Simulation of Tropical Cyclone Activities Over the Southwestern Indian Ocean

Abstract

The current study assessed the ability of the WRF-ARW model to forecast tropical cyclones over the Southwest Indian Ocean. The characteristics of tropical cyclones Jobo (2021), Kenneth (2019), and Idai (2019) were simulated using WRF-ARW numerical model, and 6 hourly near real-time Global Forecast System (GFS) data were used as initial and boundary conditions. Kain-Fritsch and newly modified Tiedtke schemes were used to investigate the impacts of cumulus parameterizations on the forecasted tropical cyclone's characteristics. The microphysics sensitivity experiments used the complex and simple WRF-ARW single-moment microphysics schemes (WRF Single Moment class 6 and WRF Single Moment 3). WRF model forecasted well the movement of the tropical cyclones to the landfall location, the tracks were closer to the observations with a lead time of up to 4 days. The average Direct Positional Error (DPE) was found to be 58 ± 6, 86 ±17.2, 146±43.7 and 240 ±145 km at 24, 48, 72, and 96 hours lead forecasts time, respectively. The values of Along Track Error (ATE) and Cross Track Error (CTE) showed that the model had a southward and a slow bias near the land. WRF forecasted central pressure (maximum wind speed) was higher (lower) than the observations. 24 hourly accumulated precipitations from the model were compared against observations on the landfall day. The result showed that the cumulus convection and microphysics of the model affect the simulated tracks, intensity, and precipitation of tropical cyclones. Kain Fritsch schemes provided the central pressure and wind evolution tendencies closer to the observations than the Modified Tiedtke scheme. The combination of the WSM6 and KF schemes gave the best estimates of the intensities of the cyclones closer to the observations compared to the MT scheme. The Tiedtke scheme produced good forecasts of the tracks of Tropical Cyclone Idai. The cyclones modelled using KF schemes were slower than in the observation causing the forecast to lag the observed location. The study showed that the WRF model had a large slowdown bias closer to the land, and the intensity during intensification and decay peaks were poorly simulated this requires further investigations. Overall, the study concludes that the WRF-ARW model simulated well the trajectory of tropical cyclones over the Southwest Indian Ocean for up to 4 days. The characteristics of tropical cyclones forecasted with the WRF-ARW model were dependent on the microphysics and cumulus convection parameterizations. Studies on other characteristics of tropical cyclones should be carried out to find the best parameterizations to be used in operational forecasting.