Cloud–Aerosol Interactions from the Micro to the Cloud Scale

The effect of suspended particles (aerosol) on clouds and precipitation from the micro to the cloud scale has been well studied through laboratory, in-situ, and remote-sensing data but many uncertainties remain. In particular, there is scant observational evidence of aerosol effects on surface precipitation. Clouds and precipitation modify the amount of aerosol through both physical and chemical processes so that a three-way interactive feedback between aerosol, cloud microphysics, and cloud dynamics must be considered. The fundamental cloud microphysical properties are driven by dynamics; vertical motions and mixing processes between the cloud and its environment determine the concentration of cloud water, a key parameter for both climate and precipitation. However, aerosol particles can signifi cantly affect the microphysics and dynamics of clouds by changing the size distribution of drops, their ability to grow to raindrops, their rates of evaporation, and their mixing with the environment. The physical system is strongly coupled and attempting to separate aerosol effects has only been done using some simple constructs, some of which will be shown to be of dubious utility. Both observations and modeling suggest that not only the magnitude, but perhaps also the sign of these effects, depends on the larger-scale meteorological context in which aerosol–cloud interactions are embedded. Some alternate approaches are considered, as we explore the possibility of self-regulation processes that may act to limit the range over which aerosol signifi cantly affects clouds. Historical Perspective Historical evidence of the effects of aerosol on atmospheric processes abounds, as seen in ancient texts all the way through to modern times. Common human 320 G. Feingold and H. Siebert experience tells us that less sunlight reaches the surface under cloudy skies and that aerosol laden skies (e.g., from biomass burning or volcanic eruptions) produce hazy skies and colorful sunsets. In 1783, Benjamin Franklin experienced a particularly cool summer in Europe and attributed it to “fog” associated with a volcanic eruption in Iceland, earlier in the year, that visibly dimmed the sunlight. Evidence that aerosol particles are complicit in cloud droplet formation came in the late 19th century, when P.-J. Coulier (1875), J. Aitken (1880), and C. T. R. Wilson (1897) showed that aerosol particles are necessary for cloud droplet formation. Subsequent work by Aitken (1923) and Köhler (1936) strengthened further the link between aerosol and cloud droplet formation. Houghton (1938) discussed the importance of collision and coalescence of cloud droplets for rain formation and even recognized the role of giant hygroscopic nuclei, a subject that is of great interest to this day. Howell (1949) developed a numerical model of the growth of a population of droplets in a rising parcel of air and laid the foundations of similar tools used today. Analytical solutions to these equations (Twomey 1959) are still in common use. Thus, although the early literature did not focus on aerosol as a means of perturbing cloud microphysical processes, the link between aerosol and clouds has existed in the scientifi c literature for over a century. Pollution and Climate Change A more direct link between pollution particles and climate change was raised by Twomey (1974), who considered the fact that the positive correlation between aerosol number concentration, Na, and drop number concentration, Nd, would result in smaller droplets and more refl ective clouds ceteris paribus; that is, all else, particularly the amount of condensed water, being equal. This constitutes a shortwave radiative forcing since low-level clouds radiate in the longwave at temperatures close to the surface temperature. The concept is a hypothetical one since it is known that feedbacks also result in modifi cations to liquid water. It is applied in global climate models by modifying the refl ectance of fi xed cloud fi elds. The recently released IPCC (2007) report still regards this “cloud albedo effect” as the single biggest unknown in climate change predictions. It is also of note that although many aspects of the effect of aerosol particles on clouds are well-known, the global radiative (and therefore climate) implications are still poorly constrained. For example, most satellite remotesensing studies quantify the change in cloud refl ectance, or cloud drop size, in response to an aerosol perturbation without stratifying for similar meteorological conditions (e.g., cloud liquid water path, LWP). Thus “ceteris paribus” is often ignored, resulting in considerable ambiguity: Are clouds more refl ective because of higher Nd and smaller drops, or because of higher liquid water content (LWC), or some combination of the two? Cloud–Aerosol Interactions from the Micro to the Cloud Scale 321 Pollution and Precipitation One of the oldest problems in cloud microphysics is the inability to describe quantitatively broad drop size distributions that are observed (compared to the narrow, modeled size distributions that assume adiabatic growth) and the onset of precipitation. Broadening has been attributed to entrainment-mixing, aerosol effects (such as giant aerosol), and turbulence effects on collision–coalescence. The fi rst experimental evidence of a link between pollution and precipitation processes known to us appeared in a paper by Gunn and Phillips (1957). They performed experiments in a giant cloud chamber and observed that when clean air was drawn into the chamber, large, precipitation-sized drops were produced; polluted air produced smaller drops and no precipitation. These observations were confi rmed in natural clouds a decade later by Warner (1968), but subsequent analysis by Warner yielded inconclusive results. The experimental evidence for this link is still a subject of enormous debate, particularly for deep convective cloud systems (e.g., Levin and Cotton 2007; see also Cotton, this volume; Ayers and Levin, this volume). The implications for climate change of aerosol effects on precipitation were considered by Albrecht (1989), who used a numerical model of a cloudy boundary layer to show that an increase in Nd results in clouds with less collision–coalescence, less precipitation, and therefore increased LWP and higher albedo. This constitutes a positive feedback to the albedo effect since Twomey (1974) had considered the radiative impacts for static clouds of the same LWP. In addition, Albrecht (1989) suggested that a suppressed precipitation process might increase the lifetime of a cloud. In the intervening decades, the precipitation suppression hypothesis has become known as the “Albrecht effect,” or “lifetime effect. ” It has also come under increasing scrutiny in recent years as observational evidence for this phenomenon (in-situ, satellite remote sensing) has been pursued. Various aspects of this linkage between aerosol, precipitation, and climate had been qualitatively confi rmed (e.g., the suppression of precipitation by aerosol in warm rain), but to date there is still considerable debate over the effects of aerosol on cloud fraction, cloud LWP, and cloud lifetime, and the quantitative relationship between aerosol and surface precipitation. Aerosol Effects on Clouds: Deconstructing the Constructs