GEO 203 Notes – 12/9/10 (Lecture #19 and 20)Ingredients for tropical cyclone:Warm (sub)tropical ocean/gulf water; 80 degrees or greaterLow vertical wind shearWeak winds aloftHigh humidity throughout troposphereConditionally unstable atmosphereDo not form over equatorial water bc Coriolis force is too weak near equator; typically developed between 5- and 20-degree latitudesKey triggers: ITCZ, tropical waves, stalled frontsUltimate energy source for hurricane: warm, moist waterWhy do hurricanes form?Organized convection theory:Surface convergence leads to rising motion of warm, moist airRising air condenses, releases huge amount of latent heatHeating of air column = air column stretches = high pressure, divergence aloftDiverging air aloft leaves air column quicker than converging surface air entering the column, causing a surface low pressure area to form and intensifyLower surface pressure further enhances surface convergenceA chain reaction develops and a hurricane is bornHeat engine theory:Heat taken in at high temperatures (from ocean surface), converted into work (kinetic energy for wind), ejected at low temperature due to radiational coolingWhy do hurricanes weaken significantly once they are over cold water or make landfall?Decreased heat and/or moistureIncreased surface frictionHurricane structure:Eye—calm, clear center (avg 10-20km)Eye wall—outer edge of eye where clouds are deepest and rain is most intenseRain-free area—band a way outward from eyeSpiral rain band—band just outside of rain-free areaSinking motion within eye = evaporation (lowest pressure within eye)Air rotates counter-clockwise, moves in toward eyeHigh pressure aloft, lowest pressure in centerLowest wind @ eye, highest @ eye wallStages of hurricane: *very likely to be on examTropical disturbanceTropical depression (20 knots) (number given to storm, i.e. TD 8)Tropical storm (35 knots) (name given to storm, i.e. TS Katrina)Hurricane (64 knots or 74 mph)Tropical cyclones:Warm-core lowsWeakens with heightSinking air in center (eye)Winds strongest at surfaceIsobars denser, more circularSmallerNo fronts associated with itMidlatitude cyclones:Cold coreIntensifies with heightRising air at centerWinds strongest in jet streamIsobars coarser, less circularBiggerAssociated with cold and warm frontsMost intense hurricane at landfall to strike US: Florida (Keys) hurricane in 1935 (cat 5; killed 408)Camille (69)Andrew (92)Katrina (05)Galveston storm (1900) killed over 6,000 peopleOnly three cat. 5s at landfall in last century (most are cat. 1s)Hurricane threats:High winds (74-150+ mph)Heavy rain (Allison produced 3ft. of rainStorm surge (Camille produced 25ft+ surge)Tornadoes (Beulah caused about 100 twisters in TX in 1967)Aftermath: downed power lines, glass, sharp metal, wild animals, illnessesStrongest winds for hurricane traveling south to north are on eastern side because storm turns counterclockwiseHurricane scale: Saffir-SimpsonCategories 1 through 5 based on wind speed (Cat. 5 is strongest)1: 74-95 mph2: 96-110 mph3: 111-130 mph4: 131-155 mph5: >155 mphNaming scheme:No names before 19531953-1977: only female names1978: male names also givenNames alternate by gender from A – Z (no Q, U, X, Y, Z)Names recurred every four years unless major hurricane occurs, in which case the name is retired (i.e. Katrina used in ’05, not reused in ’09)If all names are used, Greek letters continue (alpha, beta, gamma, etc.)How do we forecast hurricanes?Real-time dataAtmospheric modelingClimatologyLong termHurricane watch issued 24-48hrs before storm arrives; hurricane warning within 24hrs.Lecture #20: Weather ForecastingWeather forecasting: application of current technology and science to predict how the present state of the atmosphere will change for a future time and given locationAcquisition of Weather Information10,000 land-based stations, hundreds of ships and buoysUpper level: radiosondes, aircraft, satellites, radar, etc.United Nations World Meteorological Organization (located in Geneva), 175 countriesWorld Meteorological Center: Melbourne, Moscow, Washington DCNCEP, US NWSASOSMI: radiosondes launched from Detroit and GaylordMeteorological data collected from surface, rawinsonde sounding sites from around the world, transmitted to one of three World Meteorological Centers @ Melbourne, Moscow, DC; data disseminated at centers to national meteorological centers at each country where weather charts are generatedNational Oceanic and Atmospheric Administration -> National Weather ServiceNational or regional centers, weather service offices (MI: Detroit, Grand Rapids, Gaylord, Marquette), private firmsWeather forecasting tools:High speed data modeling systems: communication, storage, processing, displayDoppler radarSatellite imageryNWP forecast chartsSoundingsWind profilesMeteogram: time series of predicted weather elements at a specific locationUpper air soundings: Vertical profiles of temperature, dew point, wind speed, and direction at a given locationWeather forecasting methods:Utilize power of modern computers to numerically solve set of mathematical equations that govern motion of atmosphereComplex computer programs (forecast models) take current state of atmosphere to make prediction of future atmospheric state24-hour forecasts take millions of calculations, so numerical weather predictions are usually made on supercomputersWhy forecasts go awryEquations not precise, assumptions have to be madeInitial state is not completely known because of gaps in observational data (oceans and mountainous regions)Cannot model small-scale featuresEnsemble forecasts improve numerical weather predictionsSpaghetti model, robustEnsemble forecast: making multiple model runs w/ different conditions with slightly different initial conditions each timeOther forecasting techniques:PersistenceTrendAnalogueStatisticalWeather typeClimatologicalGEO 203 Notes – 12/7/10 (Lecture #18 and 19)FINAL: Tuesday, 10-12Tornado: Rapidly rotating column or “funnel” of high wind that spiral around very small region of low pressure beneath a thunderstormCounterclockwise rotationDiameter: 300-600mSpeed of movement: 20-70 knotsLifetime: a few minutes to an hourAverage path length: 7kmTornadoes occur anytime you have severe thunderstorms (including hurricanes)75% of tornadoes occur in US; most frequent from March to JulyLarge-scale outbreak in spring; lowest frequency in winterTornadoes occur in all fifty states; highest frequency in tornado