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We present novel remote sensing observations of cloud droplet size distributions retrieved from polarimetric observations of the wide-field airborne imaging system specMACS. The measurements were collected during the EUREC4A field campaign which took place in January and February 2020 in the trade wind region east of Barbados. We focus on observations of the cloudbow which is an optical phenomenon that results from single scattering of sunlight by liquid droplets close to the cloud top. The cloudbow signal strongly depends on the cloud droplet size distribution. By fitting model simulations (stored in a look-up table) to the cloudbow observations, both the effective radius and the effective variance (i.e., the width) of the droplet size distribution are retrieved. Traditional retrieval techniques based on total reflectance measurements are able to determine the effective radius but do not provide information on the effective variance. However, to fully understand cloud growth processes and the interaction between clouds and solar radiation, both parameters must be known. Furthermore, the cloudbow is only weakly affected by 3-D radiative transfer effects which is beneficial since these are usually a problem for traditional methods.High-resolution maps of the cloud droplet size distribution with a spatial resolution of 100 m by 100 m are presented. The maps reveal patterns within the cloud droplet size distribution at cloud top that could originate from mechanisms like entrainment or mixing processes. We further show first results of an application of the retrieval to simulated specMACS observations. The images were generated using the 3-D radiative transfer model MYSTIC and are based on realistic LES cloud field simulations. We will investigate limitations and uncertainties of the retrieval using the simulated dataset.

The cloud droplet size distribution is often described by a gamma distribution defined by the effective radius and the effective variance. 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 that forms due to the 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. The effective radius and the effective variance can be retrieved by fitting model simulations (stored in a lookup table) to polarized cloudbow observations. This study uses measurements from the wide-field polarization-sensitive camera of the spectrometer of the Munich Aerosol Cloud Scanner (specMACS) aboard the German “High Altitude and LOng range research aircraft” (HALO). Along 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 (ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte) field campaign are presented, and the cloudbow technique is demonstrated. The results are combined into maps of the effective radius and the effective variance with a 100 m × 100 m spatial resolution and large coverage (across-track swath width of 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 0.11. specMACS measures at a very high angular resolution (binned to 0.3∘) which is necessary when large droplets are present. The second case study consists of small cumulus clouds (diameters of approximately 2 km). The retrieved effective radius is 7.0 µm, and the effective variance is 0.08 (both median values). This study demonstrates that specMACS is able to determine the droplet size distribution of liquid water clouds even for small cumulus clouds, which are a problem for traditional droplet size retrievals based on total reflectances.

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).