Attenuation with distance and wind speed of HF surface waves over the ocean

Forget, Philippe; Broche, P.; Maistre, J. C. de

doi:10.1029/rs017i003p00599

Cited by 12 publications

(8 citation statements)

References 10 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…A loss of radar coverage was also noticed during strong winds, especially during the Mistral wind regime. This is consistent with the fact that propagation losses in HF ground wave propagation mode increase with sea state, leading to a decrease of the radar range, as shown both theoretically (Barrick 1971) and experimentally (Forget et al 1982). …”

Section: Setup Of the Hf Radar Systemsupporting

confidence: 87%

Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modeling

et al. 2011

View full text Add to dashboard Cite

Coastal mesoscale eddies were evidenced during a high-frequency radar campaign in the Gulf of Lions (GoL), northwestern Mediterranean Sea, from June 2005 to January 2007. These anticyclonic eddies are characterized by repeated and intermittent occurrences as well as variable lifetime. This paper aims at studying the link between these new surface observations with similar structures suggested at depth by traditional acoustic Doppler current profiler measurements and investigates the eddy generation and driving mechanisms by means of an academic numerical study. The influence of the wind forcing on the GoL circulation and the eddy generation is analyzed, using a number of idealized configurations in order to investigate the interaction with river discharge, buoyancy, and bathymetric effects. The wind forcing is shown to be crucial for two different generation mechanisms: A strong northerly offshore wind (Mistral) generates a vortex column due to the bathymetric constraint of a geostrophic barotropic current, which can surface after the Responsible Editor: Aida Alvera-Azcárate This article is part of the Topical Collection on Multiparametric observation and analysis of the Sea A. Schaeffer (B) · A. Molcard · P. Forget · P. Fraunié LSEET, Universite du Sud Toulon-Var, BP 20132,

show abstract

Section: Setup Of the Hf Radar Systemsupporting

confidence: 87%

Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modeling

et al. 2011

View full text Add to dashboard Cite

show abstract

“…Recent, careful analyses of experimental radar data [Forget et al, 1982] at 7 and 14 MHz have shown that propagation loss at these frequencies follows the theoretical results of Barrick [1971a, b] closely, indicating a clear dependence on sea state. Our experience at 25 MHz also shows pronounced sea state 320 LYONS AND BARRICK: ATTENUATION OF COASTAL RADAR SIGNALS effects: in higher seas, maximum range is decreased.…”

Section: Introductionsupporting

confidence: 60%

“…Our experience at 25 MHz also shows pronounced sea state 320 LYONS AND BARRICK: ATTENUATION OF COASTAL RADAR SIGNALS effects: in higher seas, maximum range is decreased. Forget et al [1982] found that measured attenuation rates were slightly greater than theory predicted. Unpublished measurements of one-way path loss over the ocean off the coast of Maine (taken on many frequencies from 3 to 30 MHz 15 years ago by the second author) also show close agreement with theoretical values at frequencies below about 20 MHz, but a consistent trend toward lower losses than predicted above 20 MHz.…”

Section: Introductionmentioning

confidence: 78%

Attenuation rates of coastal radar signals at 25 MHz

Lyons¹,

Barrick²

1984

Radio Science

View full text Add to dashboard Cite

The attenuation rate of the ground wave signal with range is a factor limiting the performance of coastal radars. We show that observed attenuation rates are less than theoretically predicted rates at 25 MHz. This result, contrary to earlier findings at lower frequencies, suggests the onset of tropospheric ducting above 20 MHz. The attenuation rates for various sea states and distances are tabulated to allow estimates of system performance near 25 MHz.

show abstract

“…In addition, sea state influences propagation of the EM wave (cf. Barrick 1971a, b;Forget et al 1982). Range increases with increasing sea state due to a stronger backscatter signal but at the same time attenuation increases due to loss of power by stronger backscatter at shorter ranges.…”

Section: Range Dependencymentioning

confidence: 96%

“…Note that WAM is estimating the processes at the ocean surface and does not incorporate EM scattering processes which, especially at high HF frequencies (28 MHz), will result in an additional decrease of first-order backscatter power with increasing wind speed (cf. Barrick 1971a, b;Forget et al 1982).…”

Section: Wind Speed and Bragg Frequencymentioning

confidence: 98%

Wind-speed inversion from HF radar first-order backscatter signal

et al. 2011

View full text Add to dashboard Cite

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

show abstract

Attenuation with distance and wind speed of HF surface waves over the ocean

Cited by 12 publications

References 10 publications

Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modeling

Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modeling

Attenuation rates of coastal radar signals at 25 MHz

Wind-speed inversion from HF radar first-order backscatter signal

Contact Info

Product

Resources

About