High spatial resolution retrieval of cloud droplet size distribution from polarimetric specMACS observations and application to simulated data

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.