Comparison of surface currents measured by HF Doppler radar in the western Florida Straits during November 1983 to January 1984 and Florida Current transports

Abstract

An evaluation of surface currents measured by HF radar during November 29, 1983, to January 31, 1984, with radar sites at Jupiter and Stuart on the Florida east coast is carried out in comparison with currents and transports measured by moorings and submarine cable. While an earlier analysis of currents measured in summer 1983 with radars located at Palm Beach and Jupiter (Schott et al., 1985) found significant northward shear in the northward radar currents about 20 km offshore leading to concerns about a possible bias in the radar currents, this effect was not observed in the second application farther north. It is possible that the shear in the summer 1983 field might have been real and related to the topography in the southern part of the 1983 radar field where no intercomparison current data had been available. Concerning the usefulness of radar currents as Florida Current transport indicators, which was the prime intention of their application in the context of the Subtropical Atlantic Climate Studies, this second study finds much more encouraging results than the one based on the observations of summer 1983. While the first study was inconclusive because only small transport fluctuations occurred during the summer 1983 observation period, this second study finds significant correlation. Florida Current transport fluctuations had a total range of 15 x 106 m3/s during the second observation period, and correlation with downstream radar currents, averaged zonally across the center of the radar field, was 0.85. Coherence was significant for periods longer than 5 days. Highest correlation with transport was found for radar currents farthest out, to the right of the axis of the stream. S. A. Frisch, NOAA/Wave Propagation Laboratory, 325 Broadway, Boulder, CO 80307.

Interpretation of Coastal HF Radar–Derived Surface Currents with High-Resolution Drifter Data

“…It indicates that unresolved subgrid-scale velocity variance contributes significantly to differences between velocities observed by HF radars and velocities observed with point measurement approaches such as current meters. Comparisons using single drifter observations give RMS differences near 10 cm s Ϫ1 , consistent with differences reported in previous studies comparing HF radar and current meter measurements (e.g., Janopaul et al 1982;Schott et al 1986;Graber et al 1997;Chapman et al 1997;Essen et al 2000;Emery et al 2004). Here, minimum differences between HF radar and drifter velocities are ϳ5 cm s Ϫ1 , and occur when 15-30 drifter observations are incorporated into velocity averages.…”

“…Differences between HF radar-and current meter-derived velocities near 10-15 cm s Ϫ1 have been reported by Holbrook and Frisch (1981), Janopaul et al (1982), and Schott et al (1986). More recently, Chapman et al (1997) used shipborne current meter data to suggest the upper bound of HF radar accuracy is 7-8 cm s Ϫ1 .…”

Genesis of the Gulf Stream Subseasonal Variability in the Florida Straits

“…The Florida Current (FC), the headwaters of the Gulf Stream, is an important component of the meridional overturning circulation in the subtropical North Atlantic. It is a highly variable, surface‐intensified flow, with variations spanning a broad range of timescales from days to years (e.g., Beal et al., 2008; Lee et al., 1985; Meinen et al., 2010; Schott et al., 1986). The FC has been monitored nearly continuously since 1982 using a submarine telephone cable between Florida and the Bahamas (inset of Figure 1) (e.g., Baringer & Larsen, 2001).…”

Transport properties in small-scale coastal flows: relative dispersion from VHF radar measurements in the Gulf of La Spezia

“…HF radars operate in a frequency range of 12-30 MHz with associated range of 40-100 km and space resolution of several kilometers, while VHF radars configured in a frequency range of 40-50 MHz can resolve the motion at several hundreds of meters, thereby providing information on both submesoscale and mesoscale flows. Quantitative assessments of radar velocity measurements have been provided by a number of studies, comparing radar results with in situ measurements from various platforms such as moored current meters, ADCP profilers, and drifters (Steward and Joy 1974;Schott et al 1986;Shay et al 1998aShay et al , b, 2001Shay et al , 2007Paduan and Rosenfeld 1996;Kaplan et al 2005;Paduan et al 2006).…”