1. Lecture 17
Required reading: Climate change report 2008 from the National Academies
Greenhouse gases and global warming
Global warming and H2O
Climate modeling
Air masses
Fronts
Cyclogenesis

2. Urban heat island effect

3. Notice the warmth of Pittsburgh compared to the surrounding rural area

4. Difference in hourly temps between Chicago and Aurora, IL
Difference in hourly temps between Chicago and Aurora, IL. Demonstrates Chicago’s heat island effect (mT air mass)

5. Greenhouse gases and climate change
H2O, CO2, CH4, CFCs are all gh gases
Humans have changed the amount of gh gases in the atmosphere over the past century by > 25%
Based on radiation arguments of the greenhouse effect, temperature should have increased a lot too, and it has
Skeptics say that it is hard to prove exactly why because

6. Greenhouse gases and climate change (cont)
The climate system is complex with much natural variability
Use process of elimination:
Striking warming signal since During this time
Has the output of the sun decreased? NO
Has the frequency of volcanic eruptions decreased? NO
Has to be due to anthropogenic effects

7. Climate change and atmospheric water
Human activities add little H2O to the atmosphere directly
Saturation water pressure increases very rapidly with temperature
Since H2O is a strong gh gas this will lead to still warmer temperatures, still more water vapor etc
On the other hand cloudiness may also increase, which would be a cooling effect

8. Global warming and atmospheric H2O, continued
More aerosols can mean more CCN, more droplets can form in clouds and the clouds may reflect more
Indirect aerosol effect on climate
Contrails are airplane-induced clouds

9. Stratus clouds off California, ship tracks can be seen in the clouds since their reflectance is enhanced by increased aerosols

10. Climate modeling
GCMs, global climate models consist of an atmospheric model, coupled to an ocean model, coupled to a sea ice model, land surface model. Latest versions include chemistry in atm & ocean
Give statistical estimates of future conditions
Sensitivity studies to understand processes

11. Schematic of atmospheric processes included in a GCM

12. Let’s go to the NAS report
Climate model projections for the century (until 2100)
Depend on “scenarios” for climate change. Each scenario represents plausible and internally consistent prediction of future human activites.
Predict population growth, economic development, energy and land use– social science

13. Weather – Air Mass
extremely large body of air with similar characteristics of temperature and moisture. Forms when air stagnates for long periods of time over a uniform surface
Characteristic weather of an air mass is determined by the surface above which it forms

14. Air mass types (classified based on temperature and humidity)
Polar (P): polew of 60N or S - cold
Tropical (T): equatorw of 30N or S – warm
Arctic (A): over Arctic – very cold
Continental (c): over land – dry
Maritime (m): over oceans – moist
Air mass source regions: must have light or no winds so the air has time to acquire the characteristics of the surface, uniform surface characteristics needed

15. Major air mass sources of the world

16. Air masses affecting North America

18. Hourly temperature at Madison WI during an Arctic outbreak (A air mass)

19. mT airmass from Gulf of Mexico

20. Air mass modification
An air mass may be modified when advected away from the source region. The modified air mass may be warmed/cooled from below --- remember atmospheric stability! It may also take up water vapor or be dried out by passing over certain terrain etc.

21. cP-w

24. IR image: after passage of cold front, cP air mass passes over the relatively warm Gulf stream- goes unstable

25. Winter cP air moving over warm body of water becomes unstable

26. Lake effect snows (contours show annual snowfall)

27. The air mass takes up characteristics from the surface of the lake
The air mass takes up characteristics from the surface of the lake. Heating from below  unstable. Warming + evaporationgains moisture

28. Fronts
Separate different air masses
Cold, warm, occluded fronts

29. Sfc weather associated with a cold front
Pressure T
Wind
Clouds & precip

30. Vertical slice through a cold front

31. Sfc weather with a warm front

32. Vertical structure of a warm front

33. Occluded fronts
Cold type
Warm type