AOSC 200Lesson 18

Fig. 11-1, p. 312

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. 11.7

Life cycle of an ordinary thunderstorm cell

THUNDERSTORM GROWTH AND DEVELOPMENT

• AIR NEEDS TO BE UNSTABLE• DRYLINE - LINE BETWEEN cT AND mT AIR MASSES-LEADS TO UNSTABLE AIR

• LIFTED INDEX - DIFFERENCE BETWEEN THE ACTUAL TEMPERATURE AT 500 MB, AND TEMPERATURE OF AIR PARCEL AFTER ADIABATIC EXPANSION TO 500 MB

• 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

• CAPPING INVERSION

Fig. 11-3, p. 314

A climatology of the average number of thunderstorm days in a year

Fig. 11-4, p. 315

Table 11-1, 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’.

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 te 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