Microphysical-macrophysical interactions or Why microphysics matters

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Microphysical-macrophysical interactionsorWhy
microphysics matters
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Optical properties of liquid cloudsCloud
albedo ? ? t/(t7)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
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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)
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MODIS, data courtesy of NASA
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MODIS, data courtesy of NASA
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MODIS, data courtesy of NASA
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Net cloud radiative effect from ERBE
-90 -70 -50 -40 -30 -20 -10 0
10 20 30 40
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Error in TOA net radiation in assuming no
microphysical variability (constant N)
Microphysics and radiation budget
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What determines N in warm clouds?
Cloud droplet concentration cm-3
Aerosol concentration (rgt0.1 micron) cm-3
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The Twomey Effecta.k.a first aerosol indirect
effect

• Increase N
• ? decrease re
• ? increase ? (for constant LWP)
• ? increased albedo

? ? t/(t7)t 3LWP/re
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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
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Williams et al. (2001)
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IPCC, 2007
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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)

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Summary of drizzle observations in stratiform
warm clouds from field programs
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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
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Increase precip. ? decrease cloud cover
Albrecht 1989
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Increasing N in a more sophisticated model
reduced precip. decreases LWP ! .sometimes
Ackerman et al. (2004)
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Clouds containing the ice phase
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Importance of microphysics for orographic
precipitation distribution
Ice concentration 1 liter-1 25 100
wind direction
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Homogeneous freezing sensitivity to aerosol
concentration
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Polar stratospheric clouds (PSC) and ozone
depletion
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Hamill and Toon 1991
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(No Transcript)
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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
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short timescales
long timescales
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Albrecht effect can enhance or cancel the Twomey
effect