aosc 200 lesson 18. fig. 11-1, p. 312 lifted index a parcel of air will not rise unless it is...
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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
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. 11-10, 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’.
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.
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