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Summary of Proposal OCE1044

TitleExtreme oceanic weather situation observation using TerraSAR-X data
Investigator Li, Xiaoming - Chinese Academy of Sciences, Institute of Remote Sensing and Digital Earth
Team Member
Prof. Lehner, Susanne - German Aerospace Center (DLR), Remote Sensing Technology Institute
Dr. Bruns, Thomas - Deutscher Wetterdienst, Seeschifffahrtsberatung
Mr. Brusch, Stephan - German Aerospace Center (DLR), Remote Sensing Technology Institute
Dr. Perrie, William - Bedford Institute of Oceanography, N/A
SummaryA key factor for both climate and weather in Europe, North America and Asia are tropical and extra-tropical cyclones in the North Atlantic and North-western Pacific. Active microwave radar, particularly the SAR sensors are suitable for monitoring cyclones, as it can yield two-dimensional sea surface information, e.g., sea surface wind and wave field. In this research, accurate sea surface wind field retrieved from SAR imagery, observation of higher sea state generated by typhoons and heavy rainfall caused by typhoons are the topics of interest. As TerraSAR-X is highlighted flexible imaging capability as well as high resolution, it can deepen our understanding of air-sea interactions at large and small scales under extreme weather situations. In particular, sea surface waves generated by extreme weathers, e.g., hurricanes and typhoons are less understanding, as spatial resolution of conventional SAR sensors like ERS/SAR and EVNSIAT/ASAR are limited to 25 m in image mode, which is not enough to solve the fine-scale wave field. The TerraSAR-X, e.g., the ScanSAR image with swath of 100 km and spatial resolution of 16 m can significantly improve knowledge of small scale phenomena like wind-sea and swell interaction with atmospheric turbulence.TerraSAR-X is going to be used for retrieving sea surface wind and wave field under extreme weather situations.
Final Report1. Investigation of spatial variation of surface waves in coastal zones using high spatial resolution TS-X imagery. In this study, surface wave refraction and diffraction are investigated using TSX Stripmap and Spotlight imagery acquired over the coast of Terceira island situated in the North Atlantic. Peak wavelength and wave direction are determined by SAR 2-D image spectra. They are compared to measurements of X-band marine radar and results of the WAve prediction Model (WAM). Significant wave height in the near-shore shallow water region is estimated from TSX Spotlight mode data following the wave refraction laws and using the developed XWAVE empirical algorithm. Image spectra of the TSX subscenes in the full-coverage region are given to investigate significant changes of wave direction and length. By analyzing another TSX image acquired in StripMap mode, a shadow zone in the lee side of Terceira island is identified. It is influenced jointly by wave refraction and diffraction. Furthermore, a cross-sea pattern revealed in the image spectra is investigated. The cross sea is generated by the diffracted wave rays from the northern and southern coasts of the island. Less wave directional spreading for the cross-sea situation is observed as well when compared to the image spectra at the origin of diffraction. 2. Development of an X-band Geophysical Model Function (GMF) to retrieve the sea surface wind field from TerraSAR-X data. A geophysical model function (GMF), which is denoted by XMOD2, is developed to retrieve sea surface wind field from X-band TerraSAR-X/TanDEM-X (TS-X/TD-X) data. In contrast to the previously developed XMOD1, XMOD2 consists of a nonlinear GMF, and thus, it depicts the difference between upwind and downwind of the sea surface backscatter in X-band synthetic aperture radar (SAR) imagery. By exploiting 371 collocations with in situ buoy measurements that are used as the tuning data set together with analysis wind model results, the retrieved TS-X/TD-X sea surface wind speed using XMOD2 shows a close agreement with buoy measurements with a bias of −0.32 m/s, a root-mean-square error (RMSE) of 1.44 m/s, and a scatter index (SI) of 16.0%. Further validation using an independent data set of 52 cases shows a bias of−0.17 m/s, an RMSE of 1.48 m/s, and an SI of 17.0% comparing with buoy measurements. To apply XMOD2 to TS-X/TD-X data acquired at HH polarization, we validate three X-band SAR polarization ratio models that were tuned using TS-X dual-polarization data by comparing the retrieved sea surface wind speed with buoy measurements. 3. Application of sea surface wind monitoring at the offshore wind farms In this study, we demonstrate the application of X-band TerraSAR-X (TS-X) data with high spatial resolution for studies on wind turbine wake in the near and far field of the offshore wind farm Alpha Ventus located in the North Sea. Two cases which have different weather conditions and yield different wake pattern as observed in the TS-X image are presented. Using the TS-X observations, the retrieved sea surface wind field and meteorological measurements at the FiNO1 platform jointly, we quantify the wake characteristics of wake length, wake width, maximum velocity deficit, wake merging and wake meandering. Influences of meteorological conditions on inducing wakes with different characteristics in the two cases are discussed.

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