Lowered Acoustic Doppler Current Profiler data

The analysis of Lowered Acoustic Doppler Current Profiler (LADCP) data is described in Chapter!4, but the basic steps of the initial processing are explained here. The LADCP data measurements were output in terms of depth (m). For the further analysis, depth was converted to pressure (Saunders, 1981) and the processed velocities were projected onto a 5!db grid, so as to be consistent with the CTD data.

The LADCP measures instantaneous scatterer relative velocities of the water column and these can be converted into profiles of absolute current by an elaborate processing path. The scatterer velocities are measured by utilising the Doppler frequency shift, phase changes and correlation between coded pulses transmitted and received by four transducers. Given the geometry of the transducer set, and the orientation/motion of the package, the along beam velocities are transformed into earth coordinates to give north, east and vertical current motion relative to the CTD package for each of the ensemble bins.

3.6.1 Processing

The raw velocity measurements are a combination of the true ocean currents and the motion of the instrument. Individual pings from the ADCP result in a number of overlapping velocity profiles within a range of 100!-!200 m from the instrument. These velocity profiles are relative to the unknown instrument velocity. Vertical differentiation produces a series of overlapping shear profiles. These are interpolated on to a uniform depth grid and averaged to give a composite shear profile. The data is then integrated up over the cast to produce a shear profile with zero net velocity. This process removes not only the unknown instrument velocity, but also the barotropic component.

Chapter 3: Data 52

This must be reinstated either from the ship displacement (recorded from differential GPS data) or from the relative motion of the package over the sea floor (bottom tracking). CTD data is used in the latter stages of the processing, firstly to correct the depths of the ensemble bins through the matching of the CTD data to the vertical velocity of the package as measured by the LADCP and secondly to provide in-situ sound speed values for these depths. The final velocity profile is therefore the sum of the baroclinic and barotropic components.

The method assumes that successive overlapping velocity profiles, individually covering a fraction of the water column, can be used to obtain full depth integrated profiles of absolute velocities. For all LADCP data used in this thesis the instrument was deployed as part of a CTD/rosette package and lowered through the water column during particular hydrographic casts. The data were processed using software created by Professor E. Firing of the University of Hawaii, following a now well-used method (Beal, 1997; Cunningham et al., 2003).

Following Beal (1997) the velocity measured by the LADCP can be written as three components: Ubaroclinic, the baroclinic component (calculated from the velocity shear); Ubarotropic, the unknown

barotropic component of the ocean current; and ULADCP, the unknown component from the motion of

the LADCP through the water;

Umeasured(t) = Ubarotropic + Ubaroclinic[z(t)] - ULADCP(t) 3.2

The motion of the LADCP includes Uship, the movement of the ship relative to the ground, and

Uinstrument, the motion of the instrument relative to the ship,

ULADCP = Uship + Uinstrument 3.3

The first component of equation 3.3 (Uship) is determined from the ship GPS positioning data. When

equation 3.2 is integrated in time over the period of the whole cast T, the second (unknown) component of equation 3.3 (Uinstrument) vanishes since the instrument must begin and end the cast at

the ship. Hence,

Ubarotropic= 1 T ₀Umeasured(t) T

Ú

Ú

Ú

Finally, the absolute ocean current is given by,

Uocean(t) = Ubaroclinic[z(t)] + Ubarotropic 3.5

These final velocity profiles are the sum of the baroclinic and barotropic components (over the full water column except the surface 50!m and the bottom 30!m) and are hereafter referred to as the Water Track (WT) data.

Bottom tracking was also used to obtain absolute velocity profiles when the bottom was within range of the instrument. As the LADCP approaches the ocean floor, the strong resulting backscatter

Chapter 3: Data 53

can be used to obtain a ‘bottom velocity’ which is essentially a direct measurement of the motion of the instrument relative to the stationary sea-bed. Each 2!second ensemble consists of a BT ping followed by a WT ping. BT pings are used to determine height off bottom and absolute velocity of the package; WT pings determine velocity of the water relative to the package. Absolute velocity of water over the ground is then a simple vector addition of the BT and WT velocities. Typically, near-bottom velocities can be determined between 50 and 250!m off bottom, in water depths greater than 3000m. The processing of bottom track data involves extracting the relevant position and magnetic variation from the standard LADCP data, and reading in the CTD data for that cast. The sound velocity is calculated from the CTD pressure, temperature and salinity data in order to correct the depth of the LADCP bins relative to the instrument, and to correct the measured velocities. The water depth is extracted from the LADCP processing files and the absolute near bottom water velocities are calculated by subtracting the bottom velocities from the water track velocities. All velocities are then averaged back onto the original bins (the bins are displaced by the sound speed correction). The magnetic declination is then used to correct the compass involving a simple vector rotation. The final velocity profiles are hereafter referred to as Bottom Track (BT) data.

3.6.2 Accuracy

LADCP velocities have an accumulative error which can be estimated by a random walk, if no instrument bias is assumed. This is derived when each shear profile is strung together over the depth of the cast (Firing and Gordon, 1990). A bad velocity measurement occurring in a bin at the centre of a profile will result in negatively correlated shear to either side of it (i.e. a peak). If a bad velocity occurs in the last bin of a profile, the shear is offset and doesn't recover. This offset will then affect the remaining shear data as more profiles are added.

For a 150 kHz ADCP over a profile range of around 160!m with 16!m bins, the average standard deviation of each WT and BT velocity measurement is quoted to be 1!cm!s-1 _{(RDinstruments, 1995).}

For WT data, additional uncertainty is introduced by the determination of the barotropic component of the flow. This component was derived either from the displacement of the vessel as recorded by differential GPS (water-tracking) or, when the instrument was within ~!200!m of the bottom, from the motion of the LADCP relative to the ocean floor (bottom-tracking) (Fischer and Visbeck, 1993). For BT data, if we assume that the absolute velocity error has a contribution from the bottom track and water track, and there are about 50 independent estimates of velocity in each 5!m bin, then the error of the near-bottom velocities, x, is approximately,

† x= 1 2 +12 50 =0.2 cm s -1 . 3.6

This means that errors in the BT data are an order of magnitude less than the stochastic error in a typical top to bottom profile derived from water track shear estimates (Cunningham et al., 2003).

Chapter 3: Data 54

The characteristic accuracy of the barotropic component of the LADCP flow is estimated to be 3!cm!s-1 _{by calculating the root-mean-square (rms) deviation between the WT and BT velocities over}

their common depth range. The mean deviation was 0.06!cm!s-1_{, which is not significantly different}

from zero at the 99% confidence limit.