Remote sensing of aerosol-cloud interaction

1. Remote sensing of aerosol-cloud interaction Presented by Lorraine A. Remer NASA/Goddard Space Flight Center

2. Basis of remote sensing Remote sensing of clouds and aerosols Observations of aerosol modification of cloud microphysics 3.1 Explanation Observations requiring better explanation 4.1 More explanation Observations of aerosol modification of cloud environment 5.1 Explanation 6. Aerosol effects on convection (briefly) 7. The skeptical arguments 7.1 Refuting the skeptics 8. Two examples of continuing work in this area

3. What happens when radiation interacts with a small particle?

4. What happens when radiation interacts with a small particle? The pattern of the scattered radiation depends on the relative sizes of the wavelength and the particle. The angular distribution of the scattered radiation is the “phase function”

5. What happens when radiation interacts with a small particle? It the relative size that is important. Larger particle, same wavelength produces the same effect as same particle with shorter wavelength.

6. All of the above is a simplification of a more complex problem

7. The interaction between the particle and the incident radiation can be described by: The complex refractive index (determined by particle composition) And the size distribution of the particles

8. The composition of the particle also plays a role. Here it is absorbing some of the incident radiation. However, it can be nonabsorbing and still alter the phase function. This particle property is called “complex refractive index”

9. A distribution of particles creates a radiation field. The more particles, generally the larger amounts of radiation scattered in each direction, as determined by the particles’ phase functions and particle properties.

10. Remote sensing is measuring the results of this interaction from a distance Backscattered radiation Forward scattered radiation Aerosol loading radiance radiance wavelength wavelength

11. What are clouds and aerosols? Collections of suspended particles, with different sizes and different compositions. Cloud droplet Aerosol particle

12. Measuring the result of solar radiation interacting with aerosol and cloud particles can be used to infer information about the suspended particles. The sensor is receiving solar radiation scattered from many different atmospheric and terrestrial objects. aerosol cloud

13. Result of remote sensing of aerosol from the MODIS satellite Backscattered radiation Backscattered radiation Spectral slope = particle size Aerosol loading radiance radiance wavelength wavelength

14. Remote sensing of cloud droplet size from MODIS satellite

15. Brighter Scattered radiation in a mid-IR channel (longer wavelength) is proportional to the droplet size smaller Scattered radiation in a visible channel (shorter wavelength) Is proportional to the optical thickness of the cloud Nakajima and King (1990)

16. What is going on here? These streaks are brighter at 3700 nm AVHRR 3.7 µm K. Nielson

17. All clouds are “seeded” by aerosol particles. If the aerosol particle is hygroscopic, it will collect water vapor that condenses onto the particle creating a liquid droplet. Aerosol particles that act as cloud seeds are called Cloud condensation nuclei (CCN) Without aerosol particles the air would be extremely super saturated before clouds would form. We have clouds because we have aerosols. Cloud microphysics is the name given to the process of cloud droplet formation.

18. Twomey effect. Same amount of liquid water is divided Between more CCN. Result: more but smaller cloud droplets More droplets: more light scattered (brighter clouds) Smaller droplets: more light scattered at longer wavelengths

19. Brighter smaller

20. Explanation: If there were more CCN, then more but smaller cloud droplets would form smaller droplets backscatter more solar radiation at longer wavelengths. The streaks in the cloud are the result of ships below the cloud Emitted fine hygroscopic particles from their smoke stacks. These streaks are called ship tracks and are a remote sensing proof of the Twomey effect

21. Kaufman and Nakajima 1993 Smoke from biomass burning Covers South America during the Dry season. What does it do to clouds?

22. Measured reflectance at 2.1 µm Kaufman and Nakajima 1993 As aerosol loading increases, cloud droplet size decreases. Long wavelength reflectance Using cloud free pixels to Quantify aerosol loading And cloudy pixels to retrieve Droplet size

23. Measured reflectance at 0.64 µm Kaufman and Nakajima 1993 But visible cloud reflectance decreases as aerosol loading increases. Visible wavelength reflectance Twomey’s theory says the opposite should occur

24. Explanation: Black carbon is absorbing light, making the clouds less reflective A remote sensing observation leads to a modification of Twomey’s theory

25. Kaufman and Fraser 1997 South America Wetzel and Stowe 1999 Global oceans - stratus

26. Kaufman and Fraser 1997 South America Wetzel and Stowe 1999 Global oceans - stratus

27. Because the large statistical data base available from remote sensing methods • we can both prove Twomey’s theory and also start to see caveats. • Black carbon absorption • Not all geographic regions or cloud types • Saturation at higher aerosol loading Without remote sensing how would we ever see these effects in the real world?

28. Koren et al. 2004 (South America) We have looked at cloud microphysics, let’s look at cloud coverage now.

29. When you have a lot of smoke, you don’t have clouds Koren et al 2004

30. Kaufman et al., 2005 Atlantic ocean AOD>0.3 Dust belt AOD<0.10 Stratiform droplet radius Stratiform cloud fraction Smoke belt Longitude

31. Kaufman et al., 2005 Atlantic ocean Dust belt Stratiform droplet radius Stratiform cloud fraction For a specific distance from the continent, more aerosol is associated with more clouds and smaller droplets Smoke belt Longitude

32. Puzzle: Over South America aerosols are Associated with an decrease of Cloud coverage Over the Atlantic aerosols are Associated with an increase of Cloud coverage

33. How can aerosols increase cloud coverage? MICROPHYSICS If droplets are small enough they will not fall as precipitation. Cloud liquid water stays suspended longer. Clouds last longer and cloud coverage increases. OR Clouds spread out more and cloud coverage increases. Albrecht 1989 How can aerosols decrease cloud coverage? RADIATION If the aerosol absorbs solar radiation aerosols will change the environment in which the cloud develops. Clouds can evaporate in place (Ackerman et al., 2000) Atmospheric stability can change (Davidi et al., 2009) Sensible and latent heat fluxes from the surface can be decreased (Feingold et al. 2005)

34. Aerosol effects on microphysics are strongest in pristine conditions and then saturate Aerosol effects through radiative processes are linear, strongest in low cloud fraction and build as aerosol loading builds Koren et al., (2008) microphysics radiation Aerosol optical depth

35. Koren et al. 2008 Satellite observations confirm this theory

36. Remote sensing “discovered” a contradiction in how aerosol can affect cloud coverage, And then confirmed a theoretical hypothesis that explained the contradiction as a superposition of two types of processes. Next, let’s look at how aerosols might affect convective cloud development. Unlike previous examples, we will start from theory and move to observations

37. Rosenfeld schematic of convective clouds CleanCases Glaciation Level and Vertical Profile of Droplet Size are associated with Cloud-Aerosol Interactions. Polluted Cases

38. Rosenfeld hypothesis In convective clouds, Aerosols change microphysics at cloud base, - more but smaller droplets - it takes longer for small droplets to be big enough to coalesce - the droplets do not get big enough to be rain - glaciation is postponed - mixed phase layer is extended - more lightning - more latent heat is released creating more updraft - taller clouds - more ice - more severe convection - ultimately more precipitation Rosenfeld and Woodley 2001

39. Koren et al. 2005 Increasing AOD Increasing anvils Increasing cloud cover Increasing cloud top height Decreasing droplet size

40. Many aspects of the Rosenfeld hypothesis have been observed by remote sensing measurements

41. The problems with these remote sensing studies (and perhaps why you should not believe them) Use of AOD as proxy for CCN Possibility of retrieval artifacts creating artificial associations Possibility of meteorology driving both cloud and aerosol trends together

42. Koren et al., 2010 Severe cloud mask MODIS AOD MODIS RGB MODIS AOD

43. Koren et al., 2010 Cloud fraction Inverse cloud top pressure Severe cloud clearing (pink symbols) produces lower AOD than standard (aqua), suggesting some cloud contamination in the product Even so, the relationship between AOD and cloud pressure and cloud fraction remain the same.

44. Koren et al., 2010 Correlations between AOD, Cloud fraction and cloud top pressure With 280 meteorological variables from GDAS

45. Koren et al., 2010 Clouds and aerosols are not correlated with The same meteorological variables CTP AOD

46. Criticisms of remote sensing studies are valid, but the associations between aerosol and cloud variables are very robust and cannot be dismissed.

47. TRMM–LIS Flash counts particles lightning Yuan et al. (submitted) OMI percent increase in 2005 MODIS Cloud microphysics ozone

48. Take home messages (remote sensing) : 1. Interaction between solar radiation and suspended particles changes the radiation in ways that can be modeled with accuracy. 2. The interaction between radiation and particles is dependent on the relative size of the particle and the wavelength of the incident radiation, as well as the composition of the particle and their shape. 3. Thus, by measuring the scattered radiation with sufficient wavelength range and/or number of angles, some of the particle properties can be retrieved. 4. For clouds, we can retrieve the cloud optical thickness and droplet effective radius. 5. For aerosols, by measuring intensity of the scattered radiation and making assumptions about the surface we can retrieve aerosol loading, a measure we call aerosol optical depth. 6. The process of inferring information about the particles from measuring the radiation that interacts with the particles is called “remote sensing”.

49. Take home messages (remote sensing of aerosol-cloud interaction): We use aerosol optical depth as a proxy for aerosol particle number concentration. These two quantities are not the same. 2. We look for correlations and associations between aerosol optical depth retrieved from clear sky scenes next to clouds and cloud properties retrieved from clouds. 3. We use these associations to suggest cause-and-effect. Correlation does not prove causality. 4. We have concerns about making aerosol retrievals next to clouds (cloud contamination). In such cases there would be an artificial correlation between increase of AOD and increase of cloud cover. 5. We have concerns about meteorology driving both AOD and cloud properties together so that when aerosol increase, cloud properties systematically change in the same direction. 6. Even with these concerns, we find very robust associations between variables, so that the simple explanation is causality, though not proven.

50. Take home messages (history): 1. First we saw ship tracks, that perfectly illustrated Twomey’s theory of how how aerosol particles would cause the cloud to form more but smaller droplets, and this would result in brighter clouds. 2. We saw this association between aerosol and cloud effective radius in South America, but did not see brighter clouds as a result. We explained this by darker smoke absorbing light. 3. The more we looked the more complicated the process seemed to be: now you see it, now you don’t…. Saturation of the effect when AOD was too big. 4. We were surprised to find associations between aerosols and cloud coverage, both inhibiting cloud cover and then enhancing cloud cover. 5. We explained these associations by a superposition of processes: microphysical (Twomey) and radiative (aerosol changing the cloud environment through heating). 6. We notice in convective clouds changes in cloud top height, thermodynamic phase, lightning etc that support Rosenfeld’s hypothesis. 7. Obviously the work is not yet done, but already remote sensing has played a key role in advancing our knowledge of how aerosols and clouds interact.