I thought this was going to be an interesting paper: Direct and indirect effects of anthropogenic aerosols on regional precipitation over east Asia in the most recent JGR. It turned out to be not so interesting, at least for me, as I’ll describe later.

Abstract: A regional coupled climate-chemistry-aerosol model is developed. It is used to assess the direct and indirect effects of anthropogenic sulfate and carbonaceous aerosols on regional climate over east Asia with a focus on precipitation. The simulated direct and first indirect effects for the most part reduce the solar radiation and hence decrease the surface temperature, while the second indirect effect generates both negative solar forcing and a substantial positive long-wave forcing. It decreases the precipitation, but because of the cancelling effect, surface temperature does not change very much. With the interactively model-calculated current aerosol loading and the combined direct/semidirect/first indirect effect, the simulated precipitation is reduced by about 10% in the fall and winter and by about 5% in the spring and summer. The second indirect effect has the largest impact, by itself decreasing the fall and winter precipitation from about 3% to 20%, depending on the autoconversion scheme assumed. The semidirect effect on precipitation is relatively small. An empirical orthogonal function analysis of climatological precipitation over east Asia since the last century shows a decreasing trend of the leading modes over most of China in the fall and winter, which is generally geographically consistent with the distribution of the model-simulated precipitation reduction from anthropogenic aerosols.

The first thing that should have tipped me off that this wasn’t exactly the sort of paper I was looking for is the first sentence of the abstract. This is a modelling study. Don’t get me wrong, modelling is important, but just not what I was looking for in this paper. In fact, I’m just going to skip over the whole paper right to the conclusions.

The direct, semidirect, and first indirect aerosol effects generate a surface cooling and atmospheric heating (from BC) leading to an increase in the atmospheric thermal stability and precipitation inhibition…

The second indirect effect produces a long-wave heating comparable to the negative solar cooling due to the increased cloud liquid water…

Our study has neglected the aerosol indirect effect on the microphysical processes of convective clouds and their consequent interactions with dynamical processes, and in turn the convective precipitation…

Observed temperature, precipitation, and aerosol data appear to be correlated

A review of the effects of aerosols. The first indirect effect is that more numerous smaller cloud particles reflect more solar radiation. This is because there are more cloud condensation nuclei, and the small droplets grow at the expense of the larger droplets. This is known as the Twomey effect. The second indirect effect is that smaller cloud particles decrease the precipitation efficiency that results in prolonging cloud lifetime. This is also known as drizzle suppression, or the Albrecht effect. The semi-direct effect is the absorption of solar radiation by soot, which may cause evaporation of cloud particles. (Lohmann and Feichter 2005).

One would expect that the cloud lifetime effect would have the most influence on the precipitation rates. The first indirect effect has to do with the radiative response to the aerosols, and it would be unlikely that it influences cloud precipitation rate. In this paper, they included the direct effect, first indirect effect and the semi-direct effect together. While the direct and semi-direct effect are distinct from the second indirect effect, I’m not entirely convinced that the first indirect effect is physically different. In both cases, there are smaller, more numerous droplets because of the addition of the aerosol. One describes the radiative effects of this, the other describes the liquid water response. But I do not think they are different processes controlling them. Therefore, the inclusion of the first indirect effect with the semidirect and direct effects should not have happened. If anything, it should have been included with the second indirect effect.

This study did not include the effects of aerosol on the microphysical processes in clouds because parameterizations are not yet available for regional or global models. The authors go on to explain, “the lack of the representation of aerosol indirect effects on convective microphysical processes further increases the already large uncertainty as to the aerosol effects on precipitaion.”

Lohmann, U., and Feichter (2005), Global indirect aerosol effects: A review, Atmos. Chem. Phys., 5, 715-737.

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