Friday, 3 February 2012

So what’s all this microstructure about?

In yesterday’s interview with Professor Andrew Watson, he talked about the importance of direct measurements of turbulence to the DIMES project. In today’s blog update, I spoke to Dr. John Toole from Woods Hole Oceanographic Institution and asked him about the three microstructure profilers in use on our cruise: the High-Resolution Profiler (HRP), the Deep Microstructure Profiler (DMP) and the Vertical Microstructure Profiler (VMP).

“All three of these instruments will be used on our cruise to sample the intensity of turbulent mixing in the ocean and the flow features at larger scale that may be providing the energy to support the mixing.  The HRP is a custom instrument designed and built at the Woods Hole Oceanographic Institution (WHOI).  The DMP and VMP (sister instruments) are built by Rockland Scientific, Inc.  The DMP comes from WHOI while the VMP is owned by the U.K. National Oceanography Centre (NOC).  All three instruments are un-tethered, ‘full ocean depth,’ free-fall vertical profilers which are cylindrical in shape, approximately 4m long and approximately 0.6m diameter.  The main sensor suites extend out from the lower end of each instrument.”
So how are the instruments deployed?

“For deployment, two expendable steel ‘drop-weight’ ballast cylinders are fitted to the profilers to make them sink. These weights are jettisoned at the bottom of the desired profile to enable the vehicles to return to the surface. The HRP and DMP will be deployed and recovered using a hydraulically-driven custom handling rig mounted at the starboard waist of the ship.  The rig consists of a rolling cradle and track system that is bolted to the vessel deck and a hydraulic tilting mechanism that is positioned at the rail.  The rolling cradle mates with the tilting mechanism that subsequently lifts the cradle and profiler to vertical orientation, whereupon the rig’s winch is used to lower the profiler into the water.  Upon release, the profilers sink at 30-45 m/min logging data internally.  After weight release, the HRP speeds to the surface at 2-3 m/s while the DMP and VMP rise more slowly.  During dives, the vehicles are tracked acoustically from the ship.  At the surface, a VHF radio beacon on the profilers turns on, and bearing to the instrument is determined using a Radio Direction Finder mounted above the bridge deck.  The vehicles also carry a strobe light and flag to aid visual location.  Upon sighting, the vessel is positioned alongside the floating profiler and a tag line is attached using a small hook and long pole.  The tag line is used to guide the profiler close to the deployment rig where the main lifting hook and line are attached (also using a pole).  The deployment rig winch is used to haul the profiler up to the cradle height, and the cradle is returned to horizontal position, allowing the instrument to be rolled inboard for servicing.”

And what specifically are you measuring?

“These instruments sample temperature, salinity and velocity on vertical scales from approximately 1 cm to full-ocean depth.  The so-called microstructure data (cm-scale) are used to estimate the intensity of turbulent vertical mixing.  The signals at vertical scales from ~10 m to ~1000 m are principally internal waves: time varying motions that ride on the vertical variations of ocean density (analogous to surface waves that exist at the air-sea interface).  Flows at still larger scales are chiefly associated with the major ocean currents (such as the Antarctic Circumpolar Current in our study area) and eddies spun off from those currents.  Our “instantaneous” estimates of the mixing intensity from the microstructure data will be compared to the mixing rates inferred from the vertical spread of the tracer – which is a long-time-average of the mixing intensity.”

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