Abstract. The column aerosol particle number and low cloud microphysical parameters derived from AVHRR remote sensing are compared over ocean for four months in 1990.There is a positive correlation between cloud optical thickness and aerosol number concentration, whereas the effective particle radius has a negative correlation with aerosol number. The cloud liquid water path (LWP), on the other hand, tends to be constant with no large dependence on aerosol number.

[1] The present study investigated the correlations between aerosol and cloud parameters derived from satellite remote sensing for evaluating the radiative forcing of the aerosol indirect effect. The global statistics showed that the effective particle radius and the optical thickness of low clouds correlate well with the column number concentration of the aerosol particles, indicating an aerosol indirect effect. A correlation of the cloud fraction with the aerosol number was also seen, whereas we could not find a significant correlation of the cloud-top temperature with the column aerosol number. Furthermore, the regional statistics presented that positive correlations between the cloud optical thickness and cloud fraction with the aerosol column number concentration exist in most regions consistent with the global mean statistics. However, the effective cloud particle radius showed a tendency similar to the global correlation only around the seashore regions. Using these correlations and assuming that the aerosol column number concentration has increased by 30% from the preindustrial era, the total radiative forcing of the aerosol indirect effect was evaluated to be about À0.6 to À1.2 W m À2 . The radiative forcing of the aerosol direct effect from the satellite-retrieved parameters was also evaluated as À0.4 W m À2 over the ocean. The cloud-top temperature was found to be insensitive to the change in the aerosol number, although there was a distinct negative correlation between the aerosol number and cloud temperature at which the cloud particle grows to a radius of 14 mm. This particular dependency of the cloud temperature suggests that aerosols acts on clouds so as to change cloud particle size near the cloud top, optical thickness, and fraction but to keep their cloud-top temperature without causing a significant longwave radiative forcing.

[1] Present-day global anthropogenic emissions contribute more than half of the mass in submicron particles primarily due to sulfate and carbonaceous aerosol components derived from fossil fuel combustion and biomass burning. These anthropogenic aerosols increase cloud drop number concentration and cloud albedo. Here, we use an improved version of the fully coupled climate/chemistry models to investigate cloud susceptibility and the first indirect effect of anthropogenic aerosols (the Twomey effect). We examine the correspondence between the model simulation of cloud susceptibility and that inferred from satellite measurements to test whether our simulated aerosol concentrations and aerosol/cloud interactions give a faithful representation of these features. This comparison provides an overall measure of the adequacy of cloud cover and drop concentrations. We also address the impact of black carbon absorption in clouds on the first indirect forcing and examine the sensitivity of the forcing to different representations of natural aerosols. We find that including this absorption does not change the global forcing by more than 0.07 W m À2 , but that locally it could decrease the forcing by as much as 0. Citation: Chuang, C. C., J. E. Penner, J. M. Prospero, K. E. Grant, G. H. Rau, and K. Kawamoto, Cloud susceptibility and the first aerosol indirect forcing: Sensitivity to black carbon and aerosol concentrations,

No abstract