The importance of atmosphere-ocean-wave coupling in ensemble regional convective-scale forecasts of midlatitude cyclones

<p>Convective-scale ensemble prediction systems (EPS) are critical tools to accurately forecast damaging surface winds in the short range,  capturing the local details of their variability and providing guidance on the associated forecast uncertainty. Due to computational cost, operational convective-scale EPS are atmosphere-only models, which represent ocean and wave effects through sea-state independent parametrizations, and therefore do not account for the impact of an evolving ocean and wave state during the forecast. Benefits of integrating atmosphere, ocean, and wave feedbacks into a single coupled multimodel system have been shown by global-scale deterministic systems and EPS, and convective-scale deterministic systems. These benefits lead to the question of what are the corresponding benefits of coupling in convective-scale EPS. To address this question, we present the first convective-scale regional ensemble coupled system focused on the UK domain and surrounding seas (termed RECS-UK). We demonstrate the robustness of the impact of atmosphere-ocean-wave coupling and stochastic perturbations to model physics parametrizations on forecasts of extratropical cyclone Ciara and quantify the importance of these coupling impacts relative to initial condition error. </p><p>Coupling to the ocean leads to localised reductions in the 10-m wind speeds due to cooling of sea surface temperatures, which increase the stability in the surface layer. However, these localised impacts on coupling to ocean are not apparent when comparing the ensemble strike probabilities of exceeding a storm wind threshold (set to 20 m s<sup>-1</sup><sup>)</sup> for the atmosphere-ocean-coupled and control (atmosphere-only) ensembles. In contrast, coupling additionally to waves leads to substantial reductions in wind strike probability and consistently reduces, by up to 1 m s<sup>-1</sup><sup>,</sup> the ensemble forecast median and mean of Ciara’s wind speeds at all simulation hours during which Ciara is in the model domain. Each atmosphere-ocean-wave coupled ensemble member simulates the dynamical response of wind speeds to the forced young ocean waves, with maximum reductions in high wind speed regions. The largest 10-m wind speed spread from stochastic and initial condition perturbations is found away from the strongest wind speed regions of Ciara, but the impact of coupling to waves is more enhanced in these strongest wind speed regions, and is also comparable in size there with the largest sensitivity to stochastic and initial condition perturbations. The implications of this work are that the impacts on 10-m wind speed of coupling convective-scale atmospheric models to ocean and wave models can be robust across an ensemble and be of comparable size to those of initial condition and stochastic physics perturbations. However, convection-permitting atmosphere-ocean-wave coupled EPS should be assessed in different meteorological conditions and further tested on longer timescales prior to operational implementation. </p>