Winds, Fronts, And Cyclogenesis
3/3/2010Winds, Fronts, and CyclogenesisATS 351Lecture 10Outline Atmospheric pressure Forces that affect the wind PGF, Coriolis, Centripetal, Friction Vertical Motion Fronts Mid-Latitude CyclogenesisAtmospheric Pressure Ideal Gas Law p RT p: pressure; : density; R: constant (287 J/kg/K); T:temperature It takes a shorter column of dense, cold air to exertthe same pressure as a taller column of less dense,warm air Warm air aloft is normally associated with highatmospheric pressure and cold air aloft with lowatmospheric pressure At a given level, more molecules exist above warm airthan cold air higher pressure1
3/3/2010Surface and Upper-Level Charts Sea-level pressure chart:constant height Upper level or isobaricchart: constant pressuresurface (i.e. 500mb) High heights correspondto higher than normalpressures at any givenlatitude and vice versaCold air aloft: low heights or low pressureWarm air aloft: high heights or high pressuresRidge: where isobars bulge northwardTrough: where isobars bulge southwardIn Northern Hemisphere: High pressure: anticyclone (winds blow clockwise andoutward from center) Low pressure: mid-latitude cyclone (winds blowcounter clockwise and inward towards center) 2
3/3/2010Newton’s 2nd Law of Motion 2nd Law: F ma F: net force; m: mass of object; a: acceleration At a constant mass, the force acting on the object isdirectly related to the acceleration that is produced. The object accelerates in the direction of the netforce acting on it Therefore, to identify which way the wind willblow, we must identify all the forces that affect themovement of airForces that Affect Wind Pressure gradient force (PGF) Coriolis forceCentripetal forceFrictionPressure Gradient Force Pressure gradient p/d p: difference in pressure d: distance PGF has direction & magnitude Direction: directed from high to low pressure at rightangles to isobars Magnitude: directly related to pressure gradient Tight lines (strong PGF) stronger wind PGF is the force that causes the wind to blow3
3/3/2010Coriolis Force Apparent deflection due to rotation of the Earth Right in northern hemisphere and left insouthern hemisphere Stronger wind greater deflection No Coriolis effect at the equator greatest atpoles. Only influence direction, not speed Only has significant impact over long distances Coriolis 2Ωsin 4
3/3/2010Geostrophic Winds When the force of PGF andCoriolis are balanced Travel parallel to isobars at aconstant speed An approximation sinceisobars are rarely straight inreal atmosphere but closeenough to understand windsaloft Spacing of isobars indicatesspeed Close fast, spread out slowGradient Winds & Centripetal Force Gradient wind parallel to curved isobars above thelevel of frictional influence (winds aloft) An object accelerates when it is changing speedand/or direction. Therefore, gradient wind blowing around a lowpressure center is constantly accelerating Centripetal acceleration: directed at right angles tothe wind, inward toward center of low Centripetal force: inward-directed force Results from an imbalance between the Coriolis forceand the PGF Cyclonic flow: PGF CF Anticyclonic flow: PGF CF5
3/3/2010Zonal &Meridional Winds Zonal winds: oriented in the W-E direction(parallel to latitude) Moves clouds, storms, surface anticyclonesrapidly from west to east Meridional winds: oriented in a N-Strajectory Surface storms move slowly and often intensifymajor storm systemsSurface and Upper-Level Winds Winds on Upper-level Charts Winds parallel to contour lines and flow west to east Heights decrease from north to south Surface Winds Winds normally cross isobars and blow more slowlythan winds aloft Friction: reduces the wind speed which in turn decreases theCoriolis effect Friction layer: surface to about 1000m (3300ft) Winds cross the isobars at about 30 into low pressureand out of high pressure PGF at surface is balanced by the sum offriction and Coriolis force Surface winds into low and outward fromhigh6
3/3/2010Winds & Vertical Motions Since surface winds blow into the center ofa low, they are converging and that air hasto go somewhere slowly rises Vice versa for winds blowing outward from H Therefore, a surface low has convergence atsurface and divergence aloft and a surfacehigh has the opposite.Hydrostatic Balance There is always a strong PGF directed upward Gravity balances the upward PGF When they are equal, hydrostatic equilibrium exists Good approximation for atmosphere with slowvertical movements Is not valid for violent thunderstorms andtornadoes.7
3/3/2010Air Masses of North America Continental Polar (cP) & Arctic (cA) – Cold, dry, stable air in winter– In summer, cP air mass usually brings relief from oppressive heat in centraland eastern US Maritime Polar (mP) – In winter, cP/cA air mass is carried over Pacific Ocean where moisture andwarmth is added mP at Pacific Coast is cool, moist, and conditionally unstable East of Rockies - brings fair weather and cooler temperatures(moisture has been removed by mountains) – East coast mP air mass: originates in N. Atlantic Storms may develop (heavy rain or snow, coastal flooding) Late winter, early springAir Masses and Fronts A front is a transition zone between two air masses ofdifferent densities Fronts extend both horizontally and verticallyCold Front Cold, dry stable polar air (cP) isreplacing warm, moist,conditionally unstable subtropicalair (mT) Steep vertical boundary due tosurface friction slowing down thesurface front Has strong vertical ascent along thesurface front Strong upper level westerlies pushice crystals far ahead of the front,creating Ci and Cs in advance ofthe front. Cold, dense air wedges underwarm air, forcing the warm airupward, producing cumuliformclouds Can cause strong convection,severe weather, and squalllines. Air cools quickly behind thefront8
3/3/2010Cold Front Rising motion causes decreasedsurface pressure ahead of the front– On a surface pressure map,frontal location can be seen by“kinks” in the isobars, changesin wind direction from asouthwesterly to a northwesterlywind, and decreases intemperature.– Pressure is lowest at the surfacefront.On weather maps, cold fronts areindicated by blue lines with trianglespointing in the direction of frontalmotion (towards warmer air) Cold FrontBeforeWhileAfterWindsS-SWGusty, shiftingW-NWTemperatureWarmSudden dropSteady dropPressureSteady fallMin, thensharp riseSteady riseCloudsIncreasing, Ci,Cs, CbCbCuPrecipitationBrief showersHeavy rains,severeweatherShowers, thenclearingVisibilityFair to poorPoorGoodDew PointHigh, remainssteadySharp droploweringWarm Front Occurs at the leading edge of an advancing warm, moist,subtropical air mass (mT) from the Gulf replacing aretreating cold, maritime, polar air mass from the NorthAtlantic (mP) Slowly advances as cold air recedes; moves at about halfthe speed of an average cold front– Speed may increase due to daytime mixing– Speed may decrease due to nighttime radiationalcooling Smaller vertical slope than cold front9
3/3/2010Warm Front Warmer, less-denseair rides up and overthe colder, moredense surface air– “Overrunning”– Produces cloudsand precipitationwell in advance ofthe frontWarm ureCool-coldSlowly warmingSteady riseWarmer, thensteadyPressureFallingLeveling offSlight rise,followed by fallClouds(in order) Ci, Cs,As, Ns, St, fog(Cb in summer) Stratus-typeClearing withscattered ScPrecipitationLight-tomoderateDrizzle or noneUsually noneVisibilityPoorImprovingFairDew PointSteady riseSteadyRise, thensteadyStationary Front Essentially no movement Surface winds blowparallel to front, but inopposite directions oneither side of it Separates two air masses Seen often along mountainranges when cold aircannot make it over theridge10
3/3/2010Hourly surface observations at Gage, Oklahoma showing the passage of aprimary and secondary cold front (left) and at Bowling Green, Kentuckyshowing the passage of a warm front (right).Source: Wallace and Hobbs, 2006.Occluded Fronts Cold fronts generally movefaster than warm fronts Occlusion occurs whencold front catches up to andovertakes a warm front Occlusions can be warm orcoldDry Lines Think of a dry line as a moistureboundary Separates warm, humid mT air inthe southern Great Plains fromwarm, dry cT air Drier air behind dry lines lifts themoist air ahead of it, triggeringstorms along and ahead of it– Induces lifting along front– Often produces severethunderstorms in OK & TX Unique to southern great plainsof US because of the Rockymountains and the Gulf ofMexico11
3/3/2010A guide to the symbols for weather fronts that may be found on a weather map:#1 cold front#2 warm front#3 stationary front#4 occluded front#5 surface trough#6 squall/shear line#7 dry line#8 tropical waveFeatures of a Mid-latitude Cyclone Deep low pressure areawith attached cold andwarm fronts Often an occlusionforms, the triple pointlending to theformation of severeweather Precipitation associatedwith the cold and warmfronts organizes intypical “comma cloud”structureStages in Wave CycloneDevelopment12
3/3/2010Polar Front Theory Initially, there is a stationary front that acts as theboundary separating cold, continental polar air fromwarm, maritime tropical air Winds blow parallel to this front on either side Polar Fronts are discontinuousCentral PressureCyclogenesis A wave forms on the front due to a shortwavedisturbance– Frontal Wave– Incipient Cyclone The front develops a "kink" where the wave isdeveloping Precipitation will begin to develop along the front– Overrunning and liftingStrengthening The cyclonic circulation around the low becomes moredefined The central pressure intensifies The cold front and warm front have more organizedmotion Cyclone usually pushed east or northeast by the windsaloft13
3/3/2010Mature Cyclone The cold front catches up with the warm front and anocclusion forms The cyclone is at its strongest at this point Severe weather often develops near the “ triple point”Dissipation The occlusion grows with time Eventually, the occlusion is so great that the supply of warm, moistair into the low is cut off– Cold air on both sides When this happens, the system starts to dissipate14
Air Masses and Fronts A front is a transition zone between two air masses of different densities Fronts extend both horizontally and vertically Cold Front Cold, dry stable polar air (cP) is replacing warm, moist, conditionally unstable subtropical air (mT) Steep vertical boun
o Air masses of North America Fronts o Stationary fronts o Cold fronts o Warm fronts o Fronts and the jet stream o Frontogenesis o Drylines o Occluded fronts . - transient weather systems (Rossby waves) - small scale f
Fronts are the boundaries between two air masses. Fronts are the basic building blocks of weather systems. Fronts occur where two large air masses collide at the earth's surface. Each air mass has a different temperature associated with it. Fronts are caused by winds moving one air mass
Global Winds Global winds are winds that occur in belts that go all around the planet. You can see them in Figure below. Like local winds, global winds are caused by unequal heating of the atmosphere. Global winds occur in belts around the globe. Heating and Global Winds Earth is
fronts o fronts: a boundary between air masses there are three types of fronts o cold fronts: move quickly. cold air pushes against warm air, causing the warm air to rise. cumulus clouds heavy storms a
Stationary fronts behave like warm fronts, but are more quiescent. Many times the winds on both sides of a stationary front are parallel to the front and have opposite direction. Typically stationary fronts form when polar air masses are modified significantly so as
Wind direction There are three pairs of major global wind belts on Earth: the polar easterlies, the westerlies, and the trade winds. Why Do Global Winds Curve? Remember that pressure differences can cause air to move and form winds. If Earth did not rotate, these winds
Using spectral analysis and filtering for each of the five dominant wave types in the equatorial region, Frank and Roundy (2006) created composite analyses of wave fields with respect to genesis location in each basin. About 60-70 % of tropical cyclogenesis cases occurred within the negative MJO-filtered OLR anomalies in each of the six basins.
This textbook is designed for use on ten- or twelve-week introductory courses on English phonology of the sort taught in the first year of many English Language and Linguistics degrees, in British and American universities. Students on such courses can struggle with phonetics and phonology; it is sometimes difficult to see past the new .
