Types of Thunderstorms

1. Airmass or Ordinary Cell Thunderstorms

2. Supercell / Severe Thunderstorms

•Limited wind shear•Often form along shallowboundaries of convergingsurface winds

•Precipitation does not fallinto the updraft•Cluster of cells at variousdevelopmental stages dueto cold outflow undercuttingupdraft

ORDINARY CELL THUNDERSTORMS

1. CUMULUS STAGE

• Sun heats the land

• Warm, humid air rises

• Condensation point isreached, producing acumulus cloud

• Grows quickly (minutes)because of the release oflatent heat

• Updrafts suspend droplets

• ‘Towering cumulus’ orcumulus congestus

2. MATURE STAGE

• Droplets large enoughto overcome resistanceof updrafts (rain/hail)

• “Entrainment” Drier air is drawn in

• Air descends in downdraft, due toevaporative coolingand falling rain/hail

• Anvil head when stablelayer reached (cloudfollows horizontal wind)

• Strongest stage, withlightning and thunder

Mature, ordinary cell thunderstorm with anvil head

Microbursts create aviation hazards

3. DISSIPATING STAGE

• Updrafts weaken as gust front moves away from the storm

• Downdrafts cut off thestorm’s “fuel supply”

• Anvil head sometimesremains afterward

• Ordinary cellthunderstorms may pass through all three stages in only 60 minutes

Review of Stages:Developing (cumulus), mature and dissipating

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 surfacewinds)

Conditional Instability

1. Heating within boundary layerAir trapped here due to stable layer aloftincreasing heat/moisture within boundary layer(BL).

2. External trigger mechanism forces air parcels to rise to the lifted condensation level (LCL)Clouds form and temperature follows MALR

3. Parcel may reach level of free convection (LFC). Parcel accelerates under own buoyancy.Warmer than surroundings - explosive updrafts

4. Saturated parcel continues to rise until stable layer is reached

CAPEConvective available potential energy (J/kg)

CAPE (J/kg)

0 Stable

<1000MarginallyUnstable

1000-2500Moderately Unstable

2500-3000Very Unstable

>3500Extremely Unstable

The Severe Storm Environment

1. High surface dew point

2. Cold air aloft (increases conditional instability)

3. Shallow, statically-stable layer capping the boundary layer

4. Strong winds aloft (aids tornado development)

5. Wind shear in low levels (allows for long-lasting storms)

6. Dry air at mid-levels (increases downdraft velocities)

A squall line (MCS)

Radar image of squall line

Wind shear and vertical motions in a squall line thunderstorm

Mesoscale convective complex (MCC)

Outflow Boundaries

Thunderstorm movement in an 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

Tornadogenesis

1. Mesocyclone 5-20 km wide develops2. Vortex stretching: Lower portion of

mesocyclone narrows in strong updrafts3. Wind speed increases here due to conservation

of angular momentum4. Narrow funnel develops: visible due to adiabatic

cooling associated with pressure droppage

2 hours after the Lethbridge tornado

Tornado producing supercell

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Global tornado frequency

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Waterspouts–Similar to tornadoes–Develop over warm waters –Smaller and weaker than tornadoes

Distribution of lightning strikes

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LightningSource 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 tocolder object (hailstone gets negatively charged &fall toward bottom - ice crystals get + charge)

•Many theories exist: open area of research

Development of lightning

Flashes per squarekilometre per year

                                                                           

             

Four typesof cloud-ground lightning

Most common

•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!

Cloud-ground lightning

90% induced by negatively charged leaders10% induced by positively charged leadersSometimes, there are ground to cloud leaders

Negative cloud-ground lightningLeaders branch toward the ground at about 200 km/s, with a current of 100-1000 AmperesThe return stroke produces the bright flash

•Potential difference between lower portion ofnegatively-charged leader and ground~10,000,000+ V

•As the leader nears the ground, the electricpotential breaks the threshold breakdown strength of air

•An upward-moving discharge is emitted fromthe Earth to meet with the leader

The return stroke lasts about 100 microseconds,and carries a charge of 30 kiloAmperes, producing the main flash

The temperature along the channel heats to 30,000+ K, creating an expanding high pressurechannel, producing shockwaves

Blue jet

Multiple suction vortices greatly increase damage

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