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 conse- quences 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. How- ever, 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). Both climate and climate change are intimately related to the earth's solar-radiation budget, which in turn is strongly influenced by clouds. The solar radiative interactions depend to a considerable extent on the cloud geometric structure and cloud internal op- tical properties, both of which could give rise to significantly different effects than the assumption of plane-parallel, homogeneous clouds with infinite horizontal extent. In the last two decades, the effects of finite clouds on radiative transfer have been intensively studied through Monte Carlo simulations (McKee and Cox 1974; Davies 1978; Welch and Wielicki 1985; Barker and Davies 1992; and others). Li et ai. (1994) considered a Monte Carlo model in which the effect of cloud microphysics with spatially varying single-scattering properties was employed. It is found that in cases when the cloud amount is large, inhomogeneous clouds lead to an increase in cloud solar absorption, which may be one factor to consider in the cloud absorption anomaly problem (Stephens and Tsay 1990). An important factor in the solar radiative interactions with clouds is the presence of gaseous absorbers, notably water vapour above and inside the cloud. The studies of Davies et ai. (1984) and Ramaswamy and Freidenreich (1992) emphasize that the spectral overlap between water vapour and droplets possesses a very fine structure and is of significant quantitative importance in determining the absorption in cloudy atmospheres. As a step towards understanding the effects of inhomogeneity in clouds, we consider the problem of low clouds in which there is a vertical variation of the drop microphysics. The internal vertical distribution of the drop characteristics effectively yields a vertically inhomogeneous cloud. The principal objective here is to analyse systematically the in- fluence due to the vertical inhomogeneity upon the spectrally dependent solar radiative transfer. The focus here differs somewhat from that in the study by Wiscombe et ai. (1984)