Here is another aerial image to complement those posted earlier by Falk. This photograph shows some very cool eddies, and it is also a great illustration of how thin the surface layer of dye was to the SW of inlet. The dye-free streak on the right side of the photograph was left in the path of our dye sampling WaveRunner.
Today I tried initial analyses on “vBar” pixel instruments to measure tidal currents in the inlet. These are based on Chris Chickadel’s work and seemed potentially applicable to this situation of co-aligned waves and currents in the inlet. Below shows the locations of estimates.
The figure below shows ebb current estimates for each of the three lines. Locations of zero current are mostly where pixel resolution has degraded. The cross-inlet structure and along-inlet gradients seem interesting. Of course, I have not compared against any ground truth (but hope to).
These are initial experimental results that we will work further on. We collected these data for one transect every half hour since 04/29 and added the two other transects on 05/10. We will begin bulk analysis soon-ish.
If you are interested, here are four plots illustrating the method. The first is just an example stack and shows the mix of waves propagating into the inlet (down left) and current streaks showing ebb flow (down right). This is a moderately easy case with strong current signatures.
The second figure shows the corresponding f-k spectrum (using a convention of +/- frequency and positive wavenumber). The ridge of energy going down right corresponds to the current. The curved shape in the upper half corresponds to the gravity waves and would normally be roughly parabolic but the current turns the dispersion curve. The analysis exploits the current ridge but could also exploit the wave Doppler shift (later???). (This is my coolest plot). Streaks at high k are from aliasing and are not included in the analysis.
The third figure transforms the f-k spectrum to V-k space, a al Chickadel.
The fourth figure integrates through k to give a variance versus V curve. The peak (Gaussian fitted) is the estimated current. This case is the landward-most point in west transect, so V ~ 95 cm/s.
Today (Sunday May 13th) is instrument turnaround day and general catching up. We also have a report on the dye-bomb release we did on May 11th. In short, it was an amazing dye of observing the ocean. The RIVET community had many assets in the water measuring dye, currents, temperature, and salinity. Here we’ll focus on a few observations to give a quick overview of what happenned during the day.
In contrast to the previous releases where we continuously pumped dye for 1.5-2.5 hours, this time we poured 20 gallons of Rhodamine WT at ~0800 into the inlet 300 m down from the junction with the Inter-coastal waterway. Below you can see the dye getting poured in.
As the tide was ebbing rapidly the dye gets stretched and stirred rapidly. It is quickly well mixed (confirmed with CTD+F) casts and makes its way down the inlet. As it approaches the inlet mouth, the dye has spread all the way across the inlet and (in contrast to the point releases) the dye moves out both “old” and “new” channels.
The conditions on May 11 were very benign. Very small waves and extremely light winds during the entire release. The dye then gets rapidly ejected offshore out of the two channels and the SW near-beach channel.
By shortly after 0900 dye is clear of most of the inlet. CTD+F casts offshore of the main or “new” channel reveal a surface concentrated (< 2 m) dye field of about 20 ppb with no dye at all below. This is associated with a strong halocline. This is in part due to the weak mixing that is induced due to the light winds and lack of waves and whitecapping. See the figure below.
The fresher water of the inlet simply slides on top of the saltier (and slighly colder) oceanic water. Thus forming a “lens” within which the dye is contained. The weak winds likely contribute to the maintenance of this strong water-mass gradient. These type of vertical structure conditions will be a very significant challenge for models to reproduce as a) they may not have enough vertical coverage and more importantly b) the model-induced mixing is often far too strong to maintain such a gradient.
The tide then starts to flood at ~1000, and begins to pull water back from “old channel”
Notice the wispy dye to the SW of the inlet. This is all very much surface concentrated. Often boats and jetskis driving in it left a trail of dye-free water behind. This was confirmed by the CTD casts, which show an even thinner surface layer of dye with lower max dye. Dye stratification is again is influenced largely by a strong halocine, but here there is also a stronger thermocline than in the previous set of casts
As the tide started to seriously flood, the dye patch was broken into two parts. 1) an offshore component that spread itself out from the main channel entrance to the SW and 2) a patch that got sucked back up the inlet. This mostly occured from dye in the “old” channel. As the dye approached the “S”-turn of the inlet, it converged strongly on the ever-present scum line, just as the drifters had (see previous post). See the photo below.
This sort of “front” will also be a challenge for models to reproduce as the lateral (horizontal) eddy viscosities that they have may preclude the formation of such convergence zones in these rapidly changing regions.
The ‘Maple’ group from BIO, Dalhousie and UMaine arrived Tuesday. Our objective is to characterize suspended particles (size distribution and composition) and relate it to ocean color. We have already scouted the river, taken grab samples and water samples for grain size, and have completed a drift station with a LISST and Digital Floc Camera through
the mouth of the inlet. In the next few days we plan to occupy a 13h station near the river mouth (Friday) and perform a transect along the river (Saturday). With the arrival of Emmanuel Boss, in addition to the in situ particle size spectrum, our measurements now include spectral absorption, attenuation and backscattering of both dissolved and
particulate material and hyper-spectral upwelling radiance and irradiance. Our observations will help link the in-water optical properties to remote observations.
Tim Milligan, Brent Law, Gary Bugden (BIO/DAL) and Emmanuel Boss (UMaine)
(some pics & images coming soon)
Here is a composite image from 12 days of SWIFT drifter runs through the inlet spanning many different tide/wind/wave conditions. The color scale is the surface turbulence dissipation rate, which is generally correlated with strong wave-current interactions in the two channels. Preliminary turbulence metrics and bulk wind, wave, and current values are posted for each day at http://herschel.apl.washington.edu/darla/SWIFT/, along with an xls table listing available data. More to come!
The dye release yesterday (May 8th) was very interesting. The dye release was initiated shortly after slack tide as the water started ebbing. 30 gallons of Rhodamine WT were released in 2:24 min. As usual the mounted instruments, the jetskis, NPS/RSMAS crew, REMUS, towed ET array, and the Partenavia were all measuring the dye plume. At first, as the tide was still not very low, the dye poured down the main channel and then oozed out over the southern shoal. See the aerial hyperspectral image below (taken at 1339 local time, from Luc Lenain and Nick Statom). Up the inlet near where the dye was released, the surface dye is very patchy because the dye is released about 3 m above the bed in ~9 m water depth. At times it seems like the surface dye patch has a periodic signature. The dye moves offshore of the south-western shoal in jets and squirts.
The nice thing about this image below is that it can be used to quantify Rhodamine dye concentration. As light is measured across a spectrum of wavelengths, information at absorption and emmision wavelengths of dye will be used to calibrate dye concentration against in situ sensors.
Below are some SLR camera images of the plume at similar stages. These are useful for providing context but are nowhere near as quantitative.
Here is an update on the remaining schedule for drifter and dye releases.
Thursday: There will be no dye released. Note, this is a change from the previous schedule.
Friday May 11th: Begin releasing dye at 0800: Release dye for 1.5 hrs – (to just before slack). Sampling on both ebb and flood.
Saturday May 12th: Begin releasing dye at 0830: Release dye for 1.5 hrs. Sampling on both ebb and flood.
May 14-17 (Monday-Thursday)
The imagery above is a preliminary mosaic of the New River inlet plume on May 2, at about 1150 EDT (the companion data to Gordon Farquaharson’s microASAR data in his May 4 post). Brightness in the image indicates relatively warmer temperatures, and I’ve adjusted the contrast to highlight the plume, making the warmer land white. The plume on this day (due to wind) was shooting north and the plume front(s!) are complicated – including prominent transverse features at N = 3822.5km, E = 286 to 288km. Note there are still kinks in the mapping, so image overlap is not exact, and the camera drift and angular emissivity effects have not been corrected.
REMUS Observations of rhodamine dye as it advects out of the inlet on May 8, 2012 (CORDC)