Thunderstorms and Tornadoes

By Glenn McManaway

Types of Thunderstorms

1. Airmass or Ordinary Cell Thunderstorms

2. Supercell / Severe Thunderstorms

•Limited wind shear

•Often form along shallow

boundaries of converging

surface winds

•Precipitation does not fall

into the updraft

•Cluster of cells at various

developmental stages due

to cold outflow undercutting

updraft

ORDINARY CELL THUNDERSTORMS

1. CUMULUS STAGE

• Sun heats the land

• Warm, humid air rises

• Condensation point is

reached, producing a

cumulus cloud

• Grows quickly (minutes)

because of the release of

latent heat

• Updrafts suspend droplets

• „Towering cumulus‟ or

cumulus congestus

2. MATURE STAGE

• Droplets large enough

to overcome resistance

of updrafts (rain/hail)

• “Entrainment”

Drier air is drawn in

• Air descends in

downdraft, due to

evaporative cooling

and falling rain/hail

• Anvil head when stable

layer reached (cloud

follows horizontal wind)

• Strongest stage, with

lightning and thunder

Mature, ordinary cell thunderstorm with anvil head

3. DISSIPATING STAGE

• Updrafts weaken as

gust front moves away

from the storm

• Downdrafts cut off the

storm‟s “fuel supply”

• Anvil head sometimes

remains afterward

• Ordinary cell

thunderstorms may

pass through all three

stages in only 60

minutes

Microbursts create aviation hazards

Review of Stages:

Developing (cumulus), mature and

dissipating

Converegence / DivergenceLeft Diagram: Not conducive for Thunderstorm activity (High Pressure System)

Right Diagram: Associated with Thunderstorm activity (Low Pressure System / Oragraphic lift / Air mass boundry lift / Frontal Lift

Thunderstorms

Typical conditions:

1. Conditional instability

2. Trigger Mechanism

(eg. front, sea-breeze front, mountains,

localized zones of excess surface heating,

shallow boundaries of converging surface

winds)

1. External trigger mechanism forces air parcels

to rise to the lifted condensation level (LCL)

Clouds form and temperature follows MALR

2. Parcel may reach level of free convection

(LFC). Parcel accelerates under own buoyancy.

Warmer than surroundings - explosive updrafts

3. Saturated parcel continues to rise until

stable layer is reached

A squall line

Radar image of squall line

Wind shear and vertical motions in a

squall line thunderstorm

Mesoscale convective complex (MCC)

See: http://rsd.gsfc.nasa.gov/rsd/movies/preview.html

Tornado Development

1. Pre-storm conditions:

Horizontal shaft of rotating air at altitude of

wind shift (generally S winds near surface

and W winds aloft)

2. If capping is breached and violent

convection occurs, the rotating column is

tilted toward the vertical

Supercell Thunderstorms

•Defined by mid-level rotation (mesocyclone)

Highest vorticity near updraft core

•Supercells form under the following conditions:

High CAPE, capping layer, cold air aloft, large

wind shear

Tornado producing supercell

[insert fig 11-29]

Global tornado frequency

[insert fig 11-32]

[insert table 11-2]

Lightning

Source of lightning: the cumulonimbus cloud

•Collisions between supercooled cloud particles and

graupel (or hail) cause clouds to become charged

•Most of the base of the cumulonimbus cloud

becomes negatively charged – the rest becomes

positively charged (positive electric dipole)

•Net transfer of positive ions from warmer object to

colder object (hailstone gets negatively charged &

fall toward bottom - ice crystals get + charge)

•Many theories exist: open area of research

•Heats up to 50,000 Degrees F

Distribution of lightning strikes

[insert fig 11-23]

Development

of lightning

•Intracloud Discharges

•Cloud to Ground Discharges

- death and destruction of property

- disruption of power and communication

- ignition of forest fires

- Lightning is an excellent source of soil

nitrogen!