Download Thunderstorms and Tornadoes Chapter 11

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