Types of Thunderstorms
1. Airmass or Ordinary Cell Thunderstorms
•Limited wind shear
•Often form along shallow
boundaries of converging
surface winds
2. Supercell / Severe Thunderstorms
•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
Aviation Risk even in Ordinary Cell (Airmass) Thunderstorms
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
1000-2500
Moderately
Unstable
2500-3000
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)
Thunderstorm movement in a MCC
Outflow Boundaries
See: http://rsd.gsfc.nasa.gov/rsd/movies/preview.html
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
Wind shear separates updraft from downdraft
so it can keep developing
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
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
Wall Cloud
2 hours after the Lethbridge tornado
Tornado producing supercell
[insert fig 11-29]
Multiple suction vortices greatly increase da
[insert fig 11-37]
Global tornado frequency
[insert fig 11-32]
[insert table 112]
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 ice crystals and graupel/hail surrounded
by supercooled water droplets 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)
•Result: positive charges well aloft, negative charges near the
cloud base
Development
of cloud to
ground
Lightning
Charge separation
Stepped leader approaches ground
(20% of cases)
Spark surges
up from ground
Positive charge
surges upward
from ground
Positive strikes
Particularly deadly
1. Surprise! occur outside of stormiest area
2. Tend to be stronger
Flashes per square
kilometre per year
•Summary of Lightning Facts
•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, producing
the main flash
The temperature along the channel heats to
30,000+ K, creating an expanding high pressure
channel, producing shockwaves
This results in THUNDER!