Thunderstorms

Conditions required:

1. Conditional instability

2. Trigger Mechanism (eg. front, sea-breeze front, mountains, localized zones of excess surface heating, dry ground)

1. heating within boundary layerair trapped here due to stable layer aloftincreasing heat/moisture within BL

2. External trigger mechanism forces parcel torise to the lifting 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 its surroundings.

4. Saturated parcel will continue to rise to LOC

CAPE

Convective available potential energy(shaded area in thermodynamic diagram on previous slide)

Used by meteorologists to estimate the potential intensity of thunderstorms

The Severe Storm Environment

1. High surface dew point2. Cold air aloft (increases conditional instability)3. Statically-stable layer capping the boundary layer4. Strong winds aloft (favours tornado development)

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

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

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 towardthe vertical

What is a ‘supercell’ ?

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

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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

The Fujita Scale

F-0: Light damage. Winds up to 116 km/hF-1: Moderate damage. Winds 116 to 180 km/hF-2: Considerable damage. Winds 180 to 253 km/h F-3: Severe damage. Winds 253 to 332 km/h F-4: Devastating damage. Winds 332 to 418 km/h F-5: Incredible damage. Winds above 418 km/h

F-0 and F-1 tornadoes are considered "weak" F-2 and F-3 are "strong" F-4 and F-5 are "violent"

LightningSource of lightning: the cumulonimbus cloud

Collisions between supercooled cloud particles and graupel (and hail) causes cloud 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

                                                                           

             

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

Produces the main flash

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

Produces thunder shockwaves

NESWStorm Position Storm Position