Chapter 12, water vapor, clouds and aerosols

1. Chapter 12, water vapor, clouds and aerosols Announcement: Quiz 3 will be next Monday (not this Friday). Outline for Today: Dry and wet adiabatic lapse rates Causes of lifting and cloud formation Role of clouds in climate (warming and cooling) High versus Low cloud as feedbacks Human influences on clouds? Aerosols – definition and role in climate

2. Moisture in the Atmosphere water vapor Least order Whenever matter changes state, energy is absorbed or released More ordered to less ordered, energy is absorbed; Less ordered to more, energy is released The amount of heat released or absorbed per gram during a change of state is called latent heat Implications of latent heating for Earth system, e.g. evaporation-condensation cycle – Evaporation cools earth’s surface Condensation heats earth’s atmosphere water intermediate ice most ordered Figure 12.12 Skinner et al., 1999

3. Moisture in the Atmosphere: Relative Humidity Vapor pressure = fraction of the total atmospheric pressure due to water molecules Relative humidity is the ratio of vapor pressure to saturation vapor pressure, expressed as a percent (%) Relative humidity can be changed by adding more water to air, or by changing its temperature For a given parcel of air, the temperature at which the relative humidity is 100% and condensation begins is called the dew point

4. Global water vapor distribution (current)

5. Saturation Water Vapor P vs. TemperatureWarmer air holds more moisture 1% Sea level Fig 12.13

6. Annual effective evaporation The water vapor concentration in the atmosphere also varies in space and time Water is continuously evaporating from Earth’s surface and adding moisture to the atmosphere, but at rates that vary in both space and time Precipitation depletes the atmosphere of water vapor, but at rates that are highly non-uniform in space and time Figure 12.14, Skinner et al., 1999 See also: http://www.ssd.noaa.gov/PS/PCPN/DATA/RT/na-wv-loop.htmla Highest over oceans, at lower latitudes

7. CLICKER QUESTION: If the Pressure of water vapor is 1000 Pa at 20°C, approximately what is the relative humidity of this air?A) 100%B) 50%C)25%D) 10%E) 5%

8. If the Pressure of water vapor is 1000 Pa at 20°C, what is the relative humidity of this air? At 20°C, saturated air contains 2000Pa, So relative humidity =1000/2000=.5 or 50% Also, if that air is cooled to 0°C, saturated air contains ~800 Pa, so relative humidity = 1000/800 or >100% => Condensation

9. Fig 12.16 Condensation - • when air becomes saturated with water vapor, condensation of water or ice crystals occurs • nucleation - Energy is needed to form the new surface of a drop or crystal of water – easier for a drop to form on an existing surface than to create a new one on its own • surfaces like the ground serve as nucleation sites for dew and frost to form, so do aerosols, e.g. SO2 and SO3 • one raindrop = 1 million cloud droplets

10. Condensation and Cloud Formation: Lapse Rate Adiabatic Processes: occur without the addition or subtraction of heat (an air parcel does not exchange energy with surroundings) Example: compressed air heats up (bicycle pump); expanding air cools down (air released from high pressure is cool) Warm air rising due to convection expands (because pressure decreases with altitude) and therefore cools; sinking air compresses and warms Rate of cooling of unsaturated air is ~10ºC/km and is called the dry adiabatic lapse rate When air becomes saturated condensation will start. The latent heat released will slow the cooling. The rate of cooling of rising, saturated air is called the moist adiabatic lapse rate and is on average ~5ºC/km (less than dry air because of latent heat release on condensation)

11. As air rises, it cools As an air parcel rises, the pressure around it decreases, and it expands. Work is done to expand. This energy can come from: lowering the energy of the movement (kinetic energy) of the air - decreasing temperature. extracting latent heat from the moisture in the air through condensation. http://www.geog.umn.edu/faculty/klink/geog1425/images/lapse_rates/adiab_cool1.free.gif

12. Evaporation – requires heat Condensation – releases heat Condensation of water when clouds form releases heat – hence the moist adiabatic lapse rate is not as steep as the dry adiabatic lapse rate Figure 12.15 Skinner et al., 1999

13. In the troposphere, the atmosphere gets cooler with altitude. Heating from the surface, heat is transported up by air mixing As air gets mixed upward, it cools, the saturation vapor pressure decreases, and clouds can form. Temperature at the top of the troposphere (tropopause) is well below freezing point of water; so water stays in the troposphere (gets rained out) Figure 12.5, Skinner et al., 1999

14. Fig 12.18 a. density lifting b. frontal lifting warm over cold e.g., stratus clouds, or nimbostratus c. frontal lifting cold beneath warm d. orographic lifting-topography e.g. coastal Washington state e. convergence lifting e.g. Florida peninsula, afternoon rain showers

15. CLICKER QUESTION If you wanted to design a cloud to help coolEarth’s surface, which properties would you not choose? Good reflector of sunlight Emits little IR radiation to space Emits lots of IR radiation to space Poor reflector of sunlight (B) and (D)

16. Low Stratus Clouds • made primarily of water droplets • reflect a lot of incoming sunlight (cool surface) • efficient at absorbing outgoing IR, but also at emitting IR to space: the cloud temperatures are not that much cooler than Earth’s surface. (net effect on IR small) NET EFFECT FROM LOW CLOUDS = cooling

17. High (Cirrus) Clouds • Thin, ice crystal composition • Do not reflect much sunlight back to space • Are very high up so have a low cloud-top temperature, radiation to space is low (T4) • Absorb and re-radiate surface radiation NET EFFECT OF HIGH CLOUDS =warming

18. http://icp.giss.nasa.gov/education/cloudintro/page2.html

19. One common type of cirrus cloud = Aircraft Contrails Why do contrails form? Water vapor is made from H in fuel combining with atmospheric O2 during combustion. Air leaving the engines is hot – as it cools, the water condenses (why is there not much water vapor generally in the cold upper troposphere??) Photos from Wikipedia - contrails

20. Contrails from flights going between US and Europe (over Nova Scotia, eastern Canada) http://earthobservatory.nasa.gov/NaturalHazards/Archive/Apr2002/Contrails_S2002102_lrg.jpg

21. Homework posted last Friday.Multiple feedbacks in the climate system Water vapor in troposphere Earth surface Temperature Infrared radiation Cloud cover (high versus low cloud)

22. Aerosols – particles in the atmosphere Aerosols are tiny liquid droplets or tiny solid particles that are so small that they remain suspended in the air Common liquid aerosols are water droplets in fogs Common solid aerosols are ice crystals, smoke particles from fires, sea salt crystals from ocean spray, dust from wind, volcanic emissions, pollutants from cars/factories Aerosols are everywhere in the atmosphere, particularly in the air nearest the ground

23. Southern California Wildfires, October, 2003

24. Southern California Wildfires, October, 2003

25. http://www.youtube.com/watch?v=A9SP5itTSt4&feature=related

26. Dust storm off NW Africa, carrying tons of sand to N. Atlantic

27. Climate effects of aerosols are complicated • Some aerosols directly absorb solar radiation (heat the atmosphere; soot) • Others reflect incoming radiation from space (cooling effect; sulfur-containing) • Some enable easier formation of clouds, which can reflect radiation (cooling) or cause increased absorption of infrared radiation (warming); net effect depends on whether clouds formed are high or low

28. Net effect of sulfate aerosol is cooling by increased reflection of sunlight -- but what could make this a FEEDBACK to climate change??? (Sulfate) http://saga.pmel.noaa.gov/review/dms_climate.html

29. Negative feedback Stabilizes cloud cover Amount of Incoming sunlight reaching ocean surface Number of cloud condensation nucleii (number of low clouds) Plankton production of DMS (sulfur gas that makes sulfate aerosol) http://saga.pmel.noaa.gov/review/dms_climate.html

30. Summary – material on Chapter 12that will be covered by the quiz Be able to answer review questions 8, 9,10, 13, 14, 15 and 16 at the end of Chapter 12.