Science Service System

Summary of Proposal COA0065

TitleOcean Winds from Terra SAR X
Investigator Horstmann, Jochen - NATO Undersea Research Center, N/A
Team Members
Dr. Thompson, Donald - Applied Physics Laboratory, Johns Hopkins University, Space Department
Wackerman, Christopher - General Dynamics Advanced Information Systems,
Dr. Lehner, Susanne - Deutsches Zentrum für Luft- und Raumfahrt e.V., Institut für Methodik der Fernerkundung
Dr. Ziemer, Friedwart - GKSS Research Center, Institute for Coastal Research
Dr. Mouche, Alexis A. - CLS, Radar Applications Division

Objectives: The main objective of our proposal is to develop an algorithm for extracting high resolution ocean surface wind fields from Terra SAR X data on an operational basis. The algorithm will be tested for the different modes of Terra SAR X, which are acquired at either horizontal or vertical polarization in transmit and receive.

The basic methodology will consist of two main components, a wind direction and a wind speed retrieval algorithm. SAR wind directions will be extracted from wind induced streaks, which are visible in radar images of the ocean surface. Wind speeds are retrieved from the normalized radar cross section (NRCS) of the ocean surface utilizing a geophysical model function.

SAR wind direction retrieval is based on the imaging of wind induced linear features on the ocean surface. To retrieve the orientation of these streak like features from SAR images two methods will be implemented and tested, the Local Gradient Method, which is applied in the spatial domain and the Fast Fourier Transformation Method, which is applied in the spectral domain. Both methods leave directional ambiguities, which will be removed by wind shadowing if present in the SAR image as well as the neighborhood of each SAR grid cell. In addition external sources, e.g. weather charts or numerical atmospheric models, will be considered. Furthermore, we will investigate the applicability of removing the directional ambiguities by detecting the movements of wind gusts in multiple looks of the SAR images, which are separated in time.

For wind speed retrieval, an empirical geophysical model function (GMF) relating the NRCS of the ocean surface to the local near-surface wind vector and radar geometry will be used. For Ku-band and C-band well tested GMFs already exist and have been used successfully to estimate surface wind over the ocean. As an initial candidate for the retrieval of surface winds from Terra SAR-X, we plan to investigate the GMF proposed by Masuko, et al. [1986] that relates the X-band NRCS at both VV- and HH-polarization to the near-surface wind vector and incidence angle. This model is based on airborne measurements of the X-band NRCS collected over (open ocean) buoys in the Pacific Ocean and the Sea of Japan.

For validation of the wind retrieval algorithm Terra SAR X retrieved high resolution wind fields will be compared to numerical atmospheric model results and other remote sensing sensor results, e.g. from QuickSCAT, ENVISAT ASAR and tower based X-band radars installed aboard the offshore platforms FINO I and NEPTUN, as well as to in situ measurements such as acquired at FINO I and NEPTUN.

Total duration: The project duration is 18 months and is planed to start in Sep. 2006 and end in Feb. 2008.

Data requirements: About 120 Terra SAR X images of two test sites need to be acquired during the first half of 2007 in the test sites, which are located in the western and eastern German bight (southern North Sea). All reference data will be acquired on a routine basis throughout the whole year of 2007 and the members of the project team have unlimited access to these data.

Deliverables: Comprehensive final report and online presentations for experts and potential users, describing the proposed algorithms for high resolution wind field retrieval as well as all additional important findings.

Funding: The scientific work will be financed from internal resources of the participants, who are interested in stimulating oceanic applications of SAR. The JHUAPL and GKSS have submitted two proposals for NOAA grants dealing with SAR wind retrieval and have included the investigation of Terra SAR X data in their proposals. We are planning to apply for additional funding from DLR-Bonn/BMBF for traveling, publications, and overhead costs as well as considering Terra SAR X in future EU projects.

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