by Sylvie Malardel

collaboration between RECYF   and the French Cyclone Research Cell (CRC)

published in the Météo-France 2018 Research Report (ISSN : 2116-4541)

A prototype for an Ensemble Prediction System (EPS) based on the oversea version of the limited-area French model AROME   has been developed at LACy (French Laboratory of Atmosphere and Cyclones) in 2018. Several cases which affected La Réunion and the French Caribbean islands in 2017 and 2018 have been selected for a first validation of this system.

The AROME  -oversea EPS is a direct adaptation of the AROME   ensemble system which run operationally over France. But unlike its France counterpart, which is initialised and coupled at the lateral boundaries with the French global model ARPEGE (Action de Recherche Petite Echelle Grande Echelle : Research Action Small Scale Large Scale), the initial condition and the lateral boundaries of AROME  -oversea come from the ECMWF  -IFS (European Centre for Medium-Range Weather Forecasts-Integrated Forecasting System). Another specificity of the oversea system is that it is coupled with a prognostic parametrisation of the Oceanic Mixed Layer (OML) which is initialised by MERCATOR analyses.

For this first prototype, 12 between the 50 members of the IFS-EPS are selected by a clustering method. The initial and lateral boundary conditions of the AROME  -EPS coming from the 12 selected members contain the large scale perturbation which largely controls the spread of the AROME   ensemble. The initial sea surface temperature and the initial vertical profile of the OML are also perturbed with a random large scale pattern and the Stochastically Perturbed Parametrization Tendency (SPPT) scheme is activated during the simulation.

The results for the tropical cyclone Fakir which impacted La Réunion at the end of April 2018 show several scenarios which could have alerted forecasters of the fast propagation velocity and deepening of the cyclone which had been underestimated by deterministic models and by the IFS-EPS (see figure).

Plume charts for the probability of Pmin (minimum pressure) for the EPS (ECMWF  ) and for the PE-AROME  -OI (AROME   Ensemble Prediction in Indian Ocean). Case of cyclone Fakir (S. Kébir)

This new EPS prototype has also been adapted to the Caribbean domain of AROME  -oversea. It is used for re-forecast of the 2017 hurricanes in the context of the TIREX project. It will also be used as a quasi-operational system during the field experiment campaign ReNovRisk between January and April 2019 in the SWIO (South West Indian Ocean).