Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Yuko Takeyama (1) F P Teruo Ohsawa (2) Katsutoshi Kozai (2) Charlotte Hasager (3) Merete Badger (3) Susumu Shimada (1) Tetsuya Kogaki (1)
(1) AIST, Koriyama, Japan (2) Kobe Univ., Kobe, Japan (3) RISOE, Roskilde, Denmark
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Dr. Takeyama is a senior researcher at the National institute of Advanced Industry Science and Technology (AIST) in Japan. She had studied remote sensing and marine meteorology at the Kobe University. Recently, her research theme is offshore wind resource assessment by satellite-borne radar images and numerical meteorological simulations.
Fetch effect on sea surface wind speed retrieval from sar image in coastal waters
A sea surface wind speed is thought to be one of the most important information for a site selection of an offshore wind farm. Satellite images as well as numerical meteorological models are effective methods to retrieve the global sea surface wind speed. In previous studies, it has been reported that the sea surface wind speed retrieved from Synthetic Aperture Radar (SAR) on board a satellite has negative biases against in-situ measurements in coastal waters when an offshore wind blows (toward the ocean) due to a remote sensing of sea surface roughness.
In this study, spatial variations of an indicator of the sea surface wind speed, Normalized Radar Cross Section (NRCS) from 187 SAR images are examined in the two Japanese coastal waters, Hiratsuka and Shirahama, Japan. Spatial variations of NRCSs extracted from ENVISAT ASAR images in the target areas are divided into four radar incidence angle classes. The variations are indicated as a function of a fetch (distance from a coast line). The spatial variations of NRCSs are also extracted from those SAR images divided into four sea surface wind speed classes.
Main body of abstract
It is found that NRCSs observed by IMP mode are gradually increase toward the ocean with offshore wind especially with high wind speeds. In order to clarify the tendency, gradients of NRCS from ocean to coastline are examined. In these results, the gradients with offshore wind are lower than those with onshore wind in all cases. The differences of the gradients between offshore and onshore wind are shown from 0 to -23 dB / deg. (along latitude or longitude). It seems that NRCSs are gradually growing up from coastline to offshore with offshore wind.
Additioanlly, a relation between a sea surface wave and the sea surface wind is considered. It has been reported that the sea surface roughness is steeper and a drag coefficient can be higher with offshore wind than onshore wind in coastal waters. However, an effect of atmospheric stabilities has been ignored in the previous studies because the effect is supposed to be small in the target sea areas of these studies. In Japanese coastal waters the atmospheric stability is suddenly changed and an unstable atmospheric condition is dominant. Therefore, the atmospheric stability must be considered.
Finally, the variations of NRCSs under the unstable condition are divided offshore wind with onshore wind in Shirahama. From the result, a remarkable difference between offshore and onshore wind appears and shows that NRCSs are gradually increasing (from -14 to -8 dB) toward offshore with offshore wind while those are not growing up with onshore wind under only unstable conditions.
It seems that NRCSs are gradually growing up from coastline to offshore when the offshore wind blows. On the other word, NRCSs in coastal waters are repressed as being lower than those expected by sea surface wind speeds.
Additionally, a difference between offshore and onshore wind appears and shows that NRCSs are gradually increasing toward offshore with offshore wind while those are not growing up with onshore wind under only unstable conditions.Thus, atmospheric stabilities are at least one of the factor of the negative biases on wind speeds from SAR images.
Satellite images have been utilized for the offshore wind resource assessment especially in the North Sea. Recently in other offshore areas the assessment is tried using satellite images. However a sea surface wind speed with a high accuracy, which have been shown in the North Sea, is not always acquired. This study focuses on an atmospheric stability, which had been ignored in previous studies and contributes to offshore wind resource assessments in various offshore areas.