Remote sensing of cloud droplet size distributions from the backscatter glory

The single scattering properties of clouds are mainly determined by the effective radius of the droplet size distribution. There are only few exceptions where the shape of the size distribution affects the optical properties, in particular the rainbow and the glory directions of the scattering phase function. Using observations by the Compact Airborne Spectrographic Imager (CASI) in 180 degree backscatter geometry, we found that high spatial resolution observations of the glory provide unique new information which is not available from traditional remote sensing techniques: Using only one single wavelength, 754 nm, we were able to determine not only optical thickness and effective radius, but also the width of the size distribution. In addition to providing new information, the method has the advantage of being not affected by three-dimensional cloud inhomogeneity effects because the glory is a single scattering phenomenon. Applying this technique to the ACE-2 CLOUDYCOLUMN experiment, we found that the size distributions were much narrower than previously suggested in the literature which is in agreement with in-situ observations during this campaign. While the shape of the size distribution has only little relevance for the radiative properties of clouds, it is extremely important for understanding their evolution. The potential of the method as a satellite or aircraft-based remote sensing tool will be outlined in the talk