WaveFlow KO Øyvind Breivik (MET Norway), Joanna Staneva (HZG), Jean Bidlot (ECMWF) and George Nurser (NOC) 2018-04-03

Project summary WaveFlow addresses the impact that unresolved wave-mean flow effects have on the circulation in the upper ocean. The project aims to introduce recent improvements to the physical parameterizations of wave physics in a state-of-the-art wave model used for operational wave forecasting. The wave fields and fluxes will be implemented in an openly available wave model code and tested with fields and fluxes from a decades-long wave hindcast of the soon to be completed ERA5 reanalysis. The tests will be carried out on all scales pertinent to CMEMS, ranging from vertical column one-dimensional setups to regional, high-resolution models and all the way to global ocean-only and fully coupled atmosphere-wave-ocean forecast systems. The project aims to explore the beneficial impact on the predictability of forecast systems and the potential improvement to the circulation of the upper ocean. This will be done in close collaboration with CMEMS MFCs. The project brings together a consortium with complementary competence on wave and ocean modelling and will lay the foundations for how wave and ocean models should interact.

Objectives The main objective is to introduce physically consistent surface-wave forcing in ocean-only and coupled atmosphere-wave-ocean models The following R&D objectives will help us achieve this: Assess the impact of wave effects on currents and hydrography in the upper ocean. Introduce new wave model physics in operational wave models. Assess the impact of wave-mean flow on coupled forecast systems.

Relevance to CMEMS The proposal is very relevant to CMEMS call SE 66 phase 2, Lot 1. Specifically, we address two important topics mentioned in the call: Improving the circulation in the upper ocean Increasing the predictability of the forecast systems WaveFlow aims to contribute to the development of a new generation of regional European wave-circulation operational models and to homogenize their integration into the operational systems of CMEMS MFCs by: New developments and improvements into the WAM physics that will directly contribute to improve estimation of the new wave products required for Phase 2, including spectral wave product validation. Improved wave-current routines in the NEMO model. Assessment of the relevance of wave physics parameterizations to the impact of representing ocean-wave feedback processes within CMEMS on global and regional scales. Scientific evaluation of the new implementations aiming to demonstrate the added value in improving the open ocean and coastal/regional seas’ forecasts together with skill assessment, relevant for delivery and integrations into the CMEMS Services. Propose new methodologies for validation of surface waves (incl. spectral data) and coastal ocean currents data sets (eg., Lagrangian drifters and HF radars) relevant for MFCs and different TACs.

Strategic relevance and timeliness WaveFlow will contribute to the CMEMS Service Evolution Strategy in the following ways: Improvement in wave forecasts and support the production of more consistent ocean-marine weather information, as often requested by users Responding on the increasing demand for model information with a more complete representation of dynamical processes of the turbulent ocean A consistent way of providing the surface fluxes and winds used to drive wave and ocean models For maritime and coastal applications, information on ocean surface parameters is of great importance and in great demand WaveFlow is timely because it will contribute to reducing the uncertainties and inconsistencies within the existing MFC systems in CMEMS. The proposal is also highly relevant because it will help the integration of model systems (e.g. coupling wave and circulation models) that is now planned in CMEMS Phase 2.

Methodology The methodology is structured as follows. NEMO-WAM regional domain The methodology is structured as follows. A new open-source wave model is developed which contains new physical parameterizations. Then, a wave hindcast is performed which yields wave forcing fields that can be used for testing the ocean model. The forced ocean model integrations can now be performed. This will be done on various domains and resolutions. A fully coupled system is now tested in order to evaluate the impact on the full atmosphere-wave-ocean system, with special attention to the predictability of upper-ocean currents. The quality of the wave hindcasts is assessed in terms of new spectral metrics. The ocean circulation will be evaluated with the full gamut of drifter and current observations available to the project partners. Integration with existing CMEMS MFCs will be facilitated through interaction with regional and the global MFCs once the results from the project start to emerge.

WP1: Implementation of new wave physics in an open-source WAM code Lead-HZG, MET Norway, ECMWF We will incorporate recent work at ECMWF where the calculation of momentum and energy fluxes are treated in a consistent manner to high frequencies. Also, the new physics package with improved numerical efficiency will be implemented and tested against the existing WAM physics. Subtask 1.1: Implementation of new wave model physics in WAM: The implementation of new physics (Ardhuin et al, 2010) in WAM and ECWAM will be carried out using efficient numerics. Subtask 1.2: Assessment of spectral performance and skill: High-resolution buoy data will be used to assess the impact of the new parameterizations on the different spectral bands from high-frequency wind waves (2-4 seconds) waves and down to long-periodic swell (longer than 14 seconds). Subtask 1.3 CMEMS implementation: Compare results from new WAM model with MFC products to identify a baseline of intercomparison between new products and existing CMEMS MFC products. Subtask 1.4: Open source wave model code: The model physics will be implemented in an open source version of WAM, maintained by HZG. The code will be provided to the regional MFCs. Subtask 1.5: Wave hindcast: Multiple hindcasts on the different physics packages.

WP2: Impact of new wave fields on regional, single column and global forced ocean models Lead-NOCS, MET Norway, HZG Wave forcing has recently been tested on regional versions of NEMO. This setup will be used as a test bed to assess the importance of the high-frequency contribution to wave stress, Stokes drift and TKE fluxes on water level, mixing and upwelling. The inclusion of the Stokes drift in the tracer advection equation will also be investigated. Additionally we will assess the sensitivity of the OSMOSIS OBL model to driving by the HF wave fields rather than to wave fields parameterised from the Pierson-Moskowitz spectrum. Subtask 2.1: Testing the impact of Stokes drift on tracer advection: Particle tracking with and without the Stokes drift and Stokes-Coriolis force will be performed both in the regional and global model. Subtask 2.2: Testing the single-column setup on Station Papa: A single-column NEMO model with the OSMOSIS-OBL model will be run for Station Papa with new wave forcing fields. The seasonal cycle of mixed-layer depth and temperature, and the near-surface velocity fields will be investigated. Subtask 2.3: The regional, high-resolution setup for the North Sea and the Baltic Sea: The high-resolution setup is important as this is the resolution at which operational CMEMS models are run. Subtask 2.4: Testing the global medium resolution model: A decadal integration of a global forced NEMO model with the OSMOSIS-OBL model will investigate the impact of the wave forcing fields (WP1) on global scale mixed-layers and upper ocean circulation. Subtask 2.5 CMEMS implementation: The simulations will be validated and compared with CMEMS MFC systems.

WP3: Ocean predictability - Impact of wave forcing on the coupled ocean-atmosphere system Lead-MET Norway, ECMWF The IFS run by ECMWF is a fully coupled medium-range forecast system also used for monthly predictions. The work will involve case studies of phenomena that are sensitive to upper ocean mixing and thermodynamic feedback, such as tropical cyclones. Subtask 3.1 Assessment of coupled model impact on analysis and forecasts: The impact of new wave model physics on the circulation and ocean mixing in coupled systems will be tested. Subtask 3.2: Relevance to the GLO-MFC and Regional MFCs: The new model physics will be assessed for use in the operational regional and global MFCs.

WP4: CMEMS MFCs/TACs applications, dissemination and management Lead-MET Norway Subtask 4.1 Applying and testing of new codes and parameterizations in MFC models: Validation with operational MFC ocean and wave forecast models. Subtask 4.2 CMEMS MFC and TAC integrations: Based on interaction with CMEMS MFCs, recommendations for its integration into the CMEMS Services will be given. Subtask 4.3 Dissemination: Presentations at conferences and publications in peer-reviewed journals. Subtask 4.4 Management.

Milestones Table 1. Milestones (numbered by work package) ID Description Month M1.1 ST4 and ECWAM physics implemented in WAM M6 M1.2 Global ERA5 wave forcing fields from multidecadal hindcast completed M12 M1.3 Spectral validation completed M16 M1.4 WAM open source model completed M22 M2.1 Completed single-column model runs for Ocean Station Papa M14 M2.2 Completed regional NEMO model runs for North Sea and Baltic Sea M18 M2.3 Completed global medium resolution NEMO model run M20 M3.1 Coupled ensemble runs completed M3.2 Case studies of impact on tropical cyclones completed M4.1 Completed assessment of new model physics (waves and ocean) in CMEMS MFCs M4.2 Documentation completed

Deliverables Table 2. Deliverables (numbered by work package) D1.1 Assessment of numerical efficiency of new wave model physics vs WAM physics. M09 D1.2 Assessment of impact of new model physics on the forecast skill of a global wave model. M14 D1.3 Assessment of wave model spectral performance. M18 D1.4 Global wave forcing fields from ERA5 wave hindcast (for the different physics packages) M12 D1.5 Open-source WAM model code available on Github repository. M22 D2.1 Updated NEMO model code D2.2 Global wave forcing data set based on ERA-5 atmospheric fields D2.3 Assessment of the impact of new wave forcing fields on global, regional and one-dimensional versions of NEMO M20 D2.4 Assessment of the importance of the Stokes drift on the tracer advection equation D3.1 Assessment of the impact of wave forcing on global, coupled atmosphere-wave-ocean forecasts D3.2 Case studies to investigate the importance of wave forcing on the mixing and circulation under tropical cyclones D4.1 Final Project Report M24 D4.2 Documentation and reports

The coupled ECMWF forecast model

Testing of new wave physics based on Ardhuin et al Testing of new wave physics based on Ardhuin et al. (2010) in standalone ECWAM the closest to 1 the better the lower the better Stand alone analysis + forecast runs forced by ECMWF winds Results are very encouraging. More testing is needed in the fully coupled system