Bimodal ice spectra in frontal clouds

Aircraft observations from five Lagrangian descents in frontal clouds from around the British Isles are presented. A common bimodal ice size spectral evolution is observed using two-dimensional optical array probes, and it is demonstrated that depositional growth alone is not responsible for the observed evolution. It is suggested that the process of aggregation is chiefly responsible for the large-particle mode of bimodal ice size spectra. Synthetic bimodal spectra that are a function of depth below cloud top and ice-water content are presented.