METO200Lect19 20

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Published on October 5, 2007

Author: Belly

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Slide1:  Fig. 11-1, p. 312 Slide2:  Fig. 11.2a Visible image of a supercell thunderstorm Slide3:  Fig. 11.2b Infra-red image of a supercell thunderstorm Slide4:  Fig. 11-3, p. 314 A climatology of the average number of thunderstorm days in a year Slide5:  Fig. 11-4, p. 315 Lifted Index:  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 Slide7:  Table 11-1, p. 315 Slide8:  Fig. 11-5, p. 316 Satellite derived values of lifted index, June 22 2003 THUNDERSTORM:  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 THUNDERSTORM GROWTH AND DEVELOPMENT:  THUNDERSTORM GROWTH AND DEVELOPMENT AIR NEEDS TO BE UNSTABLE DRYLINE - LINE BETWEEN cT AND mT AIR MASSES-LEADS TO UNSTABLE AIR LIFTED INDEX MUST BE LESS THAN -3. FOR SEVERE THUNDESTORMS LIFTED INDEX IS USUALLY -6 VERTICAL WIND SHEAR CAN ‘SPIN UP’ THUNDERSTORM SEVERE THUNDERSTORMS - mT MEETS Cp. GREATEST CONTRAST - SPRING AND EARLY SUMMER Slide11:  Fig. 11.8 An ordinary airmass thunderstorm Slide12:  Fig. 11.7 Life cycle of an ordinary thunderstorm cell THUNDERSTORM CUMULUS STAGE:  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:  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:  THUNDERSTORM DISSIPATING STAGE DOWNDRAFT AND ENTRAINMENT DOMINATE NO UPDRAFT THUNDERSTORM LOSES ENERGY SOURCE Slide16:  Fig. 11.10a Squall line associate with a cold front. Slide17:  Fig. 11-10, p. 320 Schematic of a multicell thunderstorm. Red arrows represent the warm updraft, blue arrows the cool downdraft Squall Line:  Squall Line Is a set of individual intense thunderstorm cells arranged in a line. Thy 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’. Slide19:  Fig. 11.13 Mesoscale Convective Complex:  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 te atmosphere. Slide21:  Fig. 11-13, p. 322 Infrared image of a mesoscale convective complex over Kansas, July 8 1997. SUPERCELL THUNDERSTORM:  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:  TYPE OF THUNDERSTORM SINGLE-CELL THUNDERSTORM MULTICELL THUNDERSTORM MESOSCALE CONVECTIVE C0MPLEX SUPERCELL THUNDERSTORM Slide24:  Box 11-1, p. 329 Slide25:  Fig. 11-30, p. 337 Geographic distribution of the month of maximum tornado threa.t TORNADO:  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 Slide27:  Fig. 11.18 TORNADO:  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:  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. Slide30:  Fig. 11-23, p. 331 STAGES OF A TORNADO:  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 Slide32:  Table 11-3, p. 333 Slide33:  Fig. 11-25, p. 333 Slide34:  Fig. 11-33, p. 342 LIGHTNING:  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 Slide36:  Fig. 11.28 Lightning formation: Charge separation. Slide37:  Fig. 11-37, p. 346 Slide38:  Fig. 11-38, p. 346 Life cycle and path of a hailstone in a supercell thunderstorm HAIL:  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 Slide40:  Fig. 11.33 Number of days per year with Hail > .75 inch

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