Abstract
Abstract. Convective clouds play an essential role for Earth's climate as well as for regional weather events since they have a large influence on the radiation budget and the water cycle. In particular, cloud albedo and the formation of precipitation are influenced by aerosol particles within clouds. In order to improve the understanding of processes from aerosol activation, from cloud droplet growth to changes in cloud radiative properties, remote sensing techniques become more and more important. While passive retrievals for spaceborne observations have become sophisticated and commonplace for inferring cloud optical thickness and droplet size from cloud tops, profiles of droplet size have remained largely uncharted territory for passive remote sensing. In principle they could be derived from observations of cloud sides, but faced with the small-scale heterogeneity of cloud sides, “classical” passive remote sensing techniques are rendered inappropriate. In this work the feasibility is demonstrated to gain new insights into the vertical evolution of cloud droplet effective radius by using reflected solar radiation from cloud sides. Central aspect of this work on its path to a working cloud side retrieval is the analysis of the impact unknown cloud surface geometry has on effective radius retrievals. This study examines the sensitivity of reflected solar radiation to cloud droplet size, using extensive 3-D radiative transfer calculations on the basis of realistic droplet size resolving cloud simulations. Furthermore, it explores a further technique to resolve ambiguities caused by illumination and cloud geometry by considering the surroundings of each pixel. Based on these findings, a statistical approach is used to provide an effective radius retrieval. This statistical effective radius retrieval is focused on the liquid part of convective water clouds, e.g., cumulus mediocris, cumulus congestus, and trade-wind cumulus, which exhibit well-developed cloud sides. Finally, the developed retrieval is tested using known and unknown cloud side scenes to analyze its performance.
Highlights
Remote sensing of cloud and aerosol parameters is mostly done by use of multi-spectral sensors, i.e., using only a limited number of spectral bands
Ewald et al.: Retrieval of vertical profiles of cloud droplet effective radius and cloud properties caused by small-scale cloud inhomogeneity which are unresolved by the coarse spatial resolution of spaceborne platforms (Zinner and Mayer, 2006; Marshak et al, 2006b; Varnai and Marshak, 2007)
In order to observe the vertical development of convective cloud microphysics, Marshak et al (2006a) and Martins et al (2011) proposed cloud side scanning measurements while Zinner et al (2008) and Ewald et al (2013) presented concrete steps towards a cloud side retrieval for profiles of phase and particle size
Summary
Various methods exist for inferring optical properties (e.g., optical thickness and cloud droplet effective radius) from observation of cloud tops, using information about the scattered and absorbed radiation in the solar spectrum (e.g., Plass and Kattawar, 1968; King, 1987). In order to observe the vertical development of convective cloud microphysics, Marshak et al (2006a) and Martins et al (2011) proposed cloud side scanning measurements while Zinner et al (2008) and Ewald et al (2013) presented concrete steps towards a cloud side retrieval for profiles of phase and particle size. The studies of Zinner et al (2008) and Ewald et al (2013) are limited to an idealized geometry and simplified cloud microphysics They focus on a spacelike perspective for a fixed viewing zenith and scattering angle above the cloud field, where sun and sensor have the same azimuth. Extending the existing approach to realistic airborne perspectives and development of methods to test the sensitivity of reflected radiances from cloud sides to cloud droplet radius, where the observer position is located within the cloud field; 2.
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