A Study Of Ocean Winds From Spaceborne Synthetic Aperture Radar Imagery

Synthetic aperture radar(SAR)has relatively higher resolution,compared with other microwave sensors.It is capable of monitoring the sea state regardless of the light and weather.So far,the TerraSAR-X at X band launched by German and GF-3 at C band launched by China could provide the products with the resolution up to 1 m.The sea surface wind is an important parameter on the air-sea interface,as the direct driving force of many ocean dynamic processes,and also has significant effect on global climate change.Therefore,it is crucial to retrieve the sea surface wind and explore the inner mechanisms of common features in SAR images related to sea surface winds,e.g.,sea surface wind streaks,offshore wind farm turbine wakes.In this thesis,the high-resolution SAR data is used to retrieve sea surface winds,explore the appearing mechanisms of sea surface winds,and investigate different features of the offshore wind farm turbine wakes.The contents are organized as follows.(1)A preliminary study on the sea surface wind retrieval and validation from the GF-3 data.Along with explaining the basic principles of wind retrieval algorithm in detail,a total of 5 GF-3 images were used to retrieve the sea surface wind,acquired from the North Sea,the South China Sea,and the central Pacific,respectively.According to the comparison of the wind speed retrieved from the SAR data and from the ECMWF model data,the wind speed retrieved from GF-3 is slightly larger in general,with the maximum speed difference 3.3 m/s.In addition,for the retrieval results of an image,the internal difference of wind speed can reach 4.0 m/s.The reason for the error is probably related to the inaccuracy of the calibration constant used in the radiometric correction.(2)The appearing laws of sea surface wind streaks in Terra SAR-X images.Wind streaks are features that are commonly observed on SAR images,and it is also important for extracting the wind direction from SAR images.However,wind streaks do not always appear in SAR imagery.In this section,the basic statistical laws of wind streaks are summarized and its formation mechanism on SAR images is studied.Based on the 227 TerraSAR-X/TanDEM-X images collected in the North Sea and Baltic Sea,approximately 48.0% SAR images displayed wind streaks,among which 67.3%,20.0%,and 12.7% occurred under unstable,neutral,and stable atmospheric conditions,respectively.The proportions indicate that wind streaks are more likely to be generated from the thermal convection.Further investigations suggest that the inflection point and the wind shear may be essential for the appearance of wind streaks in SAR images under stable atmospheric conditions.(3)A preliminary study on the wake patterns behind offshore wind turbines using spaceborne SAR data.Based on the investigation about the different wind turbine features present on SAR images,about 13% images are with jets behind the offshore wind turbines,33.7% with wakes,53.3% without both the features.When the jet occurs behind the wind turbines,the wind speed firstly decreases and then increases.Meanwhile,the turbulence still remains relatively high compared with the background,together with the increase of the wind speed.The jet cases are rarely observed on the SAR images.By contrast,about half of the cases are observed without jets or wakes.On one hand,when SAR operates at a large incidence angle,the backscattering coefficient is relatively low,which may affect the appearance of wakes or jets on the SAR images.On the other hand,when the wind speed is low,the effect of wind turbine is weaker,which causes turbulence difficult to reach the sea surface and thereby no obvious features are observed on SAR images.