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