Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models G. Myhre1,2,3, F. Stordal1,2, M. Johnsrud2, Y. J. Kaufman4, D. Rosenfeld5, T. Storelvmo1, J. E. Kristjansson1, T. K. Berntsen1,3, A. Myhre6, and I. S. A. Isaksen1 1Department of Geosciences, University of Oslo, Norway 2Norwegian Institute for Air Research, 2027 Kjeller, Norway 3Center for International Climate and Environmental Research – Oslo, 0318 Oslo, Norway 4NASA Goddard Space Flight Center, Greenbelt Maryland 20771, USA 5Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 6Telemark University College, Bø, Norway
Abstract. We have used the MODIS satellite data and two global aerosol models to
investigate the relationships between aerosol optical depth (AOD) and cloud
parameters that may be affected by the aerosol concentration. The
relationships that are studied are mainly between AOD, on the one hand, and
cloud cover, cloud liquid water path, and water vapour, on the other.
Additionally, cloud droplet effective radius, cloud optical depth, cloud top
pressure and aerosol Ångström exponent, have been analysed in a few
cases. In the MODIS data we found, as in earlier studies, an enhancement in
the cloud cover with increasing AOD. We find it likely that most of the
strong increase in cloud cover with AOD, at least for AOD<0.2, is a result
of aerosol-cloud interactions and a prolonged cloud lifetime. Large and
mesoscale weather systems
seem not to be a cause for the increase in cloud cover with AOD in this
range. Sensitivity simulations show that when water uptake of the
aerosols is not taken into account in the models the modelled cloud
cover mostly decreases with AOD. Part of the
relationship found in the MODIS data for AOD>0.2 can be explained by
larger water uptake close to the clouds since relative humidity is higher in
regions with higher cloud cover. The efficiency of the hygroscopic growth
depends on aerosol type, the hygroscopic nature of the aerosol, the relative
humidity, and to some extent the cloud screening. By analysing the
Ångström exponent we find that the hygroscopic growth of the aerosol
is not likely to be a main contributor to the cloud cover increase with AOD.
Since the largest increase in cloud cover with AOD is for low AOD (~0.2) and thus also for low cloud cover, we argue that cloud contamination
is not likely
to play a large role. However, interpretation of the complex relationships
between AOD and cloud parameters should be made with great care and further
work is clearly needed.
Citation: Myhre, G., Stordal, F., Johnsrud, M., Kaufman, Y. J., Rosenfeld, D., Storelvmo, T., Kristjansson, J. E., Berntsen, T. K., Myhre, A., and Isaksen, I. S. A.: Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models, Atmos. Chem. Phys., 7, 3081-3101, doi:10.5194/acp-7-3081-2007, 2007.