Effect of the drag coefficient on a typhoon wave model

Abstract Effects of new drag coefficient (Cd) parameterizations on WAVEWATCH III (WW3) model surface wave simulations are investigated. The new parameterizations are based on a coupled wind–wave model (CWW) and a wave tank experiment, and yields reduced Cd at high wind speeds. Numerical experiments for uniform winds and Hurricane Katrina (2005) indicate that the original Cd parameterization used in WW3 overestimates drag at high wind speeds compared to recent observational, theoretical, and numerical modeling results. Comparisons with buoy measurements during Hurricane Katrina demonstrate that WW3 simulations with the new Cd parameterizations yield more accurate significant wave heights compared to simulations with the original Cd parameterization, provided that accurate high-resolution wind forcing fields are used.

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Sea-state dependency of air-sea fluxes for high winds in ECMWF Earth System Model

10.5194/egusphere-egu2020-6074 ◽

2020 ◽

Author(s):

Jean Bidlot

Keyword(s):

Drag Coefficient ◽

Tropical Cyclones ◽

Coupled System ◽

Wave Model ◽

Earth System Model ◽

System Model ◽

Earth System ◽

Sea State ◽

Medium Range ◽

Fully Coupled

The global analyses and medium range forecasts from the European Centre for Medium range Weather Forecasts rely on a state-of-the-art Numerical Weather Prediction (NWP) system. To best represent the air-sea exchanges, it is tightly coupled to an ocean wave model.&#160; As part of ECMWF approach to Earth System Model, it is also coupled to a global ocean model for all its forecasting systems from the medium range up to the seasonal time scale.Because the feedback from and to the ocean can be significant, it is only in the fully coupled system that parameterisation for air-sea processes should be revisited. For instance, it is now accepted that the drag coefficient should generally attained maximum values for storm winds but should level or even decrease for very strong winds, namely in tropical cyclones or intense mid-latitude wind storms.A modification of the wind input source was tested, whereby the Charnock coefficient estimated by the wave model and therefore the drag coefficient sharply reduce for large winds (> 30 m/s). As a consequence, ECMWF tendency to under predict strong tropical cyclones was sharply alleviated, in better agreement with observational evidence. This change is now planned for operational implementation with the next model cycle (CY47R1, June 2020).Experimental evidences also point to a sea state/wind dependency of the heat and moisture fluxes.&#160; Following an extension of the wind wave generation theory, a sea state dependent parameterisation for the roughness length scales for heat and humidity has been tested. Again, a proper assessment of the different parameterisations warrants the fully coupled system. Experimentations so far indicate the benefit of such change. Ongoing work aims at future operational implementation.

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Assimilation of decomposed in-situ directional wave spectra into a numerical wave model on typhoon wave

Natural Hazards and Earth System Sciences Discussions ◽

10.5194/nhessd-1-3967-2013 ◽

2013 ◽

Vol 1 (4) ◽

pp. 3967-3989

Author(s):

Y. M. Fan ◽

H. Günther ◽

C. C. Kao ◽

B. C. Lee

Keyword(s):

Data Assimilation ◽

Forecast Accuracy ◽

Wave Model ◽

Sequential Data ◽

Wave Spectra ◽

Typhoon Wave ◽

Independent Observations ◽

Directional Wave ◽

Numerical Wave

Abstract. The purpose of this study was to enhance the accuracy of numerical wave forecasts through data assimilation during typhoon period. A sequential data assimilation scheme was modified to enable its use with partitions of directional wave spectra. The performance of the system was investigated with respect to operational applications specifically for typhoon wave. Two typhoons that occurred in 2006 around Taiwan (Kaemi and Shanshan) were used for this case study. The proposed data assimilation method increased the forecast accuracy in terms of wave parameters, such as wave height and period. After assimilation, the shapes of directional spectra were much closer to those reported from independent observations.

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On the Sensitivity of Typhoon Wave Simulations to Tidal Elevation and Current

Journal of Marine Science and Engineering ◽

10.3390/jmse8090731 ◽

2020 ◽

Vol 8 (9) ◽

pp. 731 ◽

Cited By ~ 1

Author(s):

Shih-Chun Hsiao ◽

Han-Lun Wu ◽

Wei-Bo Chen ◽

Chih-Hsin Chang ◽

Lee-Yaw Lin

Keyword(s):

Wave Height ◽

Tidal Current ◽

Outer Region ◽

Wave Model ◽

Tidal Currents ◽

Integrated System ◽

Wave Direction ◽

Wind Fields ◽

Tidal Elevation ◽

Typhoon Wave

The sensitivity of storm wave simulations to storm tides and tidal currents was investigated using a high-resolution, unstructured-grid, coupled circulation-wave model (Semi-implicit Cross-scale Hydroscience Integrated System Model Wind Wave Model version III (SCHISM-WWM-III)) driven by two typhoon events (Typhoons Soudelor and Megi) impacting the northeastern coast of Taiwan. Hourly wind fields were acquired from a fifth-generation global atmospheric reanalysis (ERA5) and were used as meteorological conditions for the circulation-wave model after direct modification (MERA5). The large typhoon-induced waves derived from SCHISM-WWM-III were significantly improved with the MERA5 winds, and the peak wave height was increased by 1.0–2.0 m. A series of numerical experiments were conducted with SCHISM-WWM-II and MERA5 to explore the responses of typhoon wave simulations to tidal elevation and current. The results demonstrate that the simulated significant wave height, mean wave period and wave direction for a wave buoy in the outer region of the typhoon are more sensitive to the tidal current but less sensitive to the tidal elevation than those for a wave buoy moored in the inner region of the typhoon. This study suggests that the inclusion of the tidal current and elevation could be more important for typhoon wave modeling in sea areas with larger tidal ranges and higher tidal currents. Additionally, the suitable modification of the typhoon winds from a global atmospheric reanalysis is necessary for the accurate simulation of storm waves over the entire region of a typhoon.

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Storm surge and wave simulations in the Gulf of Mexico using a consistent drag relation for atmospheric and storm surge models

Natural Hazards and Earth System Science ◽

10.5194/nhess-12-2399-2012 ◽

2012 ◽

Vol 12 (7) ◽

pp. 2399-2410 ◽

Cited By ~ 14

Author(s):

D. Vatvani ◽

N. C. Zweers ◽

M. van Ormondt ◽

A. J. Smale ◽

H. de Vries ◽

...

Keyword(s):

Wind Speed ◽

Gulf Of Mexico ◽

Drag Coefficient ◽

Storm Surge ◽

Weather Prediction ◽

Surface Wind ◽

Wave Model ◽

Water Levels ◽

Observation Data ◽

Wave Effects

Abstract. To simulate winds and water levels, numerical weather prediction (NWP) and storm surge models generally use the traditional bulk relation for wind stress, which is characterized by a wind drag coefficient. A still commonly used drag coefficient in those models, some of them were developed in the past, is based on a relation, according to which the magnitude of the coefficient is either constant or increases monotonically with increasing surface wind speed (Bender, 2007; Kim et al., 2008; Kohno and Higaki, 2006). The NWP and surge models are often tuned independently from each other in order to obtain good results. Observations have indicated that the magnitude of the drag coefficient levels off at a wind speed of about 30 m s−1, and then decreases with further increase of the wind speed. Above a wind speed of approximately 30 m s−1, the stress above the air-sea interface starts to saturate. To represent the reducing and levelling off of the drag coefficient, the original Charnock drag formulation has been extended with a correction term. In line with the above, the Delft3D storm surge model is tested using both Charnock's and improved Makin's wind drag parameterization to evaluate the improvements on the storm surge model results, with and without inclusion of the wave effects. The effect of waves on storm surge is included by simultaneously simulating waves with the SWAN model on identical model grids in a coupled mode. However, the results presented here will focus on the storm surge results that include the wave effects. The runs were carried out in the Gulf of Mexico for Katrina and Ivan hurricane events. The storm surge model was initially forced with H*wind data (Powell et al., 2010) to test the effect of the Makin's wind drag parameterization on the storm surge model separately. The computed wind, water levels and waves are subsequently compared with observation data. Based on the good results obtained, we conclude that, for a good reproduction of the storm surges under hurricane conditions, Makin's new drag parameterization is favourable above the traditional Charnock relation. Furthermore, we are encouraged by these results to continue the studies and establish the effect of improved Makin's wind drag parameterization in the wave model. The results from this study will be used to evaluate the relevance of extending the present towards implementation of a similar wind drag parameterization in the SWAN wave model, in line with our aim to apply a consistent wind drag formulation throughout the entire storm surge modelling approach.

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Cyclonic Wave Simulations Based on WAVEWATCH-III Using a Sea Surface Drag Coefficient Derived from CFOSAT SWIM Data

Atmosphere ◽

10.3390/atmos12121610 ◽

2021 ◽

Vol 12 (12) ◽

pp. 1610

Author(s):

Weizeng Shao ◽

Tao Jiang ◽

Yu Zhang ◽

Jian Shi ◽

Weili Wang

Keyword(s):

Drag Coefficient ◽

Numerical Models ◽

Sea Surface ◽

Surface Drag ◽

Wavewatch Iii ◽

Sea Surface Roughness ◽

Typhoon Wave ◽

Strong Winds ◽

Remote Sensing Techniques ◽

High Winds

It is well known that numerical models are powerful methods for wave simulation of typhoons, where the sea surface drag coefficient is sensitive to strong winds. With the development of remote sensing techniques, typhoon data (i.e., wind and waves) have been captured by optical and microwave satellites such as the Chinese-French Oceanography SATellite (CFOSAT). In particular, wind and wave spectra data can be simultaneously measured by the Surface Wave Investigation and Monitoring (SWIM) onboard CFOSAT. In this study, existing parameterizations for the drag coefficient are implemented for typhoon wave simulations using the WAVEWATCH-III (WW3) model. In particular, a parameterization of the drag coefficient derived from sea surface roughness is adopted by considering the terms for wave steepness and wave age from the measurements from SWIM products of CFOSAT from 20 typhoons during 2019–2020 at winds up to 30 m/s. The simulated significant wave height (Hs) from the WW3 model was validated against the observations from several moored buoys active during three typhoons, i.e., Typhoon Fung-wong (2014), Chan-hom (2015), and Lekima (2019). The analysis results indicated that the proposed parameterization of the drag coefficient significantly improved the accuracy of typhoon wave estimation (a 0.49 m root mean square error (RMSE) of Hs and a 0.35 scatter index (SI)), greater than the 0.55 RMSE of Hs and >0.4 SI using other existing parameterizations. In this sense, the adopted parameterization for the drag coefficient is recommended for typhoon wave simulations using the WW3 model, especially for sea states with Hs < 7 m. Moreover, the accuracy of simulated waves was not reduced with growing winds and sea states using the proposed parameterization. However, the applicability of the proposed parameterization in hurricanes necessitates further investigation at high winds (>30 m/s).

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Evaluation of numerical wave model for typhoon wave simulation in South China Sea

Water Science and Engineering ◽

10.1016/j.wse.2018.09.001 ◽

2018 ◽

Vol 11 (3) ◽

pp. 229-235 ◽

Cited By ~ 7

Author(s):

Zhi-yuan Wu ◽

Chang-bo Jiang ◽

Bin Deng ◽

Jie Chen ◽

Yong-gang Cao ◽

...

Keyword(s):

South China Sea ◽

South China ◽

Wave Model ◽

China Sea ◽

Wave Simulation ◽

Typhoon Wave ◽

Numerical Wave

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Bulk drag coefficient of a subaquatic vegetation subjected to irregular waves: Influence of Reynolds and Keulegan-Carpenter numbers

La Houille Blanche ◽

10.1051/lhb/2020015 ◽

2020 ◽

pp. 34-42

Author(s):

Thibault Chastel ◽

Kevin Botten ◽

Nathalie Durand ◽

Nicole Goutal

Keyword(s):

Drag Coefficient ◽

Wave Height ◽

Wave Attenuation ◽

Empirical Relationship ◽

Breaking Waves ◽

Irregular Waves ◽

Geometrical Parameters ◽

Flume Experiments ◽

Seagrass Meadows ◽

Artificial Vegetation

Seagrass meadows are essential for protection of coastal erosion by damping wave and stabilizing the seabed. Seagrass are considered as a source of water resistance which modifies strongly the wave dynamics. As a part of EDF R & D seagrass restoration project in the Berre lagoon, we quantify the wave attenuation due to artificial vegetation distributed in a flume. Experiments have been conducted at Saint-Venant Hydraulics Laboratory wave flume (Chatou, France). We measure the wave damping with 13 resistive waves gauges along a distance L = 22.5 m for the “low” density and L = 12.15 m for the “high” density of vegetation mimics. A JONSWAP spectrum is used for the generation of irregular waves with significant wave height Hs ranging from 0.10 to 0.23 m and peak period Tp ranging from 1 to 3 s. Artificial vegetation is a model of Posidonia oceanica seagrass species represented by slightly flexible polypropylene shoots with 8 artificial leaves of 0.28 and 0.16 m height. Different hydrodynamics conditions (Hs, Tp, water depth hw) and geometrical parameters (submergence ratio α, shoot density N) have been tested to see their influence on wave attenuation. For a high submergence ratio (typically 0.7), the wave attenuation can reach 67% of the incident wave height whereas for a low submergence ratio (< 0.2) the wave attenuation is negligible. From each experiment, a bulk drag coefficient has been extracted following the energy dissipation model for irregular non-breaking waves developed by Mendez and Losada (2004). This model, based on the assumption that the energy loss over the species meadow is essentially due to the drag force, takes into account both wave and vegetation parameter. Finally, we found an empirical relationship for Cd depending on 2 dimensionless parameters: the Reynolds and Keulegan-Carpenter numbers. These relationships are compared with other similar studies.

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