The effect of La Réunion island (Indian ocean) on tropical cyclone Dina (2002): a comparative numerical study

In this study, the French non-hydrostatic mesoscale numerical model Méso-NH is used to simulate intense tropical cyclone Dina (2002) in the south west Indian Ocean to investigate the forcing caused by the steep orography of La Réunion island (55.5°E, 20.8°S). A bogus derived from Doppler radar data is used to build a realistic vortex which leads the model to reproduce quite well the observed track. Two simulations are conducted : the first one with La Réunion island, referred to as “Island”, and the second one without it, referred to as “Ocean”. Comparisons between the 2 cases provide information about the orographic influence on cyclone structure and evolution. The presence of the island is observed to stabilize the cyclonic circulation by damping the natural elliptical eyewall rotations and forcing the flow circulation. Initially, the cyclonic flow is blocked upwind of the orography which induces a convergence associated with upward vertical velocities and maximum horizontal winds along the upwind slopes of the island. An orographic wave, generated over the highest terrains, is associated with an intrusion of stratospheric air, observed as a tongue of high potential vorticity (PV) values. On the lee side of the island, weaker winds and downward vertical velocities are simulated. When the strongest winds reach the island, the flow changed from passing around to over it. Non-dimensional flow parameters in agreement with recent theories are calculated for Dina and give some clue to explain the simulated track.