Physico-chemical measurements to investigate regional cloud-climate feedback mechanisms

On a regional scale, cloud-climate feedback mechanisms are investigated by analyzing the impact of anthropogenic sulfur emissions on the microstructure and short-wave albedo of clouds formed at a mountain-top location and simultaneously observing these clouds by AVHRR (Advance Very High Resolution Radiometer) aboard the NOAA (National Oceanographic and Atmospheric Administration) satellites. Cloud droplet number concentrations (CDNC) and ionic composition of cloud water are measured in situ along with meteorological parameters. The thickness of the overlying clouds and the 48 h back-trajectories of air masses are determined from on site measurements and Hybrid Single Particle Lagrangian Integrated Trajectory (HY-SPLIT) model. In general, more acidic clouds were found to have smaller effective radii of droplets but larger CDNC. The short-wave albedo of clouds determined from in situ measurements decreased with an increase in the cloud water pH which was largely affected by the sulfate concentration in the cloud forming air mass. For six coincidental cloud events, the AVHRR retrieved cloud albedo (channel 1, WAVELENGTH = 0.63 μm) agreed well with those determined from in situ microphysical measurements, the correlation coefficient being 0.95. These findings indicate that the sulfate contents of air masses are capable of influencing cloud-climate feedback mechanisms by changing the albedo of thin, continental clouds.