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AOSC 200 Lesson 10 Visible image of super thunderstorm from GEO satellite Fig. 11.2a IR image of the same super thunderstorm Fig. 11.2b Fig. 11-1, p. 312 THUNDERSTORM • IS A CLOUD OR CLUSTER OF CLOUDS THAT PRODUCES THUNDER, LIGHTNING, HEAVY RAIN, AND SOMETIMES HAIL AND TORNADOS • CAN DIVIDE THUNDERSTORMS INTO TWO MAIN TYPES ISOLATED THUNDERSTORMS PRODUCED WITHIN A WARM HUMID AIR MASS SEVERE THUNDERSTORMS PRODUCED BY FORCEFUL LIFTING • IN THE USA, AIRMASS THUNDERSTORMS GENERALLY OCCUR IN WARM MOIST AIR - mT • LIFTING CAN BE BY FRONTS OR OROGRAPHICALLY Lifted Index • A parcel of air will not rise unless it is unstable. • The lifted index follows a parcel of air as it is lifted from the surface and cools at the dry adiabatic lapse rate until saturation occurs, and then cools at the wet adiabatic lapse rate. • The lifted index is defined as: The environmental temperature at 500 mb minus the temperature of the parcel of air when lifted to 500 mb • If the lifted index is negative then the atmosphere is unstable. • Severe thunderstorms require a lifting index less than -3 Stepped Art Fig. 7-6, p. 175 Life cycle of an ordinary thunderstorm cell Fig. 11.7 THUNDERSTORM CUMULUS STAGE • CUMULUS STAGE • REQUIRES CONTINUOUS SOURCE OF WARM MOIST AIR • EACH NEW SURGE OF WARM AIR RISES HIGHER THAN THE LAST • STRONG UPDRAFTS • FALLING PRECIPITATION DRAGS AIR DOWN - DOWNDRAFT • ENTRAINMENT THUNDERSTORM MATURE STAGE • SHARP COOL GUSTS AT SURFACE SIGNAL DOWNDRAFTS • UPDRAFTS EXIST SIDE BY SIDE WITH DOWNDRAFTS • IF CLOUD TOP REACHES TROPOPAUSE UPDRAFTS SPREAD LATERALLY - ANVIL SHAPE • TOP OF ICE LADEN CIRRUS CLOUDS • GUSTY WINDS, LIGHTNING, HEAVY PRECIPITATION, HAIL THUNDERSTORM DISSIPATING STAGE • DOWNDRAFT AND ENTRAINMENT DOMINATE • NO UPDRAFT • THUNDERSTORM LOSES ENERGY SOURCE An ordinary airmass thunderstorm Fig. 11.8 Table 11-1, p. 315 THUNDERSTORM GROWTH AND DEVELOPMENT • AIR NEEDS TO BE UNSTABLE • DRYLINE - LINE BETWEEN cT AND mT AIR MASSES-LEADS TO UNSTABLE AIR • LIFTED INDEX IS NEGATIVE, AIR IS UNSTABLE • VERTICAL WIND SHEAR CAN ‘SPIN UP’ THUNDERSTORM • SEVERE THUNDERSTORMS - mT MEETS Cp. • GREATEST CONTRAST - SPRING AND EARLY SUMMER A climatology of the average number of thunderstorm days in a year Fig. 11-3, p. 314 Fig. 11-4, p. 315 Schematic of a multicell thunderstorm. Red arrows represent the warm updraft, blue arrows the cool downdraft Fig. 11-10, p. 320 Squall Line • Is a set of individual intense thunderstorm cells arranged in a line. • They occur along a boundary of unstable air – e.g. a cold front. • Strong environmental wind shear causes the updraft to be tilted and separated from the downdraft. • The dense cold air of the downdraft forms a ‘gust front’. Pre-frontal squall lines may form ahead of an advancing cold front as the air aloft forms waves downwind from the cold front Squall line associate with a cold front. Fig. 11.10a Fig. 11.13 Mesoscale Convective Complex • A Mesoscale Convective Complex is composed of multiple single-cell storms in different stages of development. • The individual thunderstorms must support the formation of other convective cells • In order to last a long time, a good supply of moisture is required at low levels in the atmosphere. Infrared image of a mesoscale convective complex over Kansas, July 8 1997. Fig. 11-13, p. 322 SUPERCELL THUNDERSTORM • SINGLE CELL THUNDERSTORM THAT PRODUCES DANGEROUS WEATHER • REQUIRES A VERY UNSTABLE ATMOSPHERE AND STRONG VERTICAL WIND SHEAR - BOTH SPEED AND DIRECTION • UNDER THE INFLUENCE OF THE STRONG WIND SHEAR THE ENTIRE THUNDERSTORM ROTATES • FAVORED REGION IS THE SOUTHERN GREAT PLAINS IN THE SPRING TYPE OF THUNDERSTORM • SINGLE-CELL THUNDERSTORM • MULTICELL THUNDERSTORM • MESOSCALE CONVECTIVE C0MPLEX • SUPERCELL THUNDERSTORM Tornado over College Park, 10/23/01 Box 11-1, p. 329 http://www.youtube.com/watch?v= 43VoMesUd2Qhttp://www.youtube .com/watch?v=43VoMesUd2Q Geographic distribution of the month of maximum tornado threa.t Fig. 11-30, p. 337 TORNADO • DERIVED FROM SPANISH WORD ‘TORNADA’ – THUNDERSTORM • TORNADOS ARISE FROM SEVERE THUNDERSTORMS • MOST TORNADOS IN CENTRAL US • ON AVERAGE ABOUT 770 TORNADOS ARE REPORTED ANNUALLY • OCCUR MAINLY FROM APRIL TO JUNE BUT ALL YEAR ROUND Fig. 11.18 TORNADO • LESS THAN 1.6 KM WIDE, AND SHORT LIVED • NO ONE REALLY KNOWS HOW THEY ARE FORMED • MOST DROP DOWN FROM SUPERCELL THUNDERSTORMS - ROTATING • HOWEVER THIS ROTATION CANNOT EXPLAIN THE FAST ROTATION OF A TORNADO • THE HORIZONTAL ROTATION OF THE THUNDERSTORM IS CONVERTED INTO VERTICAL ROTATION OF THE TORNADO TORNADO • TRAVELS AT ABOUT 45 KM PER HOUR AND CUTS A PATH OF ABOUT 26 KM LONG • BUT REALLY NO SUCH THING AS AN AVERAGE TORNADO • PRESSURE AT CENTER OF VORTEX AS MUCH AS 30% LOWER THAN SURROUNDINGS. • AIR RUSHES IN FROM SURROUNDINGS AND IS SPIRALED UPWARD. • CONSERVATIONOF ANGULAR MOMENTUM • BECAUSE OF TREMENDOUS PRESSURE GRADIENT WINDS CAN REACH 400 KM (250 MILES) PER HOUR. Fig. 11-23, p. 331 STAGES OF A TORNADO • ORGANIZING STAGE - FUNNEL CLOUD DROPS DOWN TO THE SURFACE • MATURE STAGE - TORNADO AT PEAK INTENSITY AND WIDTH • SHRINKING STAGE • ROPE STAGE Table 11-3, p. 333 Fig. 11-25, p. 333 Fig. 11-33, p. 342 http://www.youtube.com/watch?v=ToY11hHpAJk http://www.youtube.com/watch?v=ToY11hHpAJk LIGHTNING • .LARGE ELECTRICAL DISCHARGE THAT RESULTS FROM RISING AND SINKING MOTIONS IN A THUNDERSTORM • .SEQUENCE IS AS FOLLOWS; • . CHARGE SEPARATION - REALLY DO NOT UNDERSTAND WHY • . GROUND BECOMES POSITIVELY CHARGED • . LIGHTNIING FORMATION BEGINS - LEADERS • . LIGHTNING FLASH OCCURS Lightning formation: Charge separation. Fig. 11.28 Fig. 11-37, p. 346 Life cycle and path of a hailstone in a supercell thunderstorm Fig. 11-38, p. 346 HAIL • LARGE CLUMPS OR BALLS OF ICE • START OF AS A SMALL ICE PARTICLE • DUE TO UPDRAFT THE ICE PARTICLE DOES NOT FALL TO GROUND BUT IS RECYCLED INTO THE FREEZING PORTION OF THE THUNDERSTORM, • EACH TIME IT IS TAKEN UPWARD IT ACCUMULATES MORE ICE • CAN END UP AS LARGE AS A GRAPEFRUIT Number of days per year with Hail > .75 inch Fig. 11.33