ESCI 106 – Weather and Climate Lecture 10 Chapter 10: “Thunderstorms and Tornadoes” Jennifer D. Small Jennifer D. Small

Weather Fact of the Day: October : A F1 tornado (waterspout) came ashore and caused significant damage on the west side of Apalachicola, FL. Marina boasts sank 4 homes destroyed Part of a hospital roof collapsed. 100s of trees and power lines downed. Fortunately, no serious injuries.

National Watches and Warnings

Learning Goals for Today 1.Understand how and why thunderstorms form. 2.Understand how and why tornadoes form 3.Understand how and why hurricanes form

Thunderstorms and Tornadoes!

Thunderstorms - Intro A storm that generates lightning and thunder. Are characterized by strong “up” and “down” motions Frequently produces gusty winds, heavy rain and hail.

Thunderstorms - Intro They can form on their own They can form in conjunction with mid-latitude cyclone  Edge of a Cold-Front The can form in conjunction with hurricanes

Thunderstorms - Intro Florida has the most thunderstorms  on average per year! Commonly occur in the foothills of the Rockies Associated with Tornado Alley in the middle of the country  Associated with drylines and air with different humidities

Thunderstorms - Intro Form when warm, humid air rises in an unstable environment There are two categories: 1.Air Mass Thunderstorms – formed by unequal heating of the Earth’s surface within a maritime tropical air mass. 2.Severe Thunderstorms – formed by unequal heating & lifting of warm air along a front or mountain.

Airmass Thunderstorms Happens inside an air mass (usually mT) Usually in spring and summer Usually in mid-afternoon Not associated with a front Due to local differences in temperature

Airmass Thunderstorms Like mid-latitude cyclones T-Storms have a “Life Cycle” Stages of Development  Stage 1: Cumulus Stage  Stage 2: Mature Stage  Stage 3: Dissipating Stage

Airmass Thunderstorms Stage 1Stage 2Stage 3

Stage 1: Cumulus stage Rising air only Makes a cloud Cloud gets bigger and bigger Moisture is being added to higher and higher altitudes Needs a continuous supply of moisture to survive

Airmass Thunderstorms Stage 1Stage 2Stage 3

Stage 2: Mature Stage Precipitation forms  Mainly by the Bergeron Process Rain begins to fall  Associated with the downdraft (air going down) Most active stage  Gusty winds, lightning, heavy rain, hail Updrafts and downdrafts exist side by side

Stage 2: Mature Stage Downdrafts Downdrafts form for two reasons  Entrainment – mixing of dry air with cloud air at the edge of the cloud Causes the cloud drops to evaporate Energy from evaporation comes from the temperature of the air parcel By evaporating drops, you use up the energy and end up cooling the air parcel Temperature drops, parcel cools Cooler air sinks to the surface  Drag – Air is dragged downwards as precipitation falls.

Airmass Thunderstorms Stage 1Stage 2Stage 3

Stage 3: Dissipating Stage Cooling effect of falling precipitation and influx of colder air up top mark it’s END! Downdraft cuts off updraft Cloud stops growing You’re left with weakly descending air parcels The cloud basically KILLS itself

Airmass Thunderstorms - Summary Stage 1Stage 2 Stage 3 Stage in which updrafts dominate throughout the cloud, and growth from a cumulus to a cumulonimbus occurs. The most intense phase, with heavy rain and possibly small hail, in which downdrafts are found side by side with updrafts Dominated by downdrafts and entrainment, causing evaporation of the structure.

Severe Thunderstorms Heavy downpours, flooding, gusty straight-line winds, large hail, lightning and tornadoes! To be classified as Severe:  Must have winds in excess of 93 km/h or 58 mph  or hail 0.75 inches  or a confirmed tornado

Severe Thunderstorms Persist for many hours  Vertical wind sheer causes precipitation to fall in downdraft, allowing the updraft to retain strength.  Sometimes the updraft is so strong you get overshooting tops and anvils!

Severe Thunderstorms

Cold air of the downdrafts spread out making  “mini cold front”  Called a GUST FRONT  Called an Outflow Boundary  Can form a Roll Cloud.

Supercell Thunderstorms Causes dangerous weather a year Large, very powerful, up to 20 km in height Last many hours km in diameter

Supercell Thunderstorms Vertical wind profile may cause the updraft to ROTATE!  MESOCYCLONE – usually spawns Tornadoes! Need a lot of Latent Heat  Requires moist troposphere  An inversion a couple of km above the surface (caps moisture)  Unstable air can break through the inversion by “eroding” it via mixing.

Squall Lines Develop in the warm sector of a Mid-Latitude Cyclone in ADVANCE of the cold front. Linear in shape Can last for 10 hours or more

Squall Lines Sometimes preceded by mammatus clouds Form when the divergence and resulting lift created by the jet stream is aligned with strong persistent low level flow of humid air from the south. Also can form on drylines

Mesoscale Convective Complexes Consist of many individual T-storms organized into a large oval or circular cluster. MCCs are BIG  At least 100,000 sq km in area (39,000 sq miles) MCCs are SLOW  Can persist for up to 12 hours

Mesoscale Convective Complexes Associated with tornadoes Typically form in the Great Plains from a group of afternoon air-mass T-Storms. To transform from air-mass T- Storm to MCC you need lots of low level warm and moist air.

Microbursts Localized downdraft About 2.5 miles (4 km) across Downdraft is accelerated by evaporative cooling Typically last 2-5 minutes

Microbursts Outflow at the surface can move in excess of 160 km/hr (100 mph). Microbursts can cause quite a bit of destruction.  Tress, homes, aircraft…

Lightning and Thunder A storm is classified as a thunderstorm only after thunder is heard!! Because thunder is produced by lightning, lightning must also be present! We’ll talk about both Lightning and Thunder!

Lightning Lightning is a discharge of electrical energy. Essentially a giant “spark” between regions of positive (+) and negative (-) charge.

Lightning May occur:  Between cells in the same storm (inter-cloud lightning)  Within a cloud (intra-cloud lightning)  Cloud to air  Cloud to ground (CG)

Lightning Formation Lightning forms when a charge separation occurs in a cloud  The earth trying to equalize the electrical difference  Negative charges want to flow to the ground.

Charge Separation We don’t exactly know why it happens One theory:  Hail stones tend to have a warmer surface than ice crystals  When warm hail collides with colder ice, electrons transfer from ice to hail  Hail (-) is bigger and heavier and settles toward the bottom of the cloud  Smaller (+) ice crystals are lofted to the top.

Lightning Formation Excess electrons cause the air to ionize  Rip molecules apart (N 2 or O 2 become N+, O -…) Air is normally very insulating Ionized air is very conducting (like a metal wire)

Lightning Formation Ionized air forms tubes of ~50 m in length (150 ft) and 10 cm in diameter (4 in)  Called a LEADER  A bunch of leaders that are connected is called a STEP- LEADER.  NOTE: There still hasn’t been a flash yet!!!

Lightning Formation Each electron contains LOTS of energy When it reaches the surface the energy is RELEASED This energy is RELEASED as HEAT!! Electrons are drained at the surface first so the FLASH starts at the ground!!!

Lightning Formation AGAIN: Electrons are drained at the surface first so the FLASH starts at the ground!!! This is why lightning is said to GO UP, rather than down The movement is STILL from CLOUD to GROUND  But the “heating” happens at the surface FIRST.

Lightning Formation The electrons closest to the Earth are “discharged” first Heat that is released causes air around a step-leader to reach 8,000-30,000 K!!!!!  The warmer, the shorter the shorter the wavelength 8000 K K

Lightning Formation The emission of radiation from very hot air leads to the VISIBLE flash of lightning. Sometimes after the 1 st flash you have a left over channel that can be recharged very quickly. Can have several (3-4) in rapid succession (1/10 of a second apart). (lead by a DART LEADER)

Lightning

Lightning Occurrence in US

Thunder When air is heated quickly a shockwave forms  causes thunder Similar to planes crossing the sound barrier, firecrackers and gun shots Sound travels at 330 m/s or 1000ft/s If thunder takes 3 seconds to happen after observing lightning then the storm is:  3 s * 1000 ft/s = 3000 ft away (6/10 of a mile away).

Thunder A good “rule of thumb”:  The storm is 1 mile away for very 5 seconds in between lightning and thunder.  NOTE: Thunder actually happens at the same time the lightning strikes, but you will hear a delay because light travels much faster than sound!!

Tornadoes - Introduction A tornado is a rapidly rotating narrow region of low pressure Wind speeds from mph Pressure can be as low as 900 mb Tornadoes form during intense thunderstorms.

Tornado Development Step 1: The first object that forms is a rotating body of air at the ground  This occurs because of vertical wind sheer a)Wind speeds are higher as you increase in altitude b)Drag and Friction: Friction and drag at the surface cause the air at the ground to move more slowly.

Tornado Development Step 2: Horizontal rotating air is lifted off the ground by the Updraft of a Thunderstorm Once the horizontal rotating air mass is lifted nearly vertically it is considered a MESOCYCLONE!

Tornado Development Step 3: Mesocyclone is fully developed in the updraft of a thunderstorm  If a tornado develops it descends from the slowly rotating “wall cloud” in the lower part of the cloud.

Mesocyclone Review Remember it is a vertical cylinder of rotating air, typically 3-10 km across (2-6 miles) Develops in the updrafts of severe T-Storms Usually precedes tornadoes by 30 min

Mesocyclone Review Stretching of the mesocyclone column causes faster rotation  Just like a figure skater!!  Something that begins with a larger diameter rotating at a slow speed begins to rotate faster as the tube is elongated and the diameter decreases  No one knows why this happens!

Thunderstorm + Tornado From the wall cloud a very narrow, fast rotating structure emerges. This forms a funnel cloud (as long as the cloud does not touch the ground) As soon as the funnel cloud touches the ground it is called a tornado.

Thunderstorm + Tornado Some tornadoes have multiple suction vortices Intense areas of high winds that are part of ONE tornado There can be 4-6 suction vortices. The stronger the tornado the more vortices you’re likely to get. Weak tornadoes usually don’t have them

Tornado Occurrence in US TORNADO ALLEY!

Tornado Classification Tornados are classified as “weak,” “strong,” and “violent”  WEAK  WEAK – usually rope like and narrow  STRONG  STRONG – usually the classic funnel shape  VIOLENT  VIOLENT – usually have lots of debris associated with them and can be quite dark in color.

Fujita Scale Scale for Damage caused by tornadoes