Sensitivity of Spectral Dispersion on the Indirect Effect

In the next couple days, I’ll be summarizing / reviewing a few papers that deal with spectral dispersion and its effects on climate – mostly through the first indirect effect (Twomey 1977). I believe a brief explaination of the Twomey effect is in order. In order for water vapor in the atmosphere to condense, it needs to a small particle onto which it condenses. These particles are called cloud condensation nuclei (CCN). In the maritime regime, cloud formation is CCN limited – the number of cloud droplets is controlled by the number of CCN. Over continents, the air is generally filled with lots of dust and pollution that acts as CCN; it is thus limited in other ways, such as water content or dynamically limited.

In the pristine cloud stratocumulus cloud decks off the coasts of continents, we have a great large-scale experimental setup. These clouds, as described above, are CCN limited. If there were more CCN available, the number concentration of cloud droplets would increase. What effect would that have on the cloud? Twomey actually solved this way back in the 1970s. The ratio of the volume of the droplets to the area of the droplets will decrease if the amount of liquid water in the clouds stays constant. This means there will be more droplets of smaller diameter. In total, the droplets project a larger effective area for incoming solar radiation. Since liquid water does not absorb hardly any visible radiation these clouds with more, but smaller, droplets reflects a greater percentage of the incoming radiation than their non-polluted neighbors. This can be seen in so-called “ship tracks”.

Satellite imagery off the coast of the Northwest United States showing brighter clouds where large ships passed underneath.

Ship tracks are caused by the byproducts of combustion from large ocean-going ships. These tiny particles become additional CCN for the water vapor molecules to condense. Thus, areas where these ships passed appear brighter.

The first paper that I’ll look at in this series is by Peng and Lohmann in Geophysical Research Letters, Sensitivity study of the spectral dispersion of the cloud droplet size distribution on the indirect aerosol effect.

Abstract: To study the influence of anthropogenic aerosols on the shape of the cloud droplet size spectra (dispersion effect), we analyze observed liquid water cloud data during two Canadian field studies. Scaled by the parameter β, which is a function of the relative dispersion of cloud droplet spectra, the calculated cloud albedo shows better agreement with the independently measured cloud albedo than the cloud albedo calculated without scaling. The scaling factor β is positively correlated with the cloud droplet number concentration. A linear relationship between β and the cloud droplet number concentration obtained from different field studies is applied to the ECHAM4 general circulation model. The global mean indirect aerosol effect at the top of atmosphere including the dispersion effect is reduced by 0.2 W m−2 as compared to the reference simulation. This accounts for about 1/3 of the reduction that needed to be imposed on the simulated anthropogenic indirect aerosol effect by Lohmann and Lesins [2002] .

The relative dispersion of the cloud is the ratio of the standard deviation of the distribution to the mean radius. A low dispersion means that the particles are all around the same size; a high dispersion means the sizes are more spread out. What this study did was to compare two models of how cloud albedo (or reflectance) changes with the indirect effect. One of the models included spectral dispersion, and the other did not. When compared with data collected in two field studies, the model that included the dispersion effects was more alligned with the data. They found that the number concentration was directly proportional to a derived parameter β, which was proportional to the relative dispersion. When they included this in a global climate model with the dispersion effect included it produced a result which lowered the first indirect’s effect by 15%.

There has been discussion as to whether the relative dispersion is positively, negatively, or not correlated with the cloud drop number concentration. And it’s important to figure out which it is, because it will influence whether the Twomey effect will increase or decrease. And since the indirect effects of aerosols are important for climate calculations, we need to at least know the sign of the variation.

Lohmann, U., and G. Lesins, Stronger constraints on the anthropogenic indirect aerosol effect, Science298, 1012-1015, 2002.
Twomey, S. Influence of pollution on shortwave albedo of clouds, J. Atmos. Sci.34, 1149-1152, 1977.