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Discussion papers
https://doi.org/10.5194/os-2018-162
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2018-162
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Jan 2019

Research article | 07 Jan 2019

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Ocean Science (OS) and is expected to appear here in due course.

Evaluating the impact of atmospheric forcing resolution and air-sea coupling on near-coastal regional ocean prediction

Huw W. Lewis1, John Siddorn1, Juan Manuel Castillo Sanchez1, Jon Petch1, John M. Edwards1, and Tim Smyth2 Huw W. Lewis et al.
  • 1Met Office, Exeter, EX1 3PB, UK
  • 2Plymouth Marine Laboratory, Plymouth, PL1 3DH, UK

Abstract. Atmospheric forcing applied as ocean model boundary conditions can have a critical impact on the quality of ocean forecasts. This paper assesses the sensitivity of an eddy-resolving (1.5 km resolution) regional ocean model of the North-West European shelf (NWS) to atmospheric forcing resolution and air-sea coupling. The analysis is focused on a month-long simulation experiment for July 2014 and evaluation of simulated sea surface temperature (SST) in a shallow near-coastal region to the south-west of the UK (Celtic Sea and western English Channel). Observations above and below the sea surface at the L4 ocean buoy from the Western Channel Observatory are used to evaluate ocean and atmosphere model data.

The impacts of differences in the atmospheric forcing are illustrated by comparing results from an ocean model run in forcing mode using operational global-scale numerical weather prediction (NWP) data with a run forced by a convective scale regional atmosphere model. The value of dynamically representing feedbacks between the atmosphere and ocean state is assessed through use of these model components within a fully coupled ocean-wave-atmosphere system.

Simulated SST show considerable sensitivity to atmospheric forcing and to the impact of model coupling in near-coastal areas. A warm ocean bias relative to in-situ observations in the simulation forced by global-scale NWP (0.7 K mean difference, warmer relative to all observations in the model domain) is shown to be reduced (to 0.4 K) through use of the 1.5 km resolution atmosphere forcing. When simulated in coupled mode, this bias is further reduced by 0.2 K.

Results demonstrate much greater variability of both surface energy balance terms and near-surface winds in the higher resolution atmosphere model data, as might be expected. Assessment of the surface energy balance and wind forcing over the ocean is challenging due to a scarcity of observations. It can however be demonstrated that the wind speed over the ocean simulated by the high resolution atmosphere agreed with the limited number of observations less well than the global-scale NWP data. Further partially-coupled experiments are discussed to better understand why the degraded wind forcing does not detrimentally impact on SST results.

Huw W. Lewis et al.
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Huw W. Lewis et al.
Huw W. Lewis et al.
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Short summary
Oceans are modified at the surface by winds and by exchange of heat with the atmosphere. The effect of changing information about the atmosphere that is available to drive an ocean model of north-west Europe, which can simulate small-scale details of the ocean state, is tested. We show that simulated temperatures agree better with observations located near the coast around south-west UK when using data from a high resolution atmosphere model, and when atmosphere and ocean feedbacks are included.
Oceans are modified at the surface by winds and by exchange of heat with the atmosphere. The...
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