Transcript Lecture 20

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 layer
air trapped here due to stable layer aloft
increasing heat/moisture within BL
2.
External trigger mechanism forces parcel to
rise 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.
2.
3.
4.
High surface dew point
Cold air aloft (increases conditional instability)
Statically-stable layer capping the boundary layer
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 toward
the 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 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
The Fujita Scale
F-0: Light damage. Winds up to 116 km/h
F-1: Moderate damage. Winds 116 to 180 km/h
F-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"
Lightning
Source 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 to
colder object (hailstone gets negatively charged &
fall toward bottom - ice crystals get + charge)
Many theories exist: open area of research
Four types
of cloudground
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 100-1000 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
Produces the main flash
The temperature along the channel heats to
30,000+ K, creating an expanding high pressure
channel
Produces thunder shockwaves
SW
Storm Position
NE
Storm Position