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L'article du mois (septembre 2016)

Regionalizing Rainfall at Very High Resolution over La Réunion Island: A Case Study for Tropical Cyclone Ando

par Benjamin Pohl, Béatrice Morel, Christelle Barthe et Olivier Bousquet









Fig.1 TC Ando. Maximum velocity from IBTrACS. (b) Ando at 1253 UTC 7 Jan 2001, by the NOAA-14 satellite.

Ensemble simulations of Tropical Cyclone (TC) Ando (31 December 2000–9 January 2001) are performed
over the southwest Indian Ocean using the nonhydrostatic WRF Model. Nested domains centered over the
island of La Réunion allow for the simulation of local rainfall amounts associated with TC Ando at very high
resolution (680-m grid spacing).

The model is forced by and nudged toward ERA-Interim during the first
(1–6) day(s) of the TC’s life cycle. The nudging ends at various dates to constrain either the whole life cycle or
only parts of it. As expected, results show weakened member dispersion, as the relaxation lasts longer, with more members
producing similar cyclone tracks and intensities. The model shows reasonable skill to simulate local rainfall
amounts and distribution, as soon as the simulated TC approaches La Réunion with a realistic distance and
azimuth. Strong lower-level wind associated with the TC is forced to ascend over the slopes of the island.

The model is able to successfully simulate the extreme daily precipitation amounts (.1200 mm) and their distribution over the highest parts of La Réunion. Smaller-scale features of the rainfall field are nevertheless less
realistic in the simulations. The wind speed and direction upstream of the island are the main drivers of such
local uncertainties and errors, and they appear as an important issue to assess the local impacts of the TC over
such a complex terrain.

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