The Effect of Aerosols on Precipitation – Suppression or Enhancement?

1. The Effect of Aerosols on Precipitation – Suppression or Enhancement? Kate Cerully 4/21/09

2. 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)

3. 102 Critical %SS 101 Relative Humidity (%) 100 Critical Wet Diameter Wet Aerosol (Haze) 99 Cloud Droplet 98 0.1 1 10 Wet aerosol diameter (mm) How Do Aerosols Form Droplets? Equilibrium line for a 20 nm ammonium sulfate particle CCN = f (Size, Chemical Composition) [Moore, 2008]

4. Key Effects Impacting Formation of Precipitation

5. 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

6. 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

7. Lower A l bedo Higher A l bedo Clean Environment (fewer aerosols) Polluted Environment (more aerosols) 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]

8. Contradictions

9. 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

10. 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.”

11. How Can This Be?

12. CCN Size • 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

13. 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 0°C

14. Precipitation - Evaporation Balance

15. Radiativevs 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

17. 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

18. 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

19. References 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.

21. 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 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.