Convective precipitation embedded in stratiform clouds on the northern slope of the central Tianshan Mountains: macro- and microphysical insights from multi-source observations

Introduction Heavy rainfall in mountainous regions, such as the northern slope of the central Tianshan Mountains, is characterized by high intensity, short duration, and strong locality, often triggering secondary disasters like floods and landslides. While convective clouds are the main contributors to summer precipitation, observations in mountainous areas are scarce. This study addresses the knowledge gap by providing a detailed, multi-sensor analysis of the macro- and microphysical evolution of convective cells embedded within a stratiform cloud system. Based on multi-source observational data from a Ka-band millimeter-wave cloud radar (MCR), micro-rain radar (MRR), raindrop spectrometer (RDS), C-band radar (CBR), automatic weather stations (AWS), and FY-4A satellite products at the Tianshan Orographic Cloud Seeding Demonstration Base, this study analyzes a convective–stratiform mixed cloud precipitation event that occurred on 14 August 2023. Methods Precipitation types were classified according to radar reflectivity, with convective precipitation defined as reflectivity ≥40 dBZ and stratiform as < 40 dBZ. The justification for this threshold is elaborated in Section 3, considering local orographic effects and verification against raindrop size distribution data. The two convective precipitation periods during the event were identified as T1 (16:30–18:10) and T2 (20:40–21:30). Data from the MCR, MRR, and RDS underwent quality control and correction procedures, including removal of outliers based on raindrop physical limits and correction for raindrop shape deformation. Key microphysical parameters, including raindrop size distribution (DSD), rainfall rate (R), radar reflectivity factor (Z), liquid water content (LWC), mass-weighted mean diameter (Dm), and normalized intercept parameter (Nw), were calculated from the processed DSD data. Results The results show that the precipitation event was significantly influenced by solar radiative heating and orographic lifting, with heavy precipitation primarily occurring in the evening. The CBR and FY-4A satellite observations showed good consistency, with the convective periods T1 and T2 corresponding to peaks in both radar reflectivity (exceeding 40 dBZ) and lower cloud-top brightness temperatures. The merger of two convective cells during T2 led to explosive growth in raindrop size parameters, with maximum diameter, median diameter, and reflectivity factor reaching 2.36 mm, 1.17 mm, and 25.43 dBZ, respectively, indicating that larger drops contributed more to precipitation in this phase. The MCR observed vertically coherent echo columns extending up to 4.7 km above ground during T2, indicating strong updrafts and active ice-phase processes, with glaciated cloud tops suggested by filamentary structures. Discussion This study demonstrates the effectiveness of multi-source ground-based and satellite observations in revealing the macro- and microphysical structure of convective precipitation embedded in stratiform clouds in an arid mountainous region. Future research should focus on establishing a long-term climatology of such events and employing high-resolution numerical models to elucidate the dynamical triggers of convective initiation and cell merger.