Ocean Mixing and Stirring  
  Principal Investigator: Dr. Miles A. Sundermeyer
msundermeyer@umassd.edu • 508-999-8892
University of Massachusetts Dartmouth
School for Marine Science and Technology
 
 

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Teaching:

* Courses Taught

Research:

* Lidar Studies of Small- Scale Lateral Dispersion - LatMix
* 3D Dye Mapping using Airborne Lidar - Florida 2004 Pilot
* Lab Studies of Stirring by Small-Scale Geostrophic Motions
* Numerical Simulations of Vortical Mode Stirring
* Coastal Mixing & Optics (CMO)
(... More links coming soon!)
* The North Atlantic Tracer Release Experiment (NATRE)
(Links coming soon!)

Additional Links
* M. Sundermeyer's CV (PDF)

 

© 2005 Miles A. Sundermeyer (msundermeyer@umassd.edu)
Note: Please do not use the data, text, or images contained on this site without prior permission.

Lidar Studies of Small-Scale Lateral Dispersion
(Jointly funded by ONR and NSF, in collaboration with NAVAIR)
PIs: M. A. Sundermeyer, J. R. Ledwell and E. A. Terray
Grant Title: Lidar Studies of Lateral Dispersion in the Seasonal Pycnocline
Funding Agency: The National Science Foundation
Award: $1,088,442 (UMass:$381,652; WHOI:$706,790)
Grant Title: Lidar and Numerical Modeling Studies of Small-Scale Lateral Dispersion in the Ocean
Funding Agency: Office of Naval Research
Award: UMass:$544,808)

This project was jointly funded by the National Science Foundation and the Office of Naval Research through the "Scalable Lateral Mixing and Coherent Turbulence" (a.k.a. LatMix) Department Research Initiative. This project was a collaboration between the above PIs, and Brian Concannon of the NAVAIR lidar group.

Objectives

Our goal was to better understand lateral mixing processes in the ocean on scales of 10 m to 10 km, including the underlying mechanisms and forcing, and the temporal, spatial, and scale variability of such mixing. Our work contributes to fundamental knowledge of ocean dynamics at these scales, and efforts to parameterize sub-grid scale mixing and stirring in numerical models. Our research also enhances modeling and understanding of upper ocean ecosystems, since the flow of nutrients and plankton depends on stirring and mixing at these scales.

One objective of our work was to determine the extent to which shear dispersion - the interaction of vertical mixing with vertical shear - can explain lateral dispersion at scales of 10 m to 10 km. A second objective is to determine whether slow but persistent vortices enhance the stirring attributable to shear dispersion. We also share the overall objectives of the Lateral Mixing DRI to determine the extent to which submesoscale stirring is driven by a cascade of energy down (in wavelength) from the mesoscale, versus upwards from small mixing events (e.g., via generation of vortices). A key technical goal of our work is to develop the use of airborne LIDAR surveys of evolving dye experiments as a tool for studying submesoscale lateral dispersion.

Our approach is to release fluorescent dye tracers on isopycnal surfaces in the seasonal pycnocline, and along the Gulf Stream front, and to survey their evolution using in situ instruments as well as airborne lidar. In the June 2011 field effort, nine dye/drifter releases were conducted: two 6-day rhodamine experiments, and seven 24-36 hr fluorescein experiments. A single fluorescein experiment was also conducted in the surface mixed layer. Four lidar overflights totaling nearly 40 hrs of flight time were also conducted in June 2011 to map the spreading of the dye from the time of release to as much as 6 hrs after release. In the Feb/Mar 2012 field effort, four rhodamine dye/drifter release experiments were conducted along the north wall of the Gulf Stream, each lasting a few days. A 17-drifter cluster release was also conducted in the Sargasso Sea to examine small-scale dispersion characteristics.

In addition to the field efforts, we are also collaborating closely with M.-P. Lelong in support of her effort, "LES Modeling of Lateral Dispersion in the Ocean on Scales of 10 m - 10 km." Both the field observations and the modeling results are also being coordinated with modeling efforts of other DRI participants.

Results to Date

LatMix 2011 6-Day Rhodamine Evolutions [Top]

   
Fig. Sea surface temperature at the June 2011 study site ~300 km SouthEast of Cape Hatteras, NC.

A total of nine dye release experiments were conducted during the June 2011 field effort, two 6-day rhodamine experiments, and seven 26-36 hr fluorescien experiments. The two main rhodamine experiments served to provide a view of the larger-scale (1-10 km, up to as much as 80 km) characteristics of the mixing and strain environment. Meanwhile, the smaller and shorter lived fluorescein experiments provided snapshots of the small-scale variability and early evolution of the dye dispersal. Approximately daily surveys of the two rhodmaine experiments were conducted during the 6 days in which they were tracked. Summary maps of the dye patches for the second of the two main rhodamine experiments are shown below.

Primary analysis of both rhodamine experiments reveals diapycnal mixing rates between 2 x 10-6 and 5 x 10-6 m2 s-1 for both experiments. Elongation of the tracer patch, in the zonal direction for the first experiment, and roughly meridionally for the second, revealed strain rates of order 6 x 10-6 s-1 and up to 4 x 10-5 s-1 for the first and second experiments, respectively. These values agreed roughly with estimates derived from the drogued drifters released with the dye. Allowing for the effects of strain elongating the patch in one direction, and narrowing it in the other, lateral diffusivities inferred from dye distributions from the two experiments were also similar, ranging from 0.5-4 m2 s-1.

Beyond the above quantitative estimates of diapycnal and isopycnal dispersion rates, a major result from the dye analysis to date is that bulk dispersion estimates derived from the two main rhodamine experiments were found to be larger than could be explained by internal wave shear dispersion. Specifically, an analytical model that incorporates time dependent lateral strain, vertical shear, and a fixed diapycnal diffusivity equal to that derived from the observed dye patches was integrated in time to obtain a best fit to parameters observed in the field observations. Results of the model showed that neither low frequency or steady shears, nor near-inertial or higher frequency shears observed during the experiments, together with the observed diapycnal mixing, could explain the observed lateral spreading of the tracer patches. That high frequency internal wave shear dispersion could not explain the observed lateral dispersion is consistent with findings arrived at independently by other field PIs involved in the larger ONR LatMix effort.

 
 
Fig. Plan view maps of the first 2011 rhodamine dye experiment as surveyed using the UMass Acrobat tow package over approximately 135 hrs following release. Successive maps show the elongation and spreading of the tracer patch from its initial release of approximately 1.5 km long x 100 m wide to ~10 km long and 5 km wide. Fig. Plan view maps of the second 2011 rhodamine dye experiment as surveyed using the UMass Acrobat tow package over approximately 150 hrs following release. Successive maps show the elongation and spreading of the tracer patch from its initial release of approximately 1.5 km long x 100 m wide to >50 km long and 5 km wide.
 

LatMix 2011 0.1-6 Hr Fluorescein Evolutions

In addition to the two 6-day rhodamine releases, a total of 7 smaller fluorescein dye experiments were conducted to evaluate the short time and space evolution of the underlying mixing. Four of these releases were surveyed using airborne lidar, from 10 min. - 6 hrs after release. Summary plots of the dye evolution for the various experiments observed via airborne lidar are shown in the figures to the right. Major features are described in the following paragraphs.





Fig. Main panel: Plan view maps of ship track (bold black line), injection line (bold green line), and peak lidar returns (false color image) observed during overflights of June 10 fluorescein dye experiment approximately 3 hrs after release. In situ survey profiles where dye was found are shown (for context) as green circles. Upper left inset: Location of fluorescein patch relative to larger June 6 rhodamine dye release experiment.
Lower left inset: Mean wind speed during injection and surveys.
Lower right inset: Mean u, v velocity profiles from R/V Hatteras shipboard ADCP (150 and 600 KHz) averaged over the time of the lidar surveys, with bold green line indicating dye injection depth.

Fig. Plan view maps of lidar-derived dye signal showing evolution of the June 10 fluorescein patch over the course of the lidar surveys. Time evolution goes from left to right, with time since injection indicated above each survey. Upper panels are lidar peak intensity return (Watts). Lower panels are depth (m) of peak return for same times.
 

June 10, Fluorescein Experiment 4:

An overview of the June 10 fluorescein experiment is shown to the right. Major features are the elongation of the patch in the north-south direction, consistent with the observed velocity shear at the depth of the dye (velocity profiles, and upper panels in time evolution plot). Visual inspection of the depth of the peak lidar return (lower panels in time evolution plots) suggests that the deeper portion of the dye was sheared off to the northeast compared to the main dye streak (evident as narrow band of strong signal at southwest tip of patch in time evolutions). Also evident is a broad sinuous meander of the patch early in the evolution (3.1 hrs), as well as evidence of filamentation on the eastern edge of the patch throughout the surveys (enhanced signal on right edge of patch in all surveys, extending from lower third to middle of patch). Both of these features suggest the possibility of weak small-scale (<1 km) differential lateral advection acting on the patch. Finally, late evolution of the patch (6.3 hr) indicates a SW-NE oriented banding of the dye with wavelength of order 100 m, while the depth of the peak return (lower row in time evolution plots) shows banding oriented in the NW-SE direction. Whether this banding is the signature of internal waves, or surface waves (swell) is under investigation.





Fig. Similar to above, but for June 15 fluorescein dye experiment approximately 3.5 hr after release. Fig. Similar to above, but for June 15 fluorescein dye experiment.
 

June 15, Fluorescein Experiment 5:

An overview of the June 15 fluorescein experiment is shown in the figures to the right. Two complete surveys, plus a third incomplete survey of the patch reveal an extremely rich structure in the dye evolution for this experiment. Major features include the finger-like structures stretching westward relative to the main patch, as well evidence of the development of filamentation along both the southern and eastern sides of the patch. The fingerlike structures stretching westward appear to be consistent with variations in potential density along the track of the injection line (not shown), and hence are thought to be the result of dye being injected across internal wave crests and troughs, i.e., the injection was not perfectly along a single isopycnal. This variability of the injection enabled a mean westward differential advection of dye at shallower isopycnal depths relative to deeper, denser isopycnals. The extent of this differential advection is roughly consistent with the depths of the peak returns (lower panels in time evolution plots), as well as with mean westward shear estimated via shipboard ADCP measurements. That these fingerlike structures persisted for more than 5 hours after the injection, despite their relatively small scale (order 50 m), suggests an upper limit on the lateral dispersion acting on these scales of order 0.1 m2 s-1. Meanwhile, the 100-200 m scale filamentation observed at the southern most extent of the patch, as well as the 300 m scale curvature at the northeastern-most end of the patch both again suggest some degree of small-scale differential advection acting on the patch. Last, we again observe some suggestion of NW-SE oriented banding in the depth of the peak return (lower panels in time evolution plots), although not nearly as pronounced as in the June 10 experiment.





Fig. Similar to above, but for June 16 fluorescein dye experiment approximately 2.6 hr after release. Fig. Similar to above, but for June 16 fluorescein dye experiment.
Fig. Similar to above, but for June 16 surface dye experiment.

June 16, Fluorescein Experiments 6 and 6a:

The June 16 dye release consisted of both a pycnocline and a surface mixed layer release, the latter performed immediately following and along the same ship heading as the former. The two patches from the June 16 experiment are shown in the figure to the right. Considering first the deeper pycnocline release, three partial surveys of the patch show a broad widening of the patch, again with some evidence of small-scale structure / filamentation along the southwestern edge of the patch (upper panels in time evolution). Meanwhile, depths associated with the peak lidar returns (lower panels in time evolution) show evidence of banding of the depth of peak return oriented in the NNW-SSE directions.

Meanwhile, the surface mixed layer portion of the patch, seen in the figure to rapidly separate from the deeper patch, shows a rich structure of large eddy circulation within the mixed layer. As evident in the figure below, the surface portion of the patch rapidly (over the first 0.25 - 1.6 hrs) develops a banded structure oriented in the SW-NE direction, as it is advected downwind (SW). The banding has a wavelength of order 100 m, with deep (20 m, roughly the base of the mixed layer) tails extending upwind relative to the more rapidly advected surface (within a few m of the surface) portions of the patch. Given these characteristics, this banding appears to be consistent with some form of large eddy circulation across the depth of the mixed layer. In particular, numerical simulations by E. Skyllingstad (pers. comm.) suggest this banding may be driven by Ekman layer instability associated with a lateral mixed layer density gradient.

Ongoing Work on Lidar Inversion

As the present project represents our first major field experiment using airborne lidar to survey dye release experiments (following a proof of concept experiment performed under separate funding in 2004), a considerable part of our effort continues the development and calibration of algorithms to invert the raw lidar signal (Watts) to absolute dye concentration (ppb). Given the particular characteristics of the lidar system used in the present study (signal to noise in the backscatter vs. fluorescence channels, lidar system parameters, etc.), our approach for the present work is to use a forward model of the lidar signal and system characteristics, accounting for seawater attenuation, to invert for the dye concentration profile using nonlinear regression. An example of a synthetic dye concentration profile together with the forward backscatter and fluorescence waveforms are shown in the figure to the below, alongside a sample of the actual lidar data collected during the June 15 fluorescein dye experiment. Significant effort has been spent over the current project year testing this as well as alternate models, as well as verifying the values of relevant system parameters and how they are best incorporated into the present model.

Fig. Left panel: Synthetic dye concentration profile (ppb); Middle: fluorescence + noise (green) and backscatter (blue) returns from lidar forward model; and Right: fluorescence (green) and backscatter (blue) signals from actual observed lidar returns.

LatMix 2012 Field Experiments [Top]

Major results from the Feb/Mar 2012 field effort are that, among the four dye releases conducted within Gulf Stream, two sampled symmetric instability during strong down-front winds, one an intra-thermocline eddy, and one the separation of a filament along the north wall of the Gulf Stream. Analysis of the 2012 data is still ongoing. However, major results are as follows. For the two symmetric instability experiments, dye was released in the mixed layer at approximately 25 m depth. Within hours after release, the dye was well mixed throughout the mixed layer, and within 24.36 hrs, it was mixed beneath the mixed layer, where isopycnals were more horizontal than vertical. For both the intra-thermocline eddy, and the north wall filament experiment, dye was injected below the mixed layer, at ~120 m for the first, and ~55 m for the second. For these releases, the dye did not extend to the surface, so that the ship.s flow-through systems were unable to measure it. Nevertheless, C. Lee's (pers. comm.) Triaxus surveys of both releases revealed detailed structure of the formation of the respective events. Noteworthy in the 2012 experiments is that the conditions under which the injections and sampling were performed were among the most difficult ever performed by our group.

Also in the 2012 field effort, we conducted a series of drifter releases, including a cluster release of 17 drifters in the Sargasso Sea south of the Gulf Stream. Single particle dispersion estimates indicated a quadratic growth in the dispersion as a function of time over the first few days, followed by a gradual tendency to linear growth after of order 5 days, consistent with effective eddy diffusion at these scales.

Fig. Survey map (mainn panel) and transect views from one of four near-surface dye release experiments conducted to study mixed layer processes, symmetric instability, and isopycnal mixing along the north wall of the Gulf Stream. Inset panels are transects taken at the beginning and end of the drift, showing fluorescein, signals with isopycnals overlaid. The magenta lines at the top of the fluorescein panels show flow-through fluorometer data from R/V Atlantis. Note the subduction and spreading of the fluorescein patch from near the surface, down sloping isypycnals to >60 m depth over the ~36 hrs between the transects.

Publications / Reports

"Scalable Lateral Mixing and Coherent Turbulence," Department Research Initiative Whitepaper, DRI Planning Workshop, May 28-30, 2008, Cambridge, MA.
PDF  
 
Ledwell, J. R., M. A. Sundermeyer, E. A. Terray, L. Houghton, D. Schwartz. Development Cruise for Dye Experiments with Airborne Lidar, Cruise Report for R/V F. G. Walton Smith, Cruise 0813, 7-11 September 2008.
PDF  
 

Data Sets

The following are Matlab files from the UMass Dartmouth towed Acrobat CTD and fluorometer data collected during the May/June 2011 LatMix cruise. For more information about these data, or if you wish to use these data for your own analysis and/or publications (collaborations welcome), please contact the PI at msundermeyer@umassd.edu

018576_20110603_0414.mat     % 1 'big nothing' star line (03:31 elapsed)
018576_20110603_1526.mat     % 2 more 'big nothing' star lines (11:35 elapsed)
018576_20110603_1945.mat     % 3 another line (?) (01:05 elapsed)
018576_20110605_1350.mat     % 4 * Jun 5 rhod+flur1 W of main patch (11:35 elapsed)
018576_20110605_2328.mat     % 5 + Jun 5 rhod (08:10 elapsed) part 1 survey
018576_20110606_1003.mat     % 6 + Jun 6 rhod (09:41 elapsed) part 2 survey
018576_20110606_2225.mat     % 7 * Jun 6 rhod (07:34 elapsed) mostly complete
018576_20110607_1844.mat     % 8 - Jun 7 rhod (19:06 elapsed) complete
018576_20110608_0020.mat     % 9 - Jun 7 rhod (02:19 elapsed) last leg of prev
018576_20110608_0925.mat     % 10 * Jun 8 flur (06:33 elapsed) SW of main patch
018576_20110608_2340.mat     % 11 Jun 8 rhod (04:35 elapsed) west side + UVIC MVP
018576_20110609_0941.mat     % 12 * Jun 9 flur (06:03 elapsed) SW of main patch
018576_20110610_0927.mat     % 13 * Jun 9 rhod (11:29 elapsed) complete

018576_20110611_2334.mat     % 14 Jun 11 large-scale survey (40 km, 15:27 elapsed)
018576_20110612_2349.mat     % 15 Jun 12 large-scale survey (6 km, 02:04 elapsed)
018576_20110613_1755.mat     % 16 * Jun 13 1st trans of survey (00:10 elapsed)
018576_20110613_1812.mat     % 17 * Jun 13 2nd trans of survey (00:13 elapsed)
018576_20110613_2240.mat     % 18 * Jun 13 rhod 2 (04:25 elapsed) complete initial
018576_20110614_0446.mat     % 19 + Jun 13+14 pair with 18? (05:43 elapsed)
018576_20110614_1748.mat     % 20 * Jun 14 rhod 2 (11:42 elapsed) complete
018576_20110614_2109.mat     % 21 *Jun 14 (00:39) single transect, pair with 20
018576_20110615_2150.mat     % 22 - Jun 15 (15:33 elapsed) complete
018576_20110616_0007.mat     % 23 - Jun 15 (01:02 elapsed) 3 transects, pair with 22
018576_20110616_1843.mat     % 24 * Jun 16 (10:36 elapsed) complete
018576_20110617_1635.mat     % 25 - Jun 16+17 (20:57 elapsed) N-S line + north end
018576_20110617_1806.mat     % 26 - Jun 17 (00:51 elapsed) single transect, pair 25
018576_20110618_1736.mat     % 27 Jun 18 (09:59 elapsed) north end high res
018576_20110619_1251.mat     % 28 Jun 18+19 (12:12 elapsed) north end high res
 

Prof. Miles A. Sundermeyer
The School for Marine Science and Technology
706 South Rodney French Blvd., New Bedford, MA 02744-1221
voice: 508.999.8892 fax: 508-910-6371 e-mail: msundermeyer@umassd.edu
www.smast.umassd.edu/msundermeyer