METR Unit 4

atoms 3 components

Protons: particles with positive charge

Neutrons: particles with no charge

Electrons: particles with negative charge

ion

If # protons ≠ # electrons

atom is charged

positively charged

has more positively charged ions than negatively charged ions

negatively charged

has more negatively charged ions than positively charged ions

what is a tornado

a violently rotating column of

air that is in contact with:

◦ A cloud

◦ The ground

what types of tornado exists

Non-supercell tornadoes (commonly called

“landspouts”): typically weaker and do not

form from supercells

Supercell tornadoes: much stronger because

they form from the organized structure of

supercells

landsprouts

non supercell

weaker

form under developing clouds with only

updrafts

◦ Ordinary thunderstorms, multicell clusters, MCSs, etc.

◦ NOT supercells!

o Can cause some damage, but are usually not

destructive

o Often lack a visible funnel

◦ Like tornadoes, the circulation must be touching the

ground to be considered a landspout

◦ Can tell if they’re touching the ground by the debris they

kick up

◦ Smooth, tube-like appearance

o Often not detected by radar

◦ Since parent storms aren’t rotating (they have no

mesocyclone)

Sources: landspout

how do landsprouts form

A cloud or storm moves towards and

over pre-existing vertical vorticity at the

surface

o The storm’s updraft stretches the

vertical vorticity

o Stretching narrows the column of air

and increases vertical vorticity, producing

a landspout

o Note: only the updraft matters in this

process

Landspouts eventually weaken and

dissipate as the parent cloud (and

updraft) move away from the region of

enhanced vertical vorticity

supercell tornadoes

1. Updraft tilts horizontal

vorticity into the vertical

2. Downdraft pushes these

vortex lines (streamlines of

vorticity) downward on the

west side of the updraft

3. Vertical vorticity is brought

to the surface, tornado

forms

how do supercell tornadoes form

Tilting of horizontal vorticity into the vertical

by an updraft produces large vertical vorticity

in the middle the cloud

◦ The mesocyclone

◦ Does NOT create the intense, vertically-

oriented rotation next to the surface that is

needed for a tornado

Where do most tornadoes occur?

There are two main regions

where most tornadoes—

especially strong to violent

tornadoes—form

◦ Great Plains

◦ Southeast

The Great Plains experiences more

tornadoes overall

Many tornadoes in the Southeast occur overnight

◦ People are unaware of weather threat and can’t see the tornadoes

2. Many tornadoes in the Southeast occur in winter

◦ People aren’t expecting tornadoes

3. Many mobile homes and weak-framed homes in the Southeast

◦ Can be easily destroyed and turned into life-threatening debris

4. Greater population density than in the Great Plains

when do most tornadoes occur? Time of year? Time of day?

Tornadoes occur in every month

of the year

◦ However, they are most common in

April-July

Southeastern US: peak season in

February-May

Great Plains: peak season in

April-July

Across the US, tornadoes predominantly

occur between 3:00-8:00 PM local time

The surface is warmest during this time

frame

How are tornadoes rated?

Enhanced Fujita (EF)

Tornado rated by the maximum amount

of damage it does in its lifetime

How do tornadoes form? How certain are scientists of this mechanism?

Still just a theory

◦ Hundreds of researchers working on this

problem around the world

◦ This is also just one theory, there are multiple

o What’s the takeaway then?

o Mesocyclones: formed by horizontal

vorticity at the surface + an updraft

o Tornadoes: formed by horizontal

vorticity at the surface + an updraft +

the RFD

How are tornadoes confirmed? On radar? Visually?

The tornado is a violently rotating column of air,

and doesn’t necessarily have to have a visible

funnel

◦ Tornado will have visible dust or debris spinning

up on the ground

Two features that meteorologists look

for on radar to identify potentially

tornadic storms:

◦ The hook echo

◦ The debris ball

hook echo

The hook-like appendage on the back of a

supercell

◦ Indicates a rotating mesocyclone

◦ Does NOT indicate a tornado!

◦ However, a rotating mesocyclone is a good

indicator that a tornado may soon occur or is

occurring

denotes mesocyclonic rotation

debris ball

Region of very high reflectivity on the end of

the hook echo

◦ DOES indicate a tornado

◦ Caused by debris being lofted into the air

◦ Debris picked up by the tornado is large; reflects a LOT

of energy back to the radar

◦ Shows up as pinks, purples, even white on the

radar

denotes tornado

Debris balls don’t occur with every

tornado

◦ Only large ones that loft lots of debris

supercells

a severe thunderstorm

with a rotating updraft

◦ A “spiral” appearance is common

among many supercells due to

rotation in the mid-levels

why are supercells important

Updraft rotation can translate

into rotation at the surface!

◦ Supercells are the most prolific

tornado-producing storms

◦ Supercells also produce severe wind

and hail

axis rotation

Rotating objects and fluids spin around

an axis

◦ Called the “axis of rotation

vorticity

the rotation of a fluid

Two main mechanisms cause vorticity to

arise

A change in wind direction (see right)

◦ A change in wind speed (i.e. wind shear; see

below)

horizontal vorticity

rotation centered

around a horizontally-oriented axis

◦ Example: a spinning bicycle wheel

vertical vorticity

rotation centered

around a vertically-oriented axis

◦ Example: a record on a record player, a ceiling

fan

Vertical vorticity is needed for supercell

formation

◦ One way to produce it: tilting horizontal vorticity

into the vertical!

o Vorticity can be advected (moved around by

the wind), just like other atmospheric

quantities

stretching

It “tightens up”

◦ It spins faster

Stretching strengthens vorticity

◦ Think of a figure skater pulling their arms in

vorticity and supercells

step 1

vertical wind shear creates

horizontal vorticity

◦ Example: speed shear generating horizontal

vorticity

Step 2: the updraft begins to tilt the

horizontal vorticity into the vertical

◦ Also simultaneously stretches the vorticity,

strengthening the rotation

◦ Notice that the updraft is slightly tilted

◦ Characteristic of severe thunderstorms!

Step 3: the updraft tilts the rotating air

nearly vertically and stretches it

◦ Column of rotating air is very intense here, due

to stretching effects from the strong updraft

mesocyclone

cyclonically (counter-

clockwise) rotating updraft

◦ Size: 2-10 mi wide

◦ Rotation: up to 100x faster than the rotation of

the Earth on its axis

What ingredients are necessary to form a supercell

Shear

Weak Lift

Instability

Moisture

A low-level stable layer early in the day

Remember the 4 primary ingredients of severe storms, in general, using the

acronym SLIM

Shear

Lift

Instability

Moisture

Why is it important for supercells to be isolated from one another?

If supercells are too close to one

another...

◦ They will compete for the mT air

◦ Outflow from one storm may get too close to

another

◦ That cold air at the surface can kill the updraft

◦ May disrupt surface wind patterns, therefore disrupting

surface rotation and updraft tilting of that rotation

◦ None of these supercells can grow as strong as

they would if they were further apart

What features are associated with supercells? Where are they located in the storm?

Why are mesocyclones important to supercells?

What pressure is associated with the mesocyclone

A small low pressure center forms in the

middle of the mesocyclone

◦ Strong rotation produces low-pressure

Where in the atmosphere is the mesoscyclone the strongest?

vertical vorticity in the

mesocyclone is strongest in the midlevels

of the storm

The “tube” of vorticity becomes most

vertical far above the ground

◦ Typically, it’s most “vertical” in the middle of

the storm

between 3-7 km off the ground

downburst

a column of rapidly descending

air that can form in the downdraft of any

precipitating cloud

◦ From a severe thunderstorm to a weak rain shower

how do downbursts form

Downbursts form when rain falls from

the lower part of a storm or shower

into dry air below cloud base

◦ Partially or fully evaporates (or sublimates,

for ice and hail) into that dry air

◦ Evaporation/sublimation REQUIRES latent

heat

◦ Cools the air!

◦ Colder air = more dense air

◦ Cold air sinks rapidly downward

◦ Precipitation loading can also make the air fall faster

via frictional “dragging

microbursts vs macrobursts

Microbursts: downbursts that are ≤ 4 km across

Macrobursts: downbursts that are > 4 km across

dry vs wet bursts

dry: all precipitation evaporates/sublimates before hitting the ground

◦ Hard to see, because there’s nothing to see!

Wet: not all precipitation evaporates/sublimates before hitting the

ground

◦ Can see a large “blob” of rain hitting the surface

Both equally dangerous!

How does downburst damage compare to tornado damage?

Magnitude of damage caused can be similar

Tornado: damage occurs in a spiral shape

◦ Caused by the winds circulating around the

tornado

Downburst: damage radiates outward from

point of impact

◦ Caused by the winds spreading outward from

the center of the downburst

Sources: NWS La Crosse

How can one tell the

difference between downbursts and tornadoes

Downbursts and tornadoes look DECEIVINGLY SIMILAR on radar velocity

displays

◦ Both show red right next to blue

tornado: rotation around vertical axis yellow dot horizontal

downburst: diverge outward yellow dot vertical

What are hazards presented by downbursts?

Plane approaches downburst,

experiences headwind

◦ Provides extra lift

2. Plane enters core of downburst

◦ Horizontal wind speed is zero

◦ Decreases lift

◦ Sinking air pushes plane downward

3. Plane experiences a tailwind as it

exits downburst

◦ Decreases lift

◦ High potential of crashing into the

ground, since already so close to the

ground after step 2

virga

rain that evaporates

before it hits the ground

oThese clouds are a visual

clue that...

1. There is a pocket of

dry air aloft

2. Significant

evaporation is

happening

3. A downburst is

possible

Mesoscale Convective Systems (MCSs)

organized groups of severe thunderstorms

◦ Individual severe storms within an MCS are so close

together that their precipitation merges into one big line

MCSs? What are their characteristics? What shapes can they take?

MCSs can vary in shape

◦ MCSs can be “curved”

◦ MCSs can be highly “linear”

◦ Called a squall line

◦ Typically form along cold fronts, where forcing is

very linear in nature

What hazards do MCSs produce?

MCSs account for up to ~70% of

spring and summer precipitation in the

Plains

◦ Produce large swaths of hundreds of severe

weather reports

◦ Primarily severe hail and severe winds

o Also produce flash floods

◦ Due to heavy, persistent rain

o Can also produce tornadoes

◦ Vorticity, or “spin/rotation” is created along

the cold pool’s leading edge.

◦ This vorticity is then lifted into an updraft

and produces a tornado.

what are cold pools? How are they related to MCSs?

Cold pools are pockets of cooler air that form near the surface due to the evaporation and melting of precipitation within a thunderstorm. As rain falls, some of it evaporates, cooling the surrounding air. This cool, dense air then spreads outward as a gravity-driven flow, often creating gusty winds near the surface.

Cold pools play a crucial role in Mesoscale Convective Systems (MCSs). In these large, organized clusters of thunderstorms, cold pools help sustain and propagate the system by lifting warm, moist air ahead of the storm. This lifting can trigger new convection, reinforcing the MCS and allowing it to persist for several hours or even longer. Cold pools also contribute to the formation of gust fronts, which can enhance low-level wind shear, influencing storm intensity and structure.

What ingredients lead to the formation of MCSs? How do they do so?

Same as ingredients for severe storms: SLIM

◦ Shear

◦ Lift

◦ Usually “linearly” (line) oriented

◦ Instability

◦ Moisture

How does the LLJ contribute to these ingredients?

Influence of the LLJ: contributes to parts of the SLIM ingredients

◦ Ingredient 1: mT air, fulfils the “M” of SLIM

◦ The LLJ often advects warm, moist mT air from the Gulf of Mexico into the Plains, towards developing MCSs

◦ Ingredient 2: moderate to strong vertical wind shear, fulfils the “S” of SLIM

◦ LLJ provides moderate to strong vertical wind shear

◦ Since it causes wind to greatly increase in speed by the top of the BL

◦ Ingredient 3: strong rising motion over a large area, fulfils the “L” of SLIM

◦ Large scale advection of a very low density air mass (mT) allows for large scale rising motion

o The LLJ is highly effective at creating MCSs:

◦ This is a huge reason why MCSs are common in the late evening/overnight hours

◦ LLJ strength peaks in the evening and overnight!

What is a derecho? Why are they impactful?

widespread, long-lived wind

storm

◦ Associated with squall lines/bow echoes

What are bow echoes? How do they form? What hazards are they associated with?

What impact does wind shear orientation have on MCSs?

thunderstorms

a local storm, produced by a

cumulonimbus clouds, and is always accompanied by

lightning and thunder.

how common are thunderstorms? globally? US?

~45,000 thunderstorms occur

worldwide each day

◦ Mostly in the low latitudes

◦ Adds up to over 16 million

storms per year!

◦ ~10,000 of those storms

occur in the US

vertical wind shear

change of wind speed

and/or direction with height

types of wind shear

Speed shear: winds increasing/decreasing with

speed

Directional shear: winds changing direction

two broad classifications of thunderstorms

Ordinary thunderstorms:

◦ Last 30 minutes-1 hour

◦ Do not produce damaging hail, damaging winds, and/or tornadoes

◦ Can cause flooding and produce small hail

◦ Severe thunderstorms:

◦ Persist for hours

◦ Produce an instance of damaging hail, damaging winds, and/or tornadoes

Severe thunderstorms:

◦ Persist for hours

◦ Produce an instance of damaging hail, damaging winds, and/or tornadoes

How does wind shear contribute to producing severe storms?

Vertical wind shear tilts the updraft in the vertical

without “tipping” the developing storm over

how do thunderstorm ingredients contribute to thunderstorm formation?

Thunderstorms form primarily in warm,

moist air masses

◦ Air with the lowest density.

◦ This allows it to rise easily; rising air → clouds,

precipitation, storms

What are the components of a thunderstorm?

Describe the lifecycle of a thunderstorm.

o Ordinary

o Severe

ordinary: Last 30 minutes-1 hour

severe: lasts hours

what does a thunderstorm need to have to be severe

≥ 1” diameter hail (quarter size)

o Wind gusts ≥ 58 mph (50 kts)

o A tornado

any one of these

SLIM

Shear

Lift

Instability

Moisture

o The last three produce ordinary thunderstorms

o *All four produce severe thunderstorms!

thunderstorm formation

1. Air parcel rises upward

(convects) from the

surface, expanding and

cooling as it rises

2. A cloud forms after air

   parcels in the updraft

   rise far enough to cool

   to their dewpoints and

   reach saturation

   The updraft supports the cloud

   by continuously supplying warm,

   moist air parcels that can easily

   reach saturation and condense

3. Precipitation forms within the

   cloud as the cloud grows

   • As precipitation grows, it

   becomes too heavy to be

   supported by the updraft,

   and falls out

4. he falling precipitation pulls

   air downward with it and

   helps to cool the air (through

   some evaporation, which

   absorbs energy).

   • This creates a downdraft

5. When the descending air hits the

   ground:

   • It cannot travel vertically any

   more, so it diverges

   • The diverging air is called

   the outflow

   • Outflow produces the gusty,

   often cool winds that flow out

   of thunderstorms

6. This outflow is often called the

   cold pool

   • Often colder than the

   surrounding environment

   • Occurs due to evaporating rain

   in the downdraft

   Leading edge of the outflow:

   outflow boundary (yellow dashed

   line)

   • Location of strongest surface

   winds in a thunderstorm

   • Enhances rising motion—it is a

   lifting mechanism:

   • Can go on to initiate more

   storms!