A region of an ice cloud where a sharp transition of dual-wavelength ratio occurs at a fixed height for longer than 20 min is identified. Evidence that rapid aggregation of ice particles occurred in this region is provided. This evidence comes from triple-wavelength Doppler spectrum radar data. Through quantitative comparison of the Doppler spectra from the three radars it is possible to estimate the ice particle size distribution (of particles larger than 0.75 mm) at different heights in the cloud. This allows us to investigate the evolution of the ice particle size distribution and determine whether the evolution is consistent with aggregation, riming or vapour deposition. The newly developed method allows to isolate the signal from the larger (non-Rayleigh scattering) particles in the distribution. Therefore, a particle size distribution retrieval is possible in areas of the cloud where the dual-wavelength ratio method would fail because the bulk dual-wavelength ratio value is too close to zero.