The Impact of Maritime Continent Islands on Mesoscale Convective Systems during PISTON

Convectively active regimes of the boreal summer intraseasonal oscillation (BSISO) are composed of individual organized deep convective storms. The initiation, lifecycle, and decay of these mesoscale convective systems feedback to the broader intraseasonal oscillation, influencing the propagation of the convective envelope and its intensity. The Maritime Continent archipelago, located between the tropical Indian and western Pacific Oceans, disrupts the propagation of the BSISO. It is hypothesized that this interruption is, in part, due to the impact of steep island orography and numerous coastlines on the development and evolution of individual deep convective elements within the larger convective envelope. The archipelago mountains can act as a barrier to propagating mature convective storms, and also can promote the initiation of convection through localized heating and convergence. Coastlines of the archipelago provide additional complexity. Convection may initiate along convergence boundaries that develop in association with coastal circulations (i.e., sea breezes). Furthermore, the lifecycle of mature convection may be modified by thermodynamic and kinematic coastal gradients, suppressing or enhancing convective intensity.

This research is part of the collaborative Office of Naval Research (ONR) Direct Research Initiative (DRI) Propagation of Intra-Seasonal Tropical Oscillations (PISTON). A goal of PISTON is to better understand the multiscale, air-sea, and land-atmosphere interaction processes that regulate BSISO propagation and intensity, toward improvements in the spatial and temporal prediction of the intraseasonal oscillation. This study investigates the impact of the Maritime Continent archipelago terrain and land-sea coastal gradients on the evolution of mesoscale convective systems within the active phase of the BSISO.

Preliminary results from Advanced Research Weather Research and Forecasting Model (WRF-ARW) simulations of organized convective storm case studies from the PISTON pre-field campaign (late summer 2017) will be presented. Simulations use a 36-, 3-, 1-km triply-nested horizontal grid with initial and boundary conditions from the National Center for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) available at 6-hr temporal resolution and 0.5^ox0.5^o spatial resolution. Storm-scale physical processes contributing to the development of organized deep convection, as well as successful and unsuccessful terrain and coastline crossings will be presented in the context of the ambient environment, including observations from the pre-field campaign.