Extreme precipitation event in November 2025 in Sumatra observed with high resolution in-situ observations from BAM-Net

The Maritime Continent is a complex system of islands—often with significant topography, deep oceans, and shallow seas—located in the heart of the Indo-Pacific warm pool. The region is characterized by very high average daily precipitation, strongly modulated by a pronounced diurnal cycle. Precipitation variability in the Maritime Continent, influenced by subseasonal, seasonal, and interannual modes, has long been of interest to the research community, as it can easily lead to extreme precipitation events. However, the observational network over the region is sparse, and coherent datasets capable of assessing the physical properties of the atmosphere are limited, particularly on diurnal timescales.

An example of this complex interaction can be found in Sumatra, where the diurnal evolution of convection and precipitation is characterized by two modes. Convective clouds begin developing before noon along the western, upwind slopes of the Barisan Mountains; they grow and move inland during the afternoon, advected by the mean flow in the lower to middle troposphere. However, there is also propagation in the opposite direction: offshore, upwind-moving squall lines that produce an offshore precipitation maximum throughout the evening and night. This local variability is strongly modulated by large-scale circulation variability, which in turn affects precipitation over the island. Several physical mechanisms have been proposed to explain the offshore progression of precipitating cloud systems on diurnal timescales, but these have been based primarily on high-resolution numerical modeling. Due to the lack of observational data these mechanisms remain poorly constrained.

The aim of the Barisan–Anai Meteorological Network (BAM-Net) is to fill this observational gap by providing consistent, long-term near-surface meteorological data (pressure, temperature, humidity, horizontal winds, and rainfall), as well as cloud cover and column-integrated water vapor. To date, the dataset spans over one full year of observations collected across five stations along the Anai Valley, between the Indian Ocean coast and the first mountain pass across the Barisan Mountains at 1000 m ASL. This dataset provides a unique opportunity to continuously monitor the diurnal cycle of near-surface atmospheric properties and to assess its variability associated with seasonal, intraseasonal, synoptic, and mesoscale circulations.

In this submission, BAM-Net observations are used to study variability in the diurnal cycle from day-to-day up to seasonal timescales across the five locations, focusing on November 2025 period, when unprecedented extreme precipitation event span across west and north Sumatra and southern part of Malay peninsula, associated with development of a rare near-equatorial Tropical Cyclone Senyar in Malakka Strait. This high impact event caused over 1000 fatalities, vast devastation of civil infrastructure and personal property. BAM-Net stations in West Sumatra show precipitation accumulation exceeding 1000 mm of rain in 10 days, and indicate even higher rainfall amounts in the north and north-east part of the island. BAM-Net observations provide unique insight into event’s dynamics in the up-wind slope region, including spatio-temporal variability across region and within it. This type of observations can be used in process studies of extreme events as well as high resolution model validation.