Chapter 11 Thunderstorms

Thunderstorms

Cloud/cluster of clouds that causes:
•Lightning/thunder
•Heavy precipitation
•Hail
•High winds
•Tornadoes

Thunderstorms form from what type of clouds?

Cumulonimbus

Thunderstorm types

1. air mass
2. multicell
3. supercell

Where it's warm and moist and where lifting mechanisms are present, you get _______

thunderstorms!

Thunderstorm triggers

1. Lifting mechanism
2.Buoyancy

Lifting mechanism

•Mountains
•Fronts
•Pure convection
•Surface convergence
•"Boundaries" of all types

Buoyancy

•Warm, moist air at surface; cooler, drier air aloft
•Usually air rises rapidly only after reaching saturation
•Lifting mechanisms cool air to saturation
=Results--> rising air (updraft) up to tropopause (anvil forms)

Air mass Thunderstorm life cycle

1. Updraft condenses moisture out of air
2. Precipitation forms via Bergeron et al. processes
3. Falling precipitation leads to downdraft
4. Downdraft kills updraft, thunderstorm dies
5. UPDRAFT-DOWNDRAFT-THE END!

Vertical wind shear

change of horizontal wind (in both speed and direction) as you go up

What are the causes of vertical wind shear?

•Large-scale: "environmental" winds (jet streams)
•Small-scale: flow around thunderstorm (downdraft and outflow from storm, called the "cold pool")

Air mass thunderstorm without environmental wind shear

•Weak rising motion at both leading edges of cold pool
•Why? Due to small-scale shear
•No new storms, though
•Parent thunderstorm dissipates, end of story

Multicell thunderstorm with environmental wind shear

•Strong rising motion at eastern edge of cold pool
•Why? Due to synergy of small-scale, environmental shears
•New storm forms on the downwind or "downshear" side
•Parent thunderstorm dissipates, new storm cell takes over

Multicell thunderstorm lifecycle

•Oldest, dying cells at left (usually, west)
•Youngest, growing cells at right (usually, east)
•Shear gives storm the ability to last for hours

Mesoscale convective system (MCS)

Multicell thunderstorm type

Supercell thunderstorm

• Strong shear at surface is tilted upward by updraft
• Result: oppositely spinning pair of "mesocyclones" inside the thunderstorm (demo)
• Rotating updraft is signature of supercells

Supercell thunderstorm schematics

1. Low pressure caused by tilted-upward rotation
2. Low "sucks in" more air, accelerating rotation and causing storm to move to right of upper-level winds
3. In only 30% of supercells, tornado forms (beneath mesocyclone)

Supercell jargon

• HP,LP
• FFD,RFD
• BWER
• Hail Shaft
• Splitting, V-notch
• Right-mover
• TVS

HP

high-precip (East U.S.)

LP

low-precip (West)

FFD

Forward & rear flanking downdrafts

Rain-free base (RFD)

under updraft; visual cue for possible location of tornadoes there

Bounded Weak Echo Region (BWER)

near the rain-free base, a radar signature of a tornadic thunderstorm

Hail shaft

• Greenish area in sky, column where hail falls.
• In tornadic supercells this is generally just to the left of and just prior to the possible tornado

Splitting, V-notch

updraft so strong it splits wind around it into halves, forming V on radar

Right-mover

the rightmost cyclonically spinning half of a split thunderstorm, may contain tornado

Tornado Vortex Signature (TVS)

• Doppler radar indication that winds are coming toward (negative, or green)
• away (positive, or red) from the radar in close proximity. This means a spinning vortex!

Appearance of Air mass/ ordinary single-cell

"popcorn" in visible satellite image

Vertical wind shear of Air mass/ ordinary single-cell

Small

Chance of sever weather in air maa/ordinary single-cell

Unlikely

Appearance of Multicell

1. MCC:state-size circular cloud in infrared satellite image
2. Squall line: line of thunderstorms in radar or satellite images

Vertical wind shear of Multicell

1. MCC: Small
2. Squall line: Moderate

Chance of severe weather

1. MCC: likely (nontornadic high winds)
2.Squall line: likely (20% of tornadoes from nonsupercell storms)

Appearance of Supercell

Hook echo in radar reflectivity image

Vertical wind shear of Supercell

Large

Change of Severe weather of Supercell

Very likely (80% of tornadoes from supercell)

Factors affecting thunderstorm growth and development

1. Lift(without this, forget everything else)
2. Moisture (without this, never get a cloud)
3. Stability (will the air keep rising?If not, no thunderstorm)
4. Vertical wind shear(air mass/multicell/supercell)
5. Low-level jet(moisture and energy source)
6. Capping inversion(lid on a boiling pot

What is a common way to forecast thunderstorms?

Lifted Index (LI)

Lifted Index (LI) formula

Tenvironment-Tparcel at 5.5 km

If LI > 0

Stable atmosphere

If LI 0 to -3

• marginally unstable atmosphere
• unlikely severe thunderstorm

If LI -3 to -6

• moderately unstable atmosphere
• possible severe thunderstorm

If LI -6 to -9

• very unstable atmosphere • probably severe thunderstorm

If LI < -9

• extremely unstable atmosphere • severe thunderstorm

Does Lifted Index tell us anything about tornado likelihood?

• No, it does not
• For tornadoes to occur, you often need not only a very negative LI, but you also need strong vertical wind shear
• Especially directional shear, ex: south winds

What is the key predictor of Supercells?

• Vertical wind shear in both speed and esp. in direction
• ex: south winds at surface, west winds at jet-stream level

Rule of thumb for tornadic thunderstorm forecasting

MORE VERTICAL WIND SHEAR IN DIRECTION
= BETTER CHANCE OF SUPERCELLS
= CHANCE OF DAMAGING TORNADOES

Where do severe thunderstorms occur?

1. Just ahead(east)of cold fronts, especially when you've got warm moist air at the surface,
2. cold air and wind shear up above, and
3. upper-level divergence associated with a jet stream.