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Assessing Satellite Observations for Cloud-Mediated Aerosol Climate Forcing Analysis

In this study by Daniel Rosenfeld, the limitations of current aerosol optical signals for quantifying cloud-mediated aerosol climate forcing are evaluated, addressing challenges in interpreting satellite observations and improving aerosol measurements. The importance of aerosol properties for different cloud types and radiative forcing effects are explored. The study aims to enhance understanding and refine methodologies for assessing cloud-aerosol interactions.

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Assessing Satellite Observations for Cloud-Mediated Aerosol Climate Forcing Analysis

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  1. Are we doing the right satellite observations and analyses for quantifying cloud-mediated aerosol climate forcing? By Daniel Rosenfeld The Hebrew University of Jerusalem, Israel

  2. Challenges Recent developments show the limitations of aerosol optical signals where we need them most: • Too few aerosolsfor producing discernible optical signal: for shallow marine clouds with Nd< ~100 cm-3, where few added aerosols incur large negative radiative forcing. • Too small aerosols for producing discernible optical signal: for deep convective clouds with active warm rain processes UAP < 50 nm activate aloft and invigorate the clouds, possibly with positive radiative forcing. • Correct interpretation of existing satellite observations.

  3. Shallow clouds

  4. 50% of AOD=0 -0.03 to 0.08 50% of AOD=0 0.00 to 0.06

  5. 0.06

  6. Sensitivity of CALIOP satellite Lidar for aerosol detection 158 cm-3 D=0.12 mm

  7. Rosenfeld et al., 2012 Heavy Drizzle Threshold 7

  8. 40 N 140 W 135 W 35 N Rosenfeld et al. Science 2019 MODIS, 26 June 2003, 19:40 UT

  9. The important range is CCN<100 cm-3 Open cells Closed cells Goren and Rosenfeld, Atmos. Res., 2014

  10. The components of the radiative forcing due to closing open MSC, totaling -110 Wm-2 Cloud brightening for a given cloud water path (Twomey) 26 ± 7 % Scene brightening due to increased Cloud Cover 42 ± 9 % -29 Wm -2 -46 Wm -2 -35 Wm -2 Cloud brightening due to increased Cloud Water Path 32 ± 11 % Goren and Rosenfeld, Atmos. Res., 2013

  11. Suppressed rain Rain

  12. Re - 800 Calculated by Chongching Fan

  13. Doubling Nd Doubling CRE Cloud radiative effect [W m-2] Drop number concentration [cm-3]

  14. But most of the ACI is mediated by breaking the cloud cover with dearth of aerosols Global mean droplet concentration from MODIS using Level-3 product For liquid clouds with >80% 1x1o cloud fraction For liquid clouds with >80% 1x1o cloud fraction Rob Wood, U. Washington

  15. Most of the oceans have Nd < 100 cm-3 • Too low CCN for measurable AOD Global mean droplet concentration from MODIS using Level-3 product For liquid clouds with >80% 1x1o cloud fraction For liquid clouds with >80% 1x1o cloud fraction Rob Wood, U. Washington

  16. No AI No signal MODIS Aerosol Index MODIS Aerosol Optical Depth Can’t calculate AI for AOD<~0.1

  17. All Cf Aerosol Optical Depth Nd90 [cm-3] LWP is positive for low Nd (rainout) and negative for high Nd (drop evaporation) for all Cf, but overall susceptibility is positive. l = ∂ln(LWP)/ ∂ln(Nd)

  18. l = ∂ln(LWP)/ ∂ln(Nd) MODIS Aerosol Optical Depth Cloud Drop Concentrations LWP effect is detectable at a reduced magnitude when using AOD (compared to Nd) for all Cf, when classifying by CGT.

  19. 2013 Calculated effect No LWP & Cf effects AOD Aerosol forcing depends strongly on the reference background. Limiting the sensitivity would limit the detectable forcing

  20. Deep clouds

  21. Fan, Rosenfeld et al., Science 2018

  22. Higher and colder cloud tops • Larger number concentrations • Larger mass concentration • longer living and colder anvils • More positive radiative forcing

  23. Alternative hypothesis: aerosol warming effect on deep clouds Rosenfeld et al., Science, 2008 Colder and wider anvils radiate less IR energy to space positive RF Detrained vapor add GHG aloft Growing Mature Dissipating Hail

  24. Avichay will show that the UAPs dominate the lightning Hoppel minimum Thornton et al., GRL 2017

  25. We will address the following challenges • What ACI-important aerosol properties do we still miss with the optically based instruments? • How far can we still improve the optical measurements and what are their inherent limitations? • Possibilities and limitations of addressing ACI for shallow clouds with poor direct aerosol retrievals. • What can be achieved by combining retrieved cloud microphysics, updrafts, and aerosol measurement? • Identify the information content required for quantifying ACI for a useful forward calculation of forcing? • Initiate a road map towards full measurements of ACI by retrieving simultaneously updrafts, microphysics and aerosols

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