High spatial resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

Abstract

Abstract. The cloud droplet size distribution is often described by a gamma distribution defined by the effective radius (reff) and the effective variance (veff). The effective radius is directly related to the cloud’s optical thickness which influences the radiative properties of a cloud. The effective variance affects, among other things, the evolution of precipitation. Both parameters can be retrieved from measurements of the cloudbow. The cloudbow or rainbow is an optical phenomenon, which forms by single scattering of radiation by liquid cloud droplets at the cloud edge. The polarized radiance of the cloudbow crucially depends on the cloud droplet size distribution. reff and veff can be retrieved by fitting model simulations (stored in a look-up table) to polarized cloudbow observations. This study uses measurements from the wide-field polarization-sensitive camera of the LMU spectral imager system specMACS onboard the German research airplane HALO. Together with precise cloud geometry data derived by a stereographic method, a geolocalization of the observed clouds is possible. Observations of the same cloud from consecutive images are combined into one radiance measurement from multiple angles. Two case studies of trade wind cumulus clouds measured during the EUREC4A field campaign are presented and the cloudbow technique is demonstrated. The results are combined into maps of reff and veff with a spatial resolution of 100 m by 100 m and large coverage (across-track swath width: 8 km). The first case study shows a stratiform cloud deck with distinct patches of large effective radii up to 40 µm and a median effective variance of 0.12. The second case study consists of small cumulus clouds (diameters of approximately 2 km). The retrieved reff is 6.92 µm and veff is 0.07 (both median values).