A study of the indirect aerosol effect on subarctic marine liquid low-level clouds using MODIS cloud data and ground-based aerosol measurements

Cloud microphysics is substantially affected by aerosol loading and the resulting changes in the reflective properties of the clouds can significantly affect the global radiation budget. A study of how marine low-level clouds over Barents Sea and the northern parts of the Norwegian Sea are affected by air mass origin has been performed by combining ground-based aerosol measurements with satellite cloud retrievals. Aerosol number size distributions have been obtained from measurement stations in northern Finland, and a trajectory model has been used to estimate the movement of the air masses. To identify anthropogenic influences on the clouds, the dataset has been divided according to aerosol loading. The clean air masses arrived to the investigation area from the north and the polluted air masses arrived from the south. Satellite derived microphysical and optical cloud parameters from the Moderate Resolution Imaging Spectrometer (MODIS) have then been analyzed for days when the trajectories coincided with marine low-level clouds over the investigated area. The cloud optical thickness (tau), cloud depth (H) and droplet number concentration (N-d) were significantly higher for the polluted days compared to the clean conditions, while the opposite was found for the cloud droplet effective radius (r(e)). The H and N-d were derived from the satellite retrievals of tau and r(e). Furthermore, calculations of the aerosol cloud interaction relationship (ACI), relating N-d to boundary layer aerosol concentrations, resulted in a value of 0.17, which is in line with previous remote sensing studies. The results demonstrate that ground-based aerosol measurements can be combined with satellite cloud observations to study the indirect aerosol effect, and that the microphysics of marine sub-polar clouds can be considerably affected by continental aerosols. (C) 2011 Elsevier B.V. All rights reserved