Understanding the Role of Urbanization on the Diurnal Cycle of Precipitation in a Tropical City Using an Ensemble Approach

Urban areas have an impact on precipitation. Although past studies have demonstrated such an impact in different cities, the mechanism driving it is still not well understood. Questions remain regarding the urban effect on the temporal and spatial distribution of precipitation and if the effect is to enhance or suppress precipitation. Most observational and modelling studies suggest an urban driven enhancement of precipitation over and downwind of cities, especially under convective conditions. These studies suggest larger rainfall totals, potentially increasing the frequency of heavy rainfall events. However, the opposite effect has also been reported by some authors. Singapore, which has been subject to a rapid urbanization over the last few decades, has seen an increase in heavy rainfall events during the last 20 years. The question of a possible relationship between urbanization and rainfall events remains open and requires further investigation. The present study uses an ensemble approach to model the impact of the city on the diurnal cycle of precipitation and to investigate the driving mechanisms. A high resolution tropical version of the Met Office Unified Model is used to simulate five Novembers from 2010 to 2014 from which the ensemble mean diurnal cycle is analysed. Preliminary results show a displacement of the location where wind convergence and precipitation take place when urbanization is considered. An increase by 25% of the total daily rainfall over urban areas is also found and up to 40% when the anthropogenic heat flux is included. Further analysis will explore the role of urbanization on the mechanisms driving such modifications.