Satellite-Based Characterization of Warm-Season Mesoscale Convective Systems over Southeastern Mainland China in Response to the Western Pacific Subtropical High

Abstract Subtropical highs are large-scale dominant weather systems with significant impacts on regional climate and weather patterns. However, their cross-scale influence on mesoscale convective systems (MCSs) remains insufficiently understood. This study investigates how satellite-observed MCS characteristics vary with the western Pacific subtropical high (WPSH) over southeastern mainland China (SEMC), where the WPSH exerts a most frequent influence within the Asian continent. In 7-yr warm seasons, 725 WPSH days were identified and objectively classified into distinct weather types categorized as either WPSH-periphery or WPSH-center patterns, based on SEMC’s location relative to the WPSH. Although MCSs are generally less frequent in the WPSH-center patterns than in the WPSH-periphery ones, their occurrence remains noteworthy near regional hotspots. Across all patterns, MCS occurrence consistently exhibits a diurnal peak in late afternoon and early evening. The WPSH-center patterns show a larger diurnal amplitude with more intensive MCS activity around this peak period. MCSs in the WPSH-center patterns tend to have shorter lifetimes, fewer merging/splitting processes, and a greater tendency to form locally over SEMC in the afternoon. Despite varying movement directions and orientations, MCSs across different patterns generally move along their orientation to facilitate the MCS “training” effect, especially in the WPSH-center patterns due to slower moving speeds and stronger intensities. The analysis on atmospheric conditions suggests that MCS occurrence in the WPSH-periphery patterns is more closely linked to synoptic disturbances, including enhanced moisture transport via low-level jet streams and midlevel upward motion. The convective parameters including convective available potential energy (CAPE), total column water, and K index effectively differentiate MCS-active days from MCS-inactive days for each WPSH pattern. Significance Statement This study examines how mesoscale convective systems (MCSs) behave in response to variations in the western Pacific subtropical high (WPSH) over southeastern mainland China (SEMC). The results reveal distinct characteristics of MCSs occurring at the WPSH center versus its periphery over SEMC. MCSs at the WPSH center are overall less frequent than those at the periphery, but they exhibit a stronger diurnal cycle with more concentrated occurrence around the active period and a higher potential to produce localized heavy rainfall due to slower movement and stronger intensity. Unlike the WPSH-periphery patterns, MCSs at the WPSH-center patterns often occur in weak synoptic-forcing environments of poor predictability. These findings underscore the importance of accounting for MCSs in both WPSH scenarios in forecasting, particularly as the WPSH expands under global warming, influencing future MCS trends.