Observational and numerical studies of gravity waves during tropical cyclone Ivan (2008)

Knowledge of multiscale wave processes involved in tropical cyclone (TC) dynamics enables to improve TC numerical modeling and forecasting as well as the understanding of their effects on the environment. Recently, Chane Ming et al. (2010) revealed a relationship between the intensity of two intense TCs and observed GW energy density in the upper troposphere (UT) and the lower stratosphere (LS). A 10-year climatological study in the SWIO confirmed GW energy density observed in the lower stratosphere as an index of diagnosis of TC activity (Ibrahim et al., 2010). In this study, both observation and simulation are used to investigate convective gravity-waves (GW) induced by intense TC Ivan (2008). Environmental conditions favored the birth of the tropical storm Ivan on 14 February at north of St Brandon in the tropical South-West Indian Ocean (SWIO ; 30-90°E ; 0-40°S). Classified as a TC one day later, it fastly became intense (935 hPa, maximum winds of 95 knots) and landfalled on Madagascar on early 17 February. TC Ivan dissipated when crossing over Madagascar from 17 February to 19 February and moving linearly south over the ocean from 22 February. Characteristics of the GWs observed in the UT/LS during TC Ivan were first analyzed using daily GPS winsonde data at the meteorological stations of la Réunion (21°S ; 55°E) and Madagascar (19°S ; 47°S) completed with GPS RO COSMIC dataset over the SWIO. GWs were observed with vertical wavelengths varies between 750 and 3 km, with periods of some hours and less than one day and with horizontal wavelengths of 41-570 km. GWs with horizontal wavelengths of 400-700 km were identified in ECMWF from 16 February and 17 February in the LS. TC Ivan was secondly numerically simulated with the mesoscale non-hydrostatic French model with a 4-km horizontal resolution and 55 vertical levels up to the LS at 26 km heights from 14 February at 00h UTC to 18 February at 18h UTC. Characteristics of simulated GWs are discussed in comparison with observations.