How do the clouds form?

Review of last lecture 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.

The most common atmospheric circulation structure Radiation Convection Cooling or No Heating Heating Latent/Sensible Conduction H L Imbalance of heating Imbalance of temperature Imbalance of pressure  Wind

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

Satellite 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 clouds

Vertical structure of clouds

Cloud Properties 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

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

Why do clouds constitute a wildcard for climate change? Stronger warming effect 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. Stronger cooling effect

Video: Convective cloud time lapse http://www.youtube.com/watch?v=kapTREk0gXg

Formation 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 (FB= ρenvg – ρparcelg)  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 Δp/Δz=ρenvg ρparcelg

The three types of stability When comparing the temperature btw parcel and environment, there are 3 possible outcomes: Environment Parcel Parcel Parcel Environment Environment Absolutely Stable Absolutely Unstable Conditionally Unstable Convection happens when: boundary layer air is warm and moist Environmental air above boundary layer is cold

When convection happens: Rising up of air parcel (called updraft) Formation of clouds and sometimes precipitation Heating up the environment because parcel temperature is warmer than the environment

What stops ‘unstable’ air masses from rising indefinitely ? 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 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 a 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 clouds Generated by convection inside boundary layer Convection is driven by cloud-top longwave cooling and evaporative cooling

Summary 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

Works cited http://www.atmos.washington.edu/2004Q2/547/www/ http://capita.wustl.edu/capita/datasets/modis/globfused/glob3d.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