Aerosol Pollution Impact on Precipitation

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• Aerosol Pollution Impact on Precipitation

William Cotton1 and Zev Levin2
1 Colorado State University, Dept. of
Atmospheric Science, Fort Collins, Colorado,
USA 2 Tel Aviv University, Dept. of Geophysics
Planetary Science, Tel Aviv, Israel
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• The WMO/IUGGINTERNATIONAL AEROSOL PRECIPITATION
  SCIENCE ASSESSMENT GROUP(IAPSAG)
• Aerosol Pollution Impact on Precipitation
• A Scientific Review
• Zev Levin, Chairman
• William Cotton, Vice Chairman

Approved by the WMO - May. 2007
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There were

• 13 Lead Authors
• 27 Contributors
• 17 Reviewers
• During the process we lost
• Peter Hobbs, original chairman
• Yoram Kaufman, a lead author
• Brian Ryan, a reviewer

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The effects of aerosol pollution on clouds

• Since the 1960s, measurements have provided
  numerous and consistent pieces of evidence that
  an increase in pollution leads to increases in
  Cloud Condensation Nuclei (CCN) a sub-set of
  atmospheric aerosols.

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 Note that the increase in cloud drops tapers off
as the CCN concentrations increase. In stronger
updrafts (convective clouds) the increase in
drops with increase in pollution is steeper.
Ramanathan et al., 2001, based on data from
various groups .
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The effects of air pollution on clouds can be
observed from space using new remote sensing
methods.
Rosenfeld (2000)
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Pollution Tracks
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Ship Tracks
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Ice in clouds

• Ice nuclei (IN) are a much smaller sub-set of
  atmospheric aerosols than CCN.
• Their role in precipitation formation in certain
  clouds is critical.
• Finding correlation between the concentrations of
  ice nuclei and ice crystals in clouds is
  difficult because of the low concentrations of IN
  and the numerous mechanisms by which ice crystals
  can form including ice multiplication mechanisms.
  

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• What can pollution do to ice nuclei?
• Pollution aerosols may contain IN, thus increase
  the ice concentrations in clouds (e.g. Schaefer,
  1969 Changnon, 1980).
• In some cases pollution may contaminate the IN
  and reduce their ability to form ice (e.g. Braham
  and Spyers-Duran, 1974).

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• The effects of pollution on the amount of
  precipitation on the ground

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Orographic precipitation

• Both analysis of past data and modeling studies
  suggest that in these clouds pollution could
  modify precipitation amounts owing to
• the modest liquid water contents in them.
• the relatively short time the drops and ice
  crystals spend in the clouds.
• The temporal and spacial persistence of the
  clouds.

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Pollution reducing snow from orographic clouds
The smaller cloud drops in a polluted atmosphere
reduce the riming efficiency, leading to slower
growth of the ice crystals and to lower
precipitation.
Borys et al, 2003
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Simulations of wintertime orographic clouds by
Saleeby and Cotton (2005) revealed

• At higher concentrations of CCN, the average
  cloud droplet size decreases.
• Smaller supercooled droplets more readily
  evaporate in the presence of a strong Bergeron
  process.
• LWC is reduced at higher CCN concentrations.
• Thus at high CCN concentrations not only is
  riming reduced by lower collection efficiencies
  for smaller droplets but so is LWC, further
  reducing riming and precipitation.

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Storm Peak Lab basic concepts

• Pollution aerosols impact total snow water
  equivalent (SWE) if an orographic cloud is
  present. Otherwise hygroscopic CCN are generally
  less effective in cold cloud processes.
• 2. Snow falling thru an orographic cloud
  undergoes a seeder-feeder riming process in which
  crystals pick up extra water mass as they fall
  through the orographic cloud before reaching the
  surface.
• 3. If CCN are added to the orographic cloud, the
  droplet number concentration increases, the mean
  droplet size decreases, the riming collection
  efficiency then decreases, and the total rimed
  mass decreases thus leaving us with less SWE at
  the surface.

Heavy Rime Event
Cloud LWC up to 0.7 g/m3
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Winter Simulations Grid Configuration
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Total Precipitation Change Due to Increased
Pollution Aerosols

• An increase in CCN leads to reduced precip along
  the windward slope
• and highest plateau, and increased snowfall to
  the lee of the Divide.
• 2. A reduction in riming decreases the average
  snow crystal size and fall
• speed, thus, leading to a blow-over advection
  effect that shifts the snowfall
• spatial distribution. (Hindman et al. 1986)

GCCN 10-5 IFN Meyers Nucleation
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Aerosol Impacts on
orographic clouds a. Increasing the CCN
concentration alters the orographic cloud by
increasing droplet number and reducing droplet
size. b. Reduced riming efficiency leads
to a reduction in snow growth and graupel
formation within the orographic cloud. c.
Smaller, slower falling crystals tend to deposit
further downstream to the lee of the mountain
crest.
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Other Observations

• Givati and Rosenfeld (2004), Jirak and Cotton
  (2006) and Rosenfeld et al. (2006) have analyzed
  orographic precipitation data upwind and downwind
  of major urban areas vs. relatively cleans areas
  and found as much as 30 reductions, presumably
  due to pollution aerosols.
• However, independent re-analysis of the results
  in Israel (Levin et al., 2007) indicates no clear
  effects of aerosol pollution on these types of
  clouds in Israel.

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Pollution impact on Boundary Layer Clouds
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• Albrecht (1989) hypothesized that the higher
  droplet concentrations in clouds would reduce the
  rate of formation of drizzle drops by collision
  and coalescence. The reduced rate of drizzle
  formation would result in higher liquid water
  contents and higher droplet concentrations and
  lead to longer-lived clouds which by increasing
  cloud cover would lead to further enhance the
  albedo of those clouds.
• But, this is not the whole story!

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Drizzle can have complicated impacts on the
marine BL

• Drizzle falling only partway through the
  sub-cloud layer can destabilize the BL leading to
  cumulus under stratus.
• Drizzle falling through the entire sub-cloud
  layer can cool and stabilize the entire BL and
  lead to decoupling of the stratus layer from the
  surface.

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• In Jiang et al's. (2002) LES of marine
  stratocumulus, higher CCN concentrations
  suppressed drizzle which resulted in a more
  stable boundary layer, weaker penetrating cumulus
  and an overall reduction in the water content of
  the clouds. Thus cloud albedo was very little
  influenced by the increase in CCN concentrations.

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Additional studies

• Ackerman et al. (2004) also showed that increases
  in CCN do not necessarily result in increases in
  LWP in stratocumulus clouds. A primary factor
  affecting the LWP response to aerosol changes is
  the profile of humidity above the inversion. Only
  when the humidity above the inversion was high
  did increases in aerosol result in an increase in
  LWP. When dry air overlies the inversion,
  increases in aerosol tend to decrease LWP because
  of enhanced entrainment drying. Similar results
  were obtained by Lu and Seinfeld (2005).

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Aerosol Influences on entrainment in cu

• Another example of departures from the Albrecht
  hypothesis is Xue and Feingolds(2006) and Jiang
  et al.s(2006) simulations of aerosol influences
  on cumulus.
• They found that increasing concentrations of CCN
  and droplets, produced smaller droplets and
  suppressed drizzle and led to enhanced
  evaporation of droplets by entrainment.
• Because, for a given LWC, smaller droplets
  evaporate more readily than larger droplets,
  entrainment induced evaporative cooling was
  enhanced when CCN concentrations were high, which
  led to greater entrainment rates, reduced cloud
  fraction, cloud size, and cloud depth.

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• These simulations highlight the nonlinearity of
  cloud systems when drizzle is present and
  suggests that increased concentrations of CCN may
  not always increase cloud water contents, cloud
  lifetimes, and cloud albedo.

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Aerosol Pollution Impacts on Deep Convective
Clouds and Precipitation
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• Seifert and Beheng 2006b showed that the effect
  of changes in CCN on mixed phase convective
  clouds is quite dependent on cloud type.
• They found that for small convective storms, an
  increase in CCN decreases precipitation and the
  maximum updraft velocities.
• For multicellular storms, the increase in CCN has
  the opposite effect namely, promoting secondary
  convection, and increasing maximum updrafts and
  total precipitation. Supercell storms were the
  least sensitive to CCN.
• Their study also showed that the most important
  pathway for feedbacks from microphysics to
  dynamics is via the release of latent heat of
  freezing.

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• Other modeling efforts by Lynn et al. 2005a,b,
  Khain et al. 2005, van den Heever et al.
  2006, van den Heever and Cotton 2007 show
  complex dynamical responses to aerosols sometimes
  leading to greater precipitation amounts and
  other times less

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Possible causes increase in latent heat release
due to the formation of ice in the upper portions
of clouds invigorated by freezing of greater
amounts of supercooled in polluted clouds and by
enhanced evaporation below clouds (cold pool)
leading to the formation of neighboring clouds.
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• Van den Heever and Cottons (2007) mesoscale
  simulations suggest that in some cases the cold
  pools may interact favorably with mesoscale
  simulations like sea breeze convergence zones and
  urban heat island convergence zones and in other
  cases the cold pools may propagate away from the
  parent mesoscale forcing leading to a decrease
  in precipitation.

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RURAL
CCN-L
GCCN-L
URBAN
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Effects of biomass burning on precipitation

• Early measurements not conclusive as to the
  relative effects of pollution as compared to the
  effects of changes in meteorological conditions.
• Recent measurements in the Amazon -- smoke from
  fires decreased cloud drop size, but formed
  taller clouds possibly with more ice and hail.
• Note widespread smoke plumes will inhibit cloud
  formation by
• (a) reducing surface radiation and thus
  reducing sensible and latent heat fluxes, and (b)
  stabilizing the atmosphere
• Integrated over the whole area affected by smoke
  -- rainfall actually increased

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Urban precipitation-METROMEX

• The METROMEX experiment (in the 1970s) examined
  the effects of urban pollution on SUMMER rainfall
  around the city of St. Louis.
• Although, the cloud drops were more numerous and
  smaller, suggesting that rainfall should
  decrease, an increase was observed.
• However, greater concentrations of GCCN were
  observed.

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Urban impact on precipitation
Five year moving averages and time trend of
Centerville (downwind of St. Louis) summer
rainfall, 1941-1968. From Changnon et al. (1971).
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Van den Heever and Cottons (2007) simulations
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Experiment Design

• In the CONTROL experiment, RAMS is initialized
  homogeneously with rural CCN and GCCN
  concentrations.
• In the sensitivity tests, a continuous source of
  urban CCN and / or GCCN concentrations are used
  within the lowest 500m over the urban region. The
  sensitivity tests are otherwise identical to the
  CONTROL experiment.
• These experiments were repeated in which the
  urban region was removed while the aerosol
  characteristics were maintained.

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AEROSOL CONCENTRATIONS

• High background
• Rural CCN 1200 cc-1 GCCN 0.1 cc-1
• Urban CCN 2000 cc-1 GCCN 0.2 cc-1
• Low background
• Rural CCN 800 cc-1 GCCN 0.01 cc-1
• Urban CCN 2000 cc-1 GCCN 0.2 cc-1

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Downwind Precipitation Low Background
Concentrations
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Accumulated Volumetric Precipitation (acre-feet)
for entire Grid 3
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The re-distribution of precipitation is
consistent with observations in St. Louis,
Atlanta and in a few other urban regions.
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van den Heever and Cottons simulations indicate

• Urban land-use (i.e. Urban heat island) has the
  dominate influence on precipitation.
• Aerosol influences are important but the strength
  of the aerosol signal depends on pollution levels
  in the surrounding regions.
• That is highly polluted background aerosols can
  mask the urban influence.

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Global effects

• Thus far we discussed the effects of pollution on
  local and regional scales. In this analysis we
  implicitly assume that the global climate remains
  unchanged.
• The effects of aerosol pollution could therefore,
  mostly modify the average regional, temporal and
  spatial distributions of rain.
• If global climate changes in response to aerosols
  the availability of water vapor can also change.

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Global effects of pollution on precipitation

• GCM-- estimates 0 to - 4.5 change in global mean
  precipitation over the last 100 years due to the
  direct and indirect aerosol effects.
• The differences among models over land range from
  -1.5 to -8.5.

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GCM combined with Ocean models show that

• Increased direct indirect effects of aerosols
  reduces the incoming radiation, thus cooling the
  surface.
• This slows down the hydrological cycle which
  reduces the atmospheric water vapor (a greenhouse
  gas) and acts to counter greenhouse gas warming.
  

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The potential influence of aerosols on climate
could be far more significant than previously
thought.

• Estimating the consequences of combining
  greenhouse gas warming and aerosol cooling in the
  future, depends on
• Uncertain estimates of future pollution
  emissions.
• Greenhouse gas releases into the atmosphere.
• The ability of the models to correctly describe
  the atmospheric processes.

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Summary

• Both observations and modeling studies show that
  pollution aerosols increase cloud drop
  concentrations and reduce average drop size.
• The effects on precipitation on the ground are
  much more difficult to quantify due to the fact
  that once the precipitation process is altered,
  the dynamics of clouds and mesoscale systems is
  also altered in a nonlinear way. Thus the effects
  of aerosols on precipitation become much less
  predictable.

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• Precipitation in an Urban environment
• Observations
• Precipitation downwind from urban areas is
  affected in a complex way by urban effects
  including pollution Both increases and no
  detectable effects were reported.
• Model simulations tell us
• The most important factors are the urban land-use
  effects which modify the dynamics of the storms
  downwind of the city and affect the spatial and
  temporal distributions of rain.
• The effects on precipitation due to increased
  pollution depend on the background aerosols,
  their size and composition and the interactions
  of secondary convection via cold pools with urban
  land-use driven circulations.
• The aerosol effect is easier to identify in a
  cleaner environment.

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• Orographic precipitation
• The biggest potential effect of pollution on
  precipitation is found in orographic clouds.
• Measurements and simulations of snowfall over
  mountains show a reduction precipitation due to
  pollution.
• These measurements and simulations need to be
  expanded to evaluate pollution aerosol impacts on
  precipitation over large basins like the Colorado
  River Basin.

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Global precipitation

• GCMs show precipitation reduction due to
  increased pollution. Greater suppression in the
  Northern Hemisphere and over land.
• The high variability in precipitation amounts
  from GCMs stresses the need to improve
  representation of aerosol and cloud processes.

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Recommendations

• Implement a series of international projects
  targeted toward unraveling the complex
  interactions among aerosols, clouds, and
  precipitation.
• WMO/IUGG should take the lead in such projects
  together with other UN and International
  Organizations. 
• Some of these could be sponsored and financially
  supported by the countries involved. For example
• Study the effects of an evolving industrial
  economy such as China on precipitation.
• Study the effects of biomass burning and dust in
  some of the African regions.

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The WMO/IUGG can play a key coordination role in
encouraging that the following recommendations
are implemented.

• 1) Better characterization of aerosols
• Emission inventories
• Size, number concentrations
• Chemical processes, physical properties and
  instrumentation
• Accurate knowledge of the chemical processes
  leading from gas pollution to CCN
• The ability of different types of particles (e.g.
  mineral dust, biomass smoke, biogenic,
  carbonaceous) to act as CCN, GCCN, and IN as a
  function of aerosol size, origin, and air mass
  history.

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• Develop new and innovative instruments and
  measurements to determine CCN, GCCN and IN
  concentrations as a function of particle size,
  composition and supersaturation.
• Emphasis should be placed on understanding the
  different modes of ice nucleation.
• Global coordination of observational networks is
  needed for more complete coverage of global
  aerosols (ground-based remote sensing methods
  e.g. AERONET) .
• More accurate assessment is needed from
  satellites of the aerosol distribution,
  concentration and properties.

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• 2) The effects on clouds and precipitation
• Design experiments to better understand the role
  of ice in precipitation development
• Multi-year measurements from space of
  precipitation patterns along with retrievals of
  cloud nucleating aerosols are needed to assess
  both regional and global impacts of aerosol
  pollution on precipitation.
• Improved satellite measurements of Liquid Water
  path (LWP) and Ice Water Path (IWP), which define
  the potential for precipitation with pollution
  modulating how much will reach the ground.

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• New methods are needed to estimate precipitation
  amounts with high enough accuracy to be able to
  resolve changes due to pollution.

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• Models should be used to provide a quantitative
  answer as to the relative effects of aerosols
  versus environmental parameters (temperature,
  Relative humidity, wind shear, land-surface
  properties, etc.) on precipitation.

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• The high variability in precipitation amounts
  from GCMs stresses the need to improve
  representation of aerosol and cloud processes to
  be able to answer with some confidence the
  question on the effects of pollution on
  precipitation.
• Detailed knowledge of ice formation in clouds is
  still lacking, requiring more laboratory,
  modeling, and field studies.