Effect of changes in relative humidity on aerosol scattering near clouds

Many investigations using satellite data have determined that aerosol optical depth and cloud cover are correlated and some have interpreted the correlation as evidence of an aerosol indirect effect on clouds. This study uses in situ aircraft observations taken during the Indian Ocean Experiment (INDOEX), February-March 1999, and mostly over the northern Indian Ocean, to show that on average, relative humidity increases as distance to the boundaries of small marine trade cumulus decreases. The increase is sufficient to cause substantial growth of hygroscopic aerosol particles and consequently greatly enhance particle scattering cross sections near clouds. The measured increase is from a relative humidity of about 90% at 1- km horizontal distance from the cloud to about 94-96% at 100 m from cloud edge. This increase would result in about a 40-80% increase in aerosol scattering cross section based on the composition used to model the aerosol. Observations of scattering in the vicinity of clouds using 30-m-resolution imagery from the Multichannel Cloud Radiometer (MCR) indicated that the increase in scattering within 1-2 km of cloud edge was about 50%, comparable to the increase calculated for the particle scattering cross sections. On the basis of these findings, global average estimates of the aerosol direct radiative effect as derived from satellite observations of cloud-free oceans is estimated to be 35-65% larger than that inferred for large (> 20 km) cloud-free ocean regions. This enhancement is consistent with those derived from satellite observations.