Abstract:

This dataset reports waves, near-surface currents and sea surface roughness derived from X-band radar aboard R/V F.G. Walton Smith cruise WS17107 during the Submesoscale Processes and Lagrangian Analysis on the Shelf (SPLASH) experiment from 2017-04-17 to 2017-04-29. The radar backscatter images were processed to derive sea surface roughness, waves and currents. It was obtained from a coherent-on-receive marine X-band (9.5 GHz) radar (Braun et al., 2008), developed at Helmholtz Zentrum Geesthacht (HZG), Germany. The data includes backscatter images collected every 2-min that can be used to identify surface features such as fronts. The data also includes current velocity maps obtained by processing series of these images using 3-D Fast Fourier Transforms (FFTs) and overlays of the current maps over averaged backscatter images. This dataset supports the publication: Lund, B., Haus, B. K., Graber, H. C., Horstmann, J., Carrasco, R., Novelli, G., Guigand, C.M., Mehta, S., Laxague, N.J.M. and Özgökmen, T. M. (2020). Marine X‐Band Radar Currents and Bathymetry: An Argument for a Wave Number‐Dependent Retrieval Method. Journal of Geophysical Research: Oceans, 125(2). doi:10.1029/2019jc015618.

Suggested Citation:

Lund, Bjoern and Brian K. Haus. 2021. X-band radar measurements collected during the Submesoscale Processes and Lagrangian Analysis on the Shelf (SPLASH) experiment aboard R/V F.G. Walton Smith cruise WS17107 in the Gulf of Mexico from 2017-04-17 to 2017-04-29. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/C1G3B907

Publications:

Lund, B., Haus, B. K., Graber, H. C., Horstmann, J., Carrasco, R., Novelli, G., … Özgökmen, T. M. (2020). Marine X‐Band Radar Currents and Bathymetry: An Argument for a Wave Number‐Dependent Retrieval Method. Journal of Geophysical Research: Oceans, 125(2). doi:10.1029/2019jc015618

Purpose:

This data had three purposes: 1) to identify oceanic frontal features from backscatter images within a 3-km radius of the RV Walton Smith, 2) to measure ocean wave spectra from wave patterns observed in the backscatter images, and 3) to map oceanic near-surface currents using temporal stacks of consecutive images.

Data Parameters and Units:

The dataset contains netCDF files reporting waves, near-surface currents and sea surface roughness derived from X-band radar, kmz files for backscatter and a quick view map of the backscatter and derived near-surface currents. The dataset also includes the movies of wave energy frequency direction spectra (Wave-Energy-Frequency-Direction-Spectra.mp4) and temporally averaged radar images (Radar-Imagery.mp4). The main parameters included are described below: Marine X-band radar near-surface currents and bathymetry data: Backscatter strength (Db, grey scale 16 bit images); ux [zonal component (west-east direction) of near-surface current, m/s]; uy [meridional component (south-north direction) of near-surface current, m/s]; k [mean wavenumber, rad/m]; kDepth [mean wavenumber for depth fit, rad/m]; longitude [longitude, degrees east]; latitude [latitude, degrees north]; startTime [start time in year days; 0.5 corresponds to 01/01/2017, 12:00 UTC (noon), days]; endTime [end time in year days; 0.5 corresponds to 01/01/2017, 12:00 UTC (noon), days]; depth [water depth, m from sea surface]; gebcoDepth [GEBCO water depth, m from sea surface]; uxProfile [zonal components (west-east direction) of near-surface current as function of wavenumber, m/s]; uyProfile [meridional components (south-north direction) of near-surface current as function of wavenumber, m/s]; kProfile [current profile wavenumbers, rad/m]; nSpec [number of contributing spectra]; minRange [minimum distance from radar antenna over all contributing images, m]; nCoords [number of fit spectral coordinates]; depthNCoords [number of depth fit spectral coordinates]; nCoordsProfile [number of fit spectral coordinates as function of wavenumber]. Marine X-band radar wave parameters: time(x)[the wave analysis start time in year days and UTC with Jan 1, 12:00p (noon) corresponding to 0.5. The wave analysis period is 20 min.]; hs [significant wave height, m]; tp[peak wave period, s]; tm01 [mean wave period, s]; dp [peak wave direction, degrees]. Marine X-band radar wave energy frequency direction spectra: Spec [Frequency direction spectrum, m^2 s]; freq [Frequency, Hz]; theta [corresponding direction, degrees]; time [the center time of the spectrum in year days and UTC with Jan 1, 12:00p (noon) corresponding to 0.5.]. Sea surface roughness data: image [(y, x), relative radar return; mask value is NAN]; xCoords [x coordinates (west-east direction), m]; yCoords [y coordinates (south-north direction), m]; time [year day; 0.5 corresponds to 01/01, 12:00 UTC (noon), days]; imageOriginPosition [geographic coordinates of image origin, i.e., (0 m, 0 m) on (x, y) grid, (degrees east, degrees north)]; readMe [text].

Methods:

A Doppler marine X-band radar (MR) developed at the Helmholtz Zentrum Geesthacht (HZG), Germany, was mounted at ~12.5 m above sea level on a mast on top of the wheel-house of R/V F. G. Walton Smith. The HZG MR is based on a commercial marine X-band (9.4 GHz) radar with a 7.5-foot HH-polarized antenna, 12 kW peak power output, and an antenna rotation period of 2 s. It was modified to operate as a coherent-on-receive system, measuring both phase and intensity of the radar backscatter (Braun et al., 2008). For more detailed methods, please refer to the "data overview" and the "methodology" sections in the associated publication Lund et al., 2020.

Instruments:

The marine X-band radar (MR) has a pulse repetition frequency of 2 kHz when operated with a 50 ns pulse length (short pulse mode), which corresponds to a range resolution of 7.5 m. The horizontal antenna beamwidth is 0.8°. The radar video signal (in the form of in-phase and quadrature components) is linearly amplified and sampled at 20 MHz, yielding a range pixel spacing that matches the radar’s range resolution. Each pulse is sampled up to a maximum range of ~3.1 km and with an image depth of 13 bits.

Error Analysis:

The MR–drifter current comparison presented in Lund et al., 2018 (JTECH) is of unprecedented size and scope. A total of 4130 data pairs (from 240 MR analysis periods corresponding to ~5.8 days of measurements) are available, covering winds from 2-12 m/s. The comparison statistics indicate a measurement accuracy that is significantly better than any previously reported results, with RMS errors of 3.5 cm/s for u and 4.0 cm/s for v (Braun et al., 2008).

Provenance and Historical References:

Braun, N., Ziemer, F., Bezuglov, A., Cysewski, M., & Schymura, G. (2008). Sea-Surface Current Features Observed by Doppler Radar. IEEE Transactions on Geoscience and Remote Sensing, 46(4), 1125–1133. doi:10.1109/tgrs.2007.910221 Lund, B., Haus, B. K., Horstmann, J., Graber, H. C., Carrasco, R., Laxague, N. J. M., Novelli, G., Guigand, C.M., & Özgökmen, T. M. (2018). Near-Surface Current Mapping by Shipboard Marine X-Band Radar: A Validation. Journal of Atmospheric and Oceanic Technology, 35(5), 1077–1090. doi:10.1175/jtech-d-17-0154.1

Individual Files: