An Algorithm for Surface Current Retrieval from X-band Marine Radar Images

A new method to determine near‐surface vertical current shear from noncoherent marine X‐band radar (MR) data is introduced. A three‐dimensional fast Fourier transform is employed to obtain the wave number‐frequency spectrum of a MR image sequence. Near‐surface currents are estimated from the Doppler‐shifted surface gravity wave signal within the spectrum. They represent a weighted mean of the upper ocean flow. The longer the ocean waves on which the current estimates are based, the greater their effective depth. The novelty lies in the wave number‐dependent retrieval method, yielding ∼100 independent current estimates at effective depths from ∼2 to 8 m per ∼12 min measurement period. First, MR near‐surface vertical current shear measurements are presented using data collected from R/V Roger Revelle during the 2010 Impact of Typhoons on the Ocean in the Pacific experiment in the Philippine Sea. Shipboard acoustic Doppler current profiler (ADCP) and anemometer measurements as well as WAVEWATCH III (WW3) model results are used to demonstrate that results are in accord with physical expectations. The wind and wave‐driven Ekman flow is obtained by subtracting ADCP‐based background currents from the radar measurements. At ∼2 m, it is on average ∼1.6% of the wind speed and ∼38.9° to the right of the wind. With increasing effective depth, the speed factor decreases and the deflection angle increases. Based on WW3 results, the MR‐sensed Stokes drift speed is ∼50% of the Ekman flow at ∼2 m and ∼25% at ∼8 m. These findings are consistent with previous observations and Ekman theory.

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“…(This step is also used by Shen et al . [] within their MR mean current retrieval algorithm.) For any

true〈 θ, ω true〉

pair, just one k value may be associated with the surface waves, according to the linear dispersion relationship (equation ).…”

Section: Methodsmentioning

confidence: 99%

A new technique for the retrieval of near‐surface vertical current shear from marine X‐band radar images

et al. 2015

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“…The MR‐drifter data pairs they analyzed were acquired near the DeSoto Canyon in the Gulf of Mexico under deep water conditions, that is,

\lim_{h \to \inf} \tanh false(k h false) = 1

. Other studies investigating MR near‐surface currents in deep water or areas with known bathymetry include Bell et al (), Gangeskar (), Hessner et al (), Ludeno et al (), and Shen et al (). This study employs measurements from a later field campaign, which also involved a massive surface drifter deployment, but focused on the much shallower continental shelf waters of the Louisiana Bight, where equation must be considered in full.…”

Section: Introductionmentioning

confidence: 99%

Marine X‐Band Radar Currents and Bathymetry: An Argument for a Wave Number‐Dependent Retrieval Method

et al. 2020

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This study evaluates shipboard marine X‐band radar (MR) near‐surface current and bathymetry measurements under shallow water conditions. The retrieval algorithm is based on the surface wave signal within three‐dimensional wave number frequency MR backscatter intensity variance spectra. The MR data were collected during a research cruise that investigated submesoscale processes and their impact on oil spill transport in the Louisiana Bight. The MR currents and bathymetry are validated using measurements from 500 GPS‐equipped surface drifters, a shipboard acoustic Doppler current profiler, and a shipboard single‐beam echo sounder. Earlier results from the same experiment but using a different set of sensors indicate strong upper ocean vertical current shear over a 3.5 hr period, despite only mild wind forcing. Here, the MR currents are derived as a function of wave number, providing a measure of vertical shear for the full duration of the cruise. Strong vertical shear is frequently observed, with a maximum difference of 0.42 m/s between the MR high (effective depth of ∼0.9 m) and low ( ∼2.4 m) wave number bins. Treating the drifter and echo sounder measurements as truth, the accuracies of the MR near‐surface currents and bathymetry are 0.04–0.07 m/s and 1.2 m (or 7% of the mean water depth). However, it is shown that ∼50% of the MR high wave number and drifter current differences is due to vertical shear. The shallow water MR near‐surface current accuracy thus matches findings from a previous deep water validation where vertical shear was much weaker.

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“…Nowadays, the X-band marine radar, which is commonly used to monitor moving vessels for its economy and popularity and has the capacity of observing the ocean surface with high spatial and temporal resolutions [5][6][7][8], also has been utilized widely to extract the wave spectrum and the variously statistical sea state parameters such as significant wave height, wave peak period, wave direction, bathymetry, sea surface elevation and sea surface currents from coastal areas or moving platforms [9][10][11][12][13][14][15][16]. The retrieving significant wave height from X-band marine radar images is one of the hot issues of ocean remote sensing in recent years.…”

Section: Introductionmentioning

confidence: 99%

Wave Height Estimation from Shadowing Based on the Acquired X-Band Marine Radar Images in Coastal Area

Wei

Pian

et al. 2017

Remote Sensing

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Abstract:In this paper, the retrieving significant wave height from X-band marine radar images based on shadow statistics is investigated, since the retrieving accuracy can not be seriously affected by environmental factors and the method has the advantage of without any external reference to calibrate. However, the accuracy of the significant wave height estimated from the radar image acquired at the near-shore area is not ideal. To solve this problem, the effect of water depth is considered in the theoretical derivation of estimated wave height based on the sea surface slope. And then, an improved retrieving algorithm which is suitable for both in deep water area and shallow water area is developed. In addition, the radar data are sparsely processed in advance in order to achieve high quality edge image for the requirement of shadow statistic algorithm, since the high resolution radar images will lead to angle-blurred for the image edge detection and time-consuming in the estimation of sea surface slope. The data acquired from Pingtan Test Base in Fujian Province were used to verify the effectiveness of the proposed algorithm. The experimental results demonstrate that the improved method which takes into account the water depth is more efficient and effective and has better performance for retrieving significant wave height in the shallow water area, compared to the in situ buoy data as the ground truth and that of the existing shadow statistic method.

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An Algorithm for Surface Current Retrieval from X-band Marine Radar Images

Cited by 37 publications

References 18 publications

A new technique for the retrieval of near‐surface vertical current shear from marine X‐band radar images

A new technique for the retrieval of near‐surface vertical current shear from marine X‐band radar images

Marine X‐Band Radar Currents and Bathymetry: An Argument for a Wave Number‐Dependent Retrieval Method

Wave Height Estimation from Shadowing Based on the Acquired X-Band Marine Radar Images in Coastal Area

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