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

Microbursts create aviation hazards

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

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 surface winds)

Conditional Instability

1. Heating within boundary layer Air trapped here due to stable layer aloft increasing 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

CAPE Convective available potential energy (J/kg)

CAPE (J/kg) 0 Stable <1000 Marginally Unstable Moderately Unstable Very Unstable >3500 Extremely 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:

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 develops 2.Vortex stretching: Lower portion of mesocyclone narrows in strong updrafts 3.Wind speed increases here due to conservation of angular momentum 4.Narrow funnel develops: visible due to adiabatic cooling associated with pressure droppage

2 hours after the Lethbridge tornado

Tornado producing supercell [insert fig 11-29]

Global tornado frequency [insert fig 11-32]

[insert table 11- 2]

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

Distribution of lightning strikes [insert fig 11-23]

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

Development of lightning

Flashes per square kilometre per year

Four types of 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 leaders 10% induced by positively charged leaders Sometimes, there are ground to cloud leaders Negative cloud-ground lightning Leaders branch toward the ground at about 200 km/s, with a current of Amperes The return stroke produces the bright flash

Potential difference between lower portion of negatively-charged leader and ground ~10,000,000 + V As the leader nears the ground, the electric potential breaks the threshold breakdown strength of air An upward-moving discharge is emitted from the 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 pressure channel, producing shockwaves

Blue jet

Multiple suction vortices greatly increase damage [insert fig 11-37]