A numerical study of orographic forcing on TC Dina (2002) in South West Indian Ocean

Abstract. Using the French non-hydrostatic mesoscale numerical model Méso-NH, intense tropical cyclone (TC) Dina (2002) is simulated to investigate the forcing caused by the steep orography of Réunion island (20.8° S, 55.5° E) in the southwest Indian Ocean. The model initialised by a bogus vortex derived from Doppler radar observations reproduces quite well the dynamical characteristics of TC Dina approaching the island and provides some clues on the orographic influence on the structure and the evolution of the TC.

The presence of the island is observed to stabilise the cyclonic circulation by damping the natural elliptical eyewall rotation and forcing the flow circulation. Initially, the cyclonic flow is blocked upwind of the orography which induces a convergence associated with upward vertical velocities, intense precipitation and maximum horizontal winds along the upwind slopes of the island. A mountain wave, generated over the highest terrains, is associated with downward motions on the lee side. When the strongest winds reach the island, the flow changes its behaviour from passing around to over the island. Non-dimensional flow parameters in agreement with recent theories are calculated to explain TC track.