From Aerosols to Cloud Microphysics

1. From Aerosols to Cloud Microphysics Paolo Laj Laboratoire de Glaciologie et Géophysique de l’Environnement Grenoble - France

2. Clouds and the global Energy budget (SW radiation)

3. Some interesting numbers Clouds increase the global reflection of solar radiation from 10 to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m². Apollo 11 image of Africa & Europe At any time, 30% of the Earth’s surface is covered by clouds

4. Clouds and the global Energy budget (LW radiation)

5. Some interesting numbers Clouds increase the global reflection of solar radiation from 10 to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m². This cooling is offset somewhat by the greenhouse effect of clouds which reduces the outgoing longwave radiation by about 31 W/m². Thus the net cloud forcing of the radiation budget is a loss of about 13 W/m² Apollo 11 image of Africa & Europe At any time, 30% of the Earth’s surface is covered by clouds

6. Different kind of Clouds Question : which kind of hydrometeors ?

7. Clouds and the redistribution of radiant energy within the atmosphere Low overcast clouds result in cooling (35 W m−2 ± 9 W m−2) Thin high clouds result in warming (20 W m−2 ± 8 W m−2)

8. Clouds and the global Energy budget (LW radiation) Objective of the lecture : 1- discuss the mechanisms by which anthropogenic activities may modify the Earth radiative budget (Cloud Radiative Forcing) 2- Focus on the aerosol/cloud interaction

9. Definition • What is an aerosol ? Aerosols Particles + Gases =

10. Different kind of hydrometeors  Hydrometeor  Shape  Size (diameter)  Number concentration  Terminal velocity Cloud droplets 1mm-100mm 100-1000 cm-3 <30cm/s Raindrops 100mm- 6mm ~1 m-3 <15cm/s Ice crystals 100mm- 3mm 1-100 l-1 <1m/s Graupel and hail particles 1mm-50mm ~1 m-3 Up to 30m/s Snowflakes 2mm-20mm ~1 m-3 <1m/s

11. Size range of aerosols

12. Seoul, Korea, April 10, 2006

13. Dust in Seoul, Korea April 8, 2006 PM10 level reached 2,070 ug/m3 .

18. Black Carbon on snow

19. Enhancement of pixel-average cloud spherical albedo sph on April 5 relative to that on April 2, as a function of LWP

20. Summary • Aerosol can scatter and absorb short-wave solar radiation • Aerosol can modify cloud microphysics and, in turn, change cloud reflectivity • Question: are these processes relevant in the global energy budget ?

21. Anthropogenic Radiative Forcing from IPCC Question : what is behind the large uncertainty for the cloud albedo effect ?

22. More than one indirect effect….. Question : how do we quantify the indirect effect ?

23. Cloud Albedo and cloud microphysical properties Cloud albedo effect (Twomey effect)

24. Cloud Albedo and cloud microphysical properties Cloud Geometry Question : what does this equation tells us ?

25. LWP and Cloud Optical depth Qext = extinction coefficient LWP= Liquid Water Path (g m-2) Reff= effective radius Adiabatic assumption

26. Cloud Albedo and Cloud Optical depth a= empirical coefficient g = assimetry parameter (0.85 for clouds) Question : implications of the R/t dependency ?

27. Cloud Albedo and Cloud Microphysics Aerosol influence on cloud albedo requires comparison not of the albedo values themselves but of the enhancement in albedo relative to that expected for the same LWP Question : can we measure it ? Which kind of clouds would you use ?

28. Cloud Albedo and Cloud Microphysics Pixel-average cloud spherical albedo as a function of vertical cloud LWP, for three satellite overpasses

29. Cloud Albedo and Cloud Microphysics Enhancement against LWP shows maximum enhancement at intermediate values of LWP, for which sensitivity to increased cloud-drop number concentration is the greatest Enhancement of pixel-average cloud spherical albedo sph on April 5 relative to that on April 2, as a function of LWP

30. Is LWP independent of CN ? Question : what can you say about this picture ?

31. Aerosol activation to cloud droplets

32. CNs and CCNs Higher hygroscopic fraction Lower hygroscopic fraction smaller size

33. Cloud droplet formationThe Köhler theory ERCA School Grenoble- January2002 Equilibrium between aqueous solution and humid air Solute (Raoult) Effect: the presence of solutes in the drop decreases the saturation vapour pressure Curvature (Kelvin) Effect: the saturation vapour pressure increases with increasing curvature

34. Cloud droplet formation IIKelvin Effect The smaller the droplet, the greater the supersaturation (with respect to a flat surface) is needed to keep the droplet from evaporating

35. Cloud droplet formation IIIRaoult Effect The vapor pressure for a solution drop is less than that for a plane of pure water The vapor pressure required to maintain equilibrium decreases as the drop radius decreases. This is opposite of the effect for curvature.

36. Cloud droplet formation IIIRaoult + Kelvin Effect We can combine the effects of curvature and solution. This curve, represented by the thick line at the right, is the Köhler curve. Initially the solution effect dominates, but as the drop gets bigger, the curvature effect takes over. When the drop is very large, neither effect dominates and the surface of the drop, to the water molecules, appears as a flat surface. Question : what can we measure in the köhler equation ?

37. Effect of a lower surface tension on critical supersaturation due to organic substances Köhler curves calculated for three aerosol dry sizes and two different aerosol chemical compositions. -inorganic aerosol with surface tension equal to that of pure water (dotted lines). -inorganic + organic aerosol and variable surface tension (solid lines).

38. Modified Kolher Equation to include the effects of slightly soluble organic compounds

39. Measurement of HGF: Principle of Tandem-DMA Derived parameter Growth Factor GF = Dp(@90%RH)/D0

40. Measurement of CCNs

41. Measurement of HGF: Principle of Tandem-DMA

42. A simplified view of the Atmospheric Aerosols

43. Hygroscopic growth of laboratory aerosol mixtures Classic growth theory (soluble fraction) –Neglecting hydrophilic organic material and surface tension effect Zdanoski-Stokes-Robinson (ZSR) approach GF = (A GFA3 + B GFB3 + …)1/3 Neglecting non-linearity of organic/inorganic mixture on water activity and suface tension

44. In-situ Characterisation of scavenging Microphysics Interstitial Phase (RJI) Interstitial + Condensed Phases (Whole air) Condensed Phase (cloud impactor) Condensed Phase CVI Question : How to characterize the scavenged aerosol fraction ?

45. Cloud Sampler IThe original Sampler

46. Cloud Sampler IPassive Sampler

47. Cloud Sampler IIIActive String collector

48. 5 m s-1 Cloud Droplet Dynamics Overal Losses 5-15% 50-80% 20µm 60-80% Settling velocity: 1-2 cm s-1 Stopping distance: 0.5 cm Relaxation time: 0.001 s-1 Stokes number: 1-2 Evaporation time : 1-5 s Analyzer

49. In-situ Characterisation of scavenging Microphysics Interstitial Phase (RJI) Interstitial + Condensed Phases (Whole air) Condensed Phase (cloud impactor) Condensed Phase CVI Question : How to characterize the scavenged aerosol fraction ?

50. Sampling cloud dropletsPrinciple of a Counter Flow Virtual Impactor