Evaluation of the Representation of Raindrop Self-Collection and Breakup in Two-Moment Bulk Models Using a Multifrequency Radar Retrieval

Using multifrequency radar observations providing raindrop size distribution evolution with high spatial and temporal resolution, this study aims to assess the ability of different parameterizations of raindrop self-collection and breakup processes applied in mesoscale models, to reproduce the statistics derived from observations. The stratiform zones of two types of precipitating systems are studied, a frontal situation that occurred over Finland in June 2014 and a squall line system observed over Oklahoma in June 2011. An analysis method for determining raindrop trajectories was used to obtain the temporal variation of the total raindrop concentration from the observations. The resulting raindrop concentration rate as a function of the mean volume diameter reveals significant differences with the parameterizations currently used in two-moment bulk microphysics schemes. These results show that even if they produce variations in raindrop concentration of the same order of magnitude as the observations, the current parameterizations diverge from the median of the observations, resulting in an overestimation of either the self-collection or the breakup process. From the median of radar observations, new parameterizations of the self-collection and breakup processes and of rain self-collection efficiency are developed and can be implemented in two-moment bulk microphysics schemes.