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THUNDERSTORMS. Types of Thunderstorms Airmass or Ordinary Cell Thunderstorms 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

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Presentation Transcript
slide2

Types of Thunderstorms

  • Airmass or Ordinary Cell Thunderstorms
  • 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
slide3

ORDINARY CELL THUNDERSTORMS

  • 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
slide4

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
slide9

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
slide10

Review of Stages:

Developing (cumulus), mature and

dissipating

slide11

Thunderstorms

  • Typical conditions:
  • Conditional instability
  • Trigger Mechanism
  • (eg. front, sea-breeze front, mountains,
  • localized zones of excess surface heating,
  • shallow boundaries of converging surface
  • winds)
slide13

Thunderstorm Development

  • 1. Heating within boundary layer
  • Air trapped here due to stable layer aloft
  • increasing heat/moisture within boundary layer
  • (BL).
    • 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
slide14

CAPE

Convective available potential energy (J/kg)

slide15

CAPE (J/kg)

0

Stable

<1000

Marginally

Unstable

1000-2500

Moderately

Unstable

2500-3000

Very Unstable

>3500

Extremely

Unstable

slide18

The Severe Storm Environment

  • High surface dew point
  • Cold air aloft (increases conditional instability)
  • 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)
  • Dry air at mid-levels (increases downdraft
  • velocities)
slide29

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
slide31

Tornado Development

  • 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
slide34

Tornadogenesis

  • Mesocyclone 5-20 km wide develops
  • Vortex stretching: Lower portion of
  • mesocyclone narrows in strong updrafts
  • Wind speed increases here due to conservation
  • of angular momentum
  • Narrow funnel develops: visible due to adiabatic
  • cooling associated with pressure droppage
slide42

Tornado producing supercell

[insert fig 11-29]

slide44

Global tornado frequency

[insert fig 11-32]

slide46
Waterspouts

Similar to tornadoes

Develop over warm waters

Smaller and weaker than tornadoes

slide48

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
slide50

Development

of cloud to

ground

Lightning

(20% of cases)

Charge separation

Stepped leader approaches ground

Positive charge

surges upward

from ground

Spark surges

up from ground

slide51

Positive strikes

  • Particularly deadly
  • Surprise! occur outside of stormiest area
  • Tend to be stronger
slide52

Flashes per square

kilometre per year

slide56

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!
slide57

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

slide58

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
slide59

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!