UNDERSTANDING THE DIURNAL CYCLE AND ITS MODULATION UNDER LARGE SCALE INFLUENCES OVER THE MARITIME CONTINENT

Abstract

The Maritime Continent (MC) is one of the most convectively active and topographically complex regions on Earth, where diurnal convection strongly influences the regional hydrological environment and global circulation through interactions with large-scale tropical circulations like the Madden-Julian Oscillation (MJO). Despite its importance, numerical weather prediction and general circulation models (GCMs) struggle to accurately represent both the timing and intensity of the diurnal cycle of convection and the propagation of the MJO across the MC, highlighting the need to improve our understanding of the physical processes governing diurnal convection, which have been shown to affect the MJO. This dissertation investigates the mechanisms that modulate the diurnal cycle over the western MC and examines how MJO-induced large-scale environmental conditions modify these processes using convection-permitting simulations with the Weather Research and Forecasting model. The first part of this dissertation examines the role of cloud-radiative interactions on the diurnal cycle of rainfall. Through the removal of cloud-radiative interactions in the model, this study demonstrates that cloud-radiative interactions suppress daytime island diurnal rainfall by reducing surface shortwave heating. This weakens surface turbulent fluxes, reducing rainfall intensity and delaying convective initiation. Diurnal rainfall sensitivity is greatest over mountainous regions, where orographic forced ascent promotes frequent low clouds that enhance cloud shading, suppressing diurnal rainfall and delaying convective initiation. This study demonstrates that erroneous representation of cloud-radiative interactions likely contributes to errors in the diurnal cycle, particularly over mountainous regions, partially explaining or exacerbating the biases in GCMs. The second part investigates how the MJO modulates the diurnal cycle of rainfall through changes in the environmental moisture and circulation upon its propagation over the western MC. A series of novel initialization and boundary condition experiments are conducted to test the sensitivity of diurnal rainfall to changes in MJO-associated moisture and/or winds. Upon the onset of the MJO enhanced phase, diurnal rainfall strengthened over the oceanic regions, driven primarily by the increase in environmental moisture, likely dampening the dry-air entrainment that suppresses convective intensity and persistence. The modulation of diurnal rainfall over the islands depended on terrain-face direction, with the western sides experiencing weaker diurnal rainfall as stronger MJO-associated turbulent mixing diluted the land-sea thermal gradient driving the diurnal circulation, while the eastern sides experienced stronger diurnal rainfall because the moister MJO environment, particularly in the mid-to-upper troposphere, enhanced convection. This study highlights the mechanisms that drive the modulation of diurnal rainfall upon the onset of the MJO enhanced phase and demonstrates how MJO-associated moisture and circulation modulate the strength of these mechanisms. Collectively, this dissertation establishes that MJO-associated cloud-radiative interactions, environmental moisture, and circulation each regulate distinct components of the MC diurnal cycle. Cloud-radiative interactions primarily control the local strength and timing of island convection, environmental moisture governs oceanic rainfall strength, and environmental winds and moisture determine the spatial distribution of island convection. These findings improve the physical understanding of the multiscale mechanisms over the MC and provide a framework for evaluating the modulation of tropical convection under large-scale changes across regional and global scales.