Sea Fetch Research Articles

Sea Fetch Observed by Synthetic Aperture Radar

Two satellite synthetic aperture radar (SAR) observations of the fetch in the Bohai Sea of China are presented. The sea surface winds derived from SAR data indicated a high wind of 15–16 m/s that occurred in the fetch zone. The winds are shown to have immediate direct mechanical forcing impacts on the significant wave heights $(Hs)$ of ocean surface gravity waves. Buoy measurements and numerical wave modeling results show that the $Hs$ increased to a maximum of 3.5 m in the semienclosed sea, 3 h after the passage of the fetch winds, and the high $Hs$ in the sea was sustained for a total of 6 h. The Weather Research and Forecasting (WRF) model implemented in our modeling simulation captured the wind field responsible for the evolution of the fetch event. The model-simulated surface horizontal winds agree with the SAR-derived winds. In addition, the vertical wind distribution reveals that the fetch wind field reached the 800 hPa level, and the event lasted less than one day. This study demonstrates the synergy of using SAR imagery and the WRF model as effective tools to investigate the lateral and vertical structures of coastal wind.

Performance Test of a 3MW Wind Turbine – Effects of Shear and Turbulence

In this study a full wind turbine performance test has been performed on a 3MW onshore wind turbine in a coastal location in Norway utilizing a Windcube v2 lidar. The test was performed following the current edition (1. ed) of the IEC 61400-12-1 wind turbine power performance measurement standard with some necessary adaptations for remote sensing anemometry, and was expanded to include wind shear and veer.10 months of data from a Leosphere windcube v2 pulsed lidar, a 3MW wind turbine and a short met-mast formed the basis for the performance test. This amount of data allows for a more detailed analysis of the influence of shear, veer and turbulence on power production than required by the standard. The coastal location of the turbine also allows us to study wind conditions from sea fetch relevant for offshore turbines.The study has demonstrated the benefits and disadvantages of using a lidar for wind turbine performance measurements. Differences between hub height wind and rotor equivalent wind speed has been shown, and results show how turbulence, shear and veer influence the power production and the power curve derived from hub height and equivalent wind speed respectively.

Along‐shore variations of offshore flow

Using eddy correlation data collected by the LongEZ research aircraft, the adjustment of atmospheric flow downstream from a coastline is examined. Along‐shore variation of the turbulence over the water is generated by the varying width of the upstream land strip between the sea and inland water. Over the coastal zone, the turbulence is strongest downstream from the widest part of land. The along‐shore variation of the turbulence decreases with increasing sea fetch due to horizontal mixing. In other terms, the footprint of the flux farther offshore includes a larger width of upstream land. Beyond 5 km sea fetch, the along‐shore variation becomes small.

Evaluation of the wind-resource estimation program WAsP for offshore applications

The increasing interest in harvesting offshore wind energy requires reliable tools for the wind resource estimation at these sites. Most commonly used for wind resource predictions on land as well as offshore is the WAsP program. It has been validated extensively for sites on land and at the coast. However, due to the lack of suitable measurements there is still a need for further validation for offshore sites. Data from ongoing measurements at prospective wind farm sites in the Danish Baltic Sea region are available now. The wind resources estimated from these measurements are compared to WAsP-predictions. They agree well. The only deviation found is for two sites, which are located at about the same distance, but on different sides of the island Lolland. Here the measurements show a difference in the wind resources, which is not predicted by WAsP. A direction-dependent comparison explains this deviation. Wind speed ratios of several pairs of stations are modeled with WAsP for 12 directional sectors and compared with the measurements. Deviations in the directional wind speed predictions were found to correspond with the length of the sea fetch: For smaller sea fetches WAsP seems to slightly over-predict the wind speed, while for long fetches of more than 30 km an under-prediction is found. An analysis of the vertical wind speed profiles at three sites indicates that a combination of different effects is responsible for the correlation between sea fetch and model behavior. Effects of atmospheric stability as well as a fetch dependent sea surface roughness have to be taken into account.

The vertical turbulence structure of the coastal marine atmospheric boundary layer

The vertical turbulence structure in the marine atmosphere along a shoreline has been investigated using data from tower and aircraft measurements performed along the Baltic coast in the southeast of Sweden. Two properties make the Baltic Sea particularly interesting. It is surrounded by land in all directions within moderate advection distances, and it features a significant annual lag in sea surface temperature as compared with inland surface temperature. The present data were collected mostly during spring or early summer, when the water is cool, i.e., with a stably or neutrally stratified marine boundary layer usually capped by an inversion. Substantial daytime heating over the land area results in a considerable horizontal thermal contrast. Measurements were made on a small island, on a tower with a good sea fetch, and with an airborne instrument package. The profile data from the aircraft is from 25 slant soundings performed in connection to low level boundary layer flights. The results from the profiles are extracted through filtering techniques on individual time (space) series (individual profiles), applying different normalization and finally averaging over all or over groups of profiles. The land‐based data are from a low tower situated on the shoreline of a small island with a wide sector of unobstructed sea fetch. Several factors are found that add to the apparent complexity of the coastal marine environment: the state of the sea appears to have a major impact on the turbulence structure of the surface layer, jet‐shaped wind speed profiles were very common at the top of the boundary layer (in about 50% of the cases) and distinct layers with increased turbulence were frequently found well above the boundary layer (in about 80% of the cases). The present paper will concentrate on a description of the experiment, the analysis methods, and a general description of the boundary layer turbulence structure over the Baltic Sea.

Noise-driven intermittency in tidal dynamics

The tide nonstationarity in the Adriatic Sea is described. We show how this phenomenon can be related to the stochasticity of sea fetch by a nonlinear noise-forced oscillator model. The experimental series correlation dimension reveals a strong lifting with respect to the astronomic tide. The lifting is related to space-time intermittency.