Heinz-Hermann Essen

Wellen Radar (WERA): a new ground-wave HF radar for ocean remote sensing

HF radars can be used to measure surface currents and wave spectra. The Coastal Radar (CODAR) used by the University of Hamburg was designed for current mapping only. It has been operated for 15 field experiments during the past 15 years. Recently, a new HF radar called Wellen Radar (WERA) has been developed at the University of Hamburg. One main advantage of the system is the possibility of connecting different configurations of receive antennas. When operated with a linear array, information on the sea state can be obtained via second-order spectral bands. A further advantage is the flexibility in range resolution between 0.3 and 1.2 km, instead of the fixed resolution of about 2 km of CODAR. This is achieved by transmitting frequency-modulated continuous wave (FMCW) chirps instead of continuous wave (CW) pulses. In addition, this technique avoids the blind range of about 3 km in front of the CODAR. The technical design of WERA is described and first experimental results are presented.

Microseismological evidence for a changing wave climate in the northeast Atlantic Ocean

One possible consequence of a change in climate over the past several decades is an increase in wave heights, potentially threatening coastal areas as well as the marine industry. But the difficulties in observing wave heights exacerbates a general problem of climate-change detection: inhomogeneities in long-term observational records owing to changes in the instruments or techniques used, which may cause artificial trends. Ground movements with periods of 4–16 seconds, known as microseisms, are associated with ocean waves and coastal surf , and have been recorded continuously since the early days of seismology. Here we use such a 40-year record of wintertime microseisms from Hamburg, Germany, to reconstruct the wave climate in the northeast Atlantic Ocean. For the period 1954–77, we detect an average of seven days per month with strong microseismic activity, without a significant trend. This number increases significantly in the second half of the record, reaching approximately 14 days of strong microseisms per month. The implied increase in northeast Atlantic wave height over the past 20 years parallels increased surface air temperatures and storminess in this region, suggesting a common forcing.

An Empirical Method to Derive Ocean Waves From Second-Order Bragg Scattering: Prospects and Limitations

High-frequency (HF) radar wave processing is often based on the inversion of the Barrick-Weber equations, introduced in 1977. This theory reaches its limitations if the length of the Bragg-scattering wave raises to the order of the significant waveheight, because some assumptions are no longer met. In this case, the only solution is moving to lower radar frequencies, which is not possible or desirable in all cases. This paper describes work on an empirical solution which intends to overcome this limitation. However, during high sea state, the first-order Bragg peaks sometimes could not be clearly identified which avoids the access to the second-order sidebands. These cases cause problems to the algorithm which have not been solved yet and currently limit the maximum significant waveheight to about the same values as reported for the integral inversion method. The regression parameters of the empirical solution calibrated from the European Radar Ocean Sensing (EuroROSE) data set are constant values for the complete experiment and when applied to the HF radar data they reconstruct the measurements by a colocated wave buoy quite well. When including a radar-frequency-dependent scaling factor to the regression parameters, the new algorithm can also be used at different radar frequencies. The second-order frequency bands used for the empirical solution are sometimes disturbed by radio interference and ship echoes. Investigations are presented to identify and solve these situations

On the performance of a shipborne current mapping HF radar

High-frequency (HF) radars have been developed to map surface currents offshore by means of land-based stations. Presently available radar systems use frequencies between 25 and 30 MHz and allow a spatial resolution of 1 km and ranges of up to 50 km. This paper reports on the experience with a shipborne radar and discusses problems which arise for the azimuthal resolution on a metal ship, the correction for the ships speed, and limitations due to pitch-and-roll motions. Current measurements during cruises to the North Atlantic are presented. It has been found that, with the support of the satellite-supported Global Positioning System, the shipborne HF radar can measure surface current velocities with an accuracy of some 5 cm/spl middot/s/sup -1/.

Tidal and wind-driven parts of surface currents, as measured by radar

In December 1982, surface current measurements by means of an HF radar station were carried out from the Federal German island of Sylt in the North Sea. Two carrier frequencies (25.25 MHz and 29.85 MHz) were tested with respect to their applicability. Hourly sampled 70-hours time series of (1-dimensional) radial currents at different distances and directions from the radar are analyzed with the objective of determining the tidal and wind-driven parts. Assuming a homogeneous current field, semidiurnal tidal ellipses are synthesized and compared with currents, as documented in the “Atlas der Gezeitenstrome in der Deutschen Bucht” (Deutsches Hydrographisches Institut [1983]).

On near-shore surface current measurements by means of radar

A radar system is presented which allows the measurement of surface currents in a coastal area of about 50 km×50 km. The basic theoretical ideas of this system are described as well as the measuring equipment and date processing developed by Barrick, Evans and Weber [1977] from NOAA (National Oceanic and Atmospheric Administration). Radar data are available from the German Bight for a 26-hour period during MARSEN 1979 (Marine Remote Sensing Experiment in the North Sea). The data have been evaluated in terms of surface currents and compared with a record from a moored current meter, 7 metres below the surface. According to the comparison surface currents as observed by radar differ from the conventionally measured subsurface currents not more than 15 cm s−1 in speed. With regard to the current direction the agreement between both independent measurements seems to be best (within 10 degrees) when the surface-current speed exceeds 30 cm s−1. However, these comparative numbers do not take into account near-surface vertical shears. Thus, currents measured by means of radar are probably more accurate than those numbers indicate.

CODAR in Germany--A status report valid November 1985

The use of CODAR by the University of Hamburg has extended to a wide variety of experimental and oceanographic activities over the last three years. These have ranged from Arctic studies from land and ships to observations of the Dead Sea, all yielding surface current data. Hardware improvements have been investigated, including IF amplifier changes and loop-antenna arrays for shipboard operation.

Ekman portion of surface currents, as measured by radar in different areas

Surface currents, as measured by the HF-radar CODAR (COstal raDAR), are investigated with respect to their dependence on wind. Time series of wind are available from single weather stations, while CODAR yields current velocities on a grid with a resolution of some 3 km. As part of various research programs experiments have been carried out by the University of Hamburg (Germany) in different areas. Two of the areas under consideration are located in the Baltic Sea, two in the North Sea, and one covering the northern part of the Dead Sea. Time series of about two weeks with 2-hourly sampling are available for some 50 grid points of each area. Vector-correlation techniques are used to determine the linear relation of surface currents on wind velocity and also windstress. In both cases, significant correlation of about the same order has been found. 35% to 60% of the variance in surface currents may be explained by linear forcing from the vectors of wind velocity or windstress. Current-to-wind ratios range from 0.015 to 0.025, and a veering to the right of currents against wind has been observed. The decomposition of horizontal two-dimensional current fields into empirical orthogonal eigenfunctions (EOF) yields higher amounts of variance in the 1. EOF as may be explained by linear windforcing. Two possible mechanisms for wind-driven currents are discussed, Ekman circulation and Stokes drift. Assuming the vertical eddy viscosity to be independent on wind velocity and water depth, it may be estimated. Values of about 5×10−4 m2s−1 are found in the Baltic and 20×10−4 m2s−1 in the North Sea, which are in reasonable agreement with those used in numerical models.

Does microseisms in Hamburg (Germany) reflect the wave climate in the North Atlantic

The possibility of monitoring climate variability on decadal scales by means of microseisms is investigated. For this purpose, digital microseismic data from Hamburg (Germany) of the six winters 1992/93 to 1997/98 are compared with model ocean-wave fields of the North Atlantic. A correlation coefficient of aboutr= 0.7 was found between the linear ocean-wave amplitude at the Norwegian coast and the square-root of the microseismic amplitude. Considering monthly means of the energy of both ocean waves and microseisms the correlation exceedsr= 0.8 in special areas. A correlation coefficient ofr = 0.6 was found between the monthly winter index of the North Atlantic Oscillation (NAO) and the respectively averaged microseismic energy. Encouraged by these results, further investigations are planned for analysing earlier microseisms which has been recorded in Hamburg since 1905.

Measurement of ocean wave height and direction by means of HF radar: An empirical approach

High-frequency (HF) radar has been used for 20 years for remote sensing of ocean surface currents and ocean waves. Backscattered Doppler spectra contain two discrete lines, whose frequencies (Bragg frequency) determine the current speed, and four continuous side bands enabling inversion techniques to be used for retrieving ocean wave spectra. Recently, a new HF radar has been developed at the University of Hamburg (Germany). Data of a 34-day experiment reveal a high correlation between the standard deviation of the Bragg frequencies and the significant wave height weighted by an azimuthal function. Applying empirical regression curves it is possible to determine the significant wave height and the mean wave direction from intersecting beams of two radar stations. Compared to inversion techniques, the new method is applicable to data with a lower signal-to-noise ratio, i.e. it allows larger ranges. Two radar sites are required for current measurements. The optimum distance between two 30 MHz radars is about 20 km and, with the new method, needs not be reduced for the purpose of simultaneous wave measurements.