Kate Cerully4/21/09

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)

Wet Aerosol(Haze)

0.1 1 10

Wet aerosol diameterm

98

99

100

101

102

Rel

ativ

e H

um

idit

y (

%)

Equilibrium line fora 20 nm ammoniumsulfate particle

Critical Wet Diameter

Critical %SS

Cloud Droplet

CCN = f (Size, Chemical Composition) [Moore, 2008]

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

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

Clean Environment

Lower Albedo

(fewer aerosols)

Polluted Environment (more aerosols)

Higher Albedo

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]

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

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.”

Submicrometer CCN aerosols decrease precipitation from shallow clouds and invigorate deep convective rain clouds with warm (> ~15°C) 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

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 cloudsCan 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 0°C

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

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

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

Andreae, M.O., and Rosenfeld, D. (2008). Aerosol–cloud–precipitation interactions. Part 1. The nature and sources of cloud-active aerosols, Earth-Science Reviews, 89, pp 13–41.

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 15–45.

 Rosenfeld, D., Lohmann, U., Raga, G.B. O’Dowd, 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?title=Cloud_seeding&oldid=284460362, revised 13 April 2009.

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 water—that 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.