DSD Characteristics with Different Synoptic Patterns in the Southeastern Tibetan Plateau Region

Mêdog, located at the entrance of the water vapour channel of the Yarlung Zangbo Grand Canyon, and it has the highest rainfall and lowest elevation on the Tibetan Plateau (TP). The droplet size distribution (DSD) and microphysical processes associated with rainfall usually exhibit different characteristics under different synoptic patterns. In this study, an objective classification method is used to categorize the synoptic patterns that affect heavy rainfall (daily rainfall amounts > 10 mm) in Mêdog into four patterns: southwest airflow (SWA), southern-branch trough (SBT), intense baroclinicity (IBC), and terrain-forced precipitation (TFP). SWA occurs most frequently (approximately 70%) with a mean daily rainfall of ~22 mm, while TFP has the lowest occurrence frequency (7.7%) but the highest mean daily rainfall (29 mm). Both SBT and IBC exhibit occurrence frequencies around 12%. Among these patterns, the SWA pattern predominantly occurs during the monsoon season with abundant moisture and the lowest concentration of small raindrops. In contrast, the TFP pattern exhibits the highest concentration of large raindrops and the widest DSD spectrum, which can be attributed to the frequent convective activities in this area. As a result, compared with those of the other three synoptic patterns, the TFP pattern exhibits a larger mass-weighted mean diameter (D_m) and higher rain rate (R). For stratiform rainfall, the difference in D_m among the four synoptic patterns can be neglected. The largest (smallest) average lgN_W-value is observed in the SWA (IBC) pattern. Regarding convective rainfall, IBC dominated by northerly cold air exhibits mixed-phase processes characterized by larger raindrops and lower concentrations, resembling continental-like rainfall. In contrast, SWA occurring in monsoon season shows high concentrations of small raindrops, deeming it similar to maritime-like rainfall. In terms of the derived relationships, there are significant differences in the D_m–R and µ–Λ relationships among the four synoptic patterns. In addition, the diurnal variation in the DSD is analyzed in terms of the four synoptic patterns. These findings can improve the understanding of the microphysical processes of heavy rainfall events under different synoptic patterns and provide a reference for microphysical parameterizations of numerical models.