Week 5 - clouds and precipitation, atmospheric stability, convection, humidity

Types of clouds:

• Stratus

• Cumulus

• Nimbus

• Cirrus

• Stratocumulus

• Nimbostratus

• Altostratus

• Cirrocumulus

• Cirrostratus

Cloud condensation nuclei (CCN)

Particle for the vapour to condense on to

Water vapour condenses onto CCN and fall once they become too heavy to be held in the cloud

Cloud formation needs:

• Water vapour

• Particles for the vapour to condense on to (CCN)

White clouds

• Clouds white due to scattering of light in all directions from cloud droplets

Grey rain clouds

• Rain clouds appear greyer because thicker, less light penetrating

Precipitation

Any form of liquid that falls from a cloud and reaches the ground

Rain drops form through the collision and coalescence of water vapour onto CCN

Why can clouds contain water and ice crystals even when below freezing temp?

Both water vapour and ice crystals (even when they’re below freezing)

Due to a lack of CCN to freeze onto

Water vapour is SUPERCOOLED

Water vapour supercooled

When water vapour exists in cloud even though temp is below freezing

Drizzle

Smaller droplets (less than 0.5mm in diameter) with lower velocities

Virga

Precipitation that evaporates before reaching the ground

Snow

Frozen water falling from the sky

Snowflakes occur in many shapes and sizes as a function of the freezing level and temperature profile, the concentration of CCN and ice crystals and the fall speeds

Sleet

In Australia: Mix of rain and snow that occurs when the freezing level is near the surface

In North America: ice particles that have melted as they’ve fallen then refrozen

Freezing rain

Rain falls onto frozen surface creating a layer of ice

Hail

Updrafts take hail above the freezing level in the cloud, where liquid water freezes.

This process repeats to form layering on hail and increase the size of hailstones

Lots of energy in usually cumulonimbus clouds

Graupel

Small snow covered in a thin layer of ice

Snow grains

Snow grains are the snow equivalent of drizzle - don’t bounce or shatter

Snow pellets

White opaque pieces of ice that are larger than snow grains - bounce and shatter, crunchy underfoot

Lapse rate

Rate at which air temperature changes with altitude

Tells you how quickly air cools (or warms) as you go up into the atmosphere

Lapse rate = temp change/change in altitude

Instability (negative damped feedback)

Clouds reflecting sunlight → cooling surface → dampens system change

Instability (positive amplified feedback)

• Warm ocean → more convection → more warming locally → amplifies system change

• Melting at the edge of ice lowers the albedo, therefore warms the surface more, which melts more ice…

• The shift form equilibrium is amplified in this instance

Dry adiabatic heating and cooling

Air parcel cools if it expands

Air parcel warms if it compresses

Assumption: no energy input into the parcel, then the energy for the parcel expansion can only come from the parcel itself. It has to come from the speed of the molecules in the parcel, which reduces.

Moist adiabatic heating and cooling

Saturated (moist) adiabatic heating and cooling is an adiabatic process for which the air is saturated

Static stability

Measure of the gravitational resistance of the atmosphere to vertical displacements of air parcels

• concept can be applied to looking at how likely it is for clouds and thunderstorms to form

• Stability depends to a large extent on change in temperature with height in the atmosphere - lapse rate

Stable

Air resists vertical movement → fog, calm conditions, stratus clouds

If a parcel of air is lifted, it is cooler than the surrounding air, so it sinks back down. Little cloud development, calm weather, stratus clouds, fog

Unstable

Air rises easily → thunderstorms, heavy convective rain

If a parcel of air is lifted, it is warmer than the surrounding air, so it continues to rise. Strong convection, tall cumulonimbus clouds, thunderstorms, heavy rainfall

Buoyancy

When warm air rises, it is surrounded by colder air, so therefore rises until it is no longer warmer than its surroundings

• Unless it is being forced to rise even further

Saturated (or moist) adiabatic lapse rate (SALR)

Condensation reduces lapse rate because in clouds there is more condensation aloft, which warms the parcel

Saturated (moist) adiabatic heating and cooling is an adiabatic process for which the air is saturated

Unsaturated (or dry) adiabatic lapse rate (DALR)

Air parcel cools if it expands

Air parcel warms if it compresses

Assumption: no energy input into the parcel, then the energy for the parcel expansion can only come from the parcel itself. It has to come from the speed of the molecules in the parcel, which reduces.

SALR < ELR < DALR

This means that we can have stable conditions in dry air below a cloud, but unstable conditions within a cloud where there is moisture

Convection

Describes air parcels moving freely upwards

Convection initiation

• Need lift!

• Surface heating

• Uplift of air over mountains (orographic)

• Pushing upwards of air by cold fronts

• Convergence of winds at the surface

• Turbulence (irregular air motion dye to eddies and vertical currents)

• “Free convection” (actual unforced rising air motion due to buoyancy forces)

• An air parcel simply being warmer and therefore less dense than the surrounding air (‘free convection’)

• An air parcel being ‘lifted’ upwards by an obstacle or other mechanism

Forced convection

• Fronts

• Cold fronts are steeper, as cold air moving undercuts the warmer air and pushes it up

• This steeper gradient results in more vigorous convection, clouds with greater vertical development, and more extreme precipitation

Free Convection

Cold surface winds tend to become increasingly unstable as they move equator wards

Lower part of air warmed by oceans

Difference in temperature between lower and upper part of air increases

Environmental lapse rate increases

Instability develops

Water vapour

The gas form of water

Transforms into cloud droplets and ice crystals - particles that grow in size and fall to the earth as precipitation

Humidity

Amount of water vapour in the air

Not constant through time or space

Evaporation

The sun’s energy transforms enormous quantities of liquid water into water vapour

Some evaporation is always occurring, even if the surface temperature is less than the boiling point

Condensation

Water vapour changes state into liquid water, forming cloud droplets

Precipitation

Rain, snow, or hail

Saturation

Eg. The cloud in bottle experiment

Total number of molecules escaping from liquid (evaporating) is balanced by the number returning (condensing)

Variety of ways to specify the amount of water vapour in the air

1. Specific humidity - mass of water vapour divided by the total mass of air (both water vapour and other gases)

2. Mixing ratio - mass of water vapour divided by the mass of dry air

3. Relative humidity - amount of water vapour divided by the maximum possible amount (saturation value), expressed as a percentage

4. Vapour pressure - partial pressure of water vapour (that is, the amount of total atmospheric pressure contributed only by water vapour)

5. Saturation vapour pressure - the pressure exerted by vapour molecules necessary to make the air saturated at a given temperature

Dew point

The temperature at which the atmosphere must be cooled for saturations, and hence condensation, to occur

More moisture dew point temp would have to be higher

Relative humidity

Air is saturated inside the cloud so relative humidity is 100%

Beneath the cloud, even though it is raining, the air is not saturated

Dew

Condensed water vapour at cold surfaces on ground

On cold clear nights

Frost

Forms when dew point temperature is at or below freezing

On cold clear calm mornings

Fog

Stratus cloud at the surface

• Cooling - air is cooled to its saturation point (dew point)

• Evaporation and mixing - water vapour is added to the air by evaporation, and the moist air mixes with relative dry air

Haze

Mix of nitrogen dioxide or smoke - pollution

Types of fog

• Radiation fog

• Advection fog

• Upslope fog

Radiation fog (valley fog)

Produced by the Earth’s radiational cooling at night, as the air above the ground cools and stabilises

Advection fog (sea fog)

Warm, moist air moves over a sufficiently colder surface; the moist air may cool to its saturation point

Upslope fog

Forms as moist air flows up along an elevated plain, hill or mountain

Change of temp up a slope can cause fog