Winds Decide Where Rain Grows and Dies Over Padang’s Coastal Mountains

Every afternoon, rain sweeps in from the Indian Ocean onto the coast of Padang in West Sumatra, pushed ashore by moist winds and lifted by the Barisan mountain range that runs like a spine along the island. Some of those storms intensify as they cross the shoreline, tapping into the collision of sea breezes and orographic lift; others weaken rapidly once they move inland, starved of moisture or sheared apart. A new study published in Theoretical and Applied Climatology has now quantified, pixel by pixel and event by event, where precipitation over this coastal–orographic zone tends to intensify and where it tends to die, and how that balance shifts with the direction and speed of the winds steering the rain.

The research, carried out by Naila Az Zahra and Adi Mulsandi of the Sekolah Tinggi Meteorologi Klimatologi dan Geofisika in Indonesia, applies a technique known as growth and decay analysis, or GD, to five years of satellite rainfall data spanning 2020 to 2024. Instead of asking simply how much rain fell at a given location, the method asks how the intensity of a rain system changes as it travels. Positive growth and decay values mark places where precipitation strengthens; negative values mark where it weakens. This distinction matters because the location of the heaviest rainfall is not necessarily the location where a storm gained its strength, a subtlety that conventional rainfall maps completely miss.

The technical machinery behind the analysis is a semi-Lagrangian framework. Half-hourly precipitation estimates from the IMERG Final Run V7B product, produced for the Global Precipitation Measurement mission, were combined with wind fields at the 850-hectopascal level, roughly 1.5 kilometers above the surface, taken from the ERA5 reanalysis. The winds were used to estimate where each parcel of rainfall would be displaced during the interval between satellite snapshots, allowing the researchers to compare rain intensity at a system’s origin with its intensity at its destination. Topography from the ETOPO1 global relief model provided the physical context, tracing the transition from the coastline through the coastal plain and up into the Bukit Barisan range, while daily rain-gauge observations from Indonesia’s Meteorology, Climatology and Geophysics Agency, BMKG, anchored the satellite estimates to the ground.

The first major finding is diagnostic: for individual rainfall events, the maximum rain intensity, the region where the storm grew, and the region where it decayed frequently do not overlap in space. A downpour measured over the foothills may owe its strength to processes that occurred tens of kilometers away over the coastal waters. That separation is precisely why the authors argue that growth and decay diagnostics add genuine value to precipitation studies in complex coastal terrain, where the interplay of land–sea breezes, moisture convergence and terrain blocking reshapes storms on very short horizontal distances.

Averaged over five years, the picture becomes organized around the coastal–orographic transition itself. Positive growth and decay values cluster near the western coastal sector of Padang, indicating that this narrow strip is where rain systems most often intensify, consistent with the convergence of sea breezes and low-level onshore flow. Farther inland and around the elevated terrain of the Barisan range, the values weaken or turn negative, showing that storms which survive the crossing tend to lose intensity as they climb and move away from their maritime moisture source. The coastline, in effect, acts as an intensification line, while the interior acts as a decay zone.

Crucially, this pattern is not static. When the authors sorted the valid growth and decay samples by the direction of the 850-hectopascal steering flow, westerly and southwesterly winds emerged as the dominant regimes, and under these flows the contrast between coastal growth and inland decay was at its sharpest. The strong low-level westerlies typical of the region appear to funnel oceanic moisture directly into the coastal zone, feeding convection exactly where the terrain begins to rise. This flow dependence gives forecasters a potentially useful rule of thumb: the prevailing steering direction tells you where the day’s storms are most likely to strengthen.

Wind speed, however, complicates the story. Under fast-flow conditions, the spatial extent of negative growth and decay values expands, suggesting that strong advection can carry precipitation systems across the coastal–orographic zone too quickly for them to fully develop. Rapid transport shortens the residence time of convective cells over the region where sea-breeze convergence and orographic lift are most effective, so the rain is swept inland and elsewhere before it can intensify to its potential. In other words, it is not only where the wind blows but how fast that determines whether Padang’s coastal strip acts as a rain factory or a mere way station.

The growth and decay signal also varies with the clock and the calendar. Positive values showed clear diurnal and seasonal variation, reflecting the powerful land–sea breeze cycles of the Indonesian Maritime Continent and the seasonal migration of the large-scale circulation. Sumatra’s rainfall is famously shaped by the diurnal cycle, with convection often initiating offshore or along the coast before propagating inland, and previous research has linked Padang’s extreme rainfall episodes to equatorial waves such as convectively coupled Kelvin waves riding across the basin. The new analysis adds a layer to this understanding by showing that the growth phase of precipitation itself is timed and positioned by these rhythmic drivers, not merely the rainfall totals that result.

The overall growth and decay distribution over the coastal–orographic zone turned out to be nearly balanced, with only a weak positive multi-year mean accompanied by large variability. That balance is itself informative: it implies that intensification and weakening processes over Padang largely compensate across many events, even though any single day can be dominated by one or the other. For hydrologists and disaster managers in West Sumatra, where flooding driven by coastal and orographic rainfall is a recurring hazard, the practical message is that rainfall maxima on a map are an incomplete guide to hazard. Understanding where systems grow, and under which wind regimes they grow fastest, may improve nowcasting of rapidly developing storms in this densely populated coastal corridor.

Methodologically, the study demonstrates that a relatively simple semi-Lagrangian diagnostic, built entirely from openly available satellite precipitation, reanalysis winds and global topography, can resolve physically meaningful patterns of storm intensification in one of the most complex rainfall regimes on Earth. The authors acknowledge the ground-truth limitations inherent in satellite estimates, which is why daily BMKG gauge data were used to support the IMERG accumulations at the daily scale. As the technique matures, extending it with radar observations and higher-resolution wind analyses could sharpen the picture further. For now, the work establishes a clear principle for Sumatra’s western coast: the life story of every rainstorm there is written in the wind, and reading it requires tracking rain from birth to decay, not just measuring where it falls.

Subject of Research: Growth and decay of precipitation over the coastal–orographic zone of Padang, West Sumatra, as a function of steering-flow direction and speed

Article Title: Flow-dependent growth and decay of precipitation over the coastal–orographic zone of Padang, West Sumatra

Article References: Zahra, N. A., & Mulsandi, A. (2026). Flow-dependent growth and decay of precipitation over the coastal–orographic zone of Padang, West Sumatra. Theoretical and Applied Climatology, 157(11), Article 721. https://doi.org/10.1007/s00704-026-06653-6

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06653-6

Keywords: Padang, West Sumatra, precipitation growth and decay, semi-Lagrangian analysis, IMERG satellite rainfall, ERA5 reanalysis, orographic precipitation, coastal meteorology, Bukit Barisan, steering flow, diurnal cycle, Indonesian Maritime Continent