On the consistency of the drag between air and water in meteorological, hydrodynamic and wave models

Abstract

For the design, assessment and flood control of water defences, hydraulic loads in terms of water levels and wave conditions are required and often obtained from numerical models. For these hydraulic loads to be reliable, accurate atmospheric forcing is required. Waves and surges are typically forced by surface stress. However, in most cases, the input for these models consists of 10-m wind velocities that are internally converted to surface stress by applying a particular drag relation. This procedure generally leads to inconsistencies, since the hydrodynamic, wave and atmospheric models often apply different drag relations. By means of a case study, we explored the consequences of this inconsistency in the drag formulation for a North Sea storm wave and surge hindcast. This was done by forcing the hydrodynamic and wave models using both the 10-m wind velocity and the surface stress fields computed by the atmospheric model. Our study results show significant differences between the wave parameter values and water levels computed with surface stress input and 10-m wind velocity input. Our goal is not to assess different drag parameterizations but to raise awareness for this issue and to plea for the use of a consistent drag relation in meteorological and hydrodynamic/wave models. The consistent use of one drag formulation facilitates the identification of problems and the eventual improvement of the drag formulation. Furthermore, we suggest using the socalled pseudo-wind, which is a translation of the surface stress to the 10-m wind speed using a reference drag relation.

The Impact of a Reduced High‐Wind Charnock Parameter on Wave Growth With Application to the North Sea, the Norwegian Sea, and the Arctic Ocean

“…It is quite reasonable to think that too strong wave growth has compensated for weak winds in coarser atmospheric models. The CTRL experiment (Section 4, see also Figure A2) shows that the impact of the (nearly unbiased) winds in NORA3 is significant as H s exceeds 10 m and starts to become really excessive near 15 m. It is also clear that the response of an uncoupled system such as NORA3 will be different from that of coupled atmosphere‐wave models (ECMWF, 2020; Li et al., 2021) where the wind field will adjust to the sea surface roughness and where the Charnock parameter is exchanged, yielding identical roughness for the atmosphere and the wave model (van Nieuwkoop et al., 2015).…”

An appraisal of the value of simulated weather data for quantifying coastal flood hazard in the Netherlands

“…Furthermore, model-simulated stress can be used directly for forcing of hydrodynamic models, avoiding the precarious step of deriving stress from wind (e.g. Van Nieuwkoop et al, 2015).…”

Recurrence intervals for the closure of the Dutch Maeslant surge barrier

“…It is likely that this underestimation is caused by the fact that WAQUA/DCSMv5 uses a fixed Charnock parameter, whereas the ECMWF 5 uses a time-varying Charnock parameter. For high winds the ECMWF Charnock parameter exceeds the value of 0.032 used by WAQUA/DCSMv5, which leads to underestimation of high surges(Zweers et al, 2010;Van Nieuwkoop et al, 2015).7Ocean Sci.Discuss., doi:10.5194/os-2017Discuss., doi:10.5194/os- -5, 2017 Manuscript under review for journal Ocean Sci. Discussion started: 24 February 2017 c Author(s) 2017.…”