Sub-daily rainfall patterns in the mountainous regions of the Island of Tahiti: Insights from a one-year rain gauge network expansion

Study region: The Island of Tahiti in French Polynesia (17.5 degrees S,149.5 degrees W), with a focus on the mountainous regions of the island interior.Study focus: The high topography of Tahiti creates an obstacle for wet air masses traveling along the surface of the South Tropical Pacific Ocean, which forces air flow uplift and triggers orographic rainfall. Although the main physical processes behind orographic rain enhancement on tropical islands were identified, the ways they shape Tahitian rain fields are still not fully understood, particularly due to a lack of observations. To address the limitations posed by this shortage of rain data, this study describes the temporary expansion of the rain gauge network of Tahiti towards the mountainous island interior, and uses the resultant dataset to investigate the spatial and temporal patterns of orographic rainfall at sub-daily resolution.New hydrological insights for the region: The analysis of the rain gauge observations on daily and shorter time-scales leads to the identification of three main types of rainfall patterns: (1) a Dry Season Trade Winds type linked to a persistent Trade Winds Inversion (TWI) and characterized by moderate orographic rainfall in windward slopes, dry conditions in most leeward areas, and localized mid-afternoon showers in the mountains downwind of the main summit of the island; (2) a Transition Season Trade Winds type linked to atmospheric disturbances weakening the TWI, and characterized by intense orographic rainfall in windward slopes and mountains, and frequent showers in leeward areas by overshoot of the main crest; and (3) a SPCZ-related type occurring when the South Pacific Convergence Zone (SPCZ) crosses Tahiti during the wet season, and characterized by intense and widespread rainfall throughout the island with clear diurnal cycle and peak intensities recorded in the mid-afternoon.