The Effect of Aerosols on Precipitation

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The Effect of Aerosols on Precipitation
Suppression or Enhancement?

• Kate Cerully
• 4/21/09

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How Do Aerosols Form Droplets?

• Cloud droplets and ice particles form in the
  atmosphere by condensation of supersaturated
  water vapor on aerosol particles
• Those particles that have the potential to
  nucleate liquid cloud droplets are called cloud
  condensation nuclei (CCN), while aerosol
  particles that can induce the formation of ice
  crystals are named ice nuclei (IN)

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How Do Aerosols Form Droplets?
Equilibrium line for a 20 nm ammonium sulfate
particle
CCN f (Size, Chemical Composition)
Moore, 2008
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Key Effects Impacting Formation of Precipitation
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Radiative Effects

• Radiative effects of aerosols on clouds mostly
  act to suppress precipitation
• Aerosols decrease amount of solar radiation that
  reaches land surface
• Less heat to be available for evaporating water
  and energizing convective rain clouds
• Fraction of radiation not reflected back to space
  by the aerosols is absorbed into the atmosphere,
  leading to heating of the air above the surface
• Stabilizes the low atmosphere and suppresses the
  generation of convective clouds
• Warmer and drier air thus produces circulation
  systems that redistribute the remaining
  precipitation

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Microphysical Effects

• Clouds formed with elevated concentrations of CCN
  contain higher concentrations of smaller cloud
  droplets
• More CCN slows the conversion of cloud drops into
  raindrops by the need to nucleate larger number
  concentrations of smaller drops - slower to
  coalesce into raindrops
• Created the expectation that polluted areas would
  suffer from reduced rainfall

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Aerosol Indirect Effects
1st Indirect Effect Polluted clouds will have
smaller and more numerous droplets, which makes
the cloud denser and able to reflect more
incoming radiation 2nd Indirect Effect Smaller
droplets increase cloud lifetime by suppressing
precipitation
Moore, 2008
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Contradictions
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Microphysical Effects

• Cloud Seeding
• Expected that accelerating the conversion of
  cloud water to precipitation (i.e., increasing
  the autoconversion rate) would enhance rainfall
  amounts
• It turns out, however, that polluted areas are
  not generally drier, and rain enhancement by
  cloud seeding remains inconclusive

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Satellite Observations

• Observe the larger picture of aerosol effects on
  clouds and precipitation
• Agreement - more CCN ? suppression
• Urban and industrial air pollution plumes were
  observed to completely suppress precipitation
  from 2.5-km deep clouds over Australia
• Heavy smoke from forest fires was observed to
  suppress rainfall from 5-km-deep tropical clouds
• Disagreement - more CCN ? enhancement
• Clouds appeared to regain precipitation
  capability when ingesting giant CCN salt
  particles from sea spray and salt playas
• World Meteorological Organization and the
  International Union of Geodesy and Geophysics
  report concluded that it is difficult to
  establish clear causal relationships between
  aerosols and precipitation and to determine the
  sign of the precipitation change in a
  climatological sense. Based on many observations
  and model simulations the effects of aerosols on
  clouds are more clearly understood (particularly
  in ice-free clouds) the effects on precipitation
  are less clear.

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How Can This Be?
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CCN Size

• Submicrometer CCN aerosols decrease precipitation
  from shallow clouds and invigorate deep
  convective rain clouds with warm (gt 15C) cloud
  base
• Impact on the overall rainfall amount is not
  easily detectable
• Observations supported by a large number of
  cloud-resolving model studies
• Simulations also show that adding giant CCN to
  polluted clouds accelerates the autoconversion,
  mainly through nucleating large drops that
  rapidly grow into precipitation particles by
  collecting the other smaller cloud droplets
• Autoconversion rate is not restored to that of
  pristine clouds

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Precipitation - Evaporation Balance

• Fundamentally, the amount of precipitation must
  balance the amount of evaporation at a global
  scale
• Consequence of aerosols suppressing precipitation
  from shallow clouds must be an increase in
  precipitation from deeper clouds
• Can occur at global scale and cloud scale (clouds
  can grow to heights where aerosols no longer
  impede precipitation)
• Suggests that slowing the rate of cloud droplet
  coalescence into raindrops delays the
  precipitation of the cloud water, so that more
  water can ascend to altitudes where the
  temperature is colder than 0C

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Precipitation - Evaporation Balance
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Radiative vs Microphysical Effects

• Considered separately and independently
• Derived a point as a function of SS and
  concentration where invigoration effect is at its
  maximum
• More CCN depression less CCN enhancement

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Additional Factors

• Conclusions from numerical study
• Decreasing the solubility of the aerosol
  particles also reduces the dependency of rain
  formation on the initial number of aerosol
  particles
• Number and solubility of aerosol particles
  present at the beginning of the simulation seem
  to be the main parameters which influence
  precipitation formation, as they both determine
  the number of activated CCN and hence the size of
  the drops and the efficiency of collection
  processes

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Hygroscopicity

• Aerosol hygroscopic properties are dependent on
  particle size and chemical composition
• The typical size range of atmospheric aerosols
  spans several orders of magnitude (a few
  nanometers to tens of micrometers) and particles
  may contain any of the hundreds of different
  organic species, the eight or so major inorganic
  species (Na, K, NH4, Ca2, Mg2, SO42-, Cl-,
  NO3-) and water.
• Aerosol composition correlates for some chemical
  species to wet droplet size
• Adding to the complexity, gas- and aerosol-phase
  chemical reactions in the atmosphere change the
  hygroscopic properties of aerosols over time

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References

• Andreae, M.O., and Rosenfeld, D. (2008).
  Aerosolcloudprecipitation interactions. Part 1.
  The nature and sources of cloud-active aerosols,
  Earth-Science Reviews, 89, pp 1341.
• Leroy, D., Monier, M., Wobrock, W., and
  Flossmann, A.I. (2006). A numerical study of the
  effects of the aerosol particle spectrum on the
  development of the ice phase and precipitation
  formation, Atmospheric Research, 80, pp 1545.
•  
• Rosenfeld, D., Lohmann, U., Raga, G.B. ODowd,
  C.D., Kulmala, M., Fuzzi, S., Reissell, A., and
  Andreae, M.O. (2008). Flood or Drought How Do
  Aerosols Affect Precipitation?, Science, 321, pp
  1309-1313.
• Wikipedia, The Free Encyclopedia (2009). Cloud
  Seeding, http//en.wikipedia.org/w/index.php?titl
  eCloud_seedingoldid284460362, revised 13 April
  2009.

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How Cloud Seeding Works

• Predicated upon the fact that the equilibrium
  vapor pressure is lower over ice than over water
• When ice particles form in supercooled clouds,
  this fact allows the ice particles to grow at the
  expense of liquid droplets
• Common chemicals - silver iodide, dry ice
  (frozen carbon dioxide), liquid propane, or
  hygroscopic materials such as salt
• Seeding of clouds requires that they contain
  supercooled liquid waterthat is, liquid water
  colder than zero degrees Celsius. Introduction of
  a substance will induce freezing nucleation. Dry
  ice or propane expansion cools the air to such an
  extent that ice crystals can nucleate
  spontaneously from the vapor phase.