Numerical simulation of the diurnal evolution of tropical island convection over the maritime continent

Abstract

Numerical simulations of the diurnal evolution of tropical island convection observed during the Maritime Continent Thunderstorm Experiment (MCTEX) are performed using the Meteorological Research Institute nonhydrostatic model (MRI NHM). The MRI NHM is double-nested within a form of the Australian Bureau of Meteorology Research Centre's Limited-Area Assimilation and Prediction System specially operated for the MCTEX period. Excellent agreement is found between the simulation and observed evolution of the convective clouds over the Tiwi Islands on 27 November 1995. A transition from horizontal convection occurring during the morning to vertical convection in afternoon is evident. In the morning, the sea breeze appears along the coastlines, with a clear contrast evident in structure between the windward and leeward sides. At the windward coast, the sea breeze intrudes inland more rapidly, where the larger surface heat flux modifies the lowest air mass and makes the sea breeze front (SBF) indistinct. On the other hand, at the leeward coast, the upward motion at the head of the SBF is larger and deeper. Shallow convective clouds therefore have a preference for alignment along the leeward SBF. Over the interior of the islands ahead of SBFs, shallow convective clouds corresponding to the Rayleigh-Benard convection occur at corners of open polygonal shaped cells and seem randomly distributed. Within the SBFs, organization of convection characteristic of horizontal convective rolls (HCRs) is evident. These HCRs are preferred at the windward coast and occur within cloud-free regions. Clouds associated with the SBFs appear to develop preferentially at the cross points of the SBFs and HCRs where the surface convergence is enhanced. Following further inland propagation of SBFs, weak precipitation starts and the Rayleigh-Benard convection is disturbed by resulting outflows. At the merging stage, the clouds organize at the leeward central part of the islands in the form of an east-west line. In this convergence zone between the two SBFs. explosive growth of convection occurs and cloud top reaches the tropopause. In the case simulated here, the associated downdrafts are not strong compared with the upward motion due to a lack of the midlevel dry air necessary to enhance evaporative cooling. The inclusion of ice phase physics in the simulation produces little qualitative difference in storm development and the associated surface rainfall distribution, but yields stronger updrafts and higher cloud-top heights. The vertical profile of the apparent heat source (Q1) in the ice phase experiment shows double peaks corresponding to the condensation and freezing levels. Sensitivity experiments show that the orographic undulations as well as the horizontal scale of the island are important factors determining the timing of cloud merger and convective intensity. Without hills, the transition to the explosive growth in the merger stage is delayed. This results in weaker rainfall, even if the hills are relatively flat. A smaller island produced weaker convection, which means that the total rain produced by each island is not proportional to island area. These results suggest that the intensity of tropical island convection is determined not only by the convective stability of the environmental atmosphere but is influenced significantly by the island-scale circulations, that is, horizontal convection in the morning that ultimately forces the deep convection during the afternoon.