Understanding the mechanism underlying the formation of bimodal raindrop size distribution (RDSD) requires quantification of the cloud microphysical and dynamic behavior of precipitation particles within the precipitation system. Although the microphysical equilibrium state associated with collision-coalescence and breakup is considered the main mechanism for the formation of bimodal RDSD, the importance of dynamical advection effects associated with the influence of background wind fields has also been pointed out in recent years. Here, we investigated the formation of bimodal RDSD by quantifying the microphysical and dynamical processes that cause RDSD variability in a two-dimensional idealized simulation with an explicit representation of RDSD using a spectral-bin cloud microphysics scheme. Within the simulated precipitation system, bimodal RDSDs formed by horizontal and vertical advection and collision-coalescence breakups were present in similar proportions. The coalescence breakup-type bimodal RDSD appeared when the updrafts in the background field were strong. In contrast, the vertical advection-type bimodal RDSD was formed when the particles at the secondary peak selectively fell out by size sorting. Furthermore, the horizontal advection type of the bimodal RDSD was formed under the influence of particle size sorting associated with horizontal wind. The bimodal RDSD formed by these processes can be classified according to the particle size of the secondary peak. This study proposes a novel comprehensive picture of the bimodal RDSD formation mechanism caused by different microphysical and dynamical processes.

Raindrop size distribution (DSD) is fundamental for understanding precipitation processes. This study utilized a two-dimensional simulation with bin cloud microphysics parameterizations to investigate the spatiotemporal variability of DSDs owing to the influence of mesoscale circulation associated with the precipitation system. The simulated multicellular convection went through developing, mature, and dissipating stages, with updraft weakening and rainfall area expanding through these stages. The width of the DSD narrowed as rainfall weakened. In addition, a significant bimodal DSD was observed during the dissipating stage. Furthermore, we investigated the spatial distribution of the number density of raindrops corresponding to the maximum, local minimum, and local maximum of the significant bimodal DSD in the dissipating stage. According to the results, the raindrops constituting the maximum, local minimum, and local maximum followed different advection processes. This size-dependent advection effect may have contributed to the bimodal DSD formation.