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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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| Award: |
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$1,088,442 (UMass:$381,652; WHOI:$706,790) |
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Grant Title:
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LIDAR and Numerical Modeling Studies of Small-Scale
Lateral Dispersion in the Ocean (ONR)
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Funding Agency:
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Office of Naval Research
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- Amount Pending - (UMass:$; WHOI:$ ) |
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We have just begun work on this project, which is jointly funded by
the National Science Foundation and the Office of Naval Research
through the "Scalable Lateral Mixing and Coherent Turbulence"
Department Research Initiative.
This project is a collaboration between the above PIs, and
Jennifer Prentice and Brian Concannon of the NAVAIR LIDAR group.
Objectives
The main objective of this work is to better understand lateral
mixing processes on scales of 10 m - 10 km in the ocean.
This includes the underlying mechanisms and forcing, as well as
the temporal, spatial, and scale variability of such mixing.
The particular goal of the present work is to visualize and
understand the processes governing lateral stirring and mixing
at these scales via high resolution dye release experiments
using airborne LIDAR. The broad impacts of this work range from
a better understanding of ocean ecosystems and hence ocean
health, to improved parameterizations in numerical ocean models,
to a variety of other practical purposes.
For this project, we will conduct a series of dye
release experiments in the seasonal pycnocline and upper ocean
to examine lateral dispersion and frontal processes on scales of
10 m - 10 km.
The vertical and horizontal dispersion and advection of the dye
patches will be monitored on spatial scales of meters to several
kilometers in the horizontal, 1-10 meters in the vertical, and
on time scales of minutes to hours, up to 4 days. Sampling of
the dye will be performed using airborne LIDAR, as well as in
situ sensors lowered and towed from a ship. Additional
measurements of optical characteristics, hydrography, currents,
and internal wave characteristics will be used to identify
particular driving mechanisms of the observed dispersion.
The dye experiments will be coordinated with AUV and
microstructure measurements proposed by other investigators
to discern forcing mechanisms responsible for the dispersion.
The field work will also be guided by numerical modeling
process studies proposed by other investigators under the DRI,
and will provide data for testing such models.
In addition to the main field effort, M. Sundermeyer will also
collaborate closely with M.-P. Lelong in support of Large Eddy
Simulation (LES) modeling
of lateral dispersion in the ocean on scales of 10 m - 10 km.
As part of the latter, additional numerical simulations
and analysis will be performed in preparation for, and to aid
interpretation of, the main field studies. These numerical
simulations will also be closely coordinated with modeling
efforts of other DRI participants.
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Sea surface chlorophyll at the September 2008
study site.
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LIDAR Engineering Field Test, Sept., 2008
In preparation for the main field experiments, an engineering
field study was conducted in Sept. 2008 off North Carolina to
test the NAVAIR LIDAR capabilities for mapping a dye patch
injected at 30 m depth in the ocean.
As part of the field test, concurrent with LIDAR overflights,
we conducted in situ measurements of inherent optical
properties, dye concentration, CTD and ADCP measurements,
and Lagrangian drifter studies to better understand both the
scientific and technical challenges of the combined airborne
and ship-based sampling program.
Operations were carried out within a few nautical miles of
33.8 N, 74.5 W, southeast of Cape Hatteras and offshore of the
Gulf Stream in oligotrophic waters (right).
Approximately 7.5 kg of fluorescein dye were injected at a depth
of 30 m in a box pattern whose corners were marked by 4 drifters
equipped with drogues centered at 30 m (below and right).
Subsequent to the injection, the dye was surveyed via two
tow-yo transects through the patch, after which the boat
stood off while the aircraft conducted its surveys.
Following the aircraft overflights, the dye patch was again
sampled from the ship via multiple tow-yo transects.
Examples of profiles of dye concentration as a function of
depth for the three surveys are shown below.
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Sample dye concentration profiles as a function of depth
taken during the first (left), second (middle), and third
(right) surveys of the dye patch showing variability of
dye concentration as a function of depth and decreasing
absolute dye concentrations as a function of time.
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Ship track (blue line), drifter tracks
(dashed lines), aircraft way-lines (yellow/black dashed)
relative to the center of mass of the drogues. Green
portion of ship track corresponds to dye injection period.
Black polygon indicates mean position of drifter box
in which dye was injected.
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Downwelling radiation profiles from the SPMR,
Profile 2, 9/9/08. The 442 nm profile has been offset by
1 unit to the right; the 509 nm profile one unit to the
left for clarity. Optical depths for diffuse radiation
can be estimated roughly from the depth over which the
natural log of the radiation decreases by 1 unit.
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Absorption and attenuation of the water in the upper 30 meters
were measured at nine wavelengths with a WetLABS AC-9.
Apparent optical properties of the water were also measured
using a Satlantic SPMR Radiation Profiler a few hours preceding
the dye injection in order to better understand in situ
conditions for the LIDAR, and to use in the inversion of the
LIDAR signal for absolute dye concentration. The SPMR measures
downwelling irradiance (ED) at 411, 442, 490, 509, 554, 665,
and 684 nm, and upwelling radiance (LU) at nominally the same
wavelengths. A reference radiometer was mounted on the roof
of the winch house on the boat to read the incident downwelling
irradiance at the surface (ES). The figure right shows
the results from Profile 2, taken on 9/9/09, in semilog
coordinates for the three frequencies of most interest.
Frequencies 442 nm and 490 nm bracket the frequency of the
LIDAR, which is at 470 nm. 509 nm is near the wavelength
of the fluorescein emission of 515 to 520 nm.
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A major result from the field test is that the NAVAIR airborne
LIDAR was readily able to detect the dye patch in both the
backscatter signal at 470 nm and in the fluorescence channel at
515 nm. The figure below shows results from the LIDAR fluorescence
channel. Bands on the left are possibly the signature of
internal waves. The signal from the patch on the right was
so strong as to saturate the detector. A more detailed analysis
of both the LIDAR and in situ data sets is ongoing.
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LIDAR signal from an overflight. The flight path was from
left to right. Horizontal axis: along track distance, in
meters; Vertical axis: cross-track distance, in meters.
Color: the maximum signal recorded, regardless of depth,
in the green, fluorescence, channel. The deep red at the
right of the figure indicates saturated signal due to dye,
such that the peak is off scale. The depth of the stripes
on the left was 20 to 25 m; the depth of the red saturated
patch on the right was 10 to 20 m.
Graphic courtesy of Jeffery Lee (APL/JHU)
and Brian Concannon (NAVAIR).
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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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