Modulation of the Diurnal Cycle of Precipitation near the Southwestern Coast of Sumatra by Mixed Rossby-Gravity Waves

In this study, we investigated the impact of mixed Rossby-gravity waves (MRGWs) on the diurnal cycle of precipitation over the southwestern coastal area of Sumatra using data captured during a pilot field campaign of the Years of the Maritime Continent (YMC) project. The study focused on a 19-day period from 24 November to 12 December 2015, using data from intensive surface observations, radiosondes, and a C-band polarimetric radar (collected aboard the research vessel Mirai at 4°4′S, 101°54′E), as well as data from a global objective analysis. The results indicated a relationship between oscillations with periods of several days in the intensity of diurnal precipitation and the wind field. Wind oscillations were attributed to several westward-propagating MRGWs traversing the study site. Diurnal convection and precipitation over the land and ocean were enhanced (suppressed) when MRGW-induced offshore (onshore) wind perturbations dominated. Large-scale low-level convergence and upper-level divergence, stronger sea-breeze flow, and colder land-breeze flow were also observed with the intensification of MRGW-induced offshore wind perturbations. However, diurnal precipitation displayed a similar well-defined phase and propagation pattern over the land and ocean, coherent with the regular evolution of seaand land-breeze circulations, regardless of wind perturbations induced by MRGWs. The results suggest that local convergence induced by the land–sea contrast is mainly responsible for driving the diurnal cycle. Notwithstanding, MRGWs exert a significant impact on the amplitude of diurnal convection and precipitation by modulating the large-scale dynamic structure of the atmosphere and the intensity of local seaand land-breeze circulations.