C-Band SAR Winds for Tropical Cyclone Monitoring and Forecast in the South-West Indian Ocean

Abstract

Tropical cyclone (TC) monitoring and forecast in the South West Indian Ocean (SWIO) basin remain challenging, notably because of the lack of direct observations. During the 2018–2019 cyclone season, S-1 Sentinel SAR images were acquired, as part of the ReNovRisk-Cyclone research program, giving access to unprecedented detailed TC wind structure description without wind speed limitation. This paper assesses the quality of these data and the impact of their assimilation for TC forecasts. SAR observations are compared with analyses from a convection-permitting, limited area model AROME OI 3D-Var and with wind products used for operational TC monitoring. Their bias depends on the angle of incidence of the radar and the observation error is larger for extreme wind speed. The impact of SAR assimilation in AROME OI 3D-Var is assessed through two case studies. In the TC GELENA case, it leads to a better TC positioning and an improved representation of inner and outer vortex structures. The TC intensity reduction in the analysis propagates through subsequent analyses and it has an impact on forecasts for around 12 h. In the TC IDAI case, the 3D-Var does not manage to reproduce TC intensity captured by SAR. In both cases, the modification of the initial conditions has little influence on the intensification rate of the model forecasts. Sensitivity tests show that these results are robust to different observation errors and thinning.

Impact of Tropical Cyclones on Inhabited Areas of the SWIO Basin at Present and Future Horizons. Part 2: Modeling Component of the Research Program RENOVRISK-CYCLONE

“…on a large number of systems would enable one to propose an optimal configuration of the OWA platform for TC forecasting. As shown in Duong et al [91], SAR data obtained during RNR-CYC have started being implemented in the 3D-Var assimilation scheme of AROME in its research version. Since AROME analysis can be used as initial and lateral boundary conditions for Meso-NH, improved AROME analysis would also be a benefit to the OWA coupled simulations involving Meso-NH through a better position, intensity, and structure of the vortex in the initial state.…”

“…As shown in Bousquet et al [15], one important achievement of RNR-CYC lied in the collection of numerous space-borne synthetic aperture radar (SAR) observations within TC developing in the SWIO. As shown in the following, such data can be particularly useful to evaluate TC simulations made in the frame of the project but also to improve RSMC La Réunion real-time or post-event analyses of TC intensity and improve model forecast from assimilating SAR observations into high-resolution NWP systems [91]. During TC Idai, two exploitable SAR images were collected directly within the core of the storm, including one slightly before its landfall in Beira, on 14 March 2019 at 16:05 UTC.…”

Evaluation of a Mesoscale Coupled Ocean‐Atmosphere Configuration for Tropical Cyclone Forecasting in the South West Indian Ocean Basin

“…However, scatterometer wind measurements are limited to about 25 m/s. Recently, the experimental use in near‐real time of SAR wind data allowed a direct estimation of very strong TC winds (50–70 m/s) at a resolution of about 3 km (Duong et al., 2021).…”