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

Abstract

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. Nevertheless, smaller-scale features of the rainfall field are 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.

Landslide Processes Involved in Volcano Dismantling From Past to Present: The Remarkable Open‐Air Laboratory of the Cirque de Salazie (Reunion Island)

“…The island is regularly hit by tropical cyclones (TCs), which develop over the southwest Indian Ocean (approximately 10 TCs form in this region every year, cf., http://www.meteofrance.re/cyclone/saisons-passees). The strong orographic effect of the island influences the intensity and track of cyclones, which are apt to trigger heavy rainfall (Barbary et al., 2019; Pohl et al., 2016). As a result, Reunion Island holds world records for amounts of intense rainfall (e.g., 1,825 mm in 24 hr during TC Denise in 1966, 5,678 mm in 10 days and 6,083 mm in 15 days during TC Hyacinthe in 1980) (Barcelo et al., 1997; Chaggar, 1984; Quetelard et al., 2009).…”

Projected Characteristic Changes of a Typical Tropical Cyclone under Climate Change in the South West Indian Ocean

“…Due to the small size of La Réunion in the basin, tropical cyclones rarely make landfall, yet the island can still be impacted by passing cyclones due to far reaching spiral rainbands [42]. Combined with the island's volcanic interior and the associated orographic enhancement, rainfall presents a substantial threat to La Réunion and accounts for most TC-related damages and fatalities [7].…”

“…Additionally, rainfall over the island depends strongly on cyclone size, intensity, and the distance of the cyclone from the coast (i.e., trajectory), making it difficult to attribute changes in island rainfall rates to different future environments. Indeed, a grid spacing of less than 1 km may be necessary to accurately simulate the intensity and spatial variability of orographic precipitation over La Réunion [42]. Although orographic rainfall studies for future climates indicate an increase in precipitation with height [50], an increase in intensity [51], or a shift in the precipitating area towards the leeward side [52], high-resolution microscale modelling is necessary to estimate future changes unique to La Réunion.…”

Application of Contiguous Rain Area (CRA) Methods to Tropical Cyclone Rainfall Forecast Verification

“…Two of the greatest impacts of TCs are the flooding and landslides associated with the heavy rainfall. Even though some improvement on rainfall forecast over land and complex terrain has been made with high‐resolution cloud‐resolving models, accurate predication of rain amount in TCs is still a challenging task (Marchok et al, ; Pohl et al, ; Wang, ). Our CRA results indicate that ACCESS‐TC overestimates the rainfall in the 24‐hr period.…”