chapter 1: anatomy of a cyclone covers … basic state of the atmosphere weather maps air masses...
TRANSCRIPT
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Chapter 1: Anatomy of a cyclone
Covers …
Basic state of the atmosphereWeather mapsAir massesFronts
Much of the theory from the Norwegian School, Tor Bergeron, circa 1928.
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Weather Scales
From http://eumetrain.org/synoptic_textbook.html
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Synoptic Scale Meteorology
From http://eumetrain.org/synoptic_textbook.html
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Air can be thought of as an ideal gas
PV=Nkt Ideal gas law in fundamental form.Assumes molecules are point like (no volume) and interact only at short range
Average kinetic energy of each molecule is: <1/2 mv2> = 3/2 kTMore massive molecules (mass m) move slower on average.
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Pressure drops with height
It’s like being in a swimming pool; the mass of water per unit area above youdetermines the pressure you feel.
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Pressure changes horizontally across the Earth Winds
Newton’s laws of motion (cast for fluids as the Navier Stokes Equation),mass continuity, and the ideal gas law are used to understand fluidmotions.
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Station Model for Weather Symbols
1 Knot = 1 nautical mile per hour = 1.151 mph
Coded sea level pressure > 500, place a 9 in front.Coded sea level pressure < 500, place a 10 in front. So above, 229 becomes 1022.9 hPa = 1022.9 mbar. 1 hPa = 100 Pa.
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Upper Level Station Model
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Chapter 11Ahrens
MeteorologyReview
Air Masses and Fronts
9
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Air MassesExtremely large body of air whose
temperature and humidity are similar in any horizontal direction.
Source Regions: area where air mass originates, usually flat and uniform composition with light surface winds
Ideal source regions are usually those areas dominated by surface H.
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A cold air mass is dominating weather over much of the US
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Air MassesClassification
Classification based upon temperature and humidity related to its source region.P = polarT = tropicalA = Arcticm = maritimec = continental
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Air MassesNorth America cP and cA
Source region: N. Canada, AlaskaDry, cold, stable (A more extreme)
Topic: Lake Effect SnowcP air passes over unfrozen lake, absorbs moisture and drops snow on leeward side of lake
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Typical Air Masses around the World
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Air Masses
North American mPSource region:
North Pacific, North Atlantic
Cool, moist, unstable
North American mTSource region: Gulf
of Mexico, Caribbean, SE Pacific
Wet, warm, unstable
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Air Masses
North American cTSource Region: SW
US, Mexican Plateau
Hot, dry, stable
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Two different cold events when Arctic air intruded into the lower 48
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Solar radiation mixes atmosphere
Radiation inversions and/orsubsidence from high pressure are associated with inversions
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Cold polar air gets modified as in intrudes on warm ocean
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snow
Freezing rain Light rain
Invasion of cold, moist maritime polar air
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Called the Pineapple Express
January 1st 1997 Reno flooded: Warm tropical air brought rain on snow in themountains: example of atmospheric river. Note the low off the coast of Oregon.
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Upper level flow
Upper level trough
Upper level ridge
Maximum Temperatures
Unseasonably hot spell, Eastern US, 15-20 April 1976
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Fronts Transition zone between two air
masses of different densities Identification on Charts
1. Sharp temperature change2. Sharp change in dew point3. Shift in wind direction4. Sharp pressure change5. Clouds and precipitation
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Types of Fronts
Cold front: Cold air advancing, warm air retreating.Warm front: Warm air advancing, cold air retreating.Stationary front: Boundary between two air masses is stationary, or nearly so.Occluded front: Separates air masses that have only a small temperature contrast,typically separates cold and cool air masses.
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February 2003 Cyclone Surface Map
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February 2003 Cyclone 500 mb Level Map, Approximately 5.5 km altitude. 552=5520 meters.
T in C, Tdew as depression, height in decimeters (tens of meters). Filled circleshave dewpoint depression < 5 C, probably cloudy. Troughs are cold, ridges warm.
Short waves
Isotherms are dashed lines
Long wave pattern
Baroclinic: Isotherms crossisoheight contours. Barotropic: They are parallel.(rare).
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Shape of cold and very cold fronts
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A surface weather map showing surface-pressure systems, air masses, fronts, and isobars
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FrontsStationary
Front with no movementWinds parallel but opposite directionVariable weatherAlternating red and blue line with
blue triangles and red semi-circlesOften a cold core sits at their the
surface
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FrontsCold
Cold, dry stable air replaces warm, moist unstable air
Clouds of vertical developmentThunderstorms, squall lines (line of thunder storms)
Blue line with blue triangles
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Surface weather associated with the cold front situated in the southern United States
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Radar showing precip along frontal boundary
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Frontolysis: as temperature contrast lessens the front weakens and dissipates.Frontogenesis: if the temperature contrast increases the front strengthens.
Vertical view of the weather across the cold front
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Weak Cold Front 21 November intensifies over warm ocean water 22 November
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Unusual ‘back door’ cold front
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FrontsWarm
Warm, moist unstable air overrides cold, dry stable air
Horizontal cloud development with steady rain
Red line with red semi-circles
Topic: Dry LineNot a cold or warm front but a narrow
boundary of steep change in dew point. It separates moist air from dry air
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FrontsTopic: Wavy Warm Front
Mountain blocking path of cold air (cold air damming) causes wave shape
Occluded FrontCold front catches up to and over
takes a warm front Cold occlusion, warm occlusionPurple line with purple triangles and
semi-circles
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The formation of a cold-occluded front. The faster-moving cold front (a).
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(b) catches up to the slower-moving warm front and
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(c). forces it to rise off the ground
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d) Green-shaded area in represents precipitation.
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Warm occluded front formation:
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The formation of a warm-type occludedfront. The faster-moving cold front in (a) overtakes the slower-movingwarm front in (b). The lighter air behind the cold front rises up andover the denser air ahead of the warm front. Diagram (c) shows a surfacemap of the situation.
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Diagram (c) shows a surfacemap of the situation.
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A visible satellite image showing a mid-latitude cyclonic storm with its weather fronts over the Atlantic Ocean during March, 2005. Superimposed on the photo is the position of the surface cold front, warm front, and occluded front. Precipitation symbols indicatewhere precipitation is reaching the surface
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FrontsUpper-Air Fronts
Front aloftTropopause dips downward and folds under the Polar jet
Impacts surface weather
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Upper Air front (Upper level front) Tropopause dips downward and foldsunder the polar jet.
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Stages in the life cycle of an extra-tropical cyclone. 500 hPa contour as dashed lines
Young: Cyclones form along the polar frontDivergence occurs near the upper level short wave trough (left), promoting the cyclone,the low formsMiddle: Cold and warm fronts advance, the small wave when young amplifies to forman open wave cyclone; trough forms to the left as cold air dives south, ridge as warmair intrudes towards the north; cyclone is steered NE by the 500 mb winds; upper leveltrough is tilted to the west of the surface lowMature: Cold front advances more quickly than warm front; warm air is forced aloft; occlusion occurs; now a cold core cyclone; 500 hPa trough is centered over the surfacelow and the cyclone decays.
youngMiddle age Mature stage
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Birth of a storm
From http://eumetrain.org/synoptic_textbook.html
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Mature stage of ideal cyclone
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Occluded front
New low forming
Map of old age for front
Cold continental airadvects over warm ocean
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500 mb map showing trough slightly west of the surface lowfor this old age cyclone
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Summary of Jet Action
NORTH SOUTH
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Summary of Jet Action: Top figure shows top view from above of jet maximum.Bottom view shows vertical motions for divergence and convergence aloft.
NORTH
South
WestEast