2Winds, Fronts, and Cyclogenesis ATS 351Lecture 10
3Outline Atmospheric pressure Forces that affect the wind PGF, Coriolis, Centripetal, FrictionVertical MotionFrontsMid-Latitude Cyclogenesis
4Atmospheric Pressure Ideal Gas Law p = RT p: pressure; : density; R: constant (287 J/kg/K); T: temperatureIt takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm airWarm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressureAt a given level, more molecules exist above warm air than cold air = higher pressureHold Pressure constant: as temperature increases, the gas will become LESS denseHold temperature constant: linear relationship so as pressure increases, so does densityair at a higher pressure is more dense than air at a lower pressureAir above a region of surface HIGH is more dense than air above a surface LOW
5Surface and Upper-Level Charts Sea-level pressure chart: constant heightUpper level or isobaric chart: constant pressure surface (i.e. 500mb)High heights correspond to higher than normal pressures at any given latitude and vice versa
6Cold air aloft: low heights or low pressure Warm air aloft: high heights or high pressuresRidge: where isobars bulge northwardTrough: where isobars bulge southward
7In Northern Hemisphere: Notice wind direction around L and H on surface mapPoint out location of trough and ridge on 500mb mapPoint out that surface map is ISOBARS (pressure lines) whereas the upper-level 500mb map is HEIGHTS at 500mb (average value of 500mb is 5600)Point out how winds are flowing on the upper-level chart = blow parallel to contour lines (more on this later), whereas surface winds cross isobars (more on this later too)In Northern Hemisphere:High pressure: anticyclone (winds blow clockwise and outward from center)Low pressure: mid-latitude cyclone (winds blow counter clockwise and inward towards center)
8Newton’s 2nd Law of Motion F = maF: net force; m: mass of object; a: accelerationAt a constant mass, the force acting on the object is directly related to the acceleration that is produced.The object accelerates in the direction of the net force acting on itTherefore, to identify which way the wind will blow, we must identify all the forces that affect the movement of airObject accelerates in the direction of the NET FORCE acting on it. This is important when we begin to talk about winds and which direction they are going to blow
9Forces that Affect Wind Pressure gradient force (PGF)Coriolis forceCentripetal forceFriction
10Pressure Gradient Force Pressure gradient = ∆p/d∆p: difference in pressured: distancePGF has direction & magnitudeDirection: directed from high to low pressure at right angles to isobarsMagnitude: directly related to pressure gradientTight lines (strong PGF) stronger windPGF is the force that causes the wind to blowMAYBE DO WATER LEVEL EXPERIMENT: Shows that the greater the pressure difference, the stronger the force and the faster the water moves = same happens with air
11Point out direction and magnitude of PGF vectors Basic run-down of how to calculate PGF. Take one pressure (1024) minus the second (1016) and divide it by the distance using the scale
13Coriolis Force Apparent deflection due to rotation of the Earth Right in northern hemisphere and left in southern hemisphereStronger wind = greater deflectionNo Coriolis effect at the equator greatest at poles.Only influence direction, not speedOnly has significant impact over long distancesCoriolis = 2vΩsinM = mass ; Omega = earth’s angular spin rate; V = velocity of object; phi = latitude
14Geostrophic Winds When the force of PGF and Coriolis are balanced Travel parallel to isobars at a constant speedAn approximation since isobars are rarely straight in real atmosphere but close enough to understand winds aloftSpacing of isobars indicates speedClose = fast, spread out = slowThe wind starts to blow at point 1 and the Coriolis begins to deflect it right (in the NH) while PGF stays the same. When the strength of the Coriolis eventually equals the PGF = GEOSTROPHIC WINDS that blow parallel to the isobars.Point out how the vectors of PGF and Coriolis are the same at step 5
15Gradient Winds & Centripetal Force Gradient wind parallel to curved isobars above the level of frictional influence (winds aloft)An object accelerates when it is changing speed and/or direction.Therefore, gradient wind blowing around a low pressure center is constantly acceleratingCentripetal acceleration: directed at right angles to the wind, inward toward center of lowCentripetal force: inward-directed forceResults from an imbalance between the Coriolis force and the PGFSince gradient wind is constantly acceleration - centripetal acceleration is produced
16Cyclonic flow: PGF > CF Anticyclonic flow: PGF < CF The NET force associated with the low is greater towards the center (PGF) due to the inward centripetal force.Why is the PGF going in towards the center of the low?? Because PGF is always going from H to L pressure and therefore is directed TOWARDS the L.Therefore, the PGF in anticyclonic flow is outward because it is going from H to L. The Coriolis is pointed inwards because it is to the RIGHT of the flow which is clockwise.Cyclonic flow: PGF > CFAnticyclonic flow: PGF < CF
17Zonal & Meridional Winds Zonal winds: oriented in the W-E direction (parallel to latitude)Moves clouds, storms, surface anticyclones rapidly from west to eastMeridional winds: oriented in a N-S trajectorySurface storms move slowly and often intensify major storm systems
18Surface and Upper-Level Winds Winds on Upper-level ChartsWinds parallel to contour lines and flow west to eastHeights decrease from north to southSurface WindsWinds normally cross isobars and blow more slowly than winds aloftFriction: reduces the wind speed which in turn decreases the Coriolis effectFriction layer: surface to about 1000m (3300ft)Winds cross the isobars at about 30° into low pressure and out of high pressureThe reason winds are slower and cross isobars at the surface is FRICTION
19PGF at surface is balanced by the sum of friction and Coriolis force Surface winds into low and outward from highThe effect of surface friction is to slow down the wind so that, near the ground, the wind crosses the isobars and blows toward lower pressure.Less Coriolis means wind is not deflected as much to right……so therefore wind is going to slightly turned at an angle - ALPHA. And cross isobars. It will also slow down.This phenomenon at the surface produces an inflow of air around a low and an outflow of air around a high.Aloft, away from the influence of friction, the winds blow parallel to the lines, usually in a wavy west-to-east pattern.
20Winds & Vertical Motions Since surface winds blow into the center of a low, they are converging and that air has to go somewhere slowly risesVice versa for winds blowing outward from HTherefore, a surface low has convergence at surface and divergence aloft and a surface high has the opposite.
21Winds and air motions associated with surface highs and lows in the Northern Hemisphere.
22Hydrostatic Balance There is always a strong PGF directed upward Gravity balances the upward PGFWhen they are equal, hydrostatic equilibrium existsGood approximation for atmosphere with slow vertical movementsIs not valid for violent thunderstorms and tornadoes.Why is there a strong PGF directed upward??? Because there is higher pressure at the surface than aloft - so therefore wind will want to move from surface upwards.
23Air Masses of North America Continental Polar (cP) & Arctic (cA)Cold, dry, stable air in winterIn summer, cP air mass usually brings relief from oppressive heat in central and eastern USMaritime Polar (mP)In winter, cP/cA air mass is carried over Pacific Ocean where moisture and warmth is addedmP at Pacific Coast is cool, moist, and conditionally unstableEast of Rockies - brings fair weather and cooler temperatures (moisture has been removed by mountains)East coast mP air mass: originates in N. AtlanticStorms may develop (heavy rain or snow, coastal flooding)Late winter, early spring
24Air Masses and FrontsA front is a transition zone between two air masses of different densitiesFronts extend both horizontally and vertically
25Cold FrontCold, dry stable polar air (cP) is replacing warm, moist, conditionally unstable subtropical air (mT)Steep vertical boundary due to surface friction slowing down the surface frontHas strong vertical ascent along the surface frontStrong upper level westerlies push ice crystals far ahead of the front, creating Ci and Cs in advance of the front.Cold, dense air wedges under warm air, forcing the warm air upward, producing cumuliform cloudsCan cause strong convection, severe weather, and squall lines.Air cools quickly behind the front
26Cold FrontRising motion causes decreased surface pressure ahead of the frontOn a surface pressure map, frontal location can be seen by “kinks” in the isobars, changes in wind direction from a southwesterly to a northwesterly wind, and decreases in temperature.Pressure is lowest at the surface front.On weather maps, cold fronts are indicated by blue lines with triangles pointing in the direction of frontal motion (towards warmer air)
27Cold Front Before While After Winds S-SW Gusty, shifting W-NW TemperatureWarmSudden dropSteady dropPressureSteady fallMin, then sharp riseSteady riseCloudsIncreasing, Ci, Cs, CbCbCuPrecipitationBrief showersHeavy rains, severe weatherShowers, then clearingVisibilityFair to poorPoorGoodDew PointHigh, remains steadySharp droplowering
28Warm FrontOccurs at the leading edge of an advancing warm, moist, subtropical air mass (mT) from the Gulf replacing a retreating cold, maritime, polar air mass from the North Atlantic (mP)Slowly advances as cold air recedes; moves at about half the speed of an average cold frontSpeed may increase due to daytime mixingSpeed may decrease due to nighttime radiational coolingSmaller vertical slope than cold front
29Warm FrontWarmer, less-dense air rides up and over the colder, more- dense surface air“Overrunning”Produces clouds and precipitation well in advance of the front
30Warm Front Before While After Winds S-SE Variable S-SW Temperature Cool-coldSlowly warmingSteady riseWarmer, then steadyPressureFallingLeveling offSlight rise, followed by fallClouds(in order) Ci, Cs, As, Ns, St, fog (Cb in summer)Stratus-typeClearing with scattered ScPrecipitationLight-to- moderateDrizzle or noneUsually noneVisibilityPoorImprovingFairDew PointSteadyRise, then steady
31Stationary Front Essentially no movement Surface winds blow parallel to front, but in opposite directions on either side of itSeparates two air massesSeen often along mountain ranges when cold air cannot make it over the ridge
32Hourly surface observations at Gage, Oklahoma showing the passage of a primary and secondary cold front (left) and at Bowling Green, Kentucky showing the passage of a warm front (right).Source: Wallace and Hobbs, 2006.
33Occluded Fronts Cold fronts generally move faster than warm fronts Occlusion occurs when cold front catches up to and overtakes a warm frontOcclusions can be warm or cold
34Dry Lines Think of a dry line as a moisture boundary Separates warm, humid mT air in the southern Great Plains from warm, dry cT airDrier air behind dry lines lifts the moist air ahead of it, triggering storms along and ahead of itInduces lifting along frontOften produces severe thunderstorms in OK & TXUnique to southern great plains of US because of the Rocky mountains and the Gulf of Mexico
35A 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 wave
38Features of a Mid-latitude Cyclone Deep low pressure area with attached cold and warm frontsOften an occlusion forms, the triple point lending to the formation of severe weatherPrecipitation associated with the cold and warm fronts organizes in typical “comma cloud” structure
40Polar Front TheoryInitially, there is a stationary front that acts as the boundary separating cold, continental polar air from warm, maritime tropical airWinds blow parallel to this front on either sidePolar Fronts are discontinuous
41Cyclogenesis A wave forms on the front due to a shortwave disturbance Central PressureA wave forms on the front due to a shortwave disturbanceFrontal WaveIncipient CycloneThe front develops a "kink" where the wave is developingPrecipitation will begin to develop along the frontOverrunning and lifting
42StrengtheningThe cyclonic circulation around the low becomes more definedThe central pressure intensifiesThe cold front and warm front have more organized motionCyclone usually pushed east or northeast by the winds aloft
43Mature CycloneThe cold front catches up with the warm front and an occlusion formsThe cyclone is at its strongest at this pointSevere weather often develops near the “triple point”
44Dissipation The occlusion grows with time Eventually, the occlusion is so great that the supply of warm, moist air into the low is cut offCold air on both sidesWhen this happens, the system starts to dissipate