Daily and Subdaily Wind and Divergence Variations Observed by a Shipborne Doppler Radar off the Southwestern Coast of Sumatra

Abstract

Abstract This study investigated the daily cycle of the wind and divergence fields observed off the southwestern coast of Sumatra during a field campaign of the Years of the Maritime Continent pilot study. An algorithm was developed to retrieve kinematic variables from the single-Doppler data collected aboard the Research Vessel Mirai from 24 November to 13 December 2015. The observed daily cycles of the wind and divergence fields consisted of diurnal, semidiurnal, and short-term variations. Diurnal wind variation was characterized by deep and three-dimensional circulation. There was an approximate phase locking of the semidiurnal variation to the diurnal variation, both in the wind and divergence fields. The short-term wind variation occurred at a time scale of ∼1–3 h, and this pattern was associated with density currents or mesoscale gravity waves. Up to 73% of the daily vertical motion variance can be attributed to the diurnal and semidiurnal vertical motion variations with comparable strengths. Concurrently, precipitation propagated offshore in phase with density currents and mesoscale gravity waves. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Additionally, it appears that the daily precipitation cycle is modulated by multiple-time-scale wind variabilities of less than a day, which is also responsible for the development of strong nocturnal convection off the southwestern coast of Sumatra. Significance Statement To improve our understanding of the daily wind and divergence cycle off the southwestern coast of Sumatra, we examined wind data collected by a shipborne Doppler radar. The observed daily cycles of the wind and divergence fields consisted of diurnal and semidiurnal variations, as well as a 1–3-h variation associated with a density current or mesoscale gravity wave. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Thus, we highlight the role of multiple-time-scale wind variabilities of less than a day in modulating the daily precipitation cycle off the southwestern coast of Sumatra.