How do AerosolPrecipitation Interactions affect the Energetics of the Climate System

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How do Aerosol-Precipitation Interactions affect
the Energetics of the Climate System?

• Changes in cloud cover
• Redistribution of the latent heat vertical
  profile
• Redistribution of aerosols and trace gases
  detrainment of ice crystals and water vapor into
  the UT/LS
• Effect on circulation systems of various scales

A somewhat biased, not all-inclusive introduction
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Changes in Cloud Cover/Lifetime

• Cloud lifetime may increase with increasing
  aerosol loading due to smaller drops, less
  drizzle (Albrecht, 1989)
• Greater lifetimes increase cloud cover for
  potentially larger radiative impact than cloud
  albedo (Twomey) effect. (Larger albedo change)
• Semi-direct effect strongly absorbing aerosols
  may heat atmosphere and reduce cloud cover
  (Hansen et al., 1997 Ackerman et al., 2000)

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Evidence for Cloud Lifetime Effect
Rosenfeld et al. 2006
Sekiguchi et al. 2003

• Satellite studies show aerosol loading is
  positively correlated with low cloud fraction
• Interpreted as a result of longer cloud lifetimes
  caused by increasing aerosol number and reduced
  drizzle rates

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Changes in Cloud Cover May Dominate other
Indirect Effects Kaufman et al., 2005
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Confounding Influences on Observations of Cloud
Cover Effect

• A. Apparent increases in aerosol optical depth in
  partly cloudy regions
• 1) "Clear air" pixels contaminated by small
  clouds
• 2) 3-D effects light scattered from sides of
  clouds (Wen et al. 2007 Marshak et al.,
  submitted)

B. Possible real aerosol optical depth
increases 1) Increased humidity near clouds
causes aerosols to grow 2) Aerosols acting as CCN
may undergo chemical reactions in clouds, so
larger particles are released upon evaporation 3)
Cloud detraining regions are sites for new
particle formation These all should cause
apparent or real increases in aerosol optical
depth near clouds and be more pronounced as
cloudiness increases...but not necessarily a
result of CCN effects on clouds. (Cloud effects
on aerosols!)
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Hygroscopic Growth of Aerosol Near Clouds

• Analysis of 145 Cloud Fields, 9 Flights, small
  trade wind cumulus
• In-situ data shows humidity field is enhanced
  near clouds
• Corresponds to 16 increase in diameter and
  60 change in extinction coefficient (Twohy et
  al, in prep)
• Radke and Hobbs (91) observed "halos" Koren et
  al (07) saw similar ramp-up in optical
  depth--Aeronet data

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Cold Clouds Latent Heat/Dynamical Effects

• Aerosol effects on mixed phase clouds, which
  yield the vast majority of global precipitation,
  are not well understood
• Release of latent heat due to condensation or
  freezing means a convective cloud parcel becomes
  warmer and more buoyant
• Polluted clouds Lack of removal of LW through
  precip takes more, smaller drops to higher levels
  where they freeze and release additional latent
  heat, which can invigorate clouds and produce
  additional precip (ice phase)
• Evaporation of precip in sub-cloud layer produces
  cold dense air that spreads and can influence
  neighboring cloud development

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Studies of Latent Heat/Dynamical Effects

• Modeling Khain et al. (2004, 2005) Zhang et al
  (2005) Lynn et al. (2005) Van den Heever et al.
  (2006), Siefert and Beheng (2006), others
• Hurricanes Cotton et al. (2007) Rosenfeld et
  al. (2007)
• Observations Williams et al. (2002), Andreae et
  al. (2004), Koren et al. (2005)
• WMO Rpt Chap 8 "complex dynamical responses to
  pollution aerosols found in modeling studies,
  thus requiring greater quantitative understanding
  of the behavior of cumulus clouds and their
  interaction with each other, with the boundary
  layer, and with larger-scale weather systems as
  well"

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Interaction of Warm and Cloud Cloud Effects
Latent Heat Effect
Lifetime Effect
Dynamic Effect
Albedo Effect
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CCN, IN and GCCN All Important
Predicted changes in w due to increased aerosol
of different types (Van den Heever et al., 2005)
Early stages CCN dominates
Mature stages IFN dominate (Dissipating GCCN
impt)
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Dust as CCN?

• Fresh dust primarily insoluble but may contain
  slightly soluble components can acquire soluble
  material in atmosphere
• Levin et al (1993, 2005) found dust internally
  mixed with sulfur and with sea-salt in the Med
  and studied GCCN and IN effects
• Rosenfeld (2001) showed evidence of dust as CCN

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Measurements of Saharan Dust and Clouds in
NAMMADroplets only sampled evaporated with
Counterflow Virtual Impactor (CVI)

• 1/3 of low-level cloud drops contain dust or dust
  mixed with soluble material! Small amounts of
  soluble material--at least some dust acts as CCN.
• Concentrations 25-100 cm-3 drops with dust. If
  1 act as condensation IN (T -25C, DeMott), ice
  concentrations 250-1000 lit-1 could result
• Additional CCN and IN may have impact on warm
  rain, crystal size, latent heat distribution and
  storm intensity. Effects on hurricanes?

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Light Absorbing Carbon in Clouds (PACDEX)
CVI-Soot Photometer (Subramanian, Kok,
Baumgardner)
LAC is present and measurable in both liquid and
ice clouds (red barred data). DeMott CFDC data
shows IN conc correlated with LAC concentration.
Need to understand IN activity, radiative impact.
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Needs

• Understanding of ice nuclei types and mechanisms
• Dust and carbonaceous aerosols as CCN, IN, giant
  CCN and competing effects
• Interpretation of satellite measurements of
  aerosol/cloud interactions (cloud lifetime)
• Deep convection Modeling suggests complex
  effects that need to be verified by more remote
  and in-situ observations
• Others...