Atlantic Remote Sensing Land Ocean Experiment Research Articles

Tidal and storm-surge measurements with single-site CODAR

In this paper we describe the results of a detailed analysis of CODAR measurements made from a single site during the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE). The passage of a storm front generated a surge followed by wind and current reversal, superimposed on the normal semi-diurnal tides. These data have been used previously to illustrate methods for the extraction of surface-current velocities from CODAR data [1]. Here we examine the unique spatial detail provided by CODAR out to 25 km and supported by other instrumentation during this storm. A picture of the water dynamics emerges that is entirely consistent with the physical processes expected over these spatial scales.

CALCULATION OF DIRECTIONAL WAVE SPECTRA BY THE MAXIMUM ENTROPY METHOD OF SPECTRAL ANALYSIS

Two analysis techniques for calculating directional wave spectra from measured pressure and biaxial current components were intercompared using data from the 25 October 1980 Atlantic Remote Sensing Land Ocean Experiment (ARSLOE) storm. The two methods are the conventional Fast Fourier Transform (FFT) method and a Maximum Entropy Method (MEM). The MEM is a nonlinear data adaptive method of spectral analysis which is capable of generating higher resolution spectral estimates from shorter data records than conventional FFT methods. The MEM has shown good agreement with the frequency and directional wave spectra calculated using conventional methods.

Least-squares methods for the extraction of surface currents from CODAR crossed-loop data: Application at ARSLOE

Least-squares methods are demonstrated that extract surface current radial velocities from first-order Coastal Ocean Dynamics Applications Radar (CODAR) sea-echo Doppler spectra for the compact crossed-loop/monopole antenna system. Based on the known physics of first-order sea scatter at HF, these techniques, implemented as software, are objective and automatic in that they: a) determine from the sea-echo phase and amplitude correction factors for the antenna elements; b) separate the first-order spectrum from the surrounding continuum for arbitrarily varying current conditions; c) using statistical hypothesis testing, select and use either a single or dual-angle model for radial current patterns, whichever best fits the data; d) calculate angles associated with given radial velocities; e) combine the data into a polar-coordinate map of radial velocity versus position; and f) calculate radial velocity uncertainties at each point on the map. In addition, as interpretive aids, two methods are evaluated and compared that provide total current vectors from single-site CODAR data, along with their uncertainties: model fitting and the application of the equation of continuity. It is shown how these methods can be applied to the older, CODAR 4-element antenna system, however, the following advantages of the crossed-loop/monopole system are discussed: it is physically more compact; analysis procedures are more efficient; resulting current velocities are more accurate, because there are no side-lobe problems; and finally, it also gives the ocean wave-height directional spectrum. These methods are tested and optimized against data taken during the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE) storm (October 23-27, 1980), when surface currents varied in speed between 0-50 cm/s and over nearly 300° in angle. Current velocities were measured to a range of 36 km from the radar. Standard deviations in angle are typically 1°-3°; these translate to 2-3 cm/s rms radial velocity uncertainties over most of the coverage area, with decreased accuracy in angular sectors nearest the coast. Total current velocity vectors in strips parallel to shore obtained from model fitting have typical speed and angle uncertainties of 4 cm/s and 12°, respectively. Of the several formulations for the equation of continuity evaluated here, the best gave uncertainties of 5 cm/s, 12° at the closest range cells; these values increase rapidly with range to exceed 20 cm/s, 30° for distances greater than 20 km. The surface currents were observed to follow the wind throughout most of the storm at ARSLOE, but the current was almost always more closely parallel to the shore than the wind. An interesting exception occurred when the onshore storm wind that had prevailed for two days ceased; there was a rush of surface current directly offshore as the storm-surge sea level dropped. The surface current speed measured by CODAR in the upper meter of the ocean was, on the average, 2.1 percent of the windspeed.

Accuracy characteristics of the electronics of waverider buoys used in the ARSLOE

During the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE), October-November, 1980, a large number of meteorological and oceanographic instruments, especially wave-measurement instruments, were deployed in the ocean near the Army Corps of Engineers Pier, Duck, NC. About nine Waverider buoys were deployed in an approximately rectangular pattern, about 30 km wide by 40 km seaward. The Waveriders were an accepted reference for study of other wave-measurement instrument systems. Almost all of the Waverider buoys were calibrated on the rotating-arm facility of the NOAA, Engineering Support Office. Pre- and post-deployment calibrations formed the basis for tables and equations for correction of the Waverider measurements. The calibrations discussed here established that the Waveriders were measuring lower than specification, averaging about 5 percent low (10 percent for variance spectra coefficients.) Corrections for effects of fluctuations in water temperature on Waverider sensitivity were required and are provided. The manufacturer's modification to prevent future drift in sensitivity is also described.

Directional wave spectra measured with a cloverleaf buoy during ARSLOE

The cloverleaf buoy is designed to determine the directional wave spectra with high directional resolution by measuring the vertical acceleration, surface slope, and curvature of the ocean wave surface. This paper describes the properties of the directional wave spectra measured with the cloverleaf buoy during the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE). It is shown that the directional wave spectra measured under relatively constant wind agree fairly well with the similarity spectrum reported previously, but some differences are found in the spectral parameters. The differences in the scale parameters are attributed to unstable atmospheric conditions, though reasonable explanations for those in the shape parameters are difficult now.

Performance of the CCIW wave direction buoy at ARSLOE

The performance at the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE) of the Canada Centre for Inland Waters (CCIW) wave direction buoy is examined. The waveheight and period data show good agreement with data from neighboring buoys. Nondimensional energy and fetch for wind sea cases agree well with data collected from fixed sensors in Lake Ontario. Direction measurements for a particularly well-defined swell agree well with the Experimental Environmental Research Buoy (XERB) measurements. Meteorological data are compared with that collected at the XERB. Windspeed estimates are reasonably well-correlated and much of the scatter is due to the 24-kin separation between the buoys. There is some systematic bias in the wind direction estimates.

Comparison of offshore wave measurements

Preliminary results of a comparison between several wave measurement systems used during the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE) are presented. Measurements in the offshore region 12-40 km offshore, taken during a storm that occurred October 23-26, 1980, are compared. Disagreement between significant waveheights resulting from the different measurement systems in some cases are larger than can be explained by random variability.