Lecture 6 Role of clouds on climate and climate change

1. Lecture 6Role of clouds on climate and climate change

3. Effects of clouds on temperature profiles • Clouds absorb LW • Clouds reflect SW • Which effect “wins?”  • Depends on emitting T • For low clouds, sT4 ~ sTs4 , so SW effect is greater • For high clouds, sT4 << sTs4 so LW effect “wins” • High clouds: warm • Low clouds: cool Details are sensitive to optical properties and distributions of clouds, but remember the basic conclusions

4. Observed Mean Cloud Amount Source: http://isccp.giss.nasa.gov/

5. Observed Mean Cloud Top Temp. Source: http://isccp.giss.nasa.gov/

6. Observed Cloud Top Pressure Source: http://isccp.giss.nasa.gov/

7. Observed Mean Cloud Fraction high clouds( < 440 mb) • High clouds mostly due to tropical convection (Amazon, Congo, Indonesia, W. Pacific) • Low clouds (stratocumulus) over eastern parts of subtropical ocean basins • Cold SST • Subsiding air • Strong inversion low clouds( > 680 mb) all clouds Unstable vs. stable environment

8. Annual Mean Cloud Forcing D OLR • “Cloud forcing” is defined as the difference between a “clear sky” and “all sky” measurement • At the surface, (a) is all warming, and (b) is all cooling • Net effect of clouds is to cool the surface, but changes can go either way D solar abs D Rnet

9. Global Mean Cloud Radiative Forcing • Clouds increase planetary albedo from 15% to 30% • This reduces absorbed solar by 48 W m-2 • Reduced solar is offset by 31 W m-2 of LW warming (greenhouse) • So total cloud forcing is –17 W m-2 - - - - -

10. Cloud effects on LW at TOA Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

11. Cloud effects on SW at TOA Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

12. Therefore, Clouds cool the surface climate. Duh!?

14. (a)太陽短波輻射(100) 氣體吸收:16 雲吸收: 3 被空氣散射回太空: 6 被雲反射:20 被地面反射: 4 地面吸收:51 (b) 長波輻射(進入太空的量) 地面放射:21 15被氣體吸收, 6直接進入太空 大氣放射:38 雲放射:26 出去長波輻射 出去短波輻射 (c) 對大氣而言: 吸收 = 16 + 3 + 15 = 34 放射 = 38+ 26 = 64 不夠的量 = 30, => 必須透過地表蒸發、對流送到高處，冷却達飽和凝結，釋放潛熱(23)和可感熱(7)加熱大氣而獲得能量補充

15. How might this role changes (current cloud forcing = -17 watt•m-2) if cloudiness change (increase or decrease) underG.W.?

16. Temporal change of global mean anomaly of net SW at TOA, Surface, and Atmos (1983 to 2011) (1) a decrease at the surface and TOA (atmosphere as well) produced by the Mt. Pinatubo volcanic aerosols in 1991-92; (2) an overall increase at TOA and the surface, but not in the atmosphere Effect of Pinatubo Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

17. Temporal change of global mean anomaly of net LW at TOA, Surface, and Atmos (1983 to 2011) (1) decreases at the surface and increases in the atmosphere, but not at TOA; (2) In late 1990s, a small decrease at TOA and in the atmosphere and a larger increase at the surface. Source: http://isccp.giss.nasa.gov/projects/browse_fc.html

18. ISCCP global average of Tsfc change (1983-2009)

19. ISCCP zonally average of Tsfc change (1983-2009)

20. ISCCP global average of cloud amount change (1983-2009)

21. ISCCP zonally average of cloud amount change (1983-2009)

22. ISCCP global average of cloud top pressure change (1983-2009)

23. ISCCP zonally average of cloud top pressure change (1983-2009)

24. Cloud Classification

25. ISCCP global average of high-level cloud amount change (1983-2009)

26. ISCCP global average of middle-level cloud amount change (1983-2009)

27. ISCCP global average of low-level cloud amount change (1983-2009)

28. ISCCP day-time stratus cloud amount change in over water

29. ISCCP day-time stratus cloud amount change in over land

30. ISCCP global day-time deep convective cloud amount change

31. ISCCP day-time middle-level cloud amount change in Arctic

32. ISCCP day-time deep convective cloud amount change in Antarctic

33. Trend of sea-ice cover N.H. S.H.