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Lidar Studies of Small-Scale Lateral Dispersion
(Jointly funded by ONR and NSF, in collaboration with NAVAIR)
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| PIs: |
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M. A. Sundermeyer, J. R. Ledwell
and E. A. Terray |
| Grant Title: |
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Lidar Studies of Lateral
Dispersion in the Seasonal Pycnocline
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| Funding Agency: |
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The National Science Foundation
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Grant Title:
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Lidar and Numerical Modeling Studies of Small-Scale Lateral Dispersion in the Ocean
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Funding Agency:
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Office of Naval Research
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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.
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Results to Date
LatMix 2011 6-Day Rhodamine Evolutions
[Top]
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Fig. Sea surface temperature at the June 2011 study site ~300 km SouthEast of Cape Hatteras, NC.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Fig. Similar to above, but for June 15 fluorescein dye experiment
approximately 3.5 hr after release.
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Fig. Similar to above, but for June 15 fluorescein dye experiment.
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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.
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Fig. Similar to above, but for June 16 fluorescein dye experiment
approximately 2.6 hr after release.
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Fig. Similar to above, but for June 16 fluorescein dye experiment.
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Fig. Similar to above, but for June 16 surface dye experiment.
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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.
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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.
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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.
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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.
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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.
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Publications / Reports
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"Scalable Lateral Mixing and Coherent Turbulence,"
Department Research Initiative Whitepaper,
DRI Planning Workshop, May 28-30, 2008, Cambridge, MA.
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PDF
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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.
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PDF
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Data Sets
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