Coupling waves and currents in POLCOMS with ... - Surfouest

instrument maintenance, process studies and CTD surveys. The Observatory has been ... feasibility study for the development of a joint surge and wave model.
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Coupling waves and currents in POLCOMS with measurements from the Coastal Observatory J.Wolf1

2 WAVE-CURRENT COUPLED MODEL

Abstract POLCOMS (Proudman Oceanographic Laboratory Coastal Ocean Modelling System) is being developed as a generic oceanographic modelling system. A wave module, based on a shallow water version of WAM developed in the PROMISE project, has now been coupled to this system and applied on a high-resolution model of the Irish Sea. Various interaction processes between the wave and current model are examined, including air-sea coupling, bottom friction and depth and current refraction. The one-way coupling of effects of the current model on the wave model has been implemented and the two-way coupling will be implemented next. The model system will be used in conjunction with the Pilot Coastal Observatory which is being developed by POL in Liverpool Bay. Various wave and current measurement systems are now being installed including ADCP, Waverider, HF and X-band radar.

1 INTRODUCTION The POLCOMS modelling system is described on the POL web-site, [1]. It includes a full 3D baroclinic hydrodynamic model and various ecosystem modules can be included. Tidal and atmospheric forcing are included. The need for coupled wave and current models was identified at POL in 1988 ([2]) and we have been working towards a coupled model system using the WAM wave model ([3]) and in-house hydrodynamic models. Current refraction was implemented in WAM [4]. We also identified a need for wave measurement systems. To this end we acquired an X-band radar 1989-1990. Now we also have an HF radar, a Triaxys directional wave buoy plus bottommounted S4DW, ADV and ADCP for measuring waves and currents, which are being deployed in Liverpool Bay as part of the Coastal Observatory. A coupled (1way) wave module has been implemented in POLCOMS. This system is being implemented for parallel computers. Here we briefly describe the progress of the wave-current model coupling and its implementation for Liverpool Bay and the Coastal Observatory. 1

Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside, CH43 7RA, U.K. e-mail: [email protected]

Various effects of the coupling of currents and waves can be identified. The present POLCOMS coupled model system addresses the one-way coupling of currents on waves, including: • Wave propagation i.e. refraction of waves by currents, Doppler shift of wave frequencies • Bottom friction – the wave friction factor is modified in the presence of currents Other effects which need to be investigated are: • Apparent wind – the effective wind-speed will be modified in the presence of a surface current. • Current shear – the existence of a near-surface current profile will affect waves of different wavelengths The effects of waves on the mean circulation will also be incorporated. These include: • Surface stress – change in drag coefficient due to the presence of waves, the goal of consistent stress between atmosphere, wave and current models is being pursued • Bottom friction – waves enhance the bottom drag coefficient • Radiation stress and Stokes’drift– these can contribute a component to the mean flow especially nearshore. The one-way coupled model has been implemented in the Irish Sea on a 1.85km grid [4].

3 COASTAL OBSERVATORY The POL pilot Coastal Observatory in Liverpool Bay [8] will consist of many in-situ and remote sensing systems as well as instrumented ferries, returning data in real-time.. Regular cruises are carrying out instrument maintenance, process studies and CTD surveys. The Observatory has been operational since late 2002 and is still under development. An X-band radar deployment [5] on Hilbre Island will soon be operational, measuring the local wave field and bathymetry in the vicinity of the sandbanks at the mouth of the Dee. An HF radar installation is being planned, measuring the wider area surface current and wave fields. Likely configurations for the HF radar installation are shown in Figure 2. The data and model results are being displayed on the POL Coastal Observatory web site, [7], which can be viewed for updated information.

4 PRELIMINARY RESULTS An example of coupled (one-way) model results is given in Figure 1 for the Irish Sea. Up to 10% increase in wave height is predicted due to the effects of currents... Some results from an application of POLCOMS plus SWAN on a 200m grid for Liverpool Bay are shown in Figure 3.

5 SUMMARY Several years’ work is now reaching fruition in a coupled model system as well as combined wave and current measurements as part of the Coastal Observatory. For continued development of coastal oceanographic modelling we need to compare models and observations in new and imaginative ways. A new initiative in coastal wave data assimilation will address these issues.

5 REFERENCES [1] http://www.pol.ac.uk/ [2]Wolf, J., Hubbert, K.P. and Flather, R.A. “A feasibility study for the development of a joint surge and wave model.” POL Report no. 1, 1988

[3]Komen, G.J., Cavaleri, L., Donelan, M., Hasselmann, K., Hasselmann, S. and Janssen, P.A.E.M. 1994 Dynamics and modelling of ocean waves. Cambridge University Press. 532pp. [4] Hubbert and Wolf, “Numerical investigation of depth and current refraction of waves.,” JGR, 96(C2), 2737-2748, 1991 [5] Monbaliu, J., Padilla-Hernandes, R., Hargreaves, J.C., Carretero Albiach, J.C., Luo, W., Sclavo, M. and Gunther, H. 2000 The spectral wave model, WAM, adapted for applications with high spatial resolution. Coastal Engineering, 41, 1-3, 41-62. [5] Wolf, J. and Bell, P.S. “Waves at Holderness from X-band radar.” Coastal Engineering, 43, 3-4, pp. 247-263, 2001 [6]Wolf, J., Wakelin, S.L. & Holt, J.T. “A coupled model of waves and currents in the Irish Sea.” Proceedings of the Twelfth (2002) International Offshore and Polar Engineering Conference, Kitakyushu, Japan, May 26–31, 2002. Vol. 3, pp.108-114, 2002 [7]http://cobs.pol.ac.uk/ [8]Proctor, R. & Howarth, M.J. 2003. The POL Coastal Observatory. Proceedings of the EuroGOOS conference, Athens, December 2002.

Figure 1: Coupled model results for the Irish Sea. Left: current vectors, centre: wave height, right: wave height differences (coupled minus uncoupled)

Figure 2: Planned implementation of HF radar in Liverpool Bay

Figure 3: Coupled wave and current model for Liverpool Bay. Left: current vectors and sea level, right wave height (including current effects)