esci 106 – weather and climate lecture 10 chapter 10: “thunderstorms and tornadoes” 10-27-2011...
TRANSCRIPT
ESCI 106 – Weather and ESCI 106 – Weather and ClimateClimate
Lecture 10Lecture 10
Chapter 10:Chapter 10: “Thunderstorms and “Thunderstorms and
Tornadoes”Tornadoes”
10-27-201110-27-2011
Jennifer D. Small Jennifer D. Small
Weather Fact of the Day: October 27
2006: 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.
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 - 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 80-100 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
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
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
DowndraftsDowndrafts 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.
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 1 Stage 2 Stage 3Stage 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
2000-3000 a year
Large, very powerful, up to 20 km in height
Last many hours
20-50 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
100-300 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 (N2 or O2 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 K30000 K
Lightning Formation
The emission of radiation from very hot air leads to the VISIBLE flash of lightning.
Sometimes after the 1st 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)
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 70-300 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 Classification
Tornados are classified as “weak,” “strong,” and “violent”
WEAKWEAK – usually rope like and narrow
STRONGSTRONG – usually the classic funnel shape
VIOLENTVIOLENT – usually have lots of debris associated with them and can be quite dark in color.