AOSC 200 Lesson 10

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AOSC 200Lesson 10
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Visible image of super thunderstorm from GEO
satellite
Fig. 11.2a
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IR image of the same super thunderstorm
Fig. 11.2b
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Fig. 11-1, p. 312
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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

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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

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Stepped Art
Fig. 7-6, p. 175
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Life cycle of an ordinary thunderstorm cell
Fig. 11.7
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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

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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

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THUNDERSTORM DISSIPATING STAGE

• DOWNDRAFT AND ENTRAINMENT DOMINATE
• NO UPDRAFT
• THUNDERSTORM LOSES ENERGY SOURCE

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An ordinary airmass thunderstorm
Fig. 11.8
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Table 11-1, p. 315
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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

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A climatology of the average number of
thunderstorm days in a year
Fig. 11-3, p. 314
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Fig. 11-4, p. 315
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Schematic of a multicell thunderstorm. Red arrows
represent the warm updraft, blue arrows the cool
downdraft
Fig. 11-10, p. 320
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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.

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Pre-frontal squall lines may form ahead of an
advancing cold front as the air aloft forms waves
downwind from the cold front
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Squall line associate with a cold front.
Fig. 11.10a
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Fig. 11.13
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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.

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Infrared image of a mesoscale convective complex
over Kansas, July 8 1997.
Fig. 11-13, p. 322
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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

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TYPE OF THUNDERSTORM

• SINGLE-CELL THUNDERSTORM
• MULTICELL THUNDERSTORM
• MESOSCALE CONVECTIVE C0MPLEX
• SUPERCELL THUNDERSTORM

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Tornado over College Park, 10/23/01
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Box 11-1, p. 329
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http//www.youtube.com/watch?v43VoMesUd2Qhttp//w
ww.youtube.com/watch?v43VoMesUd2Q
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Geographic distribution of the month of maximum
tornado threa.t
Fig. 11-30, p. 337
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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

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Fig. 11.18
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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

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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.

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Fig. 11-23, p. 331
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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

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Table 11-3, p. 333
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Fig. 11-25, p. 333
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Fig. 11-33, p. 342
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• http//www.youtube.com/watch?vToY11hHpAJk
  http//www.youtube.com/watch?vToY11hHpAJk

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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

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Lightning formation Charge separation.
Fig. 11.28
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Fig. 11-37, p. 346
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Life cycle and path of a hailstone in a supercell
thunderstorm
Fig. 11-38, p. 346
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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

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Number of days per year with Hail gt .75 inch
Fig. 11.33