The Effects of Very Large Drops on Cloud Absorption. Part I: Parcel Models

Abstract In an effort to bring more realism cloud-radiation calculations, arising-parcel model of cloud microphysics and a 191 waveband model of atmospheric radiation (ATRAD) have been brought to bear on the problem of cloud absorption of solar radiation, with emphasis on the effect of drops greater than 40–50 μm in radius. The earlier conclusions of Welch and others that such large drops can produce cloud absorptivities in excess of 30% have not been substantiated. Instead we find large-drop enhancements of only 0.02–0.04 in cloud and total atmospheric absorptivities. However, several other, more important influences were uncovered: 1) Large drops make it necessary to know the second and third moments of the drop distribution in order to parameterize the shortwave effect of clouds; parameterizations based only on the third moment (liquid water content) do not consider a wide enough range of variation of drop distribution. 2) Large drops cause a precipitous fall in both cloud and planetary albedo if the s...