Observation of ice particle habits and orientation from ground-based remote sensing measurements

Passive remote sensing provides invaluable information about ice cloud optical and microphysical properties. Missing knowlegde about particle habit, however, introduces an uncertainty of at least a factor 2 in optical thickness retrievals. Any information about particle habit would therefore be extremely valuable for improving ice cloud remote sensing and for better quantifying the effect of ice clouds on the radiation budget and thus on climate. Ice particle shape and orientation produce distinct signals in the sky radiance distribution, which can be observed in a variety of halo displays. 22 degree-halos are a clear indication of randomly oriented hexagonal prisms while sundogs are produced by oriented hexagonal plates. The brightness contrast of the 22 degree-halo may therefore be used to retrieve quantitative information about the fraction of hexagonal ice crystals. Whereas the brightness of the sundog in comparison to the halo should further allow to determine the fraction of oriented particles.

To provide a continuous time series of observations of different halo displays a sun-tracking wide-angle camera system was installed on the MIM (Meteorological Institute Munich) roof platform, which allows to study the long-term variability of the fraction of hexagonal and oriented ice crystals qualitatively. In order to estimate the quantitative fraction of randomly oriented hexagonal ice crystals, calibrated ground-based spectral radiance measurements are used to calculate the contrast of the 22 degree-halo, i.e. the ratio of the measured radiance at the scattering angles 22 degrees and 18 degrees. Since the contrast of the 22 degree-halo is not only a function of the fraction of hexagonal ice crystals but also of the optical thickness and effective radius of the particles, further information is needed. For the retrieval of cirrus optical thickness and effective radius, sun-photometer measurements may be used in combination with spectral radiance measurements in the near-infrared.

The spectral observations are complemented by cloud radar and lidar observations: while radar provides the geometrical position of the cloud and the fall speed of the particles (via the Doppler signal), the lidar depolarization channel allows to retrieve the non-sphericity of the particles. Combining spectral radiance, radar, and lidar measurements, which together form the newly installed Munich Aerosol and Cloud Scanner (MACS), we hope to gain more knowledge about ice particle habit and orientation and thus to improve common retrievals of ice cloud optical properties. As an important pre-requisite for studying the radiative effects of ice particle habits and orientations, the radiative transfer model MYSTIC (Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres) was extended for oriented ice particles using a Monte Carlo-based raytracing technique. First simulation results will be shown as well as a first evaluation of data from spectral and imaging observation systems together with lidar and radar observations.