Lecture 13Outline of Last Lecture I. Mid-Latitude CycloneII. Upper-Level Low or TroughIII. Surface lowIV. Satellite AppearanceV. Extratropical CyclonesVI. Air Mass MovementVII. WeatherVIII. BlizzardIX. Blizzard IngredientsX. Chicago Blizzard 2011XI. Nor’EastersXII. Coastal FloodingXIII. Northeast Blizzard (2013)Outline of Current Lecture XIV. What is the Jet Stream?XV. Review of PressureXVI. Changing the SurfaceXVII. ConvergenceXVIII. DivergenceXIX. Convergence and DivergenceXX. Surface Low Pressure SystemsXXI. Surface High Pressure SystemsXXII. Net DivergenceATMS 100 1st EditionXXIII. Net ConvergenceXXIV. Strengthening and WeakeningXXV. Mid-Latitude Cyclone DevelopmentXXVI. Jet Stream- 300 mb MapXXVII. Upper Air Troughs and Surface LowsXXVIII. Vertical MotionXXIX. Hydrostatic BalanceCurrent LectureXXX. What is the Jet Stream?a. The jet stream is a river of fast moving air in the upper atmosphereb. exists near tropopause leveli. about 10 k above sea levelii. 300-200 mbc. flow is geostrophic (parallel to height contours)d. flow is not the same everywhere along the jet stream axisXXXI. Review of Pressureb. Recall that pressure is force (weight) per unit areac. more force over same area yields greater pressurei. weight is a forced. more air molecules over a point yield higher pressuree. less air molecules over a point yield low pressuref. ** there ar emore air molecules above a surface high than above a sur-face low**XXXII. Changing the Surface Pressureb. to change the surface pressure, a column of air must gain or lose air moleculesi. add or substract molecules from tubes of water to change pres-sureii. this changes the weight of the column and hence the pressurec. add molecules --> net convergence of airi. increases pressured. lose molecules --> net divergence of airi. decreases pressureoXXXIII. Convergenceb. Bringing things togetherc. length of arrows proportional to wind speedi. long=fast flowii. short=slow flowd. fast to slowXXXIV. Divergenceb. slow flow to fast flow i. slow going into region, fast coming outXXXV. Convergence and Divergenceb. Recall around a surface lowi. air spirals counter-clockwise and inward to center of lowii. air then converges and rises iii. then air diverges into the airc. Recall around the surface highi. air diverges d. Usually opposite between surface and alofti. upper-air divergence generally above surface convergence and vise versii. in, up, and out (lows)iii. in, down, and out (highs)iv. Net Divergence1. divergence > convergencev. Net Convergence 1. convergence > divergenced. recall that at the surface, air flows counter-clockwise and inward to-ward the center of lowsiv. surface convergencev. increases pressuree. lows strengthen when the surface pressure decreasesiv. requires net divergencev. divergence required to develop a surface low cannot be caused by the surface low; it must be aloftf. remember graident wind balance!g. recall that flow aloft is faster than geostrophic around ridges alofth. flow aloft is slower than geostrophic around troughs alofti. when air flows from a ridge to a trough, it slows downiv. convergence aloftj. when air flows from a trough to a ridge, it speeds upiv. divergence aloftk. look downsream (east) of troughs in a jet stream to find upper-level di-vergenceiv. this is where surface lows will formXXXVI. Surface Low Pressure Systemsc. Cause: Divergence aloft forces surface pressures to falld. Effect: Air converges into low pressure systems at the surfaceiv. surface winds flow counter-clockwise and inward in NHv. Surface air converges, rises, cools, and water vapor condenses,forming clouds and precipitation near surface lows XXXVII. Surface High Pressure Systemsc. Cause: Covergence aloft forces surface pressures to rised. Effect: Air diverges from high pressure systems at the surfaceiv. surface winds flow clockwise and outward in NHv. air sinks, warms, and dries, inhibitng cloud formationXXXVIII. Net Divergencec. Decreases with surface pressureiv. divergence aloft greater than convergence at surface (surface lows)v. divergence at surface greater than convergence aloft (surface highs)vi. subtracts molecules from air column1. net loss in air molecules ALWAYSvii. pressure falls, so lows strengthen and highs weakenXXXIX. Net Convergencec. increases surface pressureiv. adds weight to air columnsv. convergence at surface greater than divergence aloft (surface lows)vi. convergence aloft greater than divergence at surface (surface highs)vii. pressure rise, so highs strengthen and lows weakenXL. Strengthening and Weakeningc. strengtheing low: pressures falliv. net divergences (divergence aloft > convergence surface)d. weakening low: pressure riseiv. net convergence (convergence surface > divergence aloft)e. strengthening high: pressure riseiv. net convergence (convergence aloft > divergence surface)f. weakening high: pressures falliv. net divergence (divergence surface>convergence aloft)XLI. Mid-Latitude Cyclone Developmentc. How do they form?iv. convergence and divergence are keyv. must have net divergence- net decrease of air molecules over a point on te surface1. surface pressure decreasesvi. regions of net divergence are favored for surface lowsXLII. Upper Air Troughs and Surface Lowsc. Where would you expect find a surface low pressure system?iv. expect surface lows to be east of upper-air trough axisv. east (downstream) of an upper-level troughvi. divergence aloft, surface pressure falls (decreases)d. A surface high pressure system?iv. east of an upper-level ridgev. convergence aloft, surface pressure rises (increases)e. surface lows cannot strengthen directly beneath upper-level troughsiv. no divergence aloftXLIII. Vertical Motionc. Vertical motions usually smalliv. steady-rain: cm/s (about 1 mile/day)v. faster rising air=more interesting weatherd. strong-upward directed PGF balanced by gravityiv. called hydrostatic balancev. works well most of the timevi. does not apply to thunderstorms, tornadoesXLIV. Hydrostatic