Addition to WEGA Cruise and Result Presentation - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 152

Fig. 9.7 Measuring instrument ADCP with yellow protecting above sensors ( left ) and ADCP with
four red transducer faces within ground rack ( middle ); right image shows construction of ADCP
ground rack ( 1 ), weights ( 2 ), and buoy ( 3 ) connected by cable rope ( 4 ) before dropping
travel, they ricochet off particles suspended in the moving water and reflect them
back to the instrument. Due to the Doppler effect, sound waves bouncing back from
a particle moving away from the profiler have a slightly lowered frequency when
they return. Particles moving toward the instrument send back higher frequency
waves. Then the difference in frequency between the waves the profiler sends out
and the waves it receives is called the Doppler shift. The shift is used to calculate
how fast the particle and the water around it are moving. But ADCP does not send a
single wave but several pulses—broadband technology. So, finally, it is not the
difference of frequency between the emitted wave and the reflected wave that is
measured but the variation of phase between several reflected pulses. The depth of
velocity through the measured profiles is calculated by considering the time of
return of the wave and the speed of sound. The column of water is partitioned into
vertical elements (bins), and the ADCP “listens” to the reflected echoes at different
time intervals, which correspond to given depths.
During the WEGA cruise 141, three ADCPs were sunk into the North Sea at
three different positions around the wind farm alpha ventus . They were dropped in
the morning of 11 May 2013 and collected in the morning of 13 May 2013. At 08:30
UTC, all three instruments started measures on 11 May 2013.
The ADCP worked with a broadband of 614.4 kHz with 50 pings per ensemble, a
time per ping of 12.0 00 , thus an ensemble interval of 600.0 s and a bin size in the
vertical of 0.5 m, so 60 vertical elements.
The postprocessing of ADCP data included a filtering of the tidal signal. That
was necessary to actually have a change to detect upwelling and downwelling
structures in the data set because, here, tides impose the dominant signal on each
dynamical measurement. The done tide filtering followed the principle of the
harmonic analysis.
The ADCP measurements are illustrated in Fig. 9.8 . On the one hand, data are
shown, including the tidal signal; on the other hand, the tidal signal is filtered by the
harmonic analysis. The order of vertical velocities, after tidal extraction, is higher
than in the case of model simulation, shown in Fig. 9.9 . For most of the time, the