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. Plain Language Summary: A better knowledge of atmospheric processes leading to precipitation is mandatory to better predict severe floods and mitigate their impact on human societies, in particular in the context of climate change. Some of the most uncertain rain microphysics processes are the raindrop self‐collection and breakup. These processes are represented in numerical weather prediction (NWP) models via significantly different parameterizations, based on few laboratory experiments or from purely empirical approaches. In this work, 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. In both cases, multifrequency radar observations provide raindrop size distribution with high spatial and temporal resolution. Comparisons between observations and current parameterizations show significant divergences. Hence, a new parameterization of raindrop self‐collection and breakup processes is developed based on multifrequency radar observations. Key Points: The temporal variation of raindrop concentrations as they fall is derived from zenith pointing Ka‐ and W‐band radar measurementsRaindrop concentration rate obtained from the parameterizations of microphysical processes used in mesoscale models differ from observationsA new parameterization of self‐collection and breakup processes based on the multifrequency radar observations is proposed [ABSTRACT FROM AUTHOR]