Discrepancies between measured and modeled solar and UV radiation within polluted boundary layer clouds

Abstract

Using aircraft measurements and sophisticated radiative transfer calculations, a significant gap between measured and modeled radiation inside clouds is revealed in two out of four detailed case studies of polluted boundary layer clouds. In contrast to previous aircraft studies on that subject, vertical profiles of in situ measured aerosol and drop microphysical data are included in the one-dimensional spectral radiative transfer calculations. Instead of comparing the calculated cloud absorption with the measured one, we focus on the calculated and measured vertical profiles of solar and ultraviolet (UV) downwelling and upwelling irradiances and respective albedo profiles. These profile data are much less affected by measurement uncertainties compared to the cloud absorption, which results as residual of two net irradiances at cloud top and base. The enhanced (with respect to the calculations) solar cloud absorption is estimated to reach values up to 37 W m-2. For the UV data the discrepancy between measured and calculated down- and upwelling irradiances (and albedo) inside the clouds is less pronounced, although it seems to be significant as well. The reason for the discrepancies between measured and calculated radiation inside the clouds is still unresolved. Additional radiative transfer calculations show that uncertainties of surface albedo, vertical cloud inhomogeneities, and pollution of cloud drops by inclusion of strongly absorbing material are not responsible for the gap between measurements and calculations within the cloud. There is a clear need for spectrally resolved measurements and improved model calculations taking into account cloud inhomogeneities to solve this problem. Copyright 1999 by the American Geophysical Union.