Convection-Permitting Regional Climate Modelling of the Maritime Continent and the Impact of Urban Areas

The Maritime Continent has consistently been identified as a challenging region in terms of simulating precipitation. Its intricate configuration of islands with complex topography surrounded by a warm and shallow ocean makes this tropical archipelago an unequaled setting to test potential benefits of very high-resolution models. In this study, we focus on the western Maritime Continent to determine features of precipitation that are better represented with increased horizontal resolution, as well as those characteristics that deteriorate at finer grid spacing.

We investigated the impact of spatial resolution on rainfall amounts and diurnal cycle using the Weather Research and Forecasting model. A set of simulations running over a 5-year period at resolutions of 50, 10 and 2 km were completed and compared against satellite-derived observational products. While annual biases over the mountains are larger as resolution increases, our results suggest that precipitation is physically more realistic in the convection-permitting experiment performed at 2-km grid spacing. For example, the shape and phase of the diurnal cycle in the region, which are traditionally misrepresented in models, are substantially improved at higher resolution when comparing with the observational datasets. The amplitude of the diurnal cycle is also improved over most areas, although deficiencies still exist in that the strength of the cycle is overestimated.

The influence of cities on precipitation characteristics is quantified for two major urban nuclei in the region (Jakarta and Kuala Lumpur) and results indicate that their presence locally enhances precipitation by over 30 %. This increase is mainly due to an intensification of the diurnal cycle. We analyse the impact on temperature, humidity and wind to put forward physical mechanisms that explain such changes. Cities increase near surface temperature, generating instability. They also make land-sea temperature contrasts stronger, which enhances sea breeze circulations. Together, they increase near-surface moisture flux convergence and favour convective processes leading to an over-all increase of precipitation over urban areas. The diurnal cycle of these effects is reflected in the atmospheric foot-print of cities on variables such as humidity and cloud mixing ratio and accompanies changes in precipitation.