Global Variations in Cloud Water Droplet Concentrations
Changes in the amount and nature of aerosols produced by human activities, such as clearing forests for agriculture or air pollution from fuel and forest burning, could alter Earth's climate. Since these small aerosol particles scatter and absorb sunlight, they directly affect climate by slightly modifying the radiative balance at the surface. They also act as condensation centers for cloud droplets, so that any changes in their number and physical properties could alter cloud properties that in turn could impact climate.
Clouds decrease the amount of sunlight absorbed at Earth's surface and heat radiated from the atmosphere. They bring water evaporated from the oceans to the land, thus completing the hydrological cycle. In both cases, clouds alter the energy balance that drives the atmospheric and oceanic circulations. Clouds form by vertical motions in the atmosphere through complex microphysical processes. These, in turn, depend on the cloud particle concentration, the mean cloud droplet radius, and the thickness of the cloud layer. Climate change can alter the ways in which clouds form and their physical properties, which in turn feeds back on climate.
Aircraft measurements over the past few decades have shown that aerosol concentrations over oceans are lower than over continents, by a factor of 10-100. Similarly, cloud droplet concentrations (N) are lower over oceans by a factor of 5-10. Yet the average radius (r) of the cloud droplets over oceans is usually 30-50% larger than that over continents. This has been interpreted to mean that the total liquid water content of a cloud remains approximately constant, even when aerosols change the cloud droplet concentration. The total liquid water content of a cloud (LWC) is the product of the droplet number concentration and their mass, derived from their volume and density. Therefore, if LWC remains constant but N changes, r varies as the cube root of N. However, aircraft measurements reveal that LWC actually varies, depending on the strength of atmospheric motions that produce clouds.
Figuring out these complex aerosol-cloud interactions requires global satellite observations of cloud physical properties. Han and colleagues have now completed the first near-global satellite surveys of cloud droplet radius and column concentration (Nc, the product of the average droplet concentration and the cloud layer thickness) in low-altitude water clouds, using data from the International Satellite Cloud Climatology Project.
As shown in the figure, the most striking feature is the marked contrast between maritime and continental clouds. Cloud droplet concentrations over continental areas (except over tropical rain forests) are more than two to four times the values in clouds over maritime areas, which is consistent with earlier aircraft measurements. This observation is directly linked to the expected effect of higher concentrations of land aerosols on clouds. Anthropogenic aerosol inputs, such as sulfur dioxide from fossil fuel combustion and carbon from biomass burning, are suggested by the very high Nc values in the Gulf of Mexico, near the east coasts of the U.S. and China, and over western Europe. All of these areas are known to have heavy air pollution from fossil fuel burning. Marked seasonal contrasts of Nc over tropical South America are consistent with seasonal variations in biomass burning. Burning peaks during the dry season (July and October) in southern Brazil, with droplet concentrations often exceeding 300 cm-3. On the other hand, N is typically less than 100 cm-3 during the wet season (January and April).
This satellite survey of low level water clouds clearly shows the expected increase of droplet column concentrations in continental clouds, especially over deserts, and in tropical areas during dry seasons, where biomass burning is prevalent. Although the earlier aircraft data also showed the increase of average size of continental vs. oceanic clouds, both datasets also show that cloud LWC varies. Therefore, an understanding of how aerosol changes affect cloud properties requires much more detailed studies such as these.
Han, Q-Y., W.B. Rossow, and A.A. Lacis 1994. Near-global survey of effective cloud droplet radii in liquid water clouds using ISCCP data. J. Climate 7, 465-497.
Han, Q., W.B. Rossow, J. Chou, and R.M. Welch 1998. Global survey of the relationships of cloud albedo and liquid water path with droplet size using ISCCP. J. Climate 11, 1516-1528.
Han, Q., W.B. Rossow, J. Chou, and R.M. Welch 1998. Global variation of droplet column concentration in low-level clouds. Geophys. Res. Lett. 25, 1419-1422.