A line - by - line investigation of solar radiative effects in vertically inhomogeneous low clouds

SUMMARY Using a detailed line-by-line, multiple-scattering solar radiative-transfer model, the influences due to cloud internal inhomogeneity in the vertical upon the solar radiative transfer are investigated. In particular, the consequences due to non-uniform vertical profiles of liquid water and droplet sizes within low clouds are explored in a systematic manner. The fine structure of the spectral overlap between the water droplet and water vapour optical properties, and its effects upon the radiation absorbed within the cloud layer and that reflected at the top of the cloud, are discussed. Without consideration of the in-cloud water vapour, a vertically inhomogeneous cloud with properties resembling those observed absorbs more solar radiation than an equivalent homogeneous cloud. However , consideration of the effects of the in-cloud vapour, while still leading to a slightly greater absorption for the inhomogeneous case, partly offsets the difference introduced by the vertical distribution of the drop microphysics. The vertical distribution of cloud heating rate is changed substantially because of the inhomogeneity in the mi-crophysics, with the heating rate in the top region of the cloud nearly 50% more than that due to an equivalent vertically homogeneous cloud. Vertical inhomogeneity of cloud microphysics has little influence on the broadband solar albedo, but can cause significant decreases of the cloud reflectance at specific near-infrared wavelengths i.e. wavelengths greater than 1 J,Lm, (equivalentl y, wave numbers less than 10 000 cm-1).