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RASP 2002 Mamala Bay Experiment Interpretative Report of Microstructure Measurements conducted at the Sand Island Outfall

March 21, 2003 Carl H. Gibson & Pak Tao Leung UCSD La Jolla, CA, USA

Fabian Wolk Rockland Oceanographic Services Inc. Victoria, BC, Canada

Hartmut Prandke ISW Wassermesstechnik Petersdorf, Germany

Work performed under contract NBCHF010272, Directed Technologies, Inc., prime - funded by Naval Air Systems Command.

RASP 2002 Interpretative Microstructure Report

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Table of Contents Table of Contents ........................................................................................................................ i List of Figures ............................................................................................................................ ii Objective .................................................................................................................................... 1 Status of the Microstructure Investigation ................................................................................. 1 Outfall Plume Characteristics .................................................................................................... 1

Signature and shape ............................................................................................................... 1 Current Direction and Speed.................................................................................................. 9 Horizontal plume signature .................................................................................................. 11

Mechanism of surface manifestation ....................................................................................... 16 Recommendation for future studies ......................................................................................... 23 Summary .................................................................................................................................. 25 Appendix: Hydrodynamic Phase Diagrams............................................................................. 27 REFERENCES......................................................................................................................... 29


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List of Figures Figure 1: Satellite data from ISINTECH’s October 2002 report, Figure 6.12........................... 2 Figure 2: Stations of vertical drops with the MSS on 09/02/2002............................................. 3 Figure 3: Temperature over the diffuser on 09/02/2002 (14:15h– 15:20h). ............................. 4 Figure 4: Density over the diffuser on 09/02/2002 (14:15h– 15:20h). ...................................... 5 Figure 5: Vertical section of turbidity directly over the diffuser site......................................... 6 Figure 6: Vertical section of salinity directly over the diffuser site........................................... 7 Figure 7: Vertical section of temperature gradient variance directly over the diffuser. ............ 8 Figure 8: Trajectories of drogues D90 (30 m) and D150 (50 m) on 09/02/2002....................... 9 Figure 9: Schematic of the results of the satellite data (drawn from Figure 1)........................ 10 Figure 10: Path of the towed instrument on 09/02/2002, 08:30h to 11:30 h. .......................... 11 Figure 11: Turbidity values along the first tow on 09/02/2002. .............................................. 12 Figure 12: Turbidity along the afternoon tow on 09/02/2002.................................................. 13 Figure 13: Depth of the instrument at the afternoon tow of 09/02/2002. ............................... 14 Figure 14: 10-m average of temperature gradient variance ..................................................... 15 Figure 15: Vertical profile at Station “Green 1”, SE of the diffuser, on 09/02/2002............... 16 Figure 16: Vertical profile near the end of the diffuser, on 09/02/2002. ................................. 18 Figure 17: Detailed Thorpe displacements of the vertical profile near the end of the diffuser 19 Figure 18: Hydrodynamic phase diagram points, points A-G off the end of the diffuser. ...... 20 Figure 19: Vertical profiles east of the diffuser, on 09/02/2002. ........................................... 21 Figure 20: Vertical profiles from the Green 1 station . ............................................................ 22 Figure 21: Hydrodynamic phase diagram points, points I-O from Green station 1................. 23


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Objective This document (R02AID) presents analysis and interpretation of an important part of the RASP 2002 data obtained with the towed and dropped Microstructure Sonde (MSS). The data presented were collected during the RASP 2 experiment in Mamala Bay on September 2, 2002, the day an IKONOS-2 satellite image was obtained by DTI for analysis by the ISINTECH team for surface wave anomalies due to the submerged waste field produced by the Sand Island municipal outfall, Honolulu, Hawaii. The complete MSS data set is summarized in the “RASP 2002 Report on Microstructure Data Processing” (R02MDP) presented by Rockland Oceanographic and ISW in March 2003. The goal is to provide sea truth to the satellite image analysis results (Figure 1) of the Mamala Bay waste field showing a plume with lobes to the southeast (SE) and southwest (SW).

Status of the Microstructure Investigation Data from all sensors on both instruments have been validated and checked for consistency. Inter-calibration between both MSS instruments and statistical and spectrum based quality checks were carried out to verify the quality and reliability of the measured and computed parameters. The inspection of the processed data showed a high degree of data quality and that the data sets are internally consistent (between consecutive profiles and between both MSS instruments). Details of the data processing procedures, data quality checks, and inter-calibration between both MSS instruments and different types of shear sensors used in these experiments are given in R02MDP. Sample calculations have been carried out for data from September 6, 2002, when the QUICKBIRD satellite image was scheduled and the ISINTECH helicopter images were obtained. We recommend continuation of the analysis and interpretation of the present data set and follow up studies at the same location to take advantage of the experience gained in RASP 2002.

Outfall Plume Characteristics

Signature and shape As shown in R02MDP, measurements of the MSS profiler clearly show the signature of the submerged plume water. In addition to the examples given in R02MDP, here we present the signature and the horizontal and vertical distribution of the plume water in the immediate vicinity of the diffuser from microstructure measured September 2, 2002 (data of the over flight of the IKONOS satellite) and attempt to connect our measurements with the surface signatures of the submerged plume water as obtained from the satellite image (Figure 1) using hydrophysical analysis and interpretation of the microstructure data.


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Figure 1: Satellite data from ISINTECH’s October 2002 report, Figure 6.12. The green boundary line is the outline of the waste field as marked in the original figure. The thick red line is an overlay of our afternoon tow path on 09/02/2002. Also shown are the locations of our two far field stations “Green 1” and “Green 2”. Green 1 was at 11am on 09/02/2002, and is within the green boundary outlining the range of surface wave anomalies detected in their analysis of the IKONOS-2 satellite image. The waste water diffuser pipe coming from the Sand Island treatment plant is in red at the top, to the right of the Honolulu airport landing strip. Green 2 was 08/28/2002.

Data obtained from a vertical transect directly over the diffuser (Figure 2) are used to establish a characteristic signature of the plume water. On 09/02/2002, the ambient stratification was characterized by stable temperature and density stratifications (Figure 3 and Figure 4).


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Figure 2: Stations of vertical drops with the MSS on 09/02/2002, approximately three hours after the satellite over flight.


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Figure 3: Temperature over the diffuser on 09/02/2002 (14:15h– 15:20h). White triangles on the top indicate the positions of vertical profiles taken with the MSS instrument (from left to right at stations G4072, G4071, G4061, G0451, G4031, G4021, G4012, G4011, c.f. Figure 2). Red triangles on the bottom indicate points along the diffuser labeled “outfall 1”, “outfall 2”, and “outfall 3” in Figure 2.


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Figure 4: Density over the diffuser on 09/02/2002 (14:15h– 15:20h).

The highly turbid waste water injected into the ambient bottom water by the diffuser jets has a low Salinity of 3 PSU and a temperature of 26 oC (Dayan Vithanage, Oceanit, personal communication 2002) and mixes with a bottom dilution of about 50 to 1. The plume water (waste water diluted with sea water), therefore, is characterized by

1. Local maxima in turbidity

2. Low values in Salinity.

These characteristics are evident in Figure 5 and Figure 6. Contours of turbidity and salinity verify the onshore NW surface current and the deep SW offshore current. Furthermore, the data show that the edges of the plume are also clearly distinguished by elevated temperature gradient variance (Figure 7). It should be noted, however, that the latter parameter by itself cannot unambiguously identify the plume, because temperature gradient variance is a ubiquitous feature of any thermally stratified fluid body where turbulence patches occur.


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Figure 5: Vertical section of turbidity directly over the diffuser site.


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Figure 6: Vertical section of salinity directly over the diffuser site.


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Figure 7: Vertical section of temperature gradient variance (rms temperature gradient) directly over the diffuser site. The color index is in units of (K/m)2.

The vertical transect graphs show that the plume rises above the diffuser to a depth of 45 m below the sea surface. We will call this depth the “trapping depth”. The depth of the main thermocline most likely determines the trapping depth (c.f. Figure 3). The plume can be imagined to have the shape of an elongated mushroom or muffin on top of and aligned with the diffuser pipe. This notion explains the fact that we see only little evidence of the plume water in the profile taken station G4051 (longitude W157.905), which is north of “outfall 2”: this station is well north of the diffuser, so the instrument misses the plume entirely. The profile near “outfall 1” (G4061, W 157.908) is just slightly north of the diffuser, where the instrument descends through the “cap” of the mushroom but misses the stalk. The profile at “outfall 3” (long W157.899) is almost on top of the diffuser, so the instrument descends through the mushroom cap and stalk.

It is important to note that we find strong evidence of the plume water in a deep layer (70 m depth) to the west of the diffuser. This “deep plume” is seen in the two profiles at the two western most stations of the vertical contour (see, Figure 5, longitude W157.915).


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Current Direction and Speed On 09/02/2002 0915h we deployed two drogues at the diffuser site. The drogues were set to a depth between 25 – 27 m (drogue D90) and 40 – 45 m (drogue D150), respectively. Throughout the day (between 0915h and 1745h), both drogues traveled in SE direction (average bearing 150o), as shown in Figure 8. The drogue positions were determined by GPS fixes at 1304h and 1728h (for D150) and 1257h, 1404h, and 1742h (for D90). The average speed for D150 is 0.03 m/s and for D90 is 0.12 m/s. Figure 9 is a composite picture of the satellite analysis and the measured path of the drogues. The trajectory of the drogues is consistent with the direction of the SE lobe of the waste field. Since the maximum drogue depth was 45 m, the drogue path indicates that the source of the SE waste field stems from the upper, “trapped” plume identified from the vertical measurements.

Figure 8: Trajectories of drogues D90 (30 m) and D150 (50 m) on 09/02/2002.


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Figure 9: Schematic of the results of the satellite data (drawn from Figure 1). The blue line outlines the region where ISINTECH reports detection of surface wave anomalies from their analysis of the IKONOS-2 satellite image. Red arrows indicate the trajectories of two drogues deployed at the diffuser at 0915h. The long red arrow is for the 27 m drogue; the short red arrow is for the 42 m drogue.

The acoustic current Doppler profiler (ADCP) measurements, carried out by the Oceanit team, were compared to our drogue measurements. The ADCP instruments were deployed at stations B2 (shown in Figure 9) and B4 (approximately 5 km east of B2, not shown). Both data sets from B2 and B4 show a high depth variability of the local current field, indicating high level of current shear in Mamala Bay. The current directions obtained from B2, B4, and from drogue measurements do not agree. For example, at the drogue depths, data from B2 shows currents to the SW, while the drogues travelled to the SE. This discrepancy is not surprising given the high variability of the current field in Mamala Bay. However, satellite analysis shows that B2 and B4 are clearly outside of the plume region. Therefore, the current data from these stations are not useful to draw conclusions about the plume direction.


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Horizontal plume signature The horizontal plume signature is obtained from towed MSS measurements. Two sets of towed measurements were collected on 09/02/2002. The first tow was done just before the satellite over flight, the second one just after the over flight. The tows establish the near field of the wastewater plume in an area approximately 1 x 2 km around the diffuser. The morning tow (before the over flight) consists of mostly longitudinal transects at four different latitudes along the diffuser pipe. From north to south, the instrument depth decreased from approximately 30 m to 45 m (Figure 10). During the longitudinal sections of the tow, the depth of the instrument was constant to within 2 meters (except for the segment along latitude N21.280 in Figure 11 when the ship stopped to deploy the drogues).

Figure 10: Path of the towed instrument on 09/02/2002, 08:30h to 11:30 h. The colors along the path indicate the depth of the instrument.


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Figure 11: Turbidity values along the first tow on 09/02/2002. Data are color-coded and size coded. Large dots represent large value and vice-versa. Figure 11 shows the turbidity along the morning towpath. The two southerly horizons (tow paths along N21.279 and N21.274) show enhanced turbidity patches at depths between 35 and 40 m in the direction of the SE plume. The SE plume is also evident in the turbidity signal of the afternoon tow, shown in Figure 12. Figure 13 shows the depth of the afternoon tow. Both tows also show evidence of a western plume. In particular, the afternoon tow shows very high values of turbidity northwest of the diffuser (Figure 12). This location falls into the 1x1 km square #83 (ISINTECH October 2002 report). This square is color-coded yellow, which means a faint detection (c.f. Figure 1). Figure 14 shows the horizontal temperature gradient variance σ(Tx) = ⟨(dT/dx)2⟩, measured during the afternoon tow. This parameter shows a manifestation of the submerged plume to the north and northeast of the diffuser, even in locations where the turbidity signal does not give a clear sign of plume water. Similarly, σ(Tx) also reveals the existence of plume to the south of the diffuser.


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Figure 12: Turbidity along the afternoon tow on 09/02/2002.


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Figure 13: Depth of the instrument at the afternoon tow of 09/02/2002 (color coded only).

Elevated turbidity levels found to the SE of the diffuser were measured at depths above the trapping depth of the plume observed in the vertical profiles at the diffuser. Turbidity above the trapping depth of 45 m is also shown at station “Green 1” (Figure 15). This suggests that information of the trapped plume is radiated towards the surface by internal waves that break with sufficient frequency to cause vertical turbulent diffusion of the waste field turbidity and other properties that may trigger more turbulence and more vertical radiation of information from the resulting fossil turbulence patches (see next section). The combined “picture” of the information of turbidity and σ(Tx) from both tows on 09/02 is consistent with a “butterfly” pattern of the waste field.


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Figure 14: 10-m average of temperature gradient variance σ(Tx) = ⟨(dT/dx)2⟩ along the afternoon towpath on 09/02/2002.


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Figure 15: Vertical profile at Station “Green 1”, SE of the diffuser, on 09/02/2002.

Mechanism of surface manifestation The available microstructure data suggest the following model for the surface manifestation of the turbulence generated by the outfall:

1. Approximately 3 x 105 m3/day of effluent is discharged through 272 diffuser ports along the diffuser pipe. The discharge jet velocity is approximately 2 m/s. This is a source of strongly active, local turbulence in the immediate diffuser area necessary to cause maximum initial dilution of the low salinity wastewater with the dense bottom water (colder and more saline).

2. Diluted effluent forms a rising, buoyancy driven turbulent plume that is trapped below the sea surface forming partially fossilized turbulence microstructure patches. The patches are convected by the local current field.

3. The damped fossil turbulence eddies (fossil vorticity turbulence) oscillate at the local buoyancy frequency, Gibson (1980). The oscillation generates internal waves in the stratified water column.


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4. Internal wave energy is therefore radiated nearly vertically from fossil turbulence patches. At shallower depth horizons, where the local stratification is small compared to the trapping depth stratification, the fossil turbulence waves break and generate new, active turbulent patches which then fossilize and radiate more energy toward the surface as fossil turbulence waves. At the depth of the internal wave breaking, energy is drawn from ambient internal waves and the local shear from ambient currents in a process that has been aptly described as zombie turbulence (Hide Yamazaki personal communication 1990). The process is similar to maser or laser action where the fossil turbulence patches are pumped to metastable states by the ambient motions and then beam information about their existence to the surface as fossil turbulence waves.

5. This process of turbulence fossilization and re-generation is repeated along the vertical and stops by wave breaking at the surface, which interferes with the surface wave field. This generates the surface wave field anomalies detected by remote sensing methods.

6. Because the identified beamed zombie turbulence maser mechanism draws energy from ambient fluid motions, the transport of information about the submerged fossil turbulence persists for long times and is detectable over large areas.

We interpret the microstructure measurements using hydrodynamic phase diagrams (HPDs). HPDs are discussed in the Appendix and its references. For the present report we compute the stratification frequency N from the Thorpe reordered density profiles using internal and external values for each microstructure patch of interest. The internal N reflects the buoyancy forces affecting small scale turbulence within the patch if it is in the active-fossil quadrant, and the external value reflects the buoyancy forces affecting the largest scale motions, whether they are turbulent or fossil vorticity turbulence internal waves. We select as the averaging range 0.1 meters beyond the patch to determine the internal stratification and 0.5 LTmax as the external N range. Figure 16 shows the profiles of temperature, salinity, turbidity, viscous dissipation rate, temperature gradient and Thorpe overturning scale for Station G40601 just to the west of the diffuser section (Figure 2). Figure 17 shows a higher resolution Thorpe displacement profile for the station. Figure 13 gives the HPD positions of the patches for this station and two other stations for comparison.


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Figure 16: Vertical profile near the end of the diffuser, on 09/02/2002. The low salinity, high turbidity signature of the trapped waste field is seen at 42-50 meters depth. Microstructure patches A, B, C, D, E, F, and G were identified for analysis from the Thorpe displacement profile on the right.


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Figure 17: Detailed Thorpe displacements of the vertical profile near the end of the diffuser, on 09/02/2002.


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Figure 18: Hydrodynamic phase diagram points, emphasizing points A-G from the end of the diffuser.

Patches A and B are near the bottom and have smaller overturning scales than C and D at the trapping depth, showing the growth of the buoyant turbulence from small scales to large. Note that A is fossilized at the largest scales of the patch, but B is fully turbulent at saturated Froude number. Patch C has larger overturn scales than either A or B below, but smaller than the highly fossilized patch D just below the trapping depth, with nearly 6 meter overturn scales and a large normalized Reynolds number at beginning of fossilization Re/ReF of nearly 104. Patch E is 2 meters above the trapping depth, and may be a turbulence patch produced by the near vertically radiated fossil turbulence waves. Strong, actively turbulent patches G and H appear at the surface with Re/ReF about 5000. Dissipation rates in these patches are much larger than at the surface in the surrounding waters, and the turbulence activity extends to deeper layers, consistent with our model suggesting that this activity is radiated by fossil turbulence waves produced by the trapped wastefield below. Figure 19 shows a set of profiles taken at station G40302 up wind and east of the diffuser (Figure 2) where the surface dissipation rate should be representative of that due only to wind mixing. Patch H is


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identified near the surface and has a much smaller dissipation rate and Re/ReF value than for G and H as shown in Figure 18.

Figure 19: Vertical profiles east of the diffuser, on 09/02/2002. Patch H is chosen to be representative of the surface wind mixing dissipation rate of the region surrounding the diffuser not influenced by strong fossil turbulence waves.

From Figure 20, patch L with large Thorpe overturn scales of nearly 6 meters is a candidate for a fossil turbulence patch convected from the diffuser below the trapping depth. Its indicated Re/ReF value of about 2000 is comparable to that of patch D at nearly the same depth at the diffuser, as shown in Figure 18. The patches M, N and O may have been radiated upward and the patches I, J and K may have been radiated downward. Clearly from the turbidity profile of Figure 20 the turbidity profile from the diffuser has been mixed vertically in both directions.


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Figure 20: Vertical profiles from the Green 1 station about 3 km south of the outfall diffuser and within the ISINTECH southeast plume anomaly (Figure 1).


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Figure 21: Hydrodynamic phase diagram points, emphasizing points I-O from Green station 1 (Figure 1).

Recommendation for future studies The RASP 2002 experiment has shown that mechanism of fossilization of turbulence, internal waves generation and generation of zombie turbulence (FOTIZ mechanism) is a possible mechanism of the generation of detectable surface wave signatures of the plume’s stratified turbulence as identified by remote sensing methods. Actually, the data set does not show any evidence for other mechanism that could generate the detected surface pattern. Measurements of the concentration of surfactants in the Sand Island waste water treatment plant outflow have shown values between 2 ppm (Sept. 6) and 6 ppm (Sept. 2) (Dayan Vithanage, personnel communication). These values are near to the detection level for surfactants of 1 ppm and cannot influence the surface wave structure in a detectable level. However, the existing data set is not sufficient to provide full evidence for the FOTIZ mechanism. This is mainly due to the following reasons:

1. Since the horizontal extension of the detectable surface pattern was not known at the time of the measurement (“blind experiment”), the microstructure investigations were focused on the area near to the diffuser. No measurements were


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conducted directly in the centre of the surface pattern (some kilometres away from the outfall).

2. Due to the same reason, the “blue water” stations for reference measurements outside the region of submerged turbulence were not at the proper positions (and relabelled “Green”). Both station were either inside the submerged turbulence area (as indicated from satellite measurements) or just at its margin.

3. Since most of the measurements were carried out in the immediate vicinity of the outfall, buoyancy driven vertical movements of parts of the plume water could interfere with the FOTIZ mechanism. This may result in a modified mechanism which differs from the mechanism of surface pattern generation outside the region of buoyancy driven vertical water transport.

4. From a statistical point of view, the existing data set is not sufficient to give full evidence for the FOTIZ mechanism to be the source for the observed surface signatures of the submerged plume turbulence.

Consequently, further investigations are required to verify the mechanism of surface pattern generation of submerged stratified turbulence. An experiment that gives most benefit for the investigation of the FOTIZ mechanism with comparable low effort and risk could be conducted in the Mamala Bay. There are several reasons for choosing this site for an additional measurement campaign:

1. The RASP 2002 experiment provides a well proven data base for the planning of a measurement campaign. At any other place, measurement would have to start from a significant lower level of knowledge of the specific local conditions.

2. The low level of surfactants in the Sand Island waste water treatment plant outfall water excludes changes in the surface tension of the plume water to be the reason of the observed surface signatures.

3. Since the currents in the Mamala Bay are highly dominated by tides, the patterns of the discharge of the plume water are complicated. However, a regular pattern of plume water discharge dependent on the stage of the tides can be expected. This makes the distribution of plume water predictable and enables us to select the proper positions for the measurements with a high degree of confidence.

The proposed study to verify the findings on the mechanism for the generation of surface signature of submerged turbulence in the Mamala Bay should include the following activities: 1. Analyzing of all available satellite images from the Mamala Bay outfall region. The

currents in the region of interest are highly controlled by the tidal cycle. Therefore, a regular pattern of the surface signature of the submerged turbulence dependent on the stage of tidal cycle can be expected. Based on this analysis, proper locations and time intervals of the measurements should be selected.

2. Microstructure measurements should be carried out within the surface signature area at different distances from the outfall and at reference stations outside the signature area


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(“blue water” stations). For statistical reasons, a large number of profiles should be collected. To be most effective, the two MSS profiler should be operated from two ships. While one ship takes a time series of vertical profiles on fixed station in the centre of the signature area, the second ship measures at grid stations at different distances from the diffuser and outside the signature area.

3. Parallel to the microstructure measurements, the currents in the area of interest should be measured by Lagrangian drifters (drogues) and moored current meters (ADCP). Preferably, the drogue measurements should be carried out using a third boat for deployment, visiting, and recovery.

Since the MSS profiler is equipped with temperature and conductivity sensors with CTD quality, CTD measurements additional to the MSS measurements are not required. The proposed experiment would deliver a data set that would enable us to identify systematic differences in the vertical structure and the hydrodynamically state of the turbulence field inside and outside the surface signature area. Based on these findings, the FOTIZ mechanism can be verified or rejected to be the mechanism that generates the observed surface signatures of submerged stratified turbulence. The simultaneous measurements with two profilers would guarantee collection of a data set that could provide a high degree of confidence and statistical reliability of the obtained results.

Summary • MSS data collected directly at the Sand Island Municipal Outfall diffuser site show that

the outfall plume extends vertically from the diffuser on the sea floor (73 m depth) to a depth of 45 m. This is the “trapping depth” where the buoyant, turbulent plume density matches that of the stratified ocean and the turbulence fossilizes. Turbidity and salinity measurements at several vertical stations along the diffuser (Figure 2) are shown in Figure 5 and Figure 6, respectively. In particular, there is evidence of a “deep plume” west of the outfall (depth 70 m) corresponding to the SW lobe. This lobe appears to be caused by a deep offshore current moving through the buoyant plume sheet below its trapping depth and convecting patches of buoyant wastewater with it to the SW. These produce turbulence and fossil turbulence patches that radiate detectable information about their presence to the surface.

• Drogues deployed directly over the outfall at a depth of 42 m show a component of the ambient current in the southeast direction (hereafter SE), bearing 150o. This flow component is consistent with the SE plume lobe shown in the satellite pictures.

• One MSS instrument was towed between 30 m and 45 m depth in a 1 x 2 km area centered on the diffuser site. Data from this instrument show evidence of the SE directionality of the plume, and corroborate the notion of a “butterfly” pattern of the plume. The tow depth just above the trapping depth permits investigation of the mechanism of vertical information transport about the submerged plume to the sea surface for remote sensing by ISINTECH.


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• Microstructure turbulence data collected at the diffuser site clearly indicate the physical mechanism of the waste field manifestation at the surface, and corroborate the detected SW and SE plume lobes.

• Based on the results of the MSS data investigation, scientific recommendations were made about future studies necessary to prove the identified FOTIZ mechanism that generates a surface signature of submerged stratified turbulence.


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Appendix: Hydrodynamic Phase Diagrams Figure 22 shows a summary (Gibson 1996) of laboratory and oceanic microstructure classified according to the hydrodynamic state of the microstructure using normalized Froude versus Reynolds number of the microstructure patches; that is, Fr/Fro versus Re/ReF.

Figure 22. Hydrodynamic phase diagram for microstructure in the laboratory, seasonal thermocline, abyssal layers, seamount wakes, Arctic, and equatorial undercurrent

1. The Dillon (1984) correlation LR = LTrms shown by the horizontal dashed arrow assumes seasonal thermocline patches are actively turbulent rather than active-fossil.

2. Large equatorial patches with density overturns of twenty to thirty meters have been observed by Hebert et al. (1992) , Wijesekera and Dillon (1991) , and Peters et al. (1994) , indicating εo values in previous actively turbulent states more than 106 εF and 103-105 larger than measured values of ε for these dominant equatorial events. These are discussed in Gibson (1991b).

3. Abyssal dropsonde profiles Toole, Polzin and Schmitt (1994), are usually so sparse they fail to detect even the fossils of the dominant turbulence events, (inferred by Gibson 1982a).

4. The "mixing efficiency" Γ ≡ (DCN2/ε) = εo/13ε shown on the right (Oakey 1982) can be 104 larger than 100% for fossilized patches.

5. Measurements of powerful patches in the Arctic, Wijesekera and Dillon (1997) confirm the fact that large, actively-turbulent microstructure events exist in the ocean interior, even though they are rare.

The HPD classification method is based on turbulence defined as an eddy-like state of fluid motion where the inertial vortex forces ( v ×ω ) of the eddies are larger than any other forces that tends to damp the eddies out, Gibson (1999). Thus the cascade of irrotational fluid that supplies the kinetic energy of turbulence from large scales to small is a non-turbulent cascade


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by definition. Turbulent eddies are first formed at the Kolmogorov scale of the shear instability. The eddies pair, and the pairs of eddies pair with pairs of eddies as the scale of the turbulence grows. The scale of turbulence increases as boundary layers thicken, wakes spread and jets grow. Turbulence always cascades from small scales to large as shown by the sequence of numbers from 1 to 4 in Figure 22. Starting from the Kolmogorov scale at point 1 the Reynolds number and patch size grows till point 2 where the largest scale eddies of the cascade feel the buoyancy forces at the critical Froude number Fro. Thereafter, the Ozmidov scale of overturning LR monotonically decreases as the HDP points move down along the decay line with slope 1/3 to points 3 and 4. The size of a fossil turbulence patch does not collapse but remains relatively constant at its maximum value LRo = 1.6LTmax. If there are no ambient internal waves or shears the decay will continue from the active-fossil into the completely fossil quadrant as shown by the laboratory measurements. If the ambient stratified fluid is in motion, then the patch can extract energy from ambient motions. Tilting of the strong density gradient layers bounding the fossil turbulence patch will cause vorticity generation on the surface due to baroclinic torques. The shear layers produced will become turbulent and the patch dissipation rate and possibly the Thorpe overturn scale increased. Thus the fossil turbulence patch will come back to life by the zombie turbulence mechanism (a descriptive term coined by Hide Yamazaki, personal communication 1990) and move back up the decay line as ε increases and to the right if the overturn scale LTmax increases. As shown in Figure 22, the laboratory studies have much smaller ReO to ReF ratios than most reported microstructure in ocean layers. The first most extensive measurements of microstructure were made during the MILE expedition, where simultaneous dropsonde and towed body measurements were possible, Washburn and Gibson (1984). Since most microstructure patches found were relatively weak, Dillon (1984) proposed that possibly they were actively turbulent and that the universal constant had been incorrectly estimated in the Gibson (1980) fossil turbulence theory. Dillon's correlation is shown by the horizontal arrow. When Dillon's correlation was proposed, few patches had been observed in their actively turbulent state except by towed bodies despite laboratory confirmations of the Gibson (1980) universal constants by Stillinger et al. (1983) and Itsweire et al. (1986) in a stratified grid turbulence flow, as discussed by Gibson (1991a). However, measurements in the wake of Ampere Seamount, Gibson et al. (1994), show that oceanic microstructure indeed begins in the actively turbulent quadrant for patches sampled over the crest of the seamount, and decays into the active-fossil quadrant for patches sampled downstream of the crest, as indicated by the oval region and arrow. From Figure 17 the ocean microstructure is not likely to be found in its original actively turbulent state, but in some stage of fossilization. Wijesekera and Dillon (1997) report only one fully active patch from their Arctic Sea data (dark circles). Table 1: Values used to prepare HPDs such as shown in Figure 22.

• Length Scale (LT) is the maximum Thorpe overturn scale of the patch. • Depth (meters) is the location of the patch. • ε is the dissipation rate of the patch measured by Microstructure Profilers • εo ≈ 3LT

2 N 3 is the dissipation rate when the patch begins to fossilize.

• 230 NF νε = is the dissipation rate when the patch reaches complete fossilization.


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• 31

)(OOFr

Frεε

= is the normalized Froude number.

• )(ReRe

FF εε

= is the normalized Reynolds number.

• Väisäilä Frequency (dzdgN ρ

ρ−= ) at the depth of the patch.

REFERENCES Dillon, T. R. 1984. The energetics of overturning structures: Implications for the theory of

fossil turbulence, J. Phys. Oceanogr., 14, 541-549.

Gibson, C. H. 1980. Fossil temperature, salinity, and vorticity turbulence in the ocean, in Marine Turbulence, J. Nihoul (Ed.), Elsevier Publishing Co., Amsterdam, 221-257.

Gibson, C. H. 1986. Internal waves, fossil turbulence, and composite ocean Microstructure spectra, J. Fluid Mech. 168, 89-117.

Gibson, C. H. 1987. Fossil turbulence and intermittency in sampling oceanic mixing processes, Journal of Geophysical Research, 92, C5, 5383-5404.

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