Recent studies have indicated that many heavy rainfall events are produced by convective clouds with relatively low cloud-top heights, highlighting the importance of warm-rain processes driven by condensational growth and collision–coalescence in the lower troposphere. In contrast, for heavy rainfall associated with deeply developed convective clouds with high cloud tops, it remains unclear which cloud microphysical processes-particularly those involving graupel-contribute to rainfall production. Observations of Japanese convective clouds have reported the presence of frozen raindrops and graupel likely formed through the riming growth of frozen raindrops. This suggests the existence of a precipitation pathway involving raindrop freezing, in addition to conventional warm-rain and snow-origin cold-rain processes. However, the relative contributions of these microphysical processes to total precipitation remain poorly understood. In particular, the impacts of variations in cloud condensation nuclei (CCN) concentrations on graupel formation and heavy rainfall production through processes involving frozen raindrops have not yet been sufficiently clarified.

To address these issues, we implemented two diagnostic schemes -a process- tracking scheme and an origin-tracking scheme- into the Weather Research and Forecasting (WRF) model and conducted sensitivity experiments by varying CCN concentrations. The process-tracking scheme quantifies the mass growth of raindrops and graupel, as well as their relative contributions to precipitation through individual microphysical processes, such as the riming of cloud water by graupel and graupel formation through raindrop freezing. The origin-tracking scheme evaluates the relative contributions of different precipitation pathways, including the warm-rain process, the graupel formation process via snow riming, and processes involving raindrop freezing. The analysis focuses on a heavy rainfall event associated with a quasi-stationary linear precipitation band (senjo-kousuitai) over Fukuoka Prefecture during the July 2017 Northern Kyushu Heavy Rainfall, for which previous studies suggested a significant role of graupel processes.

Sensitivity experiments were conducted under three CCN conditions: a control experiment (CTL), a polluted experiment (POL; ten times the CCN concentration of CTL), and a clean experiment (CLEAN; one-tenth the CCN concentration of CTL). The results revealed that the spatial extent of heavy rainfall was largest in CLEAN. The amount of upper-level graupel did not show a simple linear relationship with CCN concentration; it was largest in CLEAN, followed by POL, and then CTL. Analysis of graupel mass growth indicated that the riming of cloud water by graupel and graupel formation through collisions between raindrops and ice crystals were the two dominant processes. The former was enhanced in POL, while the latter was more pronounced in CLEAN. Differences in the upward fluxes of condensates showed that the flux of cloud water was larger in POL, whereas the flux of raindrops was larger in CLEAN. In POL, the enhancement of updrafts and the maintenance of high cloud water mixing ratios in the lower levels contributed to the increased cloud water flux. In contrast, the increased raindrop flux in CLEAN was driven by higher raindrop mixing ratios. These differences arose because the smaller cloud droplet size in POL suppressed raindrop formation and increased cloud water retention, whereas raindrop formation was promoted in CLEAN. These results demonstrate that graupel processes respond nonlinearly to CCN concentration, primarily due to the combined effects of riming and raindrop-freezing processes-the importance of which has received limited attention in previous studies.

Furthermore, the origin-tracking results indicate that although this event was characterized by high cloud-top heights, warm-rain processes dominated the total precipitation. However, within the heavy rainfall region, warm-rain processes were more influential on the upwind side, whereas precipitation associated with graupel formed by raindrops freezing upon collision with ice crystals became relatively more important on the downwind side. In contrast, the contribution of cold-rain processes involving graupel formed through snow riming was suggested to be limited. These findings suggest that even in heavy rainfall events associated with deep convective clouds, warm-rain processes provide the primary contribution, while graupel formation through raindrop freezing plays a crucial role in localized heavy rainfall. The analysis further revealed that the formation heights of precipitation particles were vertically stratified, ranging from warm-rain processes in the lower levels to raindrop freezing and snow riming in the upper levels. Consequently, the strong horizontal winds characteristic of the back-building system advected particles formed at higher altitudes further downwind, resulting in the spatial separation of precipitation areas corresponding to their formation mechanisms.