Wind-speed inversion from HF radar first-order backscatter signal |
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| Authors: | Wei Shen Klaus-Werner Gurgel George Voulgaris Thomas Schlick Detlef Stammer |
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| Institution: | (1) Institute of Oceanography, University of Hamburg, Bundesstrasse 53, 20146 Hamburg, Germany;(2) Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA |
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| Abstract: | Land-based high-frequency (HF) radars have the unique capability of continuously monitoring ocean surface environments at
ranges up to 200 km off the coast. They provide reliable data on ocean surface currents and under slightly stricter conditions
can also give information on ocean waves. Although extraction of wind direction is possible, estimation of wind speed poses
a challenge. Existing methods estimate wind speed indirectly from the radar derived ocean wave spectrum, which is estimated
from the second-order sidebands of the radar Doppler spectrum. The latter is extracted at shorter ranges compared with the
first-order signal, thus limiting the method to short distances. Given this limitation, we explore the possibility of deriving
wind speed from radar first-order backscatter signal. Two new methods are developed and presented that explore the relationship
between wind speed and wave generation at the Bragg frequency matching that of the radar. One of the methods utilizes the
absolute energy level of the radar first-order peaks while the second method uses the directional spreading of the wind generated
waves at the Bragg frequency. For both methods, artificial neural network analysis is performed to derive the interdependence
of the relevant parameters with wind speed. The first method is suitable for application only at single locations where in
situ data are available and the network has been trained for while the second method can also be used outside of the training
location on any point within the radar coverage area. Both methods require two or more radar sites and information on the
radio beam direction. The methods are verified with data collected in Fedje, Norway, and the Ligurian Sea, Italy using beam
forming HF WEllen RAdar (WERA) systems operated at 27.68 and 12.5 MHz, respectively. The results show that application of
either method requires wind speeds above a minimum value (lower limit). This limit is radar frequency dependent and is 2.5
and 4.0 m/s for 27.68 and 12.5 MHz, respectively. In addition, an upper limit is identified which is caused by wave energy
saturation at the Bragg wave frequency. Estimation of this limit took place through an evaluation of a year long database
of ocean spectra generated by a numerical model (third generation WAM). It was found to be at 9.0 and 11.0 m/s for 27.68 and
12.5 MHz, respectively. Above this saturation limit, conventional second-order methods have to be applied, which at this range
of wind speed no longer suffer from low signal-to-noise ratios. For use in operational systems, a hybrid of first- and second-order
methods is recommended. |
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