Characterising the occurrence of monomodal and multimodal ice hydrometeor populations and their fall speeds in midlatitude frontal ice clouds using radar Doppler spectra

Multimodality in vertically pointing radar Doppler spectra is frequently observed in stratiform clouds, but understanding the microphysical processes causing these signatures, and indeed quantifying their occurrence, remains a challenge. In this study, we utilise Ka-band radar Doppler spectra from 23 deep stratiform clouds observed over Chilbolton, UK. We apply a peak finding algorithm to identify spectral reflectivity peaks in the Doppler spectra, which can be used to infer the presence of distinct coexisting ice particle populations, such as pristine crystals mixed with aggregate snowflakes. Using these results, we provide a quantitative estimate for the distribution of multimodal spectra with temperature. There are two clear temperature regimes where the occurrence of multimodal spectra increases sharply, indicating production of new particles; these are between −8 and −3 °C, which we attribute to secondary ice production, and between −18 and −13 °C, where the mechanism responsible for producing multimodal spectra is unclear. The peak finding algorithm also returns the velocities associated with the peaks in spectral reflectivity, which allows us to analyse the distribution of velocity with temperature for primary and secondary particle populations. We find that primary populations from monomodal spectra experience a significant reduction in vertical velocity at −13 °C. Examining the mean Doppler velocity (MDV) reveals a decrease in MDV from monomodal spectra at the same temperature. Whilst it has been suggested previously that this velocity decrease is caused by the presence of a secondary mode, our findings indicate that it is governed by the primary population; that is, falling particles are slowing down. Dendritic growth is favoured near this temperature, and we therefore hypothesise that particles such as polycrystals and aggregates, which form in colder cloud layers, precipitate to this level and form dendritic branches. This velocity decrease may then be associated with increased drag force acting against these particles which counteracts their increasing mass, thus causing the velocity to decrease.