# The Effect of Wind Forcing on Modeling Coastal Circulation at a Marine Renewable Test Site

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## Abstract

**:**

## 1. Introduction

## 2. Research Domain

## 3. Wind Data

## 4. Measurements of Water Currents

#### 4.1. ADCP Data

#### 4.2. CODAR System

## 5. Numerical Model

_{b}is bottom drag coefficient; ${k}_{w}$ is wind-stress coefficient; ${u}_{1},{v}_{1}$ refers to east-west and north-south velocity components computed at mid-height of the bottom layer respectively (m/s). The bottom drag coefficient ${c}_{b}$ is computed using:

_{w}= 2.6 × 10

^{−3}produced good results for a previous Galway Bay modeling study. This constant value of wind-stress coefficient was therefore also used in the present study.

## 6. Results

#### 6.1. Velocity Components

#### 6.2. Vector Fields of Surface Currents

## 7. Discussion

## 8. Conclusions

- (1)
- Forecasted high-resolution (HR) wind data for the offshore generally had similar trend in direction as the measured NUIG onshore wind data; however, the offshore HR wind speeds were consistently higher than the land-based NUIG measurements. The mean difference in wind speed for the 30-day simulation period was approximately 7.4 m/s with a standard deviation of 1.4 m/s. This indicates that significant variation exists between onshore and offshore wind speeds, and so where possible, offshore winds should be used as boundary conditions for coastal hydrodynamic models.
- (2)
- Analysis of ADCP data showed that multi-layered circulation occurs in the area with wind being the predominant driver of the surface layer and the tidal influence being the main driver of the lower water column layers.
- (3)
- Comparison of surface velocity components time series showed that the level of agreement between modeled surface currents and those recorded by CODAR were significantly better when wind was included in the model and that the HR model, with spatially-varying offshore winds applied, gave better agreement with CODAR data than both the NUIG model which used land-measured winds which in turn gave better agreement with CODAR than the NW model without wind included, especially during strong wind events. This wind forcing in an important boundary condition to consider.
- (4)
- Surface vector fields for four different states of the tide indicated that the surface currents within the Inner Bay at the time of inspection were strongly dominated by wind and that the numerical model was capable of simulating some of this wind influence, depending on the type of wind forcing specified. The model driven by winds, especially spatially varying HR wind fields, clearly produced better agreement with the measured surface current vectors. This agreement was most improved in the north-south surface velocity component (v).

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Abbreviations

ADCP | Acoustic Doppler Current Profiler |

ARIMA | Autoregressive Integrated Moving Average |

CODAR | Coastal Ocean Dynamics Applications Radar |

EFDC | Environmental Fluid Dynamics Code |

ECMWF | European Centre for Medium-range Weather Forecasting |

HR | High resolution |

HF | High-Frequency |

HW | High water |

ICHEC | Ireland’s High-Performance Computing Centre |

IRUSE | Informatics Research Unit for Sustainable Engineering |

LW | Low water |

MF | Mid-flood |

ME | Mid-ebb |

NUIG | National University of Ireland Galway |

NW | No wind |

OTPS | Oregon State University Tidal Prediction Software |

RMSE | Room-Mean-Squared-Error |

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**Figure 1.**Galway Bay study area (R1 and R2 indicate radar station at Mutton Island and Spiddal, respectively; A1 and A2 indicate ADCP sites; C1–C4 indicate reference points for comparison).

**Figure 2.**Comparison between modeled HR wind data and NUIG wind data at C1. (

**a**) Wind speed, (

**b**) wind direction.

**Figure 11.**(

**a**) Sample CODAR radial vector map from Mutton Island radar station at 23:00 Julian Day 208, 201; (

**b**) total vector map of surface currents at 15:00 Julian Day 292, 2011.

**Figure 12.**Comparison of model results and observations at location C3 during P1 ((

**a**,

**b**) east-west (u) and north-south (v) surface velocity components from models and measurements, respectively; (

**c**) wind speed; (

**d**) wind direction; (

**e**) local water elevation; (

**f**) wind speed and wind direction at C3).

**Figure 13.**Comparison of modeled and measured surface currents at locations C2 during P3 ((

**a**), (

**b**) east-west (u) and north-south (v) surface velocity components from models and measurements, respectively; (

**c**) wind speed; (

**d**) wind direction; (

**e**) local water elevation; (

**f**) wind speed and wind direction at C2).

**Figure 14.**(

**a**) Water elevation curve showing times of comparison on velocity vectors and (

**b**) HR wind speed and direction at the location C3.

**Figure 15.**Low water vector fields ((

**a**) depth-averaged currents from model NW; (

**b**) surface currents from model NW; (

**c**) CODAR current data and (

**d**) surface currents from model HR).

**Figure 16.**Mid-flood vector fields ((

**a**) Depth-averaged currents from model NW; (

**b**) surface currents from model NW; (

**c**) CODAR current data and (

**d**) surface currents from model HR).

**Figure 17.**High water vector fields ((

**a**) Depth-averaged currents from model NW; (

**b**) surface currents from model NW; (

**c**) CODAR current data and (

**d**) surface currents from model HR)

**Figure 18.**Mid-ebb vector fields ((

**a**) Depth-averaged currents from model NW; (

**b**) surface currents from model NW; (

**c**) CODAR current data and (

**d**) surface currents from model HR).

**Figure 19.**Modeled and measured north-south surface velocity component at C3 ((

**a**) north-south surface velocity components; (

**b**) modeled wind speeds and direction at C3).

Index | Period | Time | Duration (hours) |
---|---|---|---|

P1 | Period one | Julian Day 291 18:00 to Julian Day 294 00:00 | 55 |

P2 | Period two | Julian Day 301 12:00 to Julian Day 302 17:00 | 29 |

P3 | Period three | Julian Day 309 18:00 to Julian Day 312 00:00 | 55 |

P4 | Period four | Julian Day 312 00:00 to Julian Day 314 06:00 | 55 |

Variable | Standard Deviation | Mean Difference |
---|---|---|

Speed (m/s) | 1.4 | 7.4 |

Direction (°) | 18.7 | 0.9 |

Model | Wind Source Used |
---|---|

NW | No wind forcing |

NUIG | Temporally varying but spatially non-varying measured onshore wind |

HR | Temporally and spatially varying offshore wind forecast offshore wind |

Period | Model | Location | RMSE (u, cm/s) | RMSE (v, cm/s) | RSQ (u) | RSQ (v) |
---|---|---|---|---|---|---|

P1 | NW | C3 | 11.24 | 8.63 | 0.25 | 0.08 |

P1 | NUIG | C3 | 8.56 | 6.49 | 0.57 | 0.35 |

P1 | HF | C3 | 5.64 | 5.57 | 0.76 | 0.66 |

P3 | NW | C2 | 12.46 | 17.83 | 0.82 | 0.00 |

P3 | NUIG | C2 | 9.87 | 10.95 | 0.87 | 0.74 |

P3 | HF | C2 | 9.02 | 10.52 | 0.86 | 0.69 |

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**MDPI and ACS Style**

Ren, L.; Nagle, D.; Hartnett, M.; Nash, S.
The Effect of Wind Forcing on Modeling Coastal Circulation at a Marine Renewable Test Site. *Energies* **2017**, *10*, 2114.
https://doi.org/10.3390/en10122114

**AMA Style**

Ren L, Nagle D, Hartnett M, Nash S.
The Effect of Wind Forcing on Modeling Coastal Circulation at a Marine Renewable Test Site. *Energies*. 2017; 10(12):2114.
https://doi.org/10.3390/en10122114

**Chicago/Turabian Style**

Ren, Lei, Diarmuid Nagle, Michael Hartnett, and Stephen Nash.
2017. "The Effect of Wind Forcing on Modeling Coastal Circulation at a Marine Renewable Test Site" *Energies* 10, no. 12: 2114.
https://doi.org/10.3390/en10122114