Diurnal cycle of precipitation over coastal sea and small islands in the eastern region of Sumatra including season and Madden Julian Oscillation signatures

The Barisan Mountains influence atmospheric circulation in Sumatra and surrounding areas. Yet, precipitation patterns in eastern Sumatra, especially along the coast, are poorly understood. This study examines diurnal precipitation cycles in coastal seas and small islands of eastern Sumatra, utilizing 2015-2021 IMERG Version 06 rainfall data, data from 21 island rain gauges, and ERA5 reanalysis data to assess precipitation through three parameters: precipitation amount (PA), precipitation frequency (PF), and precipitation intensity (PI). The diurnal cycle of PA and PF is more pronounced than PI. Distinct diurnal precipitation patterns emerge between coastal seas and small islands despite their shared east coast location on Sumatra. Coastal seas exhibit early morning precipitation peaks forming Sumatras squall, influenced by offshore Sumatra and Malay Peninsula propagations, accentuated by high wind speeds. Coastal shape, land topography, and Sumatra's island size also impact coastal sea precipitation. On small islands, rainfall peaks earlier (12:00-15:00 LT) with gentle winds, indicating local convection due to land heating dominance. In addition, the low-amplitude topography of Bangka and Belitung islands also increase rainfall, so PA and PF are higher on these islands than on other small islands. Seasonal analysis reveals rising PA and PF near the equator during the dry southeast monsoon (SEM), linked to the dominant Inter-Tropical Convergence Zone (ITCZ). Conversely, areas like Bangka, Belitung, and the South Coast of Sumatra, farther from the equator, witness higher PA and PF during the wet northwest monsoon (NWM). The South Coast of Sumatra saw a substantial PA increase during NWM, about 60% higher than the southeast dry period. Despite varying cycle amplitudes, peak times for PA and PF remain consistent across seasons. MJO analysis during NWM highlights heightened PA and PF amplitudes across eastern Sumatra, especially over equator-adjacent coastal regions and small islands, reaching 40-70% above non-active MJO phases. In summary, this research examines diurnal precipitation variations in coastal seas and small islands of eastern Sumatra, enhancing understanding of regional climate patterns and factors impacting precipitation cycles in the area.