Clouds and Climate KNMI Climate Course 2011

1. Clouds and ClimateKNMI Climate Course 2011 A. Pier Siebesma KNMI & TU Delft Multiscale Physics Department The Netherlands Contact: siebesma@knmi.nl

2. Clouds play a crucial role in weather and climate • Hydrological cycle • Radiation balance

3. Central Questions:How do clouds respond to a perturbed climate and affect this climate (feedback)? Perturbations: • Increased Temperature (due to enhanced Greenhouse Gases) • Increased aerosol amounts

4. 1. What is a cloud and how do they form?

5. What is a Cloud? “just” water ! 28-06-2006 ; 12:00 Amsterdam

6. Saturation specific humidity qsat 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)

7. Cooling through rising air Rising air cools with 1 K per 100m ……….. Until it becomes so cold that it starts to condensate… and a cloud is born!!!

8. 2. What makes air to rise?

9. 1. Orography Lenticularis above Mount Etna seen from Taormina, Sicily Italy.

10. 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

11. Wolken top (~3 km) • Wolken basis (~1km) • 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 But what if the cloud breaks through the inversion?????????

12. ijs Then the cumulus can rise to the tropopause and reach the stage of a socalled cumulonimbus • Wolken top (5~8 km) • 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.

13. 3. Large Scale Lifting through fronts } Occuring at mid-latitudes }

14. Different Cloud Types

15. 3. A global view on clouds amount and cloud dynamics

16. Clouds as seen by geostationary satellites (infrared) July 1994

17. Clouds as seen by geostationary satellites (infrared) January 1994

18. Monthly global cloud cover for the period 1983-2008(source ISCCP) • Mean global cloud cover : ~66% • No clear trend observed yet….

19. 4. Importance Clouds for Climate

20. Radiative Effects of Clouds 2 main effects: • Shortwave Reflection • (cooling) “umbrella effect” • Longwave Emission • (warming) “blanket effect” Top of the atmosphere a: planetary albedo = 0.3

21. Cloud Radiative Forcing 31 W/m2 -44 W/m2 -13 W/m2 Clouds have a net cooling effect • Many factors matter: • Cloud amount: a • Cloud top height: Tc • Cloud optical depth: acld Strong correlation between cloud forcing and low clouds !

22. Latent Heating by Cumulus Convection

23. 5. Clouds in Climate Models

24. The climate system : A truly multiscale problem 1. The planetary scale Cloud cluster scale How did I get here? ~107 m ~105 m Cloud microphysical scale Cloud scale ~103 m ~10-6m - 1m

25. No single model can encompass all relevant processes mm 10 m 100 m 1 km 10 km 100 km 1000 km 10000 km Cloud microphysics  turbulence Cumulus clouds Cumulonimbus clouds Mesoscale Convective systems Extratropical Cyclones Planetary waves DNS Subgrid Large Eddy Simulation (LES) Model Cloud System Resolving Model (CSRM) Numerical Weather Prediction (NWP) Model Global Climate Model

26. Parametrization Grid-box size is limited by computational capability Processes that act on scales smaller than our grid box will be excluded from the solutions. We need to include them by means of parametrization (a largely statistical description of what goes on “inside” the box). Similar idea to molecules being summarized statistically by temperature and pressure, but much more complex!

27. Parametrization Examples for processes that need to be parametrized in the atmosphere

28. Parametrization As parametrizations are simplifications of the actual physical laws, their (necessary) use is an additional source of model uncertainty.

29. 6. Clouds in a Future Climate

30. Uncertainties in Future Climate model Predictions with different climate models IPCC 2007 2.5-4.3°C 1900 Past Future Present

31. Climate Model Sensitivity  temperature  radiative forcing With feedbacks: Water vapour Snow albedo clouds

32. Cloud feedback Surface albedo feedback Water vapor feedback Radiative effects only 2XCO2 Scenario for 12 Climate Models Dufresne & Bony, Journal of Climate 2008 Cloud effects “remain the largest source of uncertainty” in model based estimates of climate sensitivity IPCC 2007

33. Primarily due to marine low clouds Stratocumulus “Marine boundary layer clouds are at the heart of tropical cloud feedback uncertainties in climate models” (duFresne&Bony 2005 GRL) Shallow cumulus

34. Climate Model Sensitivity • Definition: temperature change resulting from a perturbation of 1 Wm-2 • Radiative forcing for 2XCO2 3.7 Wm-2 (DR) • Temperature response of climate models for 2XCO2 2~4.3 K (DT) • Climate model sensitivity: 0.5-1.2 K per Wm-2 (DT/DR) • The climate model sensitivity is not (very) dependent on the source of the perturbation (radiative forcing) • Main reason for this uncertainty are the representation of (low) clouds • Reducing uncertainty of climate models can only be achieved through a more realistic representation of cloud processes and is one of the major challenges of climate modelling

35. 7. Clouds and Aerosols

36. Radiative Forcing Components (Source IPCC 2007)

37. Droplet concentration and Radiation:"Indirect" aerosol effect

38. Direct and Indirect Aerosol effects

39. More info: Pier Siebesma (siebesma@knmi.nl) ; siebesma@knmi.nl