Numerical simulation of tropical convective systems during TOGA COARE IOP using ARPS mesoscale model

A three-dimensional nonhydrostatic mesoscale model, Advanced Regional Prediction System (ARPS), is used to investigate the structure and development of tropical convective systems in the different large scale environment during TOGA COARE IOP. Tropical convective system is simulated using TOGA COARE objective analysis which has 1 degree with latitude and longitude spatial resolution as initial and boundary conditions. The simulations are conducted for the periods of 14 November-21 November (convectively inactive) and 18 December-29 December (convectively active) with horizontal grid resolution 9 km and 36 km, respectively. Compared with objective analysis and satellite observation, coarse grid resolution simulations produce similar variability in wind, temperature, specific humidity and rain-rate. The simulation results is characterized with westerly wind burst, increase of moisture in the middle troposphere and development of nocturnal deep convection over IFA during convectively active period, while there exists dry air in the middle troposphere and afternoon convection over ocean during inactive period. The model tends to overpredict in the rain-rate and moisture. The difference of temperature and humidity between model simulations and observations is maximized in the surface layer. This is caused from surface flux algorithm in the ARPS model that is not tuned surface flux over tropical ocean. Convective system simulated with fine grid resolution during 24 December (convectively active period) resembles the squall line observed during GATE. The convective system show general characteristics of leading convective band, tilted upward motion and stratiform cloud rearward of squall line. Convective system during 14 November (convectively inactive period) develops as a single cell near surface layer in the afternoon and development is restricted vertically. Existence of negative specific humidity perturbation indicates the suppression mechanism of convection during convectively inactive period. The dry air in the middle troposphere which is intruded from subtropical region affects the development of cloud. Difference of maximum time of tropical convection according to convective period indicates that diurnal variation of tropical convection is influenced by large-scale circulation.