Triple frequency Doppler spectra closure study across the melting layer: Lessons learnt

By exploiting novel measurements from vertically pointing multi-frequency Doppler radars, this study investigates the link between rain and ice microphysics across the melting layer in stratiform rain, the main source of precipitation in the mid-latitudes. A closure study is proposed in this paper where Drop Size Distributions (DSD) are retrieved from multi-frequency radar Doppler spectra and propagated upward to predict the Particle Size Distributions (PSD) in the overlying snow. Snow PSDs are retrieved above the freezing level for several ice models from the full Doppler spectra measured at such level. The model that best matches the measured triple frequency spectra is used to infer the PSDs, which are then compared to the PSDs predicted from rain from a melting-only steady-state assumption. Overall, the predicted and the retrieved mean mass weighted diameters (Dm) of ice are highly correlated, with the retrieved Dm being on average circa 15 % larger. Although, a correlation between the precipitation rates above and below the melting zone is weaker (CC = 0.66), the melted equivalent accumulation over 6 h period is nearly perfectly matched (1 % difference only). This novel methodology can be applied to assess the validity of some assumptions like the constant precipitation rate across the melting region via long-term observations; this will advance our knowledge of the processes occurring across the melting region. This has the potential to improve space-borne radar precipitation retrievals as well, especially those from the Dual Frequency Precipitation Radar on-board of the Global Precipitation Measurement core satellite.