Numerical simulations of local circulations and pollution transport over Reunion Island
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 atmosp...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2011-01-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/29/53/2011/angeo-29-53-2011.pdf |
| Summary: | 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.
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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.
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At night, the trade-wind flow is pushed few kilometres offshore by
divergence of cooled air from the land.
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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. |
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| ISSN: | 0992-7689 1432-0576 |