type at a station.
To identify the relative size and characteristics of an extratropical cyclone.
To locate and describe the general temperature-field, the wind regime and the cloud
and precipitation pattern associated with a model extratropical cyclone.
To outline the sequence of stages in the life cycle of a migrating midlatitude wave
cyclone.
Air masses, fronts and cyclones
Outline
Introduction
• Patterns of the 500-mb flow
• An extratropical cyclone
Waves in the Westerlies
• Polar jet as part of the Westerlies
• Wavy flow of the Westerlies
• Contribution to the latitudinal heat transport
• Impact on mid-latitude weather
Air Masses
• Definition of an air mass
• Source regions of air masses
Fronts
• Definition and types synoptic scale fronts
• Structure of a frontal surfaces
• Weather related with fronts
• Temporal evolution of fronts
Synoptic weather charts
• Surface weather elements
• The synoptic surface chart
Life cycle of extratropical wave cyclones
• Development along a stationary front
• Cyclogenesis
• Occlusion
Idealized weather of wave cyclones
• Time Sequences of weather events
• Clouds and precipitation, wind shift
• Single point forecasting
Upper level structure
• Upper level charts
• Flow and divergence and convergence aloft
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Air masses, fronts and cyclones
Bullets
Introduction
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Upper air flow typically meandering
Large scale weather systems
Implies North -South air transport
Part of the latitudinal heat transport
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Polar jet integral part of the Westerlies - just the fastest part
Westerlies flow in paths with long wave lengths, such that three to six cycles fit
around the globe - so called Rossby Waves - after C.G. Rossby who first explained
these waves.
Shorter waves occur in the middle and upper troposphere, being associated with
cyclones at the surface traveling at rates up to 15o per day.
By alternating flow patterns between flat flow and strongly wavy flow the Westerlies
contribute to the heat transport to the poles (cf. the latitudinal heat budget) and mixing of air between the mid latitudes and the poles
During phases of strong meandering of the flow (large amplitudes of the waves) cold
air is transported into the mid latitudes and the South-North temperature gradient is
increased - cyclonic activity and more violent weather
Dish-pan experiments suggests that it is mainly the latitudinal temperature contrast
which generates the wavy circumpolar circulation patterns.
Waves in the Westerlies
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Air Masses
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Air mass - large body of air characterized by homogeneous physical properties (in
particular similar temperature and moisture throughout) at any given altitude
Movement of air masses will bring the conditions form the source region elsewhere
Air-mass weather - generated by the moving in of an air mass
Source regions - large physically uniform, allow for periods of longer stagnation
Typically regions dominated by stationary (or slow moving) anticyclones with their
extensive areas of calms or light winds
Regions dominated by cyclones are not likely to produce air masses due to convergence
Source regions for North America not found in the mid latitudes
Classification depends on the latitude of the source region (P -polar, A - arctic, T tropical, E - equatorial) and the area of origin - land or ocean (m -maritime, c -continental)
mA and cE usually do not form, since the arctic is ice covered (air is dry) and the
equator mainly ocean dominated (air wet anyway)
Air mass modification due to underlying surface, mixing from above and lifting processes
Modification processes imply changes in the stability and often transport of heat and
moisture to higher levels in the atmosphere
Fronts
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Fronts - transition zone between air masses of different densities 15-200km wide
Confluence line between two different air masses is called the front
Region of strong thermal contrast on the “cold air side” is called frontal zone or baroclinic zone
Fronts are labeled by the air mass which replaces the other one
Warm air in fronts above colder air - overrunning by warm air. Frontal surface always
slopes in direction of the cold side with increase in height
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Air masses, fronts and cyclones
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Frontal zone lies always below the frontal surface with the warm air mass above it.
Cold fronts do sometimes overhang.
Frontal zone trapped in the wedge below the frontal surface and can not move relative to the frontal surface and the direction and speed of the movement of the front is
determined by the winds in the frontal zone.
For analyses of sequences of synoptic charts it is important to have consistency
between the frontal movement and the wind field
Warm Fronts - warmer air replaces colder air at about 25 km/h - gradually sloped,
about 1:200
Overrunning is slow and slope is gradual thus light to moderate precipitation over a
large area and an extended period
Cold fronts - about 35 km/h - steeper sloped - about 1:100.
Thus more violent weather - the lifting is faster and can cause convection.
Thus more precipitation in a shorter period of time
Behind cold front subsidence and cool air masses, thus clearing of the sky
Occluded fronts - a “cold front catches up with a warm front” or it may rather be that it
forms as a new front what the surface low separates itself from the junction of their
warm and cold fronts.
Cold type and warm type occluded fronts
Warm type - front aloft (and so precipitation) precedes the arrival of the surface front only warm front extends to the ground. Passage of warm type occlusions decreases
stability.
Cold type - front aloft lags behind surface front - only cold front extends to the ground.
Passage of cold type occlusions increases stability
Idealization!
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Delineation of air masses
Location of fronts and determination of types
Sea level pressure lines
Movement of pressure centers
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Cyclone develops along a front and proceeds through a generally predictable life
cycle - hours to days - as suggested by the Bergen school around Vilhelm and Jacob
Bjerknes around 1920.
Initial development of the low pressure center along a stationary front on the crest of
a wavelike undulation in the shape of the front - cold polar Easterlies meet warm midlatitude Westerlies, small irregularities lead to shearing and wave formation (several
100 km long)
The wave shape results into invasion of warm air into the crest of the wave and cold
air moving equatorward behind the crest - readjustment in pressure to nearly circular
isobars and low pressure at the center at the apex of the wave
Establishment of a cyclonic circulation and end of the period of rapid development
(cyclogenesis)
Begin of occlusion as the low pressure center propagates toward cold air and deepens with an occluded front connecting the low center to the junction of the warm and
cold fronts. The cyclone enters maturity (maximum intensity) when it enters this
stage.
Warm air is displaced aloft during occlusion and the pressure gradient between cold
and warm air masses weakens eventually leading to a dissolution of the cyclonic
movement and the storm
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Synoptic weather charts
Life cycle of extratropical wave cyclones
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Air masses, fronts and cyclones
Idealized weather of wave cyclones
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Typically a cyclone needs 2-4 days to pass and on the northern hemisphere they tend
to move eastward. The passing of a cyclone is accompanied by a typical sequence of
weather patterns as envisaged in the classical Norwegian model.
First high clouds on the horizon - about 1000 km in front of the approaching warm
front. As the warm front approaches the could deck is coming lower and deepens.
After 12-24 h after the first sighting of clouds light precipitation starts due to the overriding with warm air. The precipitation is widespread and fairly uniform and reaches its
peak intensity just prior to the passage of the warm front.
After passage of the warm front the weather is in general warm, with southerly winds
and clear skies. However, depending on ground temperatures and heating fog or isolated thunderstorms are possible.
The cold front brings another well -organized cloud system. Typically a wall of clouds
due to the fast lifting produces heavy precipitation.
After passage the wind tends to shift from southeast to northwest and there is a pronounced drop in temperature. The weather is typically much brighter, but in marine
polar air convective clouds can locally produce heavy precipitation.
Upper level structure
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Little cyclonic activity occurs while the upper level flow aloft is relatively straight,
because in this case there is not much North-South transport of cold or warm air.
Meandering and formation of ridges and troughs of the upper air stream typically is
accompanied by surface cyclonic activity, as in this case cold air is transported southward behind a cold front and warm air northward pushing the warm front north-eastwards - on the Northern Hemisphere.
It is observed that winds in the middle troposphere (700-500mb) tend to act as “steering flow” for features on the surface.
Because the warmer air slopes into the colder air over cold and warm fronts, cold
fronts aloft lag the surface cold front, whereas warm fronts aloft precede the warm
front at the surface.
Links
BBC Weather http://www.bbc.co.uk/weather/
British Met Office http://www.met-office.gov.uk/
US National Weather Service home page: http://www.nws.mbay.net/home.html
Unisys weather page http://weather.unisys.com/index.html
NOAA links: http://www.lib.noaa.gov/docs/links.html
Western Regional climate center: http://www.wrcc.sage.dri.edu/
University of Illinois Online Guides:
Meteorology: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml
Air masses and fronts: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/home.rxml
Midlatitude cyclones: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/home.rxml
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Air Masses and Weather Patterns
Geopotential height on the 500-mb surface
The distribution of geopotential height on the 500-mb surface at 00 GCT 20 November 1964. Labels on
contours represent geopotential height, in tens of meters. The letters H and L denote centers of high
and low geopotential height respectively.
Air Masses and Weather Patterns
A vigorous low pressure system
A vigorous area of low pressure is producing strong winds, heavy rain and thunderstorms in the
Northeast and Mid Atlantic United Staates. Further to the west, in the colder air, snow is falling in
Ohio. Behind the cold fronts associated with this storm, temperatures are some 20 deg.F colder
Idealized westerly flow at the 500 mb level
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Cyclic changes occuring in upper-level air flow in the Westerlies
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
and
Model Extratropical Wave Cyclones
Air Masses and Weather Patterns
A cold air mass over the U.S.A.
Air Masses and Weather Patterns
Air masses influencing the climate in North America
and their source regions
Source: Ward's Natural Science Establishment, Inc. after
Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
Air mass weather in North America
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
Fronts
Fronts are boundary areas
that separate contrasting
air masses
Fronts are named by the
advancing air mass
Fronts act nearly as walls
between air masses
Air Masses and Weather Patterns
Simplified weather map and idealized structure of a middle-latitude cyclone
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
The Warm Front
Transition zone where a warm
air mass is replacing a colder
air mass
Warm fronts generally move
from SW to NE and the air
behind the front is warmer
and moister than the one in front
Simple model of a
cyclone with a warm
front extending to
the East
Warm fronts are
gently sloping
above cold air
being retarded by friction - lifting slow - light to moderate rain over
large area for extended periods
Air Masses and Weather Patterns
The Cold Front
Transition zone where a cold
air mass is replacing a warmer
air mass
Simple model of a
cyclone with a cold
front extending to
the south
Cold fronts
steeper and faster
than warm fronts
- the warm air is lifted much faster - more violent weather
Air Masses and Weather Patterns
Fronts
Warm front
Stationary front
Cold front
Occluded fronts
Cold type
Warm type
Occluded fronts and temperature isolines
Air Masses and Weather Patterns
Development of an occluded front
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
Life cycle of a wave cyclone
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
Weather patterns associated with a mid-latitude cyclone
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Masses and Weather Patterns
Distribution of geopotential height on the 500-mb surface at 12-h intervals
The distribution of geopotential height on the 500-mb surface at 12-h intervals beginning at
00 GCT 19 November 1964 in (a) and ending at 12 GCT 20 Novemver 1964 in (d). Contours are
labeled in tens of meters.
Air Masses and Weather Patterns
Support of cyclones and anticyclones by divergence
and convergence aloft
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air masses, fronts and cyclones
Summary
• Air masses and source regions
• Cold front - cold air advances on cool/warm air
• Warm front - warm air advances on cooler air
• Weather at fronts
• Polar front as region generating mid latitude cyclones
• Cyclone life cycle - generation, fronts, occlusion, dissipation
• Weather within a cyclone
• Role of upper air flow
Next: The global water cycle
Reading:
Smithson, P., K. Addison, and K. Atkinson, 2002. Chap 5, Chap. 14, 281-293
Briggs, D., P. Smithson, K. Addison, and K. Atkinson, 1997. Chap. 7, Chap. 14, pp. 234-240.
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