Raindrop size distribution (DSD) is one of the fundamental quantities that characterize precipitation and is useful for understanding precipitation microphysics because it reflects processes such as collision-coalescence, breakup, and evaporation. In addition, clarifying DSD characteristics is important for improving the accuracy of observations and numerical weather predictions because DSD is related to radar-based quantitative precipitation estimation and to the parameterization of cloud microphysical processes in numerical in numerical models. Since DSD can vary depending on region, season, and precipitation type, it is necessary to organize DSD characteristics while accounting for these differences. However, in Japan, relatively few studies have systematically compared DSD characteristics, organized using representative parameters such as the mass-weighted mean diameter Dm and the normalized intercept parameter Nw , in relation to difference in precipitation systems while considering seasonal variations.

Therefore, this study analyzed observational data from 2024 with the objectives of clarifying seasonal differences in DSD characteristics between the pre-Baiu and Baiu seasons in northern Kyusyu, and interpreting variations in surface-observed DSD in connection with microphysical processes aloft. Specifically, Dm and Nw were calculated from ground-based disdrometer observations, and DSD characteristics in the pre-Baiu and Baiu seasons were compared using the two-dimensional (2D) frequency distribution of Dm and log10Nw.

The results showed that the locations of high-frequency regions in the 2D frequency distribution of Dm and log10Nw differed between the pre-Baiu and Baiu seasons, indicating a seasonal difference in the predominant DSD characteristics. Furthermore, in the difference distribution (Baiu minus pre-Baiu), increases and decreases in occurrence frequency appeared in specific regions of the 2D frequency distribution of Dm andlog10Nw, making the seasonal differences more evident.

Next, to interpret these seasonal differences in relation to differences in precipitation systems, five precipitation types were defined based on the surface DSD characteristics for each season, and analyses were conducted for each precipitation type using XRAIN polarimetric radar observations. Specifically, contoured frequency by altitude diagrams (CFADs) of polarimetric parameters, including radar reflectivity Zℎ and differential reflectivity Zdr, were constructed to compare the vertical structures within precipitation regions among the precipitation types. As a result, a clear melting-layer signature was identified in precipitation types 3 and 4, whereas no such signature was observed in precipitation types 1, 2, and 5, indicating a systematic difference in vertical structure among the precipitation types. Accordingly, based on the presence or absence of the melting-layer signature, precipitation types 1, 2, and 5 were grouped together, and precipitation types 3 and 4 were treated as another group, and discussions then focused on the differences between the pre-Baiu and Baiu seasons within each group.

In addition, ∆Zℎ and ∆Zdr were calculated from the differences between lower and upper layers, and dominant microphysical processes for each precipitation type were inferred by rainfall-intensity class based on the combination of these quantities. The results showed that precipitation types 1, 2, and 5 were characterized overall by the predominance of collision-coalescence and coalescence-breakup equilibrium state. In contrast, precipitation types 2 and 4 exhibited different dominant processes under light-rain conditions: equilibrium dominated in type 3, whereas breakup dominated in type 4. These differences in dominant microphysical processes were considered to arise from differences in DSD characteristics among the precipitation types. This study demonstrated that seasonal differences in DSD between the pre-Baiu and Baiu seasons in northern Kyushu manifest as differences in vertical structure and dominant microphysical processes of precipitation among precipitation types.