Bruno, Miguel

Izquierdo, Alfredo

Roque, David

Vázquez, Águeda

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INSTRUMENTATION VIEWPOINTSessions22MEASURING VERTICAL VELOCITIES WITH LOW FREQUENCY ADCP INSTRUMENTS. A CASE OF STUDY IN THE STRAIT OF GIBRALTARM. Bruno (1), D. Roque (2), A. Vázquez (1), A. Izquierdo (1)(1) Departamento de Física Aplicada. Universidad de Cádiz. República Saharaui s/n, 11510, Puerto Real, Cádiz, Spain.34-956016076 miguel.bruno@uca.es.(2) Departamento de Oceanografía, ICMAN-CSIC, Cádiz, Spain.KeyWords: Current velocity, ADCP, internal waves, Strait of Gibraltar1. IntroductionAt present times, low frequency ADCP enjoys a widely extended use in oceanographic studies. However, its application is practically lim-ited to analyse the components of the current velocity in the horizon-tal plane, disregarding the vertical component. Commonly, vertical component is only accounted for with the use of higher frequency systems [1], which have a smaller range and, therefore,  may sample the whole water column, only in very shallow coastal waters. On the other hand, in some singular oceanographic regions characterised by strong currents aff ected by bottom topography, the velocity fi eld may have a clear 3D structure and in this cases, vertical component can be no longer neglected. One of these regions is the Camarinal Sill region in the Strait of Gibraltar. The fi rst reported values of vertical velocities in this area were published in [2], in this case authors used a 150 KHz ADCP. Later, these vertical velocities have been simulated by hydrodynamical numerical models and reproduced values resembles quite well the observations [3]. The objective of this contribution is to demonstrate that a low fre-quency (75 KHz) vessel mounted ADCP is able to measure meaning-ful vertical velocities in the Camarinal Sill region of the Strait of Gi-braltar.2. Results and Discussion In the fi gure 1, it is shown the velocity fi eld measured by a vessel mounted  75 KHz ADCP around Camarinal Sill, in the Strait of Gi-braltar. Vectors are actually, the resultant of summing the horizontal component in the along-strait direction plus the vertical component. By the time of the measurements, an intense tidal current is fl owing toward the Atlantic (to the left in the fi gure). This intense stratifi ed fl ow interacts with the topography and as a result, large amplitude perturbations (internal waves) in the density fi eld  (100 m amplitude) are produced in the lee side of the sill. Consequently, current veloci-ties matching these isopicnal displacements may take values as large as 1 m/s [2]. These observed values  of vertical velocities have been checked using other independent instrumental sources as vertical sections of temperature (built on the base of XBTs launching) and echo-sounder EK500 data.   Fig. 1. Vertical section of  current velocity in the along-strait direc-tion, traversing Camarinal Sill.3. ConclusionsIn the region of Camarinal Sill in the Strait of Gibraltar low frequency 75 KHz ADCP is able to measure the 3D structure of the fl ows there. It is due to the fact that values of vertical velocities takes high values as large as 1 m/s. Comparison with vertical sections of temperature a echo-sounder EK500 traversing Camarinal Sill in the along-strait di-rection, simultaneous to the ADCP records confi rm the reliability of the velocity fi elds.  4. References[1] A. E. Gargett, Observing turbulence with a modifi ed ADCP, J. Atmos. Oce-anic Tech., 11, 1592-1610, 1994.[2] M. Bruno, J. J. Alonso, A. Cózar, J. Vidal, F. Echevarría, J. Ruiz, A. Ruiz-Caña-vate. The boiling water phenomena at Camarinal sill, the Strait of Gibraltar. Deep Sea Research II 49,4097-4113, 2002.[3] A. Vázquez,  N. stashchuk, V. Vlasenko,  M. Bruno, A. Izquierdo, P. C. Gallach-er, Evidence of multimodal structure of the baroclinic tide in the Strait of Gi-braltar. Geophysical Research Letters, 33, L17605, 2006.M1

Measuring vertical velocities with low frequency ADCP instruments: a case of study in the strait of Gibraltar

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