For this study measurements made onboard the Met Office C-130 Hercules aircraft were analysed. Aerosol particle measurements included the size distribution and number concentration of optically interesting aerosol particles [instrument: PCASP], and their scattering and absorption coefficients [Nephelometer or PSAP, respectively]. Measured cloud parameters were the drop size distribution, cloud liquid water content (LWC) and macrophysical cloud structure [Fast FSSP and Nevzorov probe].
A typical scenario was observed on the 7th of September 2000: Extensive Sc cloud fields were present at altitudes below 1 km (drop concentrations of 130 cm-3, LWC around 0.3 g m-3), and a thick layer of biomass burning-originated aerosol was located further above (1.8-3.6 km altitude). The biomass burning aerosol was characterised by a typical particle number concentrations of around 2300 cm-3, a volume absorption coefficient of 0.6x10-5 at 567 nm and a volume scattering coefficient of roughly 6.8x10-5 at 550 nm wavelength; single scattering albedo values were 0.86 - 0.90.
The measured aerosol and cloud parameters were used as input to 1D radiative transfer calculations to estimate the radiative forcing by the biomass aerosol particles. Additional 3D radiative transfer calculations were used to account for possible cloud albedo changes due to their inhomogeneities and related effects onto the TOA forcing.
Due to the separation between the elevated biomass burning layer and the low-level clouds, no significant microphysical interaction between the biomass-originated aerosol and the clouds was obvious; a pronounced indirect effect did not occur. The direct effect was dominated by the aerosol scattering properties (single scattering albedo around 0.9) in combination with the rather low albedo (0.4) of the stratiform cloud layer beneath the aerosol. Initial results indicate that the strong negative TOA forcing by the aerosol, which can be expected in clear skies, was roughly offset in the presence of the observed Sc cloud layer (solar spectral range, sun zenith angle of 60°). The cloud inhomogeneity effects on cloud albedo were found to be of minor importance for that case.