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Fronts: Chapter 9: Weather Patterns Mid Latitude Cyclones: extratropical cyclones, Nor’easters -boundary surfaces that separate air masses of different densities temperature Region southern FL to Alaska moisture Lifecycle and associated weather -15-200km wide Regional Influence -line on the weather map Polar Front Theory: Fronts: -Norwegian Cyclone Model -surface slope is gradual -WWI -describes birth, growth and decay of cyclones Generates cyclone at the surface -warm air overlies cold air -air masses move at different speeds -one air mass will advance -clashing produces weather Midlatitude Cyclone: Fronts: -primary weather producers -warm air is always forced aloft -low pressure systems, 1000 km dia. -overrunning: warm air gliding on top of cold air -counterclockwise circulation toward center -warm and cold fronts -upward flow initiates precipitation 1 5 Types of Fronts: Warm Front: -warm -precipitation and temperature profile -Cold -Stationary -Occluded -Dryline Warm Front: Cold Front: -warm air mass is advancing, displaces colder air at the surface -cold air mass is advancing, displaces warmer air at the surface -red line with half circles -cold air more dense forces warm air aloft -gradual slope (1:200) -blue line with triangles -speed = 25-35 km/hr -steeper frontal boundary (1:100) -speed = 35-50 km/hr 1 1 200 100 Warm Front: Cold Front: -adiabatic cooling -precipitation follows the passage of the front -cloud sequence -steeper frontal boundary & speed: more violent weather -gradual slope & slow advance: widespread, light precip. long duration -altocumulus and cumulonimbus -precipitation precedes front -E to SW wind shift 1 1 200 100 2 Cold Front: Occluded Front: -same lifting of warm air, only quicker over a shorter distance -cold type occluded front -heavy downpours, short duration, narrow band of precipitation advancing air is colder than air mass it is overtaking -passage of front temperature drops, polar air, clear skies common east of the Rockies (cP overtakes mP) -SW to NW wind shift 1 100 Stationary Front: Occluded Front: -little to no horizontal movement across the front by either air mass -warm type occluded front -lateral motion advancing air is warmer than air mass it is overtaking -overrunning, light precipitation Pacific coast (mP overtakes cP) Occluded Front: Drylines: -rapid moving cold front overtakes a warm front -fronts based on moisture content -warm air driven aloft -not necessarily a difference in temperature -precipitation from wedging -dry air forces moist air aloft -strong temperature gradients -cT (southwest US) displaces mT (Gulf) spring and summer -intense weather -severe T-storms from Texas to Nebraska 3 Life of a Midlatitiude Cyclone (2-10 days) 6 basic stages Front develops Wave develops Cyclonic circulation established Cyclonic circulation established Warm air invades north (warm front) Cold air advances south (cold front) Low pressure at the crest Occlusion begins Occluded front developed Cyclone dissipates Cyclogenesis = cyclone formation Life of a Midlatitiude Cyclone Front develops Occlusion: beginning of the end Stationary front Cold front advances past the warm front cP on the North (easterlies) mT on the South (westerlies) Strong temperature gradients, storm intensifies Life of a Midlatitiude Cyclone Life of a Midlatitiude Cyclone Wave develops Occluded front developed wavelength (O 100 km) blizzards, strong winds wave steepens energy is being exhausted within a few days warm front driven aloft 4 Life of a Midlatitiude Cyclone Idealized Weather of a Midlatitiude Cyclone Cyclone dissipates A) Cirrus clouds cold air mass surrounds the low at the surface horizontal temperature gradient eliminated Front ~1200 km away Warm front advances, cloud base lowers (cirrostratus, altostratus, stratus) Idealized Weather of a Midlatitiude Cyclone B) Nibostratus clouds Light precipitation, gets heavier as front advances Temperatures increase Winds shift from an easterly direction to a southerly direction Idealized Weather of a Midlatitiude Cyclone Idealized Weather of a Midlatitiude Cyclone Cyclone generally move from west to east Steered by the general westerly circulation Right side of the storm passes first C) mT Air mass Warm, moist Clear skies Southerly winds 5 Idealized Weather of a Midlatitiude Cyclone Veering: winds rotate/shift in a clockwise direction South of the storm D) Cumulonimbus Clouds Skies will clear as you move into the mT region or cP region Heavy rains Violent weather as cold front approaches Idealized Weather of a Midlatitiude Cyclone Backing: winds rotate/shift in a counterclockwise direction North of the storm, pass through the occlusion E) Temperatures Drop Cold with precipitation cP Air mass Descending air Clear skies Low precipitation Wind shifts from southerly to westerly. Idealized Weather of a Midlatitiude Cyclone F-G) Occluded front region Temperature remains cool Precipitation beneath the front Type of precipitation depends on the lower temperature profile OF moves slower than the warm or cold fronts System rotates 6 Surface cyclone: centered below the jet stream downwind of a upper level trough Polar Front Theory developed from surface observations Cyclonic & Anticyclonic Circulation Wave develops Cyclones and anticyclones are typically found together 1. Topographic irregularities (Mnts) Surface divergence under an anticyclones feeds surface convergence under the cyclone 2. Temperature contrasts (land/sea) Divergence aloft must be greater than convergence at the surface under a cyclone 3. Ocean current influence (hurricanes) Regions of Cyclogenesis Conditions aloft - Topographic irregularities (Mnts) Surface cyclones are preceded by intensification of airflow aloft - Temperature contrasts (land/sea) Zonal airflow (W-E) little cyclonic activity - Ocean current influence (hurricanes) Longitudinal airflow (N-S) increase cyclonic activity 7 Storm Tracks: Patterns of Movement Warm Conveyor Belt: In general east to northeast track mT air moves toward the middle of cylone, north over mP or cP air Ascends to middle troposphere (JS) joins the general westerly flow Primary producer of precipitation Troposphere Most of the north Pacific storms that influence the west coast do not make it over the Rockies in tact (redevelop) Storm Tracks: Patterns of Movement 12km = avg. thickness 16km = tropics 09km = poles Cold Conveyor Belt: Originates at surface ahead of the warm front Flows westerly around the center and ascends, precipitation Air joins the general westerly circulation aloft Nor’easter, mP air is entrained from the North Atlantic Modern View: The Conveyor Belt Model Dry Conveyor Belt: 3 intersecting air streams (belts) Originates at the uppermost troposphere 2 belts originate at the surface and ascend Cold and dry 1 belt originates aloft and descends Splits and descends behind the cold front 8 Nor’easters 10/21/04 – 10/27/04 Hs = 3.8 to 4 m Waves T = 14 sec. Max setup = ~ 65 cm (~2ft) November 1950 La Guardia Airport FDR Drive December 1992 Ref: Bloomfield, J., M. Smith and N. Thompson, 1999. Hot Nights in the City. Environmental Defense Fund, NY. Tides 10/21/04 10/23/04 10/25/04 10/27/04 The Perfect Storm (12 UTC October 31,1991) Regions of Cyclogenesis Level 4 Alberta East Coast Nevada Colorado FL/Bahamas Gulf Davis and FitzGerald, 2004 9 General Observations by Month Nor’easter Intensity Scale 1347 Hindcast Storms Off North Carolina (1942-1984) Number of Storms: April (135) March (128) September (125) Level 3 Storms (194): March (32) January (27) December (25) Level 4 Storms (30): March, October, December (5) January (4) Relative Power = (Hosig(m))2 x Storm Duration (hr) Storm Class Hosig (m) Duration (hrs) Range (m2hr) 1 Weak 2.0 8 Power <= 71 2 Moderate 2.5 10 71 < Power <= 163 3 Significant 3.3 34 163 < Power <= 929 4 Severe 5.0 63 929 < Power <= 2322 5 Extreme 7.0 96 Power > 2322 Dolan & Davis, 1992, Journal of Coastal Research. General Observations by Winter Season (Oct-Apr) 1991 - 2005 Number of Storms: 92-93 (77) 97-98 (71) 04-05 (68) Level 3 Storms (194): 04-05 (18) 02-03 (16) 97-98 (14) Level 4 Storms (30): 02-03 (4) 93-94, 94-95, 97-98 (3) Severe Winters: 97-98 (14,3,1) 02-03 (16,4) 04-05 (18,2) Nor’easter Intensity Scale 900 800 Number of Storms 700 600 1189 Level % Hosig (m) 853 1 (Weak) 72 1.8 hrs 4 110 2 (Moderate) 9 2.5 20 194 3 (Significant) 16 3.2 39 500 30 4 (Extreme) 2-3 4.0 94 400 2 5 (Severe) <1 4.4 146 300 200 100 0 1 2 3 Level 4 5 10