Stronger constraints on the anthropogenic indirect Aerosol Effect

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Stronger constraints on the anthropogenic
indirect Aerosol Effect

• A paper from Ulrike Lohmann and Glen Lesins
• Presentation by Abdel Nnafie and Jasper Donker

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Contents

• Introduction
• Observations by Polder satellite
• Model
• Results
• Conclusion
• About the paper

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Introduction

• The amount of aerosol in the atmosphere has
  increased due to human activities
• Direct effect this concerns scattering,
  absorption and reflection of solar radiation by
  cloud
• Indirect effect aerosol act as cloud
  condensation nuclei, thereby affecting the
  initial cloud droplet number concentration (CDNC)
• ?increase in cloud albedo (cloud albedo effect)
• ?reductions in precipitation efficiency and
  increase of cloud lifetime (cloud lifetime
  effect)
• Cooling from these two effects causes a radiative
  forcing between -1 and -4.4 W/m2 (this great
  range causes large uncertainties in projections
  of future climate change) (IPCC)
• The anthropogenic indirect aerosol effect (aie)
  cannot be deduced from observations along but
  require a modeling component.
• Historical climate data together with simulations
  constrain this effect to a range of 0 to -1.2
  W/m2.
• In this paper a model is used for a more accurate
  study of the anthropogenic indirect aerosol
  effect, by using satellite observations

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IPCC
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Observations by Polder satellite

• 8 months of space-born measurements are done by
  the Polder satellite.
• These data were used to derive aerosol
  concentrations and cloud droplet effective radii
  (CDR).
• Results the cloud droplet size decreases with
  increasing aerosol index (AIthis represent the
  aerosol column number concentration)
• These data are not sufficient to quantify the
  magnitude of the global indirect aerosol effect.

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Model

• The ECHAM4 general circulation model is used in
  this research.
• 4 different simulation have been made
• Reference simulation ECHAM-CTL (includes both
  indirect effects)
• Simulation ECHAM-2ND-AIE (without the albedo
  effect)
• Simulation ECHAM-NO-AIE (without the aerosol
  indirect effect)
• Simulation ECHAM-PI (Pre Industrial simulation
  without anthropogenic aerosol)
• The outcomes of simulations are compared with
  the data from the POLDER satellite

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ResultsAI
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ResultsCDR
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Question?

• Do we need the anthropogenic aerosol indirect
  effects to explain the observed anticorrelation
  between AI and CDR?
• Or is this anticorrelation only determined purely
  by geographic variations in the liquid water
  path?

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• Oceanic slope is larger
• Polder oceanic and continental CDR converge
  approximately toward
• same limit (8 µm)
• Both aerosol indirect effects are needed to
  reproduce the outcomes
• of POLDER
• ECHAMs clouds are more susceptible to aerosols
  than the observed clouds
• ?ECHAM very likely overestimates the IAE.
• Oceanic slope overestimation 25.9/19.9 1.30
• Continental slope overestimation 21.6/7.00 3.09

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• With increasing AI CDR decreases and the liquid
  water path increases
• ?detection of anthropogenic AIE from observation
  only is complex
• ?using models together with observations is
  important.

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conclusions

• The climate model simulations including -1.8
  W/m2.
• ?longwave radiation 1.4 W/m2.
• ? net decrease 1.4 W/m2.
• ECHAM simulates an aerosol effect of -1.28 W/m2
  over oceans
• After correction -0.98 W/m2.
• Over land this is -1.62 W/m2 ? after
  correction -0.53 W/m2.
• NET GLOBLA MEAN -0.85 W/m2.
• This value is consistent with of 0 to -1.2 W/m2.

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About the paper