More Evidence for the Diversity of Climate Forcings by Aerosols

A very important research paper has appeared with respect to the effect of aerosols within the climate system. The paper by A. Khain , D. Rosenfeld, and A. Pokrovsky entitled “Aerosol impact on the dynamics and microphysics of deep convective clouds”

includes the following conclusion,

“the ‘aerosol effect’ on precipitation can be understood only in combination with the ‘dynamical effect’ of aerosols. Simulations allow us to suggest that aerosols, which decrease the precipitation efficiency of most single clouds, can contribute to the formation of very intensive convective clouds and thunderstorms (e.g. squall lines, etc.) accompanied by very high precipitation rates. Affecting precipitation, net atmospheric heating and its vertical distribution, as well as cloud depth and cloud coverage, atmospheric aerosols (including anthropogenic ones) influence atmospheric motions and radiation balance at different scales, from convective to, possibly, global ones.â€?

The full abstract reads,

“Mechanisms through which atmospheric aerosols affect cloud microphysics, dynamics and precipitation are investigated using a spectral microphysics two-dimensional cloud model. A significant effect of aerosols on cloud microphysics and dynamics has been found. Maritime aerosols lead to a rapid formation of raindrops that fall down through cloud updraughts increasing the loading in the lower part of a cloud. This is, supposedly, one of the reasons for comparatively low updraughts in maritime convective clouds. An increase in the concentration of small cloud condensation nuclei (CCN) leads to the formation of a large number of small droplets with a low collision rate, resulting in a time delay of raindrop formation. Such a delay prevents a decrease in the vertical velocity caused by the falling raindrops and thus increases the duration of the diffusion droplet growth stage, increasing latent heat release by condensation. The additional water that rises to the freezing level increases latent heat release by freezing. As a result, clouds developing in continental-type aerosol tend to have larger vertical velocities and to attain higher levels.

The results show that a decrease in precipitation efficiency of single cumulus clouds arising in microphysically continental air is attributable to a greater loss of the precipitating mass due to a greater sublimation of ice and evaporation of drops while they are falling from higher levels through a deep layer of dry air outside cloud updraughts. By affecting precipitation, atmospheric aerosols influence the net heating of the atmosphere. Simulations show that aerosols also change the vertical distribution of latent heat release, increasing the level of the heating peak.
Clouds arising under continental aerosol conditions produce as a rule stronger downdraughts and stronger convergence in the boundary layer. Being triggered by larger dynamical forcing, secondary clouds arising in microphysically continental air are stronger and can, according to the results of simulations, form a squall line. The squall line formation was simulated both under maritime (GATE-74) and continental (PRE-STORM) thermodynamic conditions. In the maritime aerosol cases, clouds developing under similar thermodynamic conditions do not produce strong downdraughts and do not lead to squall line formation.

Thus, the ‘aerosol effect’ on precipitation can be understood only in combination with the ‘dynamical effect’ of aerosols. Simulations allow us to suggest that aerosols, which decrease the precipitation efficiency of most single clouds, can contribute to the formation of very intensive convective clouds and thunderstorms (e.g. squall lines, etc.) accompanied by very high precipitation rates. Affecting precipitation, net atmospheric heating and its vertical distribution, as well as cloud depth and cloud coverage, atmospheric aerosols (including anthropogenic ones) influence atmospheric motions and radiation balance at different scales, from convective to, possibly, global ones.â€?

This study provides support for the 2005 National Research Council conclusion that

“Several types of forcings—most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry—impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received.â€?;

and that we need to,

“Improve understanding and parameterizations of aerosol-cloud thermodynamic interactions and land-atmosphere interactions in climate models in order to quantify the impacts of these nonradiative forcings on both regional and global scales.â€?

The Khain et al paper is a major contribution to this goal, as well as further showing the complex interactions within the climate system which makes multi-decadal prediction a much more challenging endeavor than is captured by the IPCC perspective.