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 Mimi at 4 degrees 4'S, 101 degrees 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 sea-and 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 sea- and land-breeze circulations.