Convection-permitting Model Simulations of the Processes Controlling the Diurnal Cycle Over the Maritime Continent During Different MJO Phases

Convection in the maritime continent is comprised of multi-scale coherent structures ranging from the Madden-Julian Oscillation (MJO) down to land and sea breezes initiated along complex coastlines. In the maritime continent the diurnal cycle is a prominent feature of the convective cloud and precipitation distributions, but this varies with the phase of the MJO. In addition to the afternoon peak in convective activity over land, many maritime continent regions have a secondary nocturnal or early-morning precipitation maximum. This secondary maximum can be linked to land breeze circulations or other offshore propagating disturbances associated with gravity waves. However, many aspects of the dynamics of these processes and their regional variation are poorly understood. Moreover, most numerical models with parameterized convection have difficulty reproducing the timing of the daytime peak in convective activity and the occurrence of the secondary maximum. These processes have broad implications for prediction models and are important to help understand the impact of the maritime continent on the MJO.

Here we use large-domain convection-permitting model simulations over the maritime continent to better understand the processes that control the diurnal cycle in the region and its sensitivity to different phases of the MJO. We present specific examples during YOTC over a region that focuses on New Guinea. We show that simulated variations in the diurnal cycle over land and water broadly reproduce the observed changes with MJO phase. Among other things, we demonstrate the critical role of offshore propagating gravity waves in the formation of early-morning offshore maxima in precipitation near New Guinea. We also show that although these waves are present in all phases of the MJO, their efficacy in modulating offshore convection varies with MJO phase.