The 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 interpret the correlation as evidence of an aerosol indirect effect on clouds. This study uses in situ aircraft observations during the 1999 intensive field phase of the Indian Ocean Experiment (INDOEX) to show that on average, relative humidity increases as distance to the boundaries of small cumulus clouds 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 percent at 1 km horizontal distance from the cloud to about 94 percent 100 m from cloud edge. This increase would result in a ~30 percent increase in aerosol scattering cross section based on the composition used to model the aerosol. No changes in particle concentrations were detected in the INDOEX aircraft observations, with the possible exception of the last 100 m before cloud edge. 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 ~50 percent, comparable to the increase calculated for the particle scattering cross sections. Based on these findings, global average estimates of the aerosol direct radiative effect as derived from satellite observations of cloud- free oceans is estimated to be as much as 60 percent larger than that inferred for large (> 20 km) cloud- free ocean regions.