Inner shelf lateral exchange
Kirincich, Anthony R.
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This data was collected by Kirincich as part of ongoing studies examining the spatial variability of the mechanisms and process that lead to the exchange of water masses across the inner part of the continental shelf. The data consists of estimates of the near-surface horizontal (East and North) ocean currents made via High Frequency (HF) radar-based remote sensing of the ocean backscatter spectrum. The dataset spans an 18-month period from February 2011 to August 2012. The effective measurement depth of the WHOI HF radars is 0.5 m below the ocean surface.
The WHOI HF radar system, as operated during the 2011-2012 time period, consisted of 3 individual HF radars located along the south coast of the island of Martha's Vineyard, MA at infrastructure of the Martha's Vineyard Coastal Observatory (MVCO). The system had the technical goal of measuring currents at scales approaching 400 m within a 20 km by 20 km domain south of Martha's Vineyard. The system is composed of three closely spaced sites with SeaSonde-type DF instruments running at operating frequencies near 25 MHz. Two of the three sites are located on land, with one placed at the MVCO Shore Meteorological Station (METS) and the second approximately 10 km to the west at the Long Point Wildlife Refuge (LPWR). The third site is located on the MVCO Air-Sea Interaction Tower (ASIT), approximately 4 km offshore and south of the island. To achieve the highest possible radial resolution (420 m) given the 350 kHz of bandwidth available at 25 MHz, all sites run at common frequencies using GPS- based timing to separate the transmissions from each site. Given the small spatial domain that can be adequately sampled at low geometric error with this configuration, as well as potential for interactions between the instruments at the land and tower sites, all sites transmit at low power (1-2 W, less than 3% of typical systems). The MVCO HFRs were configured to maximize the spatial and temporal independence of the observations. Spectral estimates of the observed Doppler-shifted velocities are collected in bursts of 1028 non-overlapping frequency sweeps with a sweep rate of 2 Hz for finer-resolution Doppler velocities than is typical for 25-MHz systems without interpolation. A maximum of three, but normally two, successive spectral estimates are averaged to create the necessary ensemble estimate every 15 min. Direction finding and azimuthal averaging into 5deg bands is performed on each ensemble and, for data processed using the standard software suite, successive radial velocity estimates are time averaged into 60-min averages every 30 min. No interpolation is used to smooth the fields or fill in radial gaps, but, as in previous works, outliers are removed before computing vector velocities. Given the dense spacing of the radial grid points, the vector averaging is performed using a 400-m grid with grid points starting approximately 600 m offshore and an averaging radius equal to the grid width. These alternative methods were used to achieve finer-resolution velocity estimates having greater spatial independence at the potential expense of increased noise. For the datasets used here, a number of steps were taken to ensure that the radial velocity estimates were of the highest quality possible. The first-order region limits utilized for the analysis were optimized for the conditions present south of Martha's Vineyard. Measured antenna patterns were obtained for each site and utilized in the DF algorithm to estimate radial currents over water. Finally, the spatial structure of the M2 tidal ellipses for the entire domain, estimated from the vector velocity time series at each grid point using T_Tide (Pawlowicz et al. 2002), were analyzed for patterns of unrealistic ellipse inclination (orientation) emanating from a particular site, which serve as an indication of potential bearing-related errors. The measured beam patterns were adjusted by smoothing and/or interpolation, similar to that described by Cosoli et al. (2010) and de Paolo and Terrill (2007), to minimize errors identified. The spatial extent of the vector velocities was limited by theoretical Geometrical Dilution of Precision (GDOP) values less than 1.75. An error estimate for the East, North, and total (norm) of the vector velocity components is given. Converted to netCDF via MATLAB by A. Kirincich
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