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 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.

Consideration of the cloud motion for aircraft-based stereographically derived cloud geometry and cloud top heights

“…An important contribution to the characterization of cloud properties is provided by active and passive remote sensing instruments which can be ground-, aircraft-or satellite-based and sensitive to different wavelengths both in the solar and terrestrial spectral range. One of them is the airborne spectrometer of the Munich Aerosol Cloud Scanner (specMACS, Ewald et al (2016); Pörtge et al (2023); Weber et al (2023)) operated onboard of the German High Altitude and LOng range research aircraft HALO (Krautstrunk and Giez, 2012). specMACS consists of two hyperspectral line cameras measuring radiances in the wavelength range between 400 nm and 2500 nm (Ewald et al, 2016) as well as two polarization resolving RGB cameras (Phoenix 5.0 MP Polarization Model) (Pörtge et al, 2023;Weber et al, 2023).…”

“…One of them is the airborne spectrometer of the Munich Aerosol Cloud Scanner (specMACS, Ewald et al (2016); Pörtge et al (2023); Weber et al (2023)) operated onboard of the German High Altitude and LOng range research aircraft HALO (Krautstrunk and Giez, 2012). specMACS consists of two hyperspectral line cameras measuring radiances in the wavelength range between 400 nm and 2500 nm (Ewald et al, 2016) as well as two polarization resolving RGB cameras (Phoenix 5.0 MP Polarization Model) (Pörtge et al, 2023;Weber et al, 2023). That way, the solar radiation reflected from clouds is measured with a high spectral and spatial resolution and a wide field of view (32.7 • and 35.5 • for the line cameras (Ewald et al, 2016) and 91 • × 117 • for the two polarization cameras combined (Weber et al, 2023)).…”

“…That way, the solar radiation reflected from clouds is measured with a high spectral and spatial resolution and a wide field of view (32.7 • and 35.5 • for the line cameras (Ewald et al, 2016) and 91 • × 117 • for the two polarization cameras combined (Weber et al, 2023)). The specMACS measurements are used for microphysical retrievals of cloud properties, both using a bispectral approach as described by Nakajima and King (1990) as well as from a polarimetric retrieval described in Pörtge et al (2023), which uses multi-angle polarized radiance observations of the cloudbow to derived cloud droplet size distributions (CDSD). In general, passive observations usually do not provide cloud top height or structure with height resolution.…”

“…Thereby, accurate cloud top heights are important as small errors in the cloud top height assigned to cloud targets will lead to wrong localizations of the targets in subsequent images. Consequently, this leads to a wrong aggregation of the cloudbow signal and thus, errors in the derived CDSD (Pörtge et al, 2023).…”

Polarization upgrade of specMACS: calibration and characterization of the 2D RGB polarization-resolving cameras

“…For example, Goloub et al (2000) and Riedi et al (2010) developed retrievals of the cloud thermodynamic phase from multi-angle polarimetric observations. Moreover, ice crystal asymmetry was derived from polarization by van Diedenhoven et al (2013), and retrievals of cloud droplet size distribution from cloudbow observations have been developed by Bréon and Goloub (1998), Alexandrov et al (2012), McBride et al (2020), and Pörtge et al (2023) to name a few examples for retrievals of cloud properties from polarization measurements. Besides that, polarization measurements have also been used to derive aerosol properties (Dubovik et al, 2019).…”