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ScienceWeek
2004 11 June C3 ATMOSPHERIC SCIENCE: AEROSOLS AND ANVIL CLOUD NUCLEI
The term "anvil cloud" refers to a cumulonimbus cloud whose top has reached the tropopause, a temperature inversion, or a strong wind shear. The consequence is that the main cloud updrafts are deflected horizontally, causing the ice crystals at that level to spread out. From the ground, the entire cloud has the appearance of a blacksmith's anvil.
The following points are made by A.M. Fridlind et al (Science 2004 304:718):
1) It is well understood that cloud drops form on existing atmospheric aerosols, such as sulfuric acid particles and dust. Thus, changes in aerosol number can lead to changes in drop number during cloud formation. Complex subsequent effects on cloud microphysical development vary depending on cloud type and environmental conditions (1). The overall impact of increasing anthropogenic aerosols on low clouds such as stratocumulus may be great, generally resulting in smaller, more numerous drops and leading to brighter, longer-lived clouds that reflect more sunlight (2,3). Because stratocumulus clouds persistently cover large global areas, it has been recognized that this aerosol-induced cooling effect partially offsets the warming effect of accumulating greenhouse gas concentrations (4).
2) Whereas low clouds such as stratocumulus alter the global solar radiative budget with little influence on the infrared budget, high clouds such as cirrus reduce both solar incoming and infrared outgoing radiative fluxes by comparable amounts. Whether the overall impact is warming or cooling depends in a sensitive manner on cloud optical depth and ice crystal effective radius (5), among other factors. Although cirrus clouds have a much lesser influence on the net radiative budget per unit area than stratocumulus, the area covered by tropical anvil clouds may respond strongly to increasing sea surface temperatures, thereby playing a major role in global climate sensitivity. However, the properties and evolution of anvil cirrus clouds remain poorly understood and weakly constrained in models. Recent observations also suggest that tropical cloud coverage may be rapidly changing in a manner not captured by current general circulation model simulations, which serves as further motivation to seek a better understanding of anvil-forming cumulonimbus clouds.
3) The Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) was coordinated by NASA with the primary goal of fully characterizing subtropical cumulonimbus anvil formation and evolution. The experiment took place throughout July 2002 over southern Florida, where simultaneous measurements were made from six aircraft and three ground stations, as well as satellite platforms, over the lifetimes of many storm systems. The data gathered included simultaneous measurements of the number and size distribution of aerosols and cloud particles throughout the full depth of developing cumulonimbus columns.
4) In summary: NASA's recent Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment focused on anvil cirrus clouds, an important but poorly understood element of our climate system. The data obtained included the first comprehensive measurements of aerosols and cloud particles throughout the atmospheric column during the evolution of multiple deep convective storm systems. Coupling these new measurements with detailed cloud simulations that resolve the size distributions of aerosols and cloud particles, the authors found several lines of evidence indicating that most anvil crystals form on mid-tropospheric rather than boundary-layer aerosols. This result defies conventional wisdom and suggests that distant pollution sources may have a greater effect on anvil clouds than do local sources.
References (abridged):
1. H. Graf, Science 303, 1309 (2004)
2. S. Twomey, Atmos. Environ. 8, 1251 (1974)
3. B. A. Albrecht, Science 245, 1227 (1989)
4. Intergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis, J. T. Houghton et al., Eds. (Cambridge Univ. Press, Cambridge, 2001)
5. G. L. Stephens, S.-C. Tsay, P. W. Stackhouse Jr., P. J. Flatau, J. Atmos. Sci. 47, 1742 (1990)
Science http://www.sciencemag.org
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ON AEROSOLS AND CLOUD MICROPHYSICS
The term "aerosol" refers to a dispersion in which a finely divided solid is suspended in air and the particles are of colloidal dimensions. The term "colloidal dimensions" refers to the range approximately 1 nanometer to 100 nanometers in diameter.
The following points are made by F-M. Breon et al (Science 2002 295:834):
1) Aerosols may reduce the degree of Earth global warming resulting from the increase of greenhouse gases in the atmosphere (1,2). They directly impact the radiative balance of Earth through a net increase of its albedo, particularly over the oceans (3,4). Aerosols can also act as cloud condensation nuclei, increasing the number of droplets in clouds, which tends to decrease the mean droplet size and may increase the cloud albedo (5), depending on the aerosol absorption and cloud optical thickness. This process, referred to as the "Twomey effect" or the "first indirect" aerosol radiative forcing, has a net cooling effect on climate.
2) A direct demonstration of the aerosol effect on cloud albedo was provided by the observation of lines of larger reflectance in cloud fields identified as tracks of ship exhaust. Indirect observations of this effect can also be made by comparing cloud droplet size and aerosol concentration. Cloud droplet effective radii were derived by using global scale advanced very high resolution radiometer measurements.
3) The results of a global application indicate a contrast in cloud droplet size of about 2 microns over land and ocean surfaces, as well as a hemispheric contrast of 1 micron, both of which support the Twomey hypothesis. Similar patterns of the aerosol optical thickness and the cloud droplet effective radius, derived from advanced very high resolution radiometer measurements, have been observed over the oceans. Cases of reduced droplet radii and suppression of rain -- the second indirect aerosol effect-- in areas of high aerosol load were identified on satellite imagery. Furthermore, several in situ measurements have shown a relationship between the aerosol concentration and the cloud droplet size distribution.
4) The authors report a study in which aerosol concentration and cloud droplet radii derived from space-borne measurements are used to explore the effect of aerosols on cloud microphysics. Cloud droplet size is found to be largest (14 microns) over remote tropical oceans and smallest (6 microns) over highly polluted continental areas. Small droplets are also present in clouds downwind of continents. By using estimates of droplet radii coupled with aerosol load, a statistical mean relationship is derived. The cloud droplet size appears to be better correlated with an aerosol index that is representative of the aerosol column number under some assumptions than with the aerosol optical thickness. This study reveals that the effect of aerosols on cloud microphysics is significant and occurs on a global scale.
5) The authors conclude: Whether the observed impact on cloud microphysics is of anthropogenic origin is a question of importance. The satellite measurement cannot unambiguously distinguish natural and human-generated aerosols. However, the analysis of the spatial and temporal patterns in the aerosol index monthly maps strongly suggests that the bulk of the aerosol load originates from slash-and-burn agriculture practices and from highly polluted areas (25). A large fraction of the observed aerosol effect on clouds is probably an anthropogenic impact.
References (abridged):
1. J. E. Penner, et al., Bull. Am. Meteorol. Soc. 75, 375 (1994)
2. R. J. Charlson, et al., Science 255, 423 (1992)
3. J. Haywood, V. Ramaswamy, B. Soden, Science 283, 1299 (1999)
4. O. Boucher and D. Tanr‚, Geophys Res. Lett. 27, 1103 (2000)
5. S. Twomey, J. Atmos. Sci. 34, 1149 (1977)
Science http://www.sciencemag.org
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CLIMATE EFFECTS OF BLACK CARBON AEROSOLS IN CHINA AND INDIA
The following points are made by S. Menon et al (Science 2002 297:2250):
1) China has been experiencing an increased severity of dust storms, commonly attributed to overfarming, overgrazing, and destruction of forests (1). Plumes of dust from north China, with adhered toxic contaminants, are cause for public health concern in China, Japan, and Korea, and some of the aerosols even reach the United States (2). Recent dust events have prompted Chinese officials to consider spending several hundred billion yuan (~$12 billion) in the next decade to increase forests and green belts to combat the dust storms (3).
2) Such measures may be beneficial in any case. However, the authors suggest that the observed trend toward increased summer floods in south China and drought in north China (4), thought to be the largest change in precipitation trends since 950 A.D.(4), may have an alternative explanation: human-made absorbing aerosols in remote populous industrial regions that alter the regional atmospheric circulation and contribute to regional climate change. The authors suggest that if interpretation is correct, reducing the amount of anthropogenic black carbon aerosols, in addition to having human health benefits, may help diminish the intensity of floods in the south and droughts and dust storms in the north.
3) Similar considerations may apply to India and neighboring regions such as Afghanistan, which have experienced recent droughts. Atmospheric aerosols, which are fine particles suspended in the air, comprise a mixture of mainly sulfates, nitrates, carbonaceous (organic and black carbon) particles, sea salt, and mineral dust. Black (elemental) carbon (BC) is of special interest because it absorbs sunlight, heats the air, and contributes to global warming (5), unlike most aerosols, which reflect sunlight to space and have a global cooling effect. BC emissions, a product of incomplete combustion from coal, diesel engines, biofuels, and outdoor biomass burning, are particularly large in China and India because of low-temperature household burning of biofuels and coal (9).
4) In summary: In recent decades, there has been a tendency toward increased summer floods in south China, increased drought in north China, and moderate cooling in China and India while most of the world has been warming. The authors used a global climate model to investigate possible aerosol contributions to these trends. The authors found precipitation and temperature changes in the model that were comparable to those observed if the aerosols included a large proportion of absorbing black carbon ("soot"), similar to observed amounts. Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the large-scale circulation and hydrologic cycle with significant regional climate effects.
References (abridged):
1. H. W. French, New York Times, 14 April 2002, p. 3
2. R. B. Husar, et al., J. Geophys. Res. 106, 18317 (2001)
3. Reuters, http://in.news.yahoo.com/020514/64/1o02h.html 14 May 2002.
4. Q. Xu, Atmos. Environ. 35, 5029 (2001)
5. J. Hansen, M. Sato, R. Ruedy, A. Lacis, V. Oinas, Proc. Natl. Acad. Sci. U.S.A. 97, 9875 (2000)
Science http://www.sciencemag.org
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