Instrument Primer (Part 1)

So far, I’ve avoided explaining exactly what measurements we’ll be making at Station Papa. Now that we’re within 2 days (!) of the site, it seems that time has come. If nothing else, it’ll help you identify by name what is being thrown into the water in the next few days’ pictures. In the last science-related post, I mentioned that we’re interested in three general features: wind, waves, and wave-breaking. Each of the instruments below measures one or more of these properties.

 

Camera System: I put this one first because it’s where I spend most of my time. This system includes the pan and tilt (which I’ve shown before), two small fixed focus cameras, and a weather/GPS unit. The whole package is mounted off the starboard rail on the top deck, above the bridge (where the captain drives the ship). I’ve previously put up videos of the pan and tilt in action, it keeps the cameras looking at a steady angle at the sea surface so we can measure the size and speed of whitecap crests independent of ship motion. The two cameras give us two views at the same waves: one camera has a wide angle lens to see the full size of the breaking waves, while the other has a narrower field of view to resolve the smallest scales of breaking. The GPS unit tells the angle and heading of the system, so it let’s us know how the pan and tilt is doing (it’s not perfect!). It’s also a small weather station, so it measures wind speed and direction, as well as temperature and pressure. It turns out that we have better instruments making these same measurements, but it’s good to have redundancy.

 

  

SWIFTs: These are instruments designed in our own lab and which Jim and company have been slowly perfecting for the past few years. Their name stands for “Surface Wave Instrument Float with Tracking.” The SWIFTs are an all-in-one wave package; they measure all three entities I mentioned above. They move with the waves, and use GPS to track their course. Because the orbital velocity of a wave depends on the amplitude and frequency of the wave, the GPS data can be used to calculate the wave frequency spectrum (it may be surprising to you that satellites can track such relatively small velocities. It was to me, too). There’s an added advantage to moving with the waves: it allows the SWIFTs to measure the turbulence produced by wave-breaking very close to the water surface. By moving with the waves, the SWIFTs filter out the large velocities associated with the orbital velocity of the waves, so they can measure the small velocities associated with turbulence. In addition, whether in the crest or trough of a wave, they always stay within a meter of the surface where the turbulence is most intense. Finally, above the water surface, the SWIFTs measure wind speed and take video with Go-Pro helmet cameras. The SWIFTs and cameras complement each other nicely, so we try to always make these measurements in tandem.

 

  

Waveriders: These more conventional wave buoys are made by a company named Datawell. They do one thing and they do it well: make wave spectrum. These guys come in two flavors. We have two small ones (~ 0.5 meter diameter) that measure waves, like the SWIFTs, using GPS velocity data. We throw these out from the ship with the SWIFTs and pick them both up later in the day. The larger one (~ 1 meter diameter) use internal gyroscopes to make their measurements, so they perform better and are more expensive. We’ve had one of these moored at Station Papa for nearly two years now without any upkeep and its batteries are running low, so we are bringing a second one out to spell the old one. Keeping a wave measurement device tethered to the ocean floor (remember, 4250 meters) without corrupting the data is actually quite tricky. We keep 4000 meters of the line in high tension with subsurface floats of large buoyancy to create a stable “false bottom,” to which we attach some more line and a stretchy rubber cord. This keeps our buoy floating within a roughly 1 km diameter circular area. We will have to spend the better part of one day at Papa bringing in the old waverider and deploying the new one.

That’s all for now, stay tuned for part 2! I know you’re excited.

– Mike

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