Observation-Based Source Terms in the Third-Generation Wave Model WAVEWATCH III: Updates and Verification

Abstract

The observation-based source terms available in the third-generation wave model WAVEWATCH III (i.e., the ST6 package for parameterizations of wind input, wave breaking, and swell dissipation terms) are recalibrated and verified against a series of academic and realistic simulations, including the fetch/duration-limited test, a Lake Michigan hindcast, and a 1-yr global hindcast. The updated ST6 not only performs well in predicting commonly used bulk wave parameters (e.g., significant wave height and wave period) but also yields a clearly improved estimation of high-frequency energy level (in terms of saturation spectrum and mean square slope). In the duration-limited test, we investigate the modeled wave spectrum in a detailed way by introducing spectral metrics for the tail and the peak of the omnidirectional wave spectrum and for the directionality of the two-dimensional frequency–direction spectrum. The omnidirectional frequency spectrum E(f) from the recalibrated ST6 shows a clear transition behavior from a power law of approximately f−4 to a power law of about f−5, comparable to previous field studies. Different solvers for nonlinear wave interactions are applied with ST6, including the Discrete Interaction Approximation (DIA), the more expensive Generalized Multiple DIA (GMD), and the very expensive exact solutions [using the Webb–Resio–Tracy method (WRT)]. The GMD-simulated E(f) is in excellent agreement with that from WRT. Nonetheless, we find the peak of E(f) modeled by the GMD and WRT appears too narrow. It is also shown that in the 1-yr global hindcast, the DIA-based model overestimates the low-frequency wave energy (wave period T > 16 s) by 90%. Such model errors are reduced significantly by the GMD to ~20%.

Distribution of Surface Wave Breaking Fronts

“…Overall, the WRT solution gives better spreading against RM10, approaching the spreading reported by Hwang et al (2000) for fully developed seas. This is a substantial improvement compared to other WRT solutions which tend to give much narrower spectra (Liu et al, 2019;Romero & Melville, 2010b).…”

“…where is a dimensionless factor with weak dependence on the wave age c p ∕u * (Resio et al, 2004, RM10). Following Liu et al (2019), is calculated from model solutions for 2.25 k p < k < 0.35 rad/m, with 0.35 rad/m corresponding to the upper limit before the noise floor of the data collected by RM10. The mean compensated spectrum is calculated according to The wave solutions are further characterized with respect the directional spreading calculated according to…”

The Wave Climate of the Southern Ocean

“…where S tot 5 S in 1 S swl 1 S nl 1 S wc and S in represents the wind input, S swl is the swell decay, S nl is the nonlinear interactions, and S wc is the white-cap dissipation (Liu et al 2019). In the above, F represents the energy density spectrum, which is a function of wavenumber k, direction u, the spatial vector,x, and time t. The intrinsic frequency is given by v 5 (gk) 1/2 .…”

“…In the above, F represents the energy density spectrum, which is a function of wavenumber k, direction u, the spatial vector,x, and time t. The intrinsic frequency is given by v 5 (gk) 1/2 . The source terms S tot are represented by the ST6 package originally described in Rogers et al (2012) and Zieger et al (2015) and recently updated by Liu et al (2019). The model was run over a global domain bounded by latitudes 788S and 788N with a spatial resolution of 0.58 3 0.58.…”

Satellite retrieved sea ice concentration uncertainty and its effect on modelling wave evolution in marginal ice zones

“…During the version upgrade of TodaiWW3-ArCS, s wind and s dissipation parameterisations and wind forcing were sanity checked against the 2016 September storm when the model and observations agreed well (Nose et al, 2018). We compared the leading packages, ST4 (Ardhuin et al, 2010;Rascle and Ardhuin, 2013) and ST6 (Rogers et al, 2012;Zieger et al, 2015;Liu et al, 2019), using ECMWF global reanalysis (ERA5) 10 m wind (U 10 ). The latter parameterisation showed marginally improved 155 agreement using default parameters; so all simulations used the ST6 parameterisation and were forced with ERA5 wind fields.…”