Florian Schirmer

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.

Experimental Determination of Properties of the Scholte Wave in the Bottom of the North Sea

An experiment was carried out in the North Sea with the objective of exciting the Scholte wave and investigating its propagation. Shots of 3 and 10 kg TNT at the sea bottom provided the seismic source. 3-component seismometers were set up at distances between 0.8 and 1.7 km from the source. The seismic signal was transmitted by radio to the recording ship. An elastic wave with a propagation velocity of 111 m/s was detected. The hodograph shows a vertical, polarized, retrograde ellipse. The observed frequencies are in the range 4.5 ± 1 Hz. The vertical component accounts for around 64% of the elastic energy as measured at the sediment/water interface. After subtraction of the geometric attenuation (Er prop. 1/R), it was possible to determine the order of magnitude of attenuation of the Scholte wave (7.10-3 dB/m).

Oberflächenströmungen vor Sylt — Radarmessungen im Herbst 1983

Vom 22. September bis zum 4. Oktober 1983 wurden mit zwei Radarstationen von Sylt aus Oberflachenstromungen gemessen. Die zeitliche Abtastrate der 274 Messungen variiert zwischen 1/2 und 2 Studen. Die 2 dimensionalen Stromungsgeschwindigkeiten liegen in einem Gitternetz mit 3 km Auflosung und einer mittleren Ausdehnung von 30 km×30 km vor. Fur die einzelnen Gitterpunkte werden Zeitserien erstellt, deren gezeiten- und windgetriebene Anteile mit der Methode der kleinsten Quadrate bestimmt werden. Neben der dominanten halbtagigen Gezeit wird ein starker vierteltagiger Anteil gefunden. Der Zusammenhang zwischen Stromungs- und Windgeschwindigkeit erweist sich als linear mit einem Proportionalitats-faktor von 0,018 fur die Nord- und 0,012 fur die Ostkomponente. Die Gezeitenellipsen der halbtagigen Gezeit zeigen eine deutliche Abhangigkeit von der Topographie.

Horizontal and temporal variability of surface currents in the “Lübecker Bucht”, as measured by radar

Surface currents, as measured by two land-based HF (high frequency) radar stations, are presented. The experiment was performed in the Lubecker Bucht (western Baltic Sea) from April 14 to 23, 1983. Maps of two-dimensional currents were obtained every 2 h. The data processing is discussed with emphasis to a method allowing the determination of horizontal derivatives. Maps of the remotely measured current velocities are shown as well as time series, which are compared with data from current meters at different depths. The ratio of current-to-wind speed is determined for both horizontal components at different positions within the measuring area. Time series of divergence and curl are presented, and the magnitude of the linear (local) and nonlinear (advective) acceleration are compared.

Evaluation of the North Sea, Joining in Situ and Remotely Sensed Data with Model Results

The complicated distribution and the rapid changes of water masses in the North Sea make it difficult to interpret data of ship survey cruises as quasi synoptic observations. Isoline maps of any concentrations derived from cruises of several weeks’ length do not show an actual situation but a mixture of the spatial distribution as well as the temporal development, which cannot be separated from each other. The plotted distribution includes effects of phytoplankton growth and the advective transport of water bodies with different constituents. Furthermore, the variability of other sources and sinks such as erosion of sediment and resuspension of particles or the input from rivers and through the atmosphere modify the distribution during the ship cruise, particularly in cases of strong meteorological events. In Chapter 4.7 Bohle-Carbonell analyzes the ZISCHSTAR cruise data set with respect to these shortcomings of ship observations. One possibility to improve the signification of observations is the additional use of remote sensing data. In the past 20 years, remote sensing techniques have been developed to observe and measure sea surface parameters such as temperature, sea state, currents and sea water constituents from aircraft, satellites and ship or ground based stations. Another possibility for assessing observed data is numerical modelling.

Schallausbreitung im Schlick

Es wird uber ein Experiment berichtet, bei dem neben der Schallgeschwindigkeit die Schalldampfung in den obersten Metern des Meeresbodens bestimmt wird. Zwei im Ostseeschlick befindliche Hydrofone registrieren dazu die Kompressions-Schallwelle einer kleinen Sprengstoff-Detonation in unterschiedlicher Entfernung. Die gemessene Schalldampfung betragt bei 1 kHz etwa 0,2 dB/m und verlauft proportional zu Frequenz hoch 2,3. Ahnliche Messungen im benachbarten Frequenzbereich werden zum Vergleich herangezogen. Die Mesfehler werden diskutiert.

Local Variability of Surface Currents Based on HF-Radar Measurements

Surface currents which transport oil and surface pollutants, whether shorewards or out to sea, depend on many factors. These include winds, tides, waves, the coastline contour and geophysical forces such as gravity and the Coriolis force. Thus, near-surface offshore currents will be highly variable from area to area and at different times. While the ultimate objective is to predict these currents as a function of the driving forces, their accurate measurement is the necessary first step in the development and testing of such a prediction model. In fact, when near-surface currents are known, it is theoretically possible to numerically calculate the currents and circulation all the way down to the ocean floor.

Schallgeschwindigkeit und Impedanz des oberen Meeresbodens in Gebieten der Nordsee, des Skagerraks, des Kattegats und der Ostsee

An frisch gezogenen Sedimentkernen wurde die Schallgeschwindigkeit der Kompressionswelle in Abhangigkeit von der Sedimenttiefe gemessen. Weiter wurden Dichte und Porositat bestimmt. Aus der Dichte und der Schallgeschwindigkeit last sich die Impedanz ermitteln. Es werden Darstellungen uber den Schallgeschwindigkeits- und Impedanzverlauf mit der Teufe fur verschiedene Seegebiete gezeigt und gebietsspezifische Unterschiede diskutiert. Auserdem wird der Zusammenhang zwischen Schallgeschwindigkeit und Dichte sowie zwischen Schallgeschwindigkeit und Porositat dargestellt. Empirisch ermittelte Formeln fur diese Zusammenhange werden angegeben.