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Ann. Geophys., 29, 53-69, 2011
https://doi.org/10.5194/angeo-29-53-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
 
06 Jan 2011
Numerical simulations of local circulations and pollution transport over Reunion Island
D. Lesouëf1, F. Gheusi2, R. Delmas1, and J. Escobar2 1Laboratoire de l'Atmosphère et des Cyclones, Université de La Reunion, CNRS/INSU UMR8105, La Réunion, France
2Laboratoire d'Aérologie, Université Toulouse III, CNRS/INSU UMR5560, Toulouse, France
Abstract. A series of high-resolution (1 km) numerical simulations with a limited-area numerical model has been performed over Reunion Island. In the dynamical context of a regular maritime flow perturbed by a major topographic obstacle such as Reunion Island, the objectives are to identify the main atmospheric circulations at local-scale over the island and to improve the understanding of local-scale transport and dispersion of pollutants emitted from local sources. To investigate the effects of topography and land surface heating on low-level flows over the island, simulations representative of austral winter were performed in idealized conditions keeping the radiative forcing plus a background east-south-easterly synoptic flux of varying strengths, typical of the prevailing trade-wind conditions.

The numerical experiments show mainly that flow splitting of the trade-wind occurs around the island, with enhanced winds blowing along the coast lines parallel to the synoptic flux, due to the lateral constriction of the flow by the island and resulting Venturi effect. Blocking occurs on the island side facing the trade-wind. The north-western area on the leeside is screened from the trade-wind by high mountains, and this enables the development of diurnal thermally-induced circulations, combining downslope and land-breeze at night, and upslope and sea breeze at daytime. Flow splitting is modulated by radiative convergence toward the island at daytime, and divergence from the island at night. Stronger winds than the large-scale trade-wind occur along the coast at daytime (Venturi effect), whereas at night, the trade-wind flow is pushed few kilometres offshore by divergence of cooled air from the land.

At night, the trade-wind flow is pushed few kilometres offshore by divergence of cooled air from the land.

Consequently, a number of processes of pollution transport and dispersion have been identified. Vortices in the wake of the island were found to cause counterflow circulation and trapping of polluted air masses near the north-western coast. These air masses may in turn be sucked by anabatic wind systems during daytime (upslope and sea breezes) in the cirques and up to the summits of the island, and especially to Piton Maïdo (2200 m) where a new observatory of the Indian Ocean background atmosphere is being built. A "cap effect" above the mountains downstream from the volcano (to the south-east of the island), and especially above Piton Maïdo, might occur in case of development of inland and upslope breezes on the west coast. In this case, air pumped from lower layers may protect the observatory from the volcanic plume forced to pass over a "cap" of low-level air clean of volcanic emissions.


Citation: Lesouëf, D., Gheusi, F., Delmas, R., and Escobar, J.: Numerical simulations of local circulations and pollution transport over Reunion Island, Ann. Geophys., 29, 53-69, https://doi.org/10.5194/angeo-29-53-2011, 2011.
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