1. How do the clouds form?

2. 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.

3. 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

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

5. Satellite observation of clouds
NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites 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.

6. Global map of clouds

7. Vertical structure of clouds

8. 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

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

10. 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

11. Video: Convective cloud time lapse

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. Low stratocumulus clouds
Generated by convection inside boundary layer
Convection is driven by cloud-top longwave cooling and evaporative cooling

19. 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

20. Works cited http://www.atmos.washington.edu/2004Q2/547/www/