Seasonal variation of diurnal rainfall characteristics over Vietnam

This study explores the characteristics and mechanisms of diurnal rainfall in Vietnam (VN), focusing on two key aspects: (1) the ability of Integrated Multi-satelliteE Retrievals for GPM (IMERG) to capture regional and seasonal sub-daily rainfall patterns; (2) the influence of multi-scale atmospheric processes in shaping these patterns. Data from 171 rain gauge stations served as a reference for the evaluation. Results indicate that while IMERG underestimates the magnitude of diurnal rainfall variation, it successfully identifies regional and seasonal differences in diurnal rainfall timing. Key findings include: (1) Southern VN predominantly experiences late-afternoon rainfall across all seasons; (2) Central VN shifts from late-afternoon rainfall dominated in spring-summer to nocturnal rainfall dominated in autumn-winter; (3) Northern VN displays mixed characteristics, with southern areas showing seasonal patterns like central VN, while northern areas are dominated by nocturnal rainfall year-round. Examining the maintenance mechanisms reveals that diurnal rainfall in southern VN is primarily influenced by local breezes (land-sea and mountain-valley breezes) interacting with topography, exhibiting minimal seasonal variation in timing. In central VN, significant seasonal changes in diurnal rainfall timing are driven by the seasonal shift in the location of moisture flux convergence, affected by specific humidity and low-level wind. In northern VN, the complex interplays between local breezes and the clockwise veering rotation of larger-scale diurnal wind circulation over the Gulf of Tonkin and the South China Sea, along with topographical influences, are crucial for diurnal rainfall formation. These findings highlight the importance of multi-scale atmospheric dynamics in modulating regional diurnal rainfall variations over East Asia.