Week 8 - Heatwaves and droughts, ordinary thunderstorms, organised thunderstorms, thunderstorm hazards, fire weather

Heatwaves

A prolonged period of excessive heat → consecutive days of extreme high temperature (at least 3-5 days)

Absolute threshold

Extremes are defined by a fixed threshold (eg T>35 degrees Celsius)

• Can be tied to limits relevant to impacts (eg physiological or engineering design limits)

• Not relevant for all locations

• Not necessarily meaningful in context of weather and climate processes

Relative threshold

• Extremes are defined relative to ‘normal’ values

• Meteorological variables conform to statistical distributions

• Comparable at all global locations

• Meaningful in a climate context

• Not necessarily impact relevant

Heatwave - health impacts

• Cause infrastructure problems (eg high temperatures cause railway buckling)

• Cause bushfires

• Cause ecological stress on land and in the ocean

Drivers of heatwaves

1. Persistent high pressure systems

1. Warmer than average conditions due to natural variability (eg El Niño Southern Oscillation)

3. Dry soil

Fire weather ingredients

1. Ignition source (lightning or other cause)

2. Fuel to burn

Fire spreads through:

• Strong gusty, changeable winds

• Low moisture content of the atmosphere (RH decreases at higher temperatures)

• Pre-existing dry conditions

• Often the most problematic fires occur after a cold front passes (extends the fire front making it harder to tackle the blazes)

Pyrocumulus → Pyrocumulonimbus

Fires on a large enough scale can also create their own weather and sometimes even cause more fires (ie a positive feedback)

What is a thunderstorm?

Cumulonimbus cloud with lightning and thunder (Convective storm)

Thunderstorm ingredient

1. Moisture: Needed for clouds! Moist air also has lower lapse rate than dry air (cools more slowly)

2. Instability: Air needs to be less dense than its surroundings to rise to high elevations

3. Lift: Something that forces air to rise (a “trigger” mechanism)

Ordinary thunderstorms

Raindrops grow in the updraft, and then fall to the ground when they become too heavy to be aloft.

The storm weakens when the downdraft becomes stronger than the updraft.

Ordinary thunderstorms last no more than 30 to 60 minutes.

Ordinary single cell thunderstorms

One updraft, one downdraft

Ordinary thunderstorms: Cumulus stage / Developing

Typically occur in a weak vertical wind shear environment (little change of wind speed or direction with height).

Small cumulus clouds bubble up within an unstable, but relatively dry atmosphere.

Cumulus clouds gradually moisten the atmosphere above for subsequently deeper clouds.

Clouds are too shallow for precipitation to form.

Lifting Condensation Level (LCL)

The level at which a parcel becomes saturated (RH=100%)

Sometimes same as the LFC (level of free convection), but not always

Ordinary thunderstorms: Mature stage

Eventually the clouds grow up towards the tropopause, forming an anvil.

As rain develops within the clouds, it may fall down through the convective updraft.

Updraft and downdraft

As the updraft mixes in, or “entrains”, some dry air from outside the clouds, we may get evaporative cooling, negative buoyancy, and generation of a downdraft.

When the downdraft reaches the surface, a gust front develops at the lading edge of the cold air as it hits the ground and spreads out.

Ordinary thunderstorm: Dissipation stage

Falling precipitation and evaporative cooling (ie downdraft processes) begins to dominate.

Updraft weakens, gust front moves away from storm, and now cool air near surface is not supportive of rising motion.

Storm dissipation occurs because there is not source of surface warm, moist air remaining to generate positive buoyancy and convective updrafts in the storm

Ordinary thunderstorms stages summary

Developing: warm air rises (updraft) and cools, forming a towering cumulus cloud, no rain yet — only updrafts present

Mature: storm is fully developed, both updrafts and downdrafts exist, heavy rain, lightning and thunder occur, the anvil may form at the top of the cloud

Dissipating: downdraft dominates, cutting off the updraft, the storm weakens and rain decreases, cloud begins to evaporate and fade away

Updraft

Column of warm, moist air within a thunderstorm; the sun heats the ground, warming the air above it, warm air is less dense than the surrounding air, so it rises

Downdraft

Downward current of cool air within or beneath a thunderstorm; when rain and hail fall through the cloud, they drag air downward, falling precipitation evaporates cooling the air even more — cause gusty winds at the surface, cause microbursts

Anvil

The anvil is the flat, wispy top of a cumulonimbus cloud that spreads outward when the rising air (updrafts) reaches the tropopause

“Overshooting top”

Seen at the top of a very strong thunderstorm cloud — dome like bulge that pushes above the anvil of cumulonimbus cloud

When updrafts is extremely strong, powerful enough to punch through the tropopause

Mammatus

Clouds that are rounded, pouch-shaped formations that hang beneath the anvil of a cumulonimbus cloud, looks like a cluster of smooth, bulging bubbles hanging from the sky

Air inside the cloud sinks slightly

Ice crystals

Tiny, frozen water particles that form in clouds when the air is cold enough, develop directly from water vapour in the atmosphere when the temp is low enough for deposition (gas → solid)

Virga

Precipitation that evaporates before hitting the ground

Organised thunderstorms

Vertical wind shear is a parameter that describes how wind speed and/or direction changes with height in the troposphere

Vertical wind shear helps organise convection.

At its simplest, vertical wind shear tilts the thunderstorm, separating the updraft from the downdraft.

Multi-cell thunderstorms

Groups or clusters of ordinary cells at different stages in their life cycles

Individual cells last 30-60 minutes, but the entire system can last many hours

Mesoscale Convective System (MCS)

Cluster of thunderstorm cells that organises into a larger group.

MCSs can last for several hours or even days, and cover areas of several hundred kilometers

Squall line

Line or band of intense thunderstorms that can stretch for hundreds of kilometers, usually forming along or ahead of a cold front where warm, moist meets cooler, drier air

Supercell thunderstorms

• Rotating updraft

• Can last several hours

• Have a single large, rotating, violent updraft

• Updrafts often so strong that region below the mesocyclone is rain free

• Rain not heavy enough to fall through the updraft

• Supercells produce heavy rain, large hail, strong surface winds, and majority of tornadoes

Vertical wind shear, horizontal wind shear vertical plane horizontal plane for ordinary or organised or multicelled or supercell !!!!

Vertical wind shear — change in wind speed or direction with height, seen in the vertical plane (looking at a side view of the atmosphere)

Horizontal wind shear — change in wind speed or direction across a horizontal distance, seen in the horizontal plane (looking from above)

Vertical plane (side view) — winds increase with height or change direction with height, causes the updraft to tilt — crucial for storm organisation

Horizontal plane (top view) — winds vary horizontally across a region (eg stronger winds on one side), can generate boundaries or convergence zones (good spots for thunderstorm initiation)

Thunderstorm hazards

• Large hail (>2 cm in diameter)

• Damaging wind gusts (>90 km/h or greater)

• Heavy rainfall conducive to flash flooding

• Tornadoes

All lightning is dangerous but all thunderstorms contain lightning, so lightning on its own does not make a thunderstorm severe

Tornadoes

• Violent rotating column of air touching the ground, usually attached to the base of a thunderstorm

• Can last few seconds to over an hour

• Can have multiple vortices

• Often produced by supercells, but can form with other thunderstorms

• Strong tornadoes, winds can exceed 200 mph (~322 km/h)

• Occur most frequently in the late spring and early summer in the US

• Strong vertical wind shear

• Warm, moist air from Gulf of Mexico near surface

• Cooler drier continental air aloft

Tornadoes in Australia

• Generally 20-50 observed per year

• Two main seasons: warm and cool season

• Warm season (eg supercells)

• Cool season (eg cold front thunderstorms)

• Relatively common with convergent thunderstorm bands around tropical cyclones

Large hail

Forms inside strong thunderstorms, especially supercells, when powerful updrafts keep ice particles suspended high in the storm long enough to grow big

Damaging wind

Damaging winds are associated with the downdraft and outflow winds

Damaging wind: Microburst

Rapid downburst of air from an intense thunderstorm, posing major hazard to aviation.

Dry and Wet microburst

Dry microburst

No precipitation

Wet microburst

Downdraft remains saturated with precipitation all the way to the surface, maximising the downwards acceleration

Heavy rain and flash flooding

Flash flooding caused by factors: rain rate and duration, surface type/soil type, soil moisture/previous precipitation, existing water levels (in rivers and streams), terrain effects

Lightning

• Charge separation enables static electricity build up to discharge via lightning