AOSC 200 Lesson 10
Jan 06, 2016
AOSC 200Lesson 10
Fig. 11.2a
Visible image of super thunderstorm from GEO satellite
Fig. 11.2b
IR image of the same super thunderstorm
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
Fig. 7-6, p. 175
Stepped Art
Fig. 11.7
Life cycle of an ordinary thunderstorm cell
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
Fig. 11.8
An ordinary airmass thunderstorm
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
Fig. 11-3, p. 314
A climatology of the average number of thunderstorm days in a year
Fig. 11-4, p. 315
Fig. 11-10, p. 320
Schematic of a multicell thunderstorm. Red arrows represent the warm updraft, blue arrows the
cool downdraft
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
Fig. 11.10a
Squall line associate with a cold front.
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.
Fig. 11-13, p. 322
Infrared image of a mesoscale convective complex over Kansas, July
8 1997.
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
Fig. 11-30, p. 337
Geographic distribution of the month of maximum tornado threa.t
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
Fig. 11.28
Lightning formation: Charge separation.
Fig. 11-37, p. 346
Fig. 11-38, p. 346
Life cycle and path of a hailstone in a supercell
thunderstorm
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
Fig. 11.33
Number of days per year with Hail > .75 inch