Role of Topography and Land-Sea Contrast on Precipitation in the Maritime Continent during Madden-Julian Oscillation Propagation
The Maritime Continent (MC) is the largest archipelago located between the Indian and western Pacific Oceans. The MC is surrounded by warm seas surface temperatures that leads to deep convection, which is also modulated by the land and its topography. The latent heat release due to condensation in the MC plays a significant role in modulating the global atmospheric circulation. The MC precipitation is also influenced by large-scale atmospheric variability such as the Madden-Julian Oscillation (MJO) which in turn is modulated by the MJO. In the recent past, the MC has gotten more attention due to a number of field campaigns yet the understanding of precipitation variation during MJO propagation and how they are modulated by the topography and land-sea contrast remains a major challenge to the atmospheric community. In this dissertation, I make an effort to understand the role of topography and land-sea contrast on the precipitation during the propagation of an MJO event using realistic and idealized regional model simulations. I conduct four simulations: (i) lower-resolution (12 km) simulations using cumulus parameterization in the presence (LR) and (ii) absence (LR-Flat) of topography, and (iii) higher-resolution (4 km) simulations without cumulus parameterization in the presence (HR) and (iv) absence (HR-Flat) of topography. The results indicate that the important physical processes for MJO-associated convection may be spatial different in different parts of the MC. For second part, I explore the role of topography by removing topography in both low and high resolution simulations. The control simulation (CTL), using observed topography, captures the timing, magnitude, and location of the diurnal cycle of precipitation over individual islands. The arrival of peak precipitation in FLAT is delayed by 1 hour compared with CTL. The largest difference in the precipitation diurnal cycle between the CTL and FLAT was found over areas with high topography (>1000 m). To understand the evolution of diurnal precipitation, a moisture diurnal budget analysis was conducted that indicates that the difference between the CTL and FLAT is primarily induced by both horizontal and vertical moisture advection. I also explore the relationship between topographic height and precipitation in detail as well as the relative contribution of different processes below and above the planetary boundary layer. Finally, I explore the specific contribution of individual islands to the precipitation over the surrounding islands during MJO propagation. I designed four experiments including a control (CTL), removal of Sumatra (E1L), removal of Borneo (E2L), and removal of New Guinea (E3L). I found a pattern that during background easterlies prior to MJO arrival, a systematic decrease in precipitation was found in the surrounding islands to the west side of the removed island. But, on the eastern side of the removed island, no systematic change in precipitation was found.