ThunderstormsThunderstorms

Review of last lectureReview of last lecture

1.1. Two types of lightning (cloud-to-cloud 80%, cloud-to-Two types of lightning (cloud-to-cloud 80%, cloud-to-ground 20%)ground 20%)

2.2. 4 steps of lightning development. 4 steps of lightning development.

3.3. How fast does thunder travel? How fast does thunder travel?

4.4. Climate impacts of lightning: nitrogen cycle, ozone, Climate impacts of lightning: nitrogen cycle, ozone, wildfirewildfire

5.5. Lightning safetyLightning safety

Convective systemsConvective systems

TornadoesTornadoes: about : about 100-600 m100-600 m, last , last 1 minute to 1 hour1 minute to 1 hour

ThunderstormsThunderstorms: about : about 10 Km10 Km, last , last 10 minutes to a 10 minutes to a couple of hours. couple of hours. 3 types: ordinary, multicell, supercell3 types: ordinary, multicell, supercell

Mesoscale convective systems (MCSs)Mesoscale convective systems (MCSs): A cloud : A cloud system that occurs in connection with an ensemble system that occurs in connection with an ensemble of thunderstorms and produces a contiguous of thunderstorms and produces a contiguous precipitation area on the order of precipitation area on the order of 100 Km100 Km or more in or more in at least one directionat least one direction, and often last for , and often last for several hours several hours to a couple of daysto a couple of days..

ThunderstormsThunderstorms

A storm containing lightning & thunderA storm containing lightning & thunder ConvectiveConvective; form when warm, humid air ; form when warm, humid air

rises in conditionally unstable environmentrises in conditionally unstable environment The warmer the rising air parcel is relative The warmer the rising air parcel is relative

to environment, the more to environment, the more buoyant forcebuoyant force is is driving it upwards (stronger convection)driving it upwards (stronger convection)

TriggerTrigger to start uplift: warming sfc, terrain to start uplift: warming sfc, terrain (orography), converging sfc winds, frontal (orography), converging sfc winds, frontal zones, divergence aloft (or combination)zones, divergence aloft (or combination)

Global distribution of thunderstormsGlobal distribution of thunderstorms

Three stages have been identified in ordinary thunderstorms:Three stages have been identified in ordinary thunderstorms:a)a) DEVELOPINGDEVELOPING: unstable atmosphere, vertical updrafts keep precipitation : unstable atmosphere, vertical updrafts keep precipitation

suspendedsuspendedb)b) MATUREMATURE:: entrainment of dry air that causes cooler air from evaporation, triggering entrainment of dry air that causes cooler air from evaporation, triggering

downdrafts and falling precipitation and gust frontsdowndrafts and falling precipitation and gust frontsc)c) DISSIPATINGDISSIPATING:: weakening updrafts and loss of the fuel source after 15-30 minutes.weakening updrafts and loss of the fuel source after 15-30 minutes.

Thunderstorms I. Ordinary StormsThunderstorms I. Ordinary Storms

An ordinary thunderstormAn ordinary thunderstorm

Cool downdrafts leaving a Cool downdrafts leaving a mature and dissipating storm mature and dissipating storm may offer relief from summer may offer relief from summer heat, but they may also force heat, but they may also force surrounding, low-level moist air surrounding, low-level moist air upward.upward.

Hence, dying storms often Hence, dying storms often trigger new storms, and the trigger new storms, and the successive stages may be successive stages may be viewed in the sky. viewed in the sky.

Thunderstorms II. Multicell StormThunderstorms II. Multicell Storm

A Multicell ThunderstormA Multicell Thunderstorm

Video: Development of a supercell Video: Development of a supercell thunderstormthunderstorm

http://www.youtube.com/watch?v=36vGiE5JQzs (90 min -> 2 min lapse)

Storms producing a minimum of Storms producing a minimum of a) 3/4 inch hail and/or a) 3/4 inch hail and/or b) wind gusts of 50 knots and/or b) wind gusts of 50 knots and/or c) tornado winds, classify as severe.c) tornado winds, classify as severe.

Thunderstorm III. Supercell StormThunderstorm III. Supercell Storm

Formation of supercell thunderstormsFormation of supercell thunderstorms

1. 1. Before thunderstorms develop, a Before thunderstorms develop, a change in wind direction and an increase change in wind direction and an increase in wind speed with increasing height in wind speed with increasing height creates an invisible, horizontal spinning creates an invisible, horizontal spinning effect in the lower atmosphere. effect in the lower atmosphere.

2. Spinning horizontal vortex tubes created 2. Spinning horizontal vortex tubes created by surface by surface wind shearwind shear may be may be tiltedtilted and and forced in a vertical path by updrafts. This forced in a vertical path by updrafts. This rising, spinning, and often stretching rising, spinning, and often stretching rotating air may then turn into a rotating air may then turn into a mesocyclonemesocyclone..

3. Most strong and violent tornadoes form 3. Most strong and violent tornadoes form within this area of strong rotation.within this area of strong rotation.

Vertical structure of a supercell thunderstormVertical structure of a supercell thunderstorm

In ordinary storms, the In ordinary storms, the downdraftdowndraft and falling precipitation cut and falling precipitation cut off the off the updraftupdraft. But in supercell storms, winds aloft push the . But in supercell storms, winds aloft push the rain away and the updraft is not weakened and the storm rain away and the updraft is not weakened and the storm can continue maturing and maintain its structure for hours.can continue maturing and maintain its structure for hours.

Cloud structure of a supercell thunderstormCloud structure of a supercell thunderstorm

Horizontal structure of a supercell Horizontal structure of a supercell thunderstormthunderstorm

Radar echo of a supercellRadar echo of a supercell

Satellite image of a supercellSatellite image of a supercell

Different types of supercell thunderstormsDifferent types of supercell thunderstorms

Low precipitationLow precipitation

High precipitationHigh precipitation

Effects of supercell Effects of supercell thunderstormsthunderstorms

Large hailsLarge hails Damaging windsDamaging winds FloodingFlooding Dangerous cloud-to-ground lightningDangerous cloud-to-ground lightning Deadly tornadoesDeadly tornadoes

Video: Multiple tornadoes from Video: Multiple tornadoes from one supercellone supercell

http://www.youtube.com/watch?v=FPfO-yjomig

SummarySummary

1.1. The general size and lifetime of mesoscale The general size and lifetime of mesoscale convective systems, thunderstorms and tornadoesconvective systems, thunderstorms and tornadoes

2.2. 3 types of thunderstorms. 3 types of thunderstorms.

3.3. 3 stages of the ordinary thunderstorms. 3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft.

4.4. Formation of multi-cell thunderstorms. Downdrafts Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions.initiate new thunderstorms in nearby regions.

1.1. 3 stages of the supercell thunderstorms. 3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened.