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Microphysical-macrophysical interactions or Why microphysics matters

Learn about microphysical-macrophysical interactions in liquid clouds, optical properties affecting cloud albedo, and the crucial role of cloud droplet size in precipitation formation and cloud radiative effects.

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Microphysical-macrophysical interactions or Why microphysics matters

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  1. Microphysical-macrophysical interactionsorWhy microphysics matters

  2. Optical properties of liquid cloudsCloud albedo  t/(t+7)t = 3LWP/reLWP is macro physical property- determined largely by meteorological contextre is a micro physical property- determined largely by properties of cloud condensation nuclei

  3. Optical properties of liquid cloudsWhat determines re ?re  (3LWC/4N)1/3for warm layer clouds N dominates (N can vary from 10-2000 cm-3 while LWC varies from 0.2-1 g m-3)

  4. MODIS, data courtesy of NASA

  5. MODIS, data courtesy of NASA

  6. MODIS, data courtesy of NASA

  7. Net cloud radiative effect from ERBE -90 -70 -50 -40 -30 -20 -10 0 10 20 30 40

  8. Microphysics and radiation budget Error in TOA net radiation in assuming no microphysical variability (constant N)

  9. What determines N in warm clouds? Cloud droplet concentration [cm-3] Aerosol concentration (r>0.1 micron) [cm-3]

  10. The Twomey Effecta.k.a first aerosol indirect effect Increase N  decrease re  increase  (for constant LWP)  increased albedo   t/(t+7)t = 3LWP/re

  11. Aerosol loading and cloud dropleteffective radius 0.0 0.1 0.2 0.3 0.4 Aerosol index from Breon et al. 2002, Science, 295, 834-838 6 8 10 12 14 Cloud droplet radius [micron]

  12. Williams et al. (2001)

  13. IPCC, 2007

  14. Cloud droplet size also impacts precipitation formation • Terminal speed of falling droplets increases strongly with size • Collection “efficiency” also increases with drop size • smaller drops tend to follow flow field around impinging larger ones, whereas larger ones are collected • Initial production of precipitation is more efficient in clouds with larger droplets (lower N, for a given LWC)

  15. Summary of drizzle observations in stratiform warm clouds from field programs

  16. However, what about feedbacks? Increase N  decrease re  decreasing collision coalescence  decrease precipitation  reduced LWC sink  increase cloud LWC  increase   increased albedo Albrecht effect a.k.a “second aerosol indirect effect”

  17. Increase precip.  decrease cloud cover Albrecht 1989

  18. Increasing N in a more sophisticated model reduced precip. decreases LWP ! ….sometimes Ackerman et al. (2004)

  19. Clouds containing the ice phase

  20. Importance of microphysics for orographic precipitation distribution Ice concentration: 1 liter-1 25 100 wind direction

  21. Homogeneous freezing sensitivity to aerosol concentration

  22. Polar stratospheric clouds (PSC) and ozone depletion

  23. Hamill and Toon 1991

  24. Indirect effect ratio RIE 1st AIE 2nd AIE For adiabatic cloud layers, Nd1/3 LWP5/6 Define RIE = 2ndAIE / 1st AIE Relative strength of the Albrecht effect compared with Twomey

  25. short timescales long timescales

  26. Albrecht effect can enhance or cancel the Twomey effect

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