Clouds and Climate

1. Clouds and Climate A. Pier Siebesma KNMI What are clouds? How do they form? Cloud climatology Clouds and Radiation Clouds in Climate models Cloud Climate Feedback

2. What is a Cloud? “just” water ! 28-06-2006 ; 12:00 Amsterdam klimaat cursus KNMI October 2005

3. How do you build a cloud ? klimaat cursus KNMI October 2005

4. Water vapour is the building material for clouds klimaat cursus KNMI October 2005

5. Saturation specific humidity qsatincreases exponential with temperature (Clausius-Clayperon) • Because of the presence of Cloud Condensation Nuclei (CCN’s) in the atmosphere condensation takes place if qv >qsat(p,T) • Usually through cooling that results from rising motion. CCN’s are hygroscopic aerosols (salt, dust, etc) klimaat cursus KNMI October 2005

6. Rising air cools with 1 K per 100m ……….. Until it becomes so cold that it starts to condensate… And a cloud is born!!! klimaat cursus KNMI October 2005

7. What makes air to rise? klimaat cursus KNMI October 2005

8. 1. Orography Lenticularis above Mount Etna seen from Taormina, Sicily Italy. klimaat cursus KNMI October 2005

9. 2. Convection The sun heats the soil so that…….. Thermals are formed…. that rise because of buoyancy…. And a cloud forms as a wig on top of an invisible man 24-07-2006 12:30 Amsterdam: cumulus humilis or “fair weather” cumulus www.sky-catcher.nl klimaat cursus KNMI October 2005

10. Condensational Heating allows cumulus to grow into a more mature stage: cumulus mediocris Wolken top (~3 km) Humidity condensates into cloud water….. And produces latent heat Which serves as onboard fuel that allows the cloud to rise further….. With ~5 m/s…. Until the cloud is stopped by a temperature inversion. Wolken basis (~1km) 24-07-2006 Amsterdam: cumulus mediocris. 15:30 klimaat cursus KNMI October 2005

11. Mean profile “Level of zero kinetic energy” Inversion Level of neutral buoyancy (LNB) non-well mixed layer height Level of free convection (LFC) Lifting condensation level (LCL) well mixed layer Poor man’s cloud model: adiabatic ascent klimaat cursus KNMI October 2005

12. Horizontal Variability and Correlation Mean profile height But what if the cloud breaks through the inversion????????? klimaat cursus KNMI October 2005

13. Then the cumulus can reach the stage of a socalled cumulonimbus Wolken top (5~8 km) ijs With vertical velocities over 10m/s Up to a height of 5~15 km So that the water becomes ice which gives the fluffy appearance of the top of the cloud and strong precipation is on the way 08-02-2006 Amsterdam: cumulonimbus. Moist Convection occurs all over the globe but is predominant in the tropics and over the subtropical oceans. klimaat cursus KNMI October 2005

14. Convection in the (sub)tropics } Hadley circulation klimaat cursus KNMI October 2005

15. Cartoon of Hadley Circulation Tropopause 10km Subsidence ~0.5 cm/s inversion 10 m/s Cloud base ~500m Shallow Convective Clouds No precipitation Vertical turbulent transport No net latent heat production Fuel Supply Hadley Circulation Stratocumulus Interaction with radiation Deep Convective Clouds Precipitation Vertical turbulent transport Net latent heat production Engine Hadley Circulation klimaat cursus KNMI October 2005

16. Stratocumulus klimaat cursus KNMI October 2005

17. Trade Wind Cumulus klimaat cursus KNMI October 2005

18. Deep Convective Towers klimaat cursus KNMI October 2005

19. Cirrus klimaat cursus KNMI October 2005

20. 3. Large Scale Lifting through fronts } Occuring at mid-latitudes } klimaat cursus KNMI October 2005

21. Summary of all types klimaat cursus KNMI October 2005

22. Global Distribution of Clouds Determined by: • Main circulation Systems • Availability of Moisture Global cloud cover climatology (1983-1990): ISCCP Data set klimaat cursus KNMI October 2005

23. Why are clouds relevant for climate ? They strongly affect the radiative fluxes throughout the atmosphere klimaat cursus KNMI October 2005

24. Radiative Effects of Clouds in the Climate System 2 main effects: • Shortwave Reflection (cooling) “umbrella effect” • Longwave Emission (warming) “blanket effect” klimaat cursus KNMI October 2005

25. Shortwave Cloud Radiative Forcing (ERBE JJA87) SWCRF = Sobs – Sclear ; mainly by solid (BL) cloud decks cooling in subtropics and extratropics Global cooling effect: ~-48 W/m2 klimaat cursus KNMI October 2005

26. Longwave Cloud Radiative Forcing (TOA ERBE JJA87) LWCRF = Lobs – Lclear ; mainly by high clouds Heating in tropics and extratropics Global warming effect: ~31 W/m2 klimaat cursus KNMI October 2005

27. Total Cloud Radiative Forcing (ERBE JJA87) Total effect Overall cooling subtropics Concentrated in subtropics and extratropics Longwave and shortwave effects almost cancel in tropics (W/m2) (W/m2) klimaat cursus KNMI October 2005

28. Why are clouds relevant for climate ? They strongly affect the radiative fluxes throughout the atmosphere Condensation processes in clouds are a dominant source of energy and influence directly the large scale dynamics and cause precipitation. klimaat cursus KNMI October 2005

29. Latent Heating by Convection klimaat cursus KNMI October 2005

30. Why are clouds relevant for climate ? Clouds are a common atmospheric feature and cover large parts of the globe They strongly affect the radiative fluxes throughout the atmosphere Condensation processes in clouds are a dominant source of energy and influence directly the large scale dynamics and cause precipitation. Clouds connected to convective processes transport heat, moisture, momentum and other atmospheric constituents over large distances in the vertical (Remember the Hadley Circulation!!) klimaat cursus KNMI October 2005

31. CLOUDS in GCM’s : What are the problems? There is a huge variety of cloud types klimaat cursus KNMI October 2005

32. CLOUDS in GCM’s : What are the problems? There is a huge variety of cloud types Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically) klimaat cursus KNMI October 2005

33. 50 km Virtually all cloud systems are not resolved by present-day “state of the art” global atmospheric models. klimaat cursus KNMI October 2005

34. CLOUDS in GCM’s : What are the problems? • There is a huge variety of cloud types • Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically) • Clouds are the result of complex interactions of a large number of processes, e.g., • Convection and turbulentie • vertical motion • microphysics klimaat cursus KNMI October 2005

35. CLOUDS in GCM’s : What are the problems? • There is a huge variety of cloud types • Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically) • Clouds are the result of complex interactions of a large number of processes, e.g., • moist convection • turbulence • Large scale vertical motion • Microphysics • Many of these processes are still poorly understood • The major cloud effect (radiative) is determined by many different cloud parameters. klimaat cursus KNMI October 2005

36. Cloud top and base height Amount of condensate Cloud fraction In-cloud conden-sate distribution Cloud-radiation interaction Phase of condensate Cloud particle shape Cloud particle size Cloud macrophysics Cloud microphysics klimaat cursus KNMI October 2005

37. Clouds in GCM’s: How? Main variables: Cloud fraction, a; Cloud condensate (cloud water and/or ice), l. Assume that cloud fills a model layer entirely in the vertical, hence cloud fraction refers to horizontal cover. Diagnostic approach Prognostic approach klimaat cursus KNMI October 2005

38. Cloud top and base height Amount of condensate Cloud fraction In-cloud conden-sate distribution Cloud-radiation interaction Phase of condensate Cloud particle shape Cloud particle size Cloud macrophysics Cloud microphysics klimaat cursus KNMI October 2005

39. Indirect Aerosol Effects Cloud 2 Cloud 1 Higher aerosol concentration => more CCN’s => smaller droplet sizes • Consequences: • Enhanced Cloud albedo => cooling effect (Twomey Effect) • Reduced precipitation => longer lifetime clouds => cooling effect klimaat cursus KNMI October 2005

40. IPCC klimaat cursus KNMI October 2005

41. How well are GCM’s doing in the present climate? Global Cloud Cover klimaat cursus KNMI October 2005

42. ECMWF Model climate - Cloud fraction JJA 1987 T63L31TCC Model - ISCCP Underestimation of Scu-decks ; Overestimation cc in ITCZ and Arctics klimaat cursus KNMI October 2005

43. Cloud cover How well are GCM’s doing for present climate? AMIP-intercomparison (10 years) OLR klimaat cursus KNMI October 2005

44. How well are GCM’s doing for future climate? G : direct radiative forcing due to CO2 doubling: 4 W/m2 DCRF : change of the effects that clouds exert on the radiative fluxes of the top of the atmosphere • Cloud feedback factor : DCRF / G : 0 no feedback >0 positive feedback <0 negative feedback klimaat cursus KNMI October 2005

45. How well are GCM’s doing for future climate? Cess et al: 1990, 1996 Cloud climate feedback Is the largest uncertainty for future climate predictions!!! klimaat cursus KNMI October 2005

46. Which clouds are responsible for this uncertainty? Bin clouds according to the mid-tropospheric vertical velocity Bony and Dufresne 2005 klimaat cursus KNMI October 2005

47. klimaat cursus KNMI October 2005

48. Future Directions • The new generation satellites will constrain: • Water vapour content (with vertical resolution) • Condensed water content (with vertical resolution • Radiative properties • Allows for the design of more accurate cloud and convection schemes and constrained radiation schemes in GCM’s and reduce the uncertainty in cloud representation in these models and hence cloud climate feedback!!! klimaat cursus KNMI October 2005

49. 1. Model Development Methodology: Large Eddy Simulation Models + Observations Single Column Model version Implementation In full 3d model klimaat cursus KNMI October 2005

50. ..\TURK_ATrain_Movie_20060822.avi klimaat cursus KNMI October 2005