How do the clouds form?How do the clouds form?

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

• Humidity indices (there are 6 total). Saturation vapor pressure increases non-linearly with temperature

• 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

CloudsClouds

• Clouds are instrumental to the Earth’s energy and moisture balances, and constitute a wild card for climate change

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.

Global map of cloudsGlobal map of clouds

Vertical structure of cloudsVertical structure of clouds

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: Convective cloud time lapseVideo: Convective cloud time lapse

• http://www.youtube.com/watch?v=kapTREk0gXg

Formation of cloudsFormation of clouds

• Most clouds form as air parcels in boundary layer are lifted and cooled to saturation.

• The air parcels could be lifted by mountains, meeting of different air masses, surface convergence, and local convection

Lifting by local convection

• Static stability – refers to atmosphere’s susceptibility to being displaced

• Stability related to buoyancy force determined by density difference btw parcel and environment (FFBB= = ρρenvenvg – g – ρρparcelparcelgg) determined by temperature difference btw parcel and environment (ρ = P/TR)

• When an air parcel rises, the cooling rate of the parcel (adiabatic lapse rate or ALR) relative to the cooling rate of surrounding atmosphere (environmental lapse rate or ELR) determines the “stability” of a parcel.

Environment

Parcel

ρρparcelparcelgg

ΔΔp/p/ΔΔz=z=ρρenvenvgg

Environment

Parcel

Environment

Parcel

The three types of stability

AbsolutelyUnstable

ConditionallyUnstable

Environment

Parcel

AbsolutelyStable

Convection happens when: (1)boundary layer air is warm and moist(2)Environmental air above boundary layer is cold

When comparing the temperature btw parcel and environment, there are 3 possible outcomes:

When convection happens:When convection happens:

1. Rising up of air parcel (called updraft)

2. Formation of clouds and sometimes precipitation

3. Heating up the environment because parcel temperature is warmer than the environment

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

In convection, an updraft is often associated with In convection, an updraft is often associated with a downdraft – Overturning of the tropospherea downdraft – Overturning of the troposphere

• Air can be cooled down by radiation, evaporation of raindrops, melting of snowflakes, etc.

• Air that is cooler than its environment tends to sink, leading to the formation of downdrafts

• Sometimes precipitation drag enhances the downdrafts

• Downdrafts cool down the environment (generally the lower troposphere)

• Downdrafts (also called downbursts) can cause significant damage at the ground

Low stratocumulus cloudsLow stratocumulus clouds

• Generated by convection inside boundary layer

• Convection is driven by cloud-top longwave cooling and evaporative cooling

3 cloud properties, 9 ISCCP cloud types

Why do clouds constitute a wildcard for climate change? Competition between greenhouse effect and albedo effect

Convection: 3 types of stability. Two factors limiting the height of clouds

Summary

Works citedWorks cited

• http://www.atmos.washington.edu/2004Q2/547/www/• http://capita.wustl.edu/capita/datasets/modis/globfused/g

lob3d.html

• http://www.ssec.wisc.edu/sos/wvsst/wvsst.html • http://www.arm.gov/news/facility/post/1025• http://apollo.lsc.vsc.edu/classes/met130/notes/chapter4/

es_temp.html

• http://www.meted.ucar.edu/oceans/currents/print.htm • http://visibleearth.nasa.gov/view.php?id=57735 • http://www.crh.noaa.gov/lmk/?n=downburst • http://cde.nwc.edu/SCI2108/course_documents/

earth_moon/earth/earth_science/convection/convection_advection.htm