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

Abstract

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⋅s −1 ) the median of the ensemble 95th percentile 10-m wind speeds from its value of about 24 m⋅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⋅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 andThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Added value of including waves into a coupled atmosphere–ocean model system within the North Sea area

“…In a next step, it has to be analyzed in more detail what the effects on the ocean and on the waves are. Also, as in the study of Gentile et al (2022) on a convective scale, ensembles can be used to estimate the uncertainty of the fully coupled system on the mesoscale, as it has already been shown that the coupling between CCLM and NEMO as well as CCLM and WAM reduce the internal model variability of CCLM (Ho-Hagemann et al, 2020;Wiese et al, 2020). The inclusion of the wave model into the system consisting of the atmospheric model and the ocean model especially reduces errors in the wind speed representation of the atmospheric model over the North Sea and, hence, also in the significant wave height of the wave model.…”

“…Also, they showed an impact of the inclusion of the wave model in the system already consisting of atmosphere and ocean on the wind speed, especially during extratropical cyclones. Furthermore, Gentile et al (2022) used a convective-scale ensemble prediction system with dynamical atmosphere-ocean-wave coupling to simulate storm Ciara in February 2020 and showed that the coupling has a consistent impact across the 18 ensemble members. They hinted that the impacts of the coupling are comparable in size to that of adding perturbations to the initial conditions and lateral boundary conditions, as well as stochastic physics perturbations to the uncoupled atmosphere-only ensemble simulation, which implies that the coupling of ocean and wave to the atmosphere is an important aspect of model uncertainty.…”

June 2023 marine heatwave over the Northwest European shelf: origins, weather feedback and future recurrence

“…ATMostia_ens and ATMclim_ens (Fig. 5, S13): Same set-up as ATMostia but run as an 18-member ensemble forecast started on June 19 th for a 5-day forecast 35 . An ensemble is best to assess impact of SST on hourly precipitation or cloud features, as changes in a single hindcast member for such a short window would be affected by internal variability.…”

Extreme Mediterranean cyclones and associated variables in an atmosphere-only vs an ocean-coupled regional model

“…However, RCMs often use coarse resolution SST from global model simulations or reanalysis datasets as lower boundary conditions. Integrating regional atmosphere and ocean model components into a coupled system is being increasingly challenged by research groups and operational centres (Wahle et al 2017;Ricchi et al 2017;Lewis et al 2018;Varlas et al 2018;Gentile et al 2022). Directly simulating the effect of the dynamical ocean state on atmospheric surface processes is expected to better simulate surface uxes, leading to improved representation of weather systems characterised by strong nearsurface wind speeds, such as in extratropical cyclones.…”