Aerosol effects, Sc microphysics and climate

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Aerosol effects, Sc microphysics and climate

• Motivation Anthropogenic aerosol effect on
  climate

IPCC 2007
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Ship Tracks,Baja California(visible reflectance)
Type I enhanced LWP in clean shallow Sc Type
II albedo increase but no LWP increase in
polluted Sc
LWP depleted in plume
Volcano tracks, South Sandwich Islands
A Albedo B Drop radius C LWP
(Gasso 2008 JGR)
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Pockets of Open Cells (POCs)
Bretherton et al (2004)
POCs reduce the mean albedo of the stratocumulus,
but form only in unpolluted regions with low
background aerosol ? positive Albrecht effect.
POCs
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POCS, cloud droplet size, and drizzle
Rob Wood (U. Washington)
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LES simulation of POCs

• Savic-Jovcic and Stevens (JAS, in press)
• UCLA-0 LES
• DYCOMS resolution, 25x25 km domain
• ND (No Drizzle) Nd 200 cm-3
• D (Drizzle) Nd 25 cm-3
• Identical initial/boundary conditions.

DS simulation shows stronger mesoscale cellular
structure of cloud and vertical velocity.
w (m/s)
Pseudo-albedo
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(Nature 2004)

• LES simulations of marine Sc-capped boundary
  layers with bin-resolved microphysics and varying
  aerosol concentrations.
• More aerosol made thin cloud layers even thinner,
  contrary to expectations.
• Primary mechanism was increased entrainment at
  higher droplet concentration

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Why sedimentation decreases entrainment efficiency

• Sedimentation depletes liquid water from cloud
  top, where entrainment occurs ? wT ? rd2 ? Nd-2/3
• Reduced evaporative enhancement of entrainment.
• More aerosol
• smaller droplets fall slower
• more efficient entrainment
• Þ thinner Sc clouds.

wT
w
(Bretherton et al. 2007)