Wednesday, 5 June 2002
Global and regional correlations between aerosol and cloud parameters
Aerosols can act as cloud condensation nuclei (CCN) to change the cloud optical properties, and hence cause a change in the cloud radiative forcing. The forcing of the aerosol indirect effect has not been well accessed and the estimate ranges from 0 to -2 W/m2. The present study investigated the correlations between aerosol column number concentration and cloud microphysical parameters derived from AVHRR satellite remote sensing. As the global correlation, it is found that the optical thickness and effective particle radius of low clouds correlate well with the column number concentration of aerosol particles, whereas we cannot verify the existence of a significant correlation of the cloud top temperature and the cloud fraction with the column aerosol number. We further investigated the regional correlation, and found that a negative correlation consistent with the global statistics between aerosol column number concentration and cloud effective radius exists in most regions. But we also found that the correlation between aerosol column number concentration and cloud optical thickness has a large regional dependence and even the sign changes from positive in some urban region to negative in most remote ocean. The present study suggests an existence of a mechanism that cloud water increases when aerosol particles increase in not-polluted region. The slope of the global correlation is larger than the average of that in each region. Using these correlation slopes and the assumption that aerosol column number concentration has increased by 15% from pre-industrial era, we calculate the indirect radiative forcing as about -0.8 to -0.2W/m2.