The sensitivity of probabilistic convective-scale forecasts of an extratropical cyclone to atmosphere-ocean-wave coupling

The benefits of dynamical atmosphere-ocean-wave coupling in probabilistic weather forecasts generated using convective-scale ensemble prediction systems are to date unknown. We investigate the respective impacts of atmosphere-ocean-wave coupling, and initial condition (IC), lateral boundary condition (LBC), and stochastic physics perturbations within a convective-scale ensemble coupled system for an extratropical cyclone case study. Towards this aim, we developed the first 18-member, 2.2 km grid spacing ensemble regional coupled system (Ensemble-RCS) with domain covering the British Isles and surrounding seas. Ensemble-RCS coupled and uncoupled simulations of cyclone Ciara (February 2020) were performed. Adding stochastic perturbations to the model physics parametrizations enhances the ensemble spread of the uncoupled atmosphere-only ensemble driven by IC and LBC perturbations, while slightly reducing (by up to 0.5 m center dot s-1) the median of the ensemble 95th percentile 10-m wind speeds from its value of about 24 m center dot s-1 at peak time. A substantial proportion of this impact is attributable to Charnock parameter perturbations alone. By coupling the atmosphere-only ensemble, with stochastic physics, to the ocean, the ensemble median and spread is mainly unaffected. However, additional coupling to waves reduces the median wind speed by 1 m center dot s-1, which leads to reductions of up to 70% in strong wind strike probability, and halving of the spatial coverage of high values (>50%) of this probability. Finally, we demonstrate the usefulness of two metrics originally developed for precipitation verification - the neighbourhood-based Fraction Skill Score (FSS) and the object-based Structure, Amplitude, Location (SAL) - for examining the spread in convective-scale ensemble forecasts. It is concluded that coupling has a consistent impact across the ensemble members. Remarkably, the impact of coupling to waves is found to be comparable in size to that of adding IC, LBC and stochastic physics perturbations to the uncoupled atmosphere-only ensemble simulation, implying that the dynamical coupling to ocean and sea-state are important aspects of model uncertainty.