Oriented particles in microwave and submillimeter radiative transfer simulations of ice clouds

Observations of Global Precipitation Measurement Microwave Imager (GMI) at 166 GHz consistently show polarized scattering signals of ice clouds. Conceptual models indicate that these signals emerge from oriented ice particles. Existing databases of scattering data of realistically shaped ice crystals for microwave and submillimeter typically assume total random orientation of ice particles. This is often a very reasonable assumption, but cannot explain the polarized ice cloud signals. Only few works considering oriented ice crystals exist, but they only consider microwave. With the upcoming Ice Cloud Imager (ICI) on board of Metop-SG B satellite, there will be additional dual-polarization measurements at 243 GHz and 664 GHz. These measurements will deliver new insights about clouds and their structure, if we know the scattering properties of oriented and realistically shaped ice crystals.

We provide publicly available scattering data for 51 different sized hexagonal plates and 18 different sized plate aggregates for 35 frequencies between 1 GHz and 864 GHz. The ice particles are assumed to be azimuthally randomly oriented with a fixed but arbitrary tilt angle. The scattering data for azimuthal random orientation is much more complex than for total random orientation. The scattering data of azimuthally randomly oriented particles depends in general on the incidence angle and two scattering angles compared to only one scattering angle for total random orientation. The scattering data is based on discrete dipole approximation simulations in combination with a self-developed orientation averaging approach.

We present detailed radiative transfer simulations of polarized GMI observations at 166 GHz and ICI observations at 243 GHz and at 664 GHz using our scattering data. The simulations of GMI recreate the observed polarization patterns. Analysis shows that not only orientation affects the polarization signal but also the hydrometeor composition. Furthermore, particle orientation also affects single polarized observations. Ignoring orientation can cause a negative bias for vertically polarized observations and a positive bias for horizontally polarized observations.