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

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

4. klimaat cursus KNMI October 2005 4 Water vapour is the building material for clouds nameSymbol Units DefinitionNear surface values Atmospheri c column spec. humidity q v [g/kg]amount of water vapour in 1kg dry air 10 g/kg20 kg/m2 Saturation spec. hum. q s [g/kg]Max. amount of water vapour in 1kg dry air 15 g/kg Liquid water q l [g/kg]amount of liquid water in 1kg dry air 1 g/kg100 g/m2

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

6. 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!!!

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

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

9. 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

10. klimaat cursus KNMI October 2005 10 Condensational Heating allows cumulus to grow into a more mature stage: cumulus mediocris 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. 24-07-2006 Amsterdam: cumulus mediocris. 15:30 Wolken basis (~1km) Wolken top (~3 km)

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

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

13. klimaat cursus KNMI October 2005 13 Then the cumulus can reach the stage of a socalled cumulonimbus 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. Wolken top (5~8 km) ijs Moist Convection occurs all over the globe but is predominant in the tropics and over the subtropical oceans.

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

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

16. klimaat cursus KNMI October 2005 16 Stratocumulus

17. klimaat cursus KNMI October 2005 17 Trade Wind Cumulus

18. klimaat cursus KNMI October 2005 18 Deep Convective Towers

19. klimaat cursus KNMI October 2005 19 Cirrus

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

21. klimaat cursus KNMI October 2005 21 Summary of all types

22. klimaat cursus KNMI October 2005 22 Global Distribution of Clouds Determined by: Main circulation Systems Availability of Moisture ISCCP Data set ISCCP globe63.1 NH59.3 SH68.7 Tropics61.2 Global cloud cover climatology (1983- 1990):

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

24. 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”

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

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

27. 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/m 2)

28. 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.

29. klimaat cursus KNMI October 2005 29 Latent Heating by Convection

30. 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!!)

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

32. 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)

33. 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.

34. 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

35. 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.

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

37. 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

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

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

40. klimaat cursus KNMI October 2005 40 IPCC

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

42. 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

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

44. 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  CRF : change of the effects that clouds exert on the radiative fluxes of the top of the atmosphere Cloud feedback factor :  CRF / G : 0 no feedback >0 positive feedback <0 negative feedback

45. 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!!!

46. 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

47. klimaat cursus KNMI October 2005 47

48. 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!!!

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

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

51. klimaat cursus KNMI October 2005 51

52. Cloud albedo bias Plane parallel cloud Scu Cloud albedo Liquid water path (LWP) x x a a(LWP) < a(LWP)

53. klimaat cursus KNMI October 2005 53 30-12-2005 8:00 30-12-2005 9:00 30-12-2005 12:00 31-12-2005 15:00

54. klimaat cursus KNMI October 2005 54 60/70’s Condensation (non- convective) Radiation effects Prescribed Albedo MicrophysicsNo 70/80’s a = f(RH) l prescribed No 80/90’s l prognostic a = f(RH) Simple Bulk microphysics ECMWF l prognostic a prognostic Complex Bulk microphysics Statistical Cloud Schemes a: cloud fraction l: cloud condensate A Brief History