Thunderstorms

What are the conditions required for thunderstorm development?

- Trigger mechanism

- Adequate moisture at low levels

- Deep layer of conditionally unstable air

Why is a trigger mechanism required for thunderstorm development?

- To push unstable saturated air aloft

- Different triggers affect how quickly air rises (length of TS)

Why is enough moisture required for thunderstorm development?

- Fuel that drives development

- Rising saturated air cooling releases latent heat to condense

- Heating promotes further instability and uplift

Why is a conditionally stable atmosphere required for thunderstorm development?

Dry air is stable and doesn't rise but the saturated air is unstable and rises (so trigger only moves unstable air)

How does latent heat help development of TS?

- Main driver

- Fuels development

- Internal trigger

- Source of instability

- Increases height of TS

How does entrainment affect the development of TS?

- Mixing of environmental air into pre-existing cloud

- Downdrafts pull cold dry air from outside of cloud

- Air in storm becomes colder and drier

- Helps with decay

What is the developing stage of TS?

- Trigger lifts air

- Condensation

- Latent heat

- Warms surrounding air

- Cloud gets bigger

- Trigger still present

- Forces more moisture into cloud

- Updrafts

What is the mature stage of TS?

- Anvil starts developing from stable upper layer

- Ingestion/blocking due to surrounding cold air stops growth

- Entrainment

- Cold air wants to sink (downdrafts)

- FZL bulges

- Precipitation on downdraft side (microbursts)

- Gust front

What is the dissipating stage of TS?

- Updrafts cease

- Bottom of cell begins to evaporate

How do thunderstorms regenerate?

- Fed additional energy

- New external trigger/multiple cells regenerating

What are convective localised thunderstorms?

- Most common

- Short life-span

- When warming and instability over strong convective currents

- Absence of wind shear (linked to area of convection)

What are convective travelling thunderstorms?

- Larger and longer than convective localised

- Wind shear at altitude

- Separated up and down draft zones

What are orographic thunderstorms?

- Air being pushed up mountain

- Confined to windward side

What are nocturnal tropical thunderstorms?

- Over oceans at night

- Retains heat from the day

- Differences in temperature in tropospheric layers cause instability

- Upper layer cools faster

What are frontal and convergence thunderstorms?

- From cold fronts

- Steeper, more aggressive trigger

- Gust front drags warm air back into cold front

- Frontal storm has squall line

- Convergence storm has haphazard line

What are surface and upper trough thunderstorms?

- Cold air aloft advected

- Steepens ELR

- Promotes instability

What are warm from embedded thunderstorms?

- Slower than cold fronts

- Active fronts producing TS behind and along boundary

How are TS identified on radar?

Doppler radar, reflected by raindrops, hail and snowflakes

Where are the severity and location of meteorological hazards in a TS?

- Turbulence in middle, underneath and in front

- Icing bad above FZL

- Wet microbursts more common in NZ

- Hail can be ejected, sides and bottom

How is turbulence a hazard?

Strengthens as TS develops, can affect wide area

How is gust front a hazard?

Strong downdraft with cold dense air, sudden wind change

How is icing a hazard?

Exposure to more icing conditions between 0°C to -12°C is the worst

How is lightning a hazard?

Damage varies, can affect instruments

How is hail a hazard?

Larger hail is worse, moves up and down in cloud and increases size

How is visibility a hazard?

Becomes poor due to showers

How are tornados a hazard?

Extreme area of cyclonic rotation, extreme winds, can form water spouts

How are microbursts a hazard?

Severe low-level wind pattern driven by strong downdrafts and evaporative cooling, air spreads out at high speeds, either wet or dry

What is the enhanced fujita scale?

Measures the amount of damage from least at EF0 to most at EF5

What causes lightning formation?

Ice particles growing and interacting, from friction forms electrical potential which must be discharged

Why is lightning dangerous?

Lots of electrical and heat energy discharged, creates shockwave due to rapidly changing pressure (thunder)

What is a transient luminous event?

Rare stratospheric and mesospheric lighting events only seen at night

- Sprites

- Jets

- Elves

What is a severe TS?

- 2cm hail

- 45 kts gusts

- Tornadoes

What is a multi-cell TS?

Multiple cells going through each different stage separately

What is a super-cell TS?

- Self-propagating (regenerating)

- Associated with mesocyclone