Amelioration of global warming via the modification of droplet concentrations in marine stratocumulus clouds

Marine stratocumulus clouds are of particular interest with regard to global warming, due to their widespread cover (about one third of the oceanic surface) and relatively high albedos. It is suggested by Latham (1990; 2002) and Bower et al. (2006) that it is possible to ameliorate the effects of global warming by deliberately modifying the microphysical properties of these marine stratocumuli. This could be done by the injection of sea-salt aerosol into the atmosphere at cloud base which act as cloud condensation nuclei (CCN), thereby increasing the cloud droplet number concentrations (N) and leading to an increased albedo. A negative radiative forcing would be generated via the first and second indirect effects, resulting in a significant cooling of the Earth's climate (Twomey, 1977).

Results will be presented from a computational study in which droplet concentrations in marine stratocumuli are enhanced, in order to assess if such global cooling could be achieved. This involves the use of both a global climate model to investigate changes in the global radiation balance and a cloud resolving model to examine the sensitivity of the stratocumulus cloud microphysics.

The global climate model used is HadGAM (the atmospheric component of the Met Office Unified Model) in which N has been increased in all regions of low-level cloud, as well as in regions of persistent marine stratocumuli. The effect on cloud droplet effective radius will be presented for different values of N, along with computations of the effect on the earth's radiation balance. The results, an extension of those presented in Latham et al., 2008, indicate that by enhancing the properties of these clouds it may be possible to yield a negative radiative forcing of more than twice that required to offset the warming produced by a doubling of carbon dioxide.

Results will also be shown from the Met Office Large Eddy Model (LEM). This is being used on a regional scale to examine the microphysical and radiational effects of altering droplet concentrations in warm stratocumuli, initialized by atmospheric profiles from the recent VAMOS Ocean-Cloud-Atmosphere-Land Study (VOCALS) program.