Clouds and rain formationClouds and rain formation

• Global water (hydrological) cycle

• Water Vapor Basics (names of different phase changes, latent heat)

• Two methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds.

Review of last lecture

The most common atmospheric The most common atmospheric circulation structurecirculation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

Satellite observation of cloudsSatellite observation of clouds

• NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites. 1983-Now, cloud top height and optical depth.

• NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.

CloudsClouds

• Clouds consist of many tiny water droplets (~10 μm) and/or ice particles

• Most clouds form as air parcels are lifted and cooled to saturation

• Clouds have a wide range of height and thickness

• Clouds are instrumental to the Earth’s energy and moisture balances

Level of saturation/condensation

1) rate of cooling of parcel (unsaturated v. saturated)• unsaturated – dry adiabatic lapse rate (DALR)• saturated – saturated adiabatic lapse rate (SALR)

2) rate of cooling of surrounding atmosphere• environmental lapse rate (ELR): an overall

decrease in air temperature with height• ELR is measured by weather balloon. Long-term

global average ELR = 0.65oC/100m.

Three possible outcomes:1) absolutely unstable air2) absolutely stable air

3) conditionally unstable air

Static Stability and the Environmental Lapse Rate

• Static stability – refers to atmosphere’s susceptibility to being displaced• Stability related to buoyancy function of temperature• The rate of cooling of a parcel relative to its surrounds determines its ‘stability’ of a parcel

Environment

ParcelEnvironment

ParcelEnvironment

Parcel

The three types of stability

AbsolutelyUnstable

ConditionallyUnstable

AbsolutelyStable

1) Entrainment• Turbulent mixing of ambient air into parcel • Leads to evaporation along cloud boundaries• Evaporation uses latent heat, cooling the cloud

reduces buoyancy

Courtesy Russ Dickerson, U. Maryland

What stops ‘unstable’ air masses from rising indefinitely ?

2) Encountering a layer of stable air (inversion)• a rising parcel may reach a stable upper air environment • the parcel cooling rate will exceed that of the ambient air• the parcel will slowly cease ascension and come to rest at some equal temperature level• three types: radiation, frontal, subsidence

1. Cloud top height/pressure

2. Cloud thickness (optical depth)

3. Cloud coverage

• When clouds comprise more than 9/10th of the sky = overcast

• When coverage is between 6/10th and 9/10th = broken

• When coverage is between 1/10th and 6/10th = scattered

• Cloud coverage less than 1/10th = clear

Cloud Properties

NASANASA’’s International Satellite Cloud Climatology Project (ISCCP) s International Satellite Cloud Climatology Project (ISCCP) Cloud Classification - commonly used in climate researchCloud Classification - commonly used in climate research

• Clouds are both good reflectors of solar radiation (cooling effect) and good absorbers of earth emitted longwave radiation (warming effect).

• The net effect (cooling or warming) depends on the type of cloud

• In a changing climate, increases in high thin clouds would promote warming, while increases in low thick clouds would cause cooling

• Climate models have difficulties in simulating clouds, especially low thick clouds (stratocumulus)

• Conclusion: Clouds cause the largest uncertainty in model simulations of future climate.

Why do clouds constitute a wildcard for climate change?

Strongerwarming effect

Stronger cooling effect

Video: The climate wild cardVideo: The climate wild card

• https://www.youtube.com/watch?v=Py1dEFKuJJU

Global distribution of precipitationGlobal distribution of precipitation

Precipitation formation - cloud drop growthPrecipitation formation - cloud drop growth

• Not all clouds precipitate due to their small sizes and slow fall rates• Balance between gravity and frictional

drag eventually become equal to achieve terminal velocity VT, which is proportional to the square root of cloud drop radius VT=c r0.5 ,where r is drop radius and c is a constant.

• For a cloud drop to fall, its terminal velocity must exceed the vertical velocity of the upward-moving air parcel. Otherwise it will be carried up.

• Cloud drop growth is required for precipitation to form

Fgravity

Fdrag

1. Collision Coalescence (warm clouds, T > 0 C, form rain)

2. Bergeron Process (cool/cold clouds, T < 0 C, form snow)

Mechanisms for cloud drops to grow larger

Cold Clouds Cool Clouds

• Process begins with larger collector drops which have higher terminal velocities

• Collector drops collide with smaller drops and merge with them (coalesce). Coalescence efficiency is generally very high, indicating that most collisions result in the two drops joining.

• If collector drop is too big: compressed air beneath falling drop forces small drops aside

• If collector drop is too small (same size as other drops) it will fall at same speed and no collision will occur

• So, collection efficiency is greatest when the size of collector drop is slightly larger than the size of the other drops

• After the collector drops become large, the larger one among them can serve as a “super-collector” to collide with other collector drops

1. Collision Coalescence: Growth in Warm Clouds

• Determined by competition between surface tension and frictional drag. Frictional drag is larger at the bottom than at the top

• Small drop (<0.08in): frictional drag << surface tension Sphere shape

• Medium-size drop (0.08in<size<0.25in): frictional drag approaches surface tension Parachute shape

• Large drop (>0.25in): frictional drag at bottom > surface tension Split (The surface tension at the top allows the raindrop to remain more spherical while the bottom gets more flattened out.)

• Maximum drop size of about 0.25in or 5 mm

Raindrop shape and maximum size

SummarySummary• Formation of clouds: 3 types of stability. Two factors

limiting the height of clouds. • 3 cloud properties. 9 ISCCP cloud types. Why do clouds

constitute a wildcard for climate change? • Forces acting on a cloud/rain droplet. Terminal velocity.

How does it change with cloud drop radius?• Growth mechanisms for rain and snow• Formation of rain: coalescence process (the collector is

larger than the cloud droplets but not too large)

Works citedWorks cited

• http://www.edudemic.com/study-finds-most-people-think-cloud-computing-is-run-on-actual-clouds/

• http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diph2o.html

• http://nyffetyff.deviantart.com/art/Raindrop-189805290 • http://www.its.caltech.edu/~atomic/snowcrystals/photos/

photos.htm • http://www.crh.noaa.gov/unr/?n=06-04-99_pg1 • http://www.clker.com/clipart-cartoon-sun.html • http://pmm.nasa.gov/node/145