[A-AS10] General Meteorology

[AAS10-14] A detailed analysis of bimodal raindrop size distributions with a bin microphysics model

*Megumi Okazaki¹, Kosei Yamaguchi², Kentaroh Suzuki³, Tomoro Yanase⁴, Yousuke Sato⁵, Eiichi Nakakita² (1.Graduate School of Engineering, Kyoto University, 2.Disaster Prevention Research Institute, Kyoto University, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Graduate School of Information Science, University of Hyogo, 5.Faculty of Science, Hokkaido University)

Keywords:raindrop size distribution, bin microphysics model, horizontal and vertical flux convergence, collision-coalescence and breakup

Drop size distribution (DSD) is the basic unit required for the calculation of cloud microphysical processes in numerical weather models. Many numerical weather simulations use simple bulk methods, where the particle size distribution function is prescribed, due to computational cost issues. On the other hand, in the real atmosphere, DSDs of various shapes are observed. Therefore, there exists a DSD of shapes that are difficult to approximate with functions. One of them is the bimodal raindrop size distribution (RDSD). Bimodal RDSD has two number density peaks at the small and intermediate particle sizes. Previous research of developing microphysical scheme of breakup process showed equilibrium process of collision–coalescence and breakup is involved the bimodal RDSD. This theory was treated as mainstream of the bimodal RDSD formation mechanism. In contrast, recent observational studies have suggested that size sorting contributes to the formation of bimodal RDSDs. Size sorting is caused by the difference in raindrop fall velocity in an environment with strong updrafts and wind shear. However, there has been no quantitative analysis of size sorting that contributes to the formation of the bimodal DSD, focusing on the movement of individual raindrops. This study aims to investigate the changes of number density of raindrops in each size bins and to reveal the physical processes behind the formation of the bimodal RDSD. We conducted an idealized simulation with a bin method as cloud microphysical parameterization. This study used a regional cloud-resolving model called SCALE-RM (Scalable Computing for Advanced Library and Environment-Regional Model). In the simulation, a convective precipitation system was generated and several bimodal RDSDs were formed. Among them, we focused on the bimodal RDSD formed in the dissipating stage when the updraft weakened. For identification of the physical processes that contributed to the formation of bimodal RDSDs, we compared the number density changes due to each physical process for the particles constituting the local minimum and maximum, respectively. The results showed that horizontal and vertical flux convergence in the region where the bimodal RDSD existed was a negative in the size bins of the local minimum, whereas a positive pattern was identified for the particles of the local maximum. In other words, horizontal and vertical flux convergence contributed to the formation of the bimodal RDSD. This can be attributed to the different advection processes associated with each fall velocities of different raindrop sizes. In the simulations described above, breakup process was not calculated. Therefore, we implemented breakup process into bin method and examined its effects. The results showed that even when the breakup process was included, bimodal RDSDs were formed due to the horizontal and vertical flux convergence processes. Additionally, bimodal RDSDs were also formed due to the contribution of collision–coalescence and breakup processes. Therefore, it is necessary to classify the bimodal RDSD by the most contributing physical process and compare the characteristics of each to comprehensively understand the mechanism. It will also be necessary in the future to verify the accuracy of the implemented breakup processes by comparing them with observation.