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10/18/14 Air Mass Classifica1on •  Air mass: regional-­‐scale (1000s-­‐km) air mass w/ uniform moisture and temp. characteris1cs –  Named based on source region they most recently hanged at Moisture, Temperature & Stability Characteris1cs •  Con1nental vs. Mari1me •  Tropical vs. Polar •  Arc1c North American Air Mass Migra1on Why do air masses migrate seasonally? During what season would there be the strongest contrasts? Air Mass Modifica1on •  Air masses modified by the surface they pass over –  Through the exchange of heat/moisture –  Radia1onal hea1ng or cooling •  cP air mass travels from Canada southward –  Sun warms snow-­‐free ground, warmer ground heats the boQom of the air mass, heat distributed ver1cally –  If enough moisture can be unstable •  cP air mass traveling over snow-­‐covered ground experiences less modifica1on –  Incoming solar radia1on is reflected, not absorbed –  Strong longwave cooling at night from snow surface 6 © AMS Tropical Air Mass Modifica1on Front: Clashing of Air Masses extra credit… •  Front: boundary between dis1nctly different air masses –  Denotes the leading edge of abrupt change •  Demarcated by density gradient (mostly temperature, humidity too) •  Trough in pressure paQern •  Surface convergence •  Change in wind direc1on •  Warm air over cold land : increases stability •  Can undergo significant modifica1on through orographic upliWing –  Rising air cools, heavy precipita1on on the windward slopes –  Leeward descent leads to adiaba1c warming and drying –  Air mass emerges considerably milder and drier 1 10/18/14 Fronts Surface weather map Front types tell us something about the air mass behind the front and/or movement of weather systems Cold front-­‐ cold air advances toward warm air Warm front-­‐ warm, moist, unstable air approaches cool air Occluded front-­‐ cold front overtakes a warm front Sta:onary front-­‐ has essen1ally no movement, air mass moves parallel to front Primary Mechanisms for Heat Transport In Mid-­‐La1tudes (30-­‐70 degrees la1tude) Frontal LiWing •  Cold Fronts (shark teeth, “bi:ng” cold” –  Cold air forces warm air aloW –  400 km wide (250 mi) –  Sharper gradient = stronger forcing Cold Air •  Warm Fronts (puppy tongues, “warm”) –  Warm air moves up and over cold air –  1000 km wide (600 mi) –  Much more gradual Warm Air Sta1onary Front Warm Front: warm air advances on cool air •  Stalled gradient in air masses (typically ini1al stages of cyclone) •  Preferen1ally forms where temperature gradient is strong (e.g., lee of mountain range, off coast of Eastern US) Light, but persistent precipita1on on cold side Clouds thicken and base drops as front approaches (1000-­‐km) Steady rain where upliW is maximized; however mainly stable air Overrunning: cloud shield © AMS © AMS 2 10/18/14 Cold Front: cold air displaces warm air Occlusion Much stronger temperature contrast across cold front + narrower region Moist air in the warm-­‐air sector plus warm surface condi1ons primes the atmosphere for instability Cold front overtakes a warm front, oWen typical of mature stage of cyclone –  Reduced temperature contrast between air behind the cold front, and air ahead of the warm front –  Cold occlusion: More prevalent in the East as cold con1nental air advances –  Warm occlusion: More prevalent in the West as polar mari1me air advances © AMS Midla1tude Cyclone •  Midla:tude cyclone: a well-­‐organized migra1ng low-­‐pressure system in the midla1tudes that contains warm and cold fronts -­‐ comma-­‐shaped cloud: mature cyclone -­‐ the primary source of precipita1on for much of the West Open Wave Cyclone: Idealized life cycle of a mid-­‐la1tude cyclone Norwegian Cyclone Model 3 10/18/14 Cyclogenesis: Birth of a Cyclone 3-­‐D View of Cyclones and An1cyclones Development: convergence at the surface, divergence aloW • 
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Occurs downstream of trough. Why??? Surface pressure falls, cyclonic mo1on begins Polar front pushes southward as a cold front west of sfc L Warm air pushes northward east of the low, Open Wave Cyclone DIVERGENCE CONVERGENCE Mature Cyclone Jet provides steering • 
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Central pressure con1nues to drop, winds strengthen Upper-­‐level trough deepens Fronts form a pronounced wave paQern and comma cloud Extensive stra1form cloudiness appears north of the warm front •  Cold front catches up to warm front, occlusion forms •  Cold air behind cold front eats up the warm air sector •  Upper level paQern shows closed circula1on and is directly over the surface low (ver1cally stacked), looses horizontal divergence Death and Rebirth triple point •  Occlusion mixes warm and cold air = no more temp gradient •  Triple point favors development of a secondary cyclone 4 10/18/14 Warm conveyor belt •  Warm + moist air moves northward to warm sector at low levels •  Ascent leads to broad region of clouds/precipita1on north of the warm front Dry conveyor belt •  Originates upstream of trough from high in the troposphere and low stratosphere, very dry air •  One branch descends southward behind the cold front; other forms the dry slot that separa1ng head & tail of comma cloud The Weather Experience Extratropical Cyclones Tracks Frontal Sectors Track B puts Chicago on the cold side with no frontal passage Cold conveyor belt •  Originates north of the warm front, rises at it moves west •  Forms the comma cloud and produces precipita1on •  Turns clockwise at upper levels and follows westerly flow aloW •  Track A puts Chicago on the warm side with passage of the warm and cold fronts •  Cyclone moves in direc1on of 500mb flow at half-­‐speed •  May die out over intermountain west and regenerate •  Nor’easters intensify off the E. coast, then track northeast 5 10/18/14 Thermal Wind and Temperature Advec1on Dashed: Temperature Isolines Solid: Height Isolines Only in regions where the isolines of geopoten1al height cut across the isotherms do we have temperature advec1on Where is CAA? Where is WAA? ∂T
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WAA CAA Drylines are not warm fronts or cold fronts, but represent a narrow boundary where there is a steep horizontal change in moisture as indicated by a rapid change in dew-­‐point temperature. A dryline separates warm, moist mari1me tropical (mT) on its eastern side from hot, dry con1nental tropical air (cT) on its western side. Veering and Backing Backing: Change of wind direc1on with height is counterclockwise. This leads to cold air advec:on. Veering: Change of wind direc1on with height is clockwise. This leads to warm air advec:on. Influence on Stability Cold Air ∇T
V500 VT VML Warm Air V1000 VML V500 V1000 Γd
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Warm advec1on destabilized VT Cold Air θ
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Cold advec1on stabilized If there is warm advec1on in the lower layer, or cold advec1on in the upper layer, or both, the sounding will become more unstable. Vor1city Defined: local spin of fluid or rota1on of parcels within a fluid Why should we care about diagnos1c and prognos1c equa1ons of vor1city? (1)  Good indicator of weather system strength and loca1on (loca1ng center instead of guessing with troughs) (2)  Fundamental variable related to •  Growth/decay of waves in atmosphere =movement of systems •  Surface pressure tendency = evolu1on of surface low/high •  Ver1cal mo1on = clouds, precip, etc. 6