Chapter 14

Thunderstorms

A storm containing lightening and thunder; convective storms

Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50kts, or tornado

Ordinary Cell ThunderstormsAir-mass thunderstorms: limited wind sheerStages: cumulus, mature, dissipatingEntrainment, downdraft, gust front

Thunderstorms Multi-cell Thunderstorms

Thunderstorms that contain a number of convection cells, each in a different stage of development, moderate to strong wind shear; tilt, over shooting top

Gust Front: leading edge of the cold air out-flowing air; shelf cloud, roll cloud, outflow boundary

Micro-bursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind; wind shear, virga

Thunderstorms

Multi-cell ThunderstormsSquall-line thunderstorms; line of multi-cell

thunderstorms, pre-frontal squall-line, derecho

Meso-scale Convective Complex: a number of individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system; summer, 10,000km2

Thunderstorms Supercell thunderstorms

Large, long-lasting thunderstorm with a single rotating updraft

Strong vertical wind shearOutflow never undercuts updraftClassic, high precipitation and low precipitation

supercellsCap and convective instabilityRain free base, low-level jetSurface, 850mb, 700mb, 500mb, 300mb

conditions

Thunderstorms Thunderstorms and the Dryline

Sharp, horizontal change in moistureThunderstorms form just east of drylinecP, mT, cT

Floods and Flash FloodsFlash floods rise rapidly with little or no

advance warning; many times caused by stalled or slow thunderstorm

Large floods can be created by training of storm systems, Great Flood of 1993

Thunderstorms

Topic: Big Thompson CanyonJuly 31, 1976, 12 inches of rain in 4 hours

created a flood associated with $35.5million in damage and 135 deaths

Distribution of ThunderstormsMost frequent Florida, Gulf Coast, Central

PlainsFewest Pacific coast and Interior valleysMost frequent hail Central Plains

Thunderstorms

Lightening and ThunderLightening: discharge of electricity in mature

storms (within cloud, cloud to cloud, cloud to ground)

Thunder: explosive expansion of air due to heat from lightening

Electrification of Clouds: graupel and hailstones fall through supercooled water, ice crystals become negatively charged

Upper cloud positive, bottom cloud negative

Thunderstorms

Observations: ElvesBlue jets, red sprite, ELVES

The Lightening StrokePositive charge on ground, cloud to ground

lighteningStepped leader, ground stroke, forked

lightening, ribbon lightening, bead lightening, corona discharge

Thunderstorms

Observation: Apple treeDO NOT seek shelter during a thunderstorm

under an isolated tree.

Lightening Detection and SuppressionLightening direction finder detects

radiowaves produced by lightening, sphericsNational Lightening Detection NetworkSuppression: seed clouds with aluminum

Tornadoes

Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground.

Tornado life cycleOrganizing, mature, shrinking, decay stage

Tornado outbreaksFamilies, super outbreak

Tornadoes Tornado Occurrence

US experiences most tornadoesTornado Alley (warm, humid surface; cold dry air

aloft)Highest spring, lowest winter

Tornado windsMeasurement based upon damage after storm

or Doppler radarFor southwest approaching storms, winds

strongest in the northeast of the storm, 220 kts maximum

Multi-vortex tornados

Tornadoes

Seeking shelterBasement or small, interior room on ground

floorIndoor vs. outdoor pressure

The Fujita ScaleBased upon the damage created by a stormF0 weakest, F5 strongestEnhanced Fujita Scale

Tornadic Formation

Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear

Supercell TornadoesWind sheer causes spinning vortex tube that

is pulled into thunderstorm by the updraftMesocyclone, BWER, rear flank downdraft,

vertical stretching, funnel cloud, rotating cloud, wall cloud

Stepped Art

Fig. 14-46, p. 402

Tornadic Formation

Nonsupercell TornadoesGustnadoesLand spoutCold-air funnels

Severe Weather and Doppler Radar Doppler radar measures the speed of

precipitation toward and away radar unit Two Doppler radars can provide a 3D

view TVS, Doppler lidar NEXRAD

Waterspouts

Rotating column of air that is connected to a cumuliform cloud over a large body of water

Tornadic waterspout