Multiscale controls on diurnal convection in the Maritime Continent: Insights from high‐resolution observations in Timika

Abstract

This study examines multiscale controls on the diurnal cycle of precipitation over Timika, southern Papua, where coastal processes, steep orography, and large‐scale tropical variability interact within a narrow coastal‐to‐mountain transition zone. Rainfall peak timing is identified using ground‐based gauge observations and the Global Precipitation Measurement (GPM) mission's Integrated Multi‐satellite Retrievals for GPM (IMERG) precipitation estimates for 2001–2022. GPM‐IMERG detects rainfall peaks 1–3 hr later than gauge observations, reflecting point‐to‐grid representativeness differences and the tendency of GPM‐IMERG to identify rainfall once precipitation systems become more spatially organized. Two dominant rainfall regimes are identified. Afternoon‐to‐evening rainfall (1500 h–2100 h local time, UTC + 0900) is the most frequent and is associated with thermally driven convection over land and the foothills, followed by relatively slow offshore propagation. In contrast, morning rainfall (0300 h–1200 h local time), although less frequent, is typically more intense and often originates over the adjacent ocean or coastal zone before propagating inland. These regimes are closely linked to the vertical structure of the meridional wind. Afternoon‐to‐evening rainfall is mainly influenced by lower‐ to mid‐tropospheric steering flow, especially within the 700–500 hPa layer, whereas morning rainfall is favored when low‐level onshore flow enhances inland moisture transport and coastal convergence. Large‐scale modes modulate these local regimes by altering the background meridional wind and moisture environment. Westward‐moving mixed Rossby–gravity (WMRG; also known as MRG or Yanai wave)‐related anomalies provide event‐scale perturbations to the steering flow, whereas the Madden–Julian Oscillation modifies the intraseasonal background state without imposing a deterministic phase‐to‐phase shift in rainfall peak timing. The seasonal monsoon provides the strongest background modulation: December–May conditions reinforce the dominant afternoon‐to‐evening regime, whereas June–November increases the likelihood of later evening to morning rainfall. El Niño–Southern Oscillation acts as a slower interannual influence, with its clearest signal in the afternoon‐to‐evening rainfall category. Overall, diurnal convection over southern Papua is governed by interacting processes in which local land–sea breeze and orographic forcing set the primary diurnal cycle, whereas larger‐scale modes modulate rainfall timing, propagation, and organization.