Scattering Properties of Snowflakes, Constrained Using Colocated Triple-Wavelength Radar and Aircraft Measurements

The scattering of microwaves by ice particles is an important problem for remote sensing applications. For example CloudSat has been shown to be a crucial tool for monitoring snowfall globally. The Ice Cloud Imager will allow us to measure ice water path with much greater precision than previously possible. Yet in both cases, the size of the particles being probed is comparable to the wavelength of the radiation - and hence the accuracy of the retrievals is quite sensitive to the scattering characteristics of the particles being probed.

In recent years there has been an explosion in the number of numerical calculations of scattering properties for ice particles. The shapes used in the computations range from pristine crystals to very large complex aggregates. The question thus arises - which scattering model should we choose for our retrievals? And on what evidence?

We report new measurements from the 2018 PICASSO field project, during which the FAAM instrumented research aircraft sampled ice particles while multiple-wavelength polarimetric radars at the Chilbolton Observatory (wavelengths ranging from 3mm up to 10cm) probed the same clouds. We use this data to directly test the fidelity of the scattering models. The radar antennas were synchronised and the scans we controlled by a tracking system fed using live position data from the aircraft, to maximise colocation. At vertical incidence (during overpasses), Doppler spectra were collected adding a further dimension to the analysis. We use this data to probe the scattering behaviour of snowflakes and link that to their physical properties.