Cloud Formation

1. Cloud Formation Matt Rogers (rogers@atmos.colostate.edu) Department of Atmospheric Science Colorado State University AIM Workshop 25 July 2006 Anchorage, Alaska

2. Water Cycle Revisited Sources of water for clouds come from evaporation and transpiration Sinks of water for clouds come in the form of precipitation and evaporation What are the mechanisms that happen in between?

3. Thirty Seconds of Thermodynamics Temperature is a measure of molecular energy - if a collection of molecules has a lot of energy, odds are, it’ll have a higher temperature as well (and vice versa.) Gases behave (well, ideally, at least) independently of each other If matter in the form of a liquid gains enough energy (which we call the latent heat of evaporation) it may undergo a phase change and become a gas. If matter in the form of a gas loses enough energy (which we call the latent heat of condensation) it may turn back into a liquid.

4. Water Saturation Thought experiment - which beaker likely has a warmer temperature? Why? The temperature of a gas (say, water vapor) is closely related to its energy The amount of water vapor and liquid water will depend on the total energy of the system - obeying the laws of thermodynamics and staying in equilibrium

5. Describing Saturation In terms of a saturation vapor pressure, which is simply the water vapor pressure (for a given temperature and atmospheric pressure) that maintains equilibrium. In terms of the Relative Humidity, which is the fraction of the current vapor pressure (whatever it might be) to the saturation vapor pressure it would have if it were in equilibrium. Or, in terms of a dewpoint temperature, which is the temperature that the vapor (at its current vapor pressure) would have to be cooled to to reach its saturation vapor pressure. Nothing here depends on the dry air!

6. Dew • Surfaces cool strongly at night • Strongest on clear, calm nights • If a surface cools below the dew point, water condenses on the surface and dew drops are formed

7. Frost • If the temperature is below freezing, the dew point is called the frost point • If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals • The resulting crystals are known as frost, hoarfrost, or white frost

8. Cloud droplet formation • If the air temperature cools below the dew point (RH > 100%), water vapor will tend to condense and form cloud/fog drops • As with dew and frost, cloud drop formation prefers to condense on a surface of some sort - we call these particles cloud condensation nuclei (CCN) • CCN surfaces offer a locally lower saturation vapor pressure that facilitates condensation • The most effective CCN are water soluble. • Without these particles clouds would not form in the atmosphere • RH of several hundred percent required for pure water drop formation

9. Aerosol Sources • Terrestrial Sources • Dust/sand/dirt particles • Smoke - volcanic, fires, and pollution (sulfates) • Pollens and spores • Oceanic Sources • Sea Salts • Chemical sources • Photodissociation • Heterogeneous/Homogeneous chemistry

10. Typical sizes

11. Cloud Formation Mechanisms …can be anything that causes water vapor cool and condense Direct cooling… …lifting of an airmass…

12. Cloud Formation Mechanisms …can be anything that causes water vapor cool and condense …buoyant lifting… …or mechanical forcing

13. Cloud classification Clouds are classified by height, appearance, precipitation, and category of vertical development • High Clouds - generally above 16,000 ft at middle latitudes • Main types - Cirrus, Cirrostratus, Cirrocumulus • Middle Clouds – 7,000-16,000 feet • Main types – Altostratus, Altocumulus • Low Clouds - below 7,000 ft • Main types – Stratus, stratocumulus, nimbostratus • Clouds of Vertical Development • Main types – Cumulus, Cumulonimbus • Nimbo- and -nimbus prefix/suffix • Denotes precipitation

14. Stratus Uniform, gray Resembles fog that does not reach the ground Usually no precipitation, but light mist/drizzle possible Stratocumulus Low lumpy clouds Breaks (usually) between cloud elements Lower base and larger elements than altostratus Nimbostratus Dark gray Continuous light to moderate rain or snow Evaporating rain below can form stratus fractus Low Clouds

15. Stratiform cloud layers

16. Stratocumulus cloud streets Stratus undulatus

17. Looking down on an eastern Atlantic stratus deck

18. Altocumulus <1 km thick mostly water drops Gray, puffy Differences from cirrocumulus Larger puffs More dark/light contrast Altostratus Gray, blue-gray Often covers entire sky Sun or moon may show through dimly Usually no shadows Middle Clouds

19. Altostratus

20. Altocumulus

21. High clouds White in day; red/orange/yellow at sunrise and sunset Made of ice crystals Cirrus Thin and wispy Move west to east Indicate fair weather Cirrocumulus Less common than cirrus Small, rounded white puffs individually or in long rows (fish scales; mackerel sky) Cirrostratus Thin and sheetlike Sun and moon clearly visible through them Halo common Often precede precipitation High Clouds

22. Cirrus

23. Cirrocumulus

24. Cirrostratus Cirrostratus with Halo

25. Cumulus Puffy “cotton” Flat base, rounded top More space between cloud elements than stratocumulus Cumulonimbus Thunderstorm cloud Very tall, often reaching tropopause Individual or grouped Large energy release from water vapor condensation Vertically developed clouds

26. Cumulonimbus with Pileaus caps

28. Mechanically Forced Clouds Lenticularis Wave clouds

29. Clouds - Why We Care • Clouds transport energy from one area to another • Evaporation takes latent heat out of warm surface waters • Condensation releases same latent heat into atmosphere in a different location • Heat has been ‘carried’ by the water inside a cloud

30. Latent Heat Release An average thunderstorm contains several thousand metric tons of water Condensing 1 kg of water releases ~ 2.26x106 J of latent heat energy An average thunderstorm containing around 1500 tons of water will release 3.45 billion Joules of energy

31. Clouds - Why We Care • Clouds affect the radiation budget by reflecting visible light from the sun (thus cooling the planet) and by trapping infrared radiation from the surface (thus warming the planet) • Reflection/trapping behavior depends on type of cloud…

32. Cloud Radiative Effects

33. Salient Tidbits • In the infrared • Clouds absorb radiation from below, re-emit at the temperature of the cloud • Cirrus - very cold, emit little radiation • Stratus - very warm, basically emit like the surface • In the visible • Clouds reflect radiation based on the amount of cloud droplets • Cirrus - thin, not as reflective as thicker water clouds • Stratus - thicker, many water droplets, highly reflective

34. High Clouds Thin, cold ice clouds reflect less sunlight Extremely cold, emits infrared at colder temperatures, prevents warmer surface infrared from escaping to space NET EFFECT: Warming

35. Low Clouds Very thick water clouds reflect large amounts of sunlight Very near the surface, temperature of the cloud effectively the same as surface. Infrared radiation is therefore about the same - almost like the cloud wasn’t there! NET EFFECT: Cooling

36. Challenges • Climate modelling with clouds - need to get both type and amount correct, which is difficult • Overestimating high clouds - too much warming • Overestimation low clouds - not enough • Understanding cloud feedbacks • What does CO2 warming do to cloud populations? • How does increased aerosol pollution affect cloud types and amounts? • Observations • Layered cloud structures not seen from satellites