Predicting the development of the Hector convective system with WRF and UM models

Strong convective systems (generally known as Hector thunderstorms) develop frequently, usually in the early afternoon, over the Tiwi Islands (Northern Australia) during the pre-monsoon season (Nov.-Dec.). The storms generated over the islands can reach heights up to 20 km. Hence, Hector events (and similarly high storms which often develop over North Australia's coastal areas in the Maritime Continent region) play an important role in vertical transport of mass and pollutants into the tropical tropopause layer and possibly the lowermost stratosphere. The understanding of the onset and development of such events has usually relied on observation data. Indeed, the main processes involved occur at scales that are not typically resolved within mesoscale models although few attempts have been made to use high-resolution models to study different aspects of the Hector storms. In this study, we focus on the application of WRF and UM (the UK Met Office's mesoscale model) in predicting the development of the Hector convective system at horizontal resolutions down to 1.6 km with WRF and 4 km with UM. Numerical simulations have been performed for a demonstrative Hector episode on November 30, 2005. This episode was well documented as part of the ACTIVE and SCOUT-O3 Tropical field campaigns (based in Darwin, Australia). Using a horizontal resolution of 5 km with WRF and 4 km with UM, the results of both simulations indicate that convection needs to be parameterized even at these resolutions in order to capture the intensity variation with time of the convective system. However, at finer resolution the Hector event was reproduced successfully by WRF (using a 1.6-km horizontal resolution) with an explicit representation of convection. The system was found to develop over the islands where land-sea breeze flows converge from the coastlines. The paper also presents a detailed comparison of the two models at different scales and discusses the requirements in terms of parameterization setup.