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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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.

Three-Dimensional Mapping of Fluorescent Dye Using a Scanning,
Depth-Resolving Airborne LIDAR
PIs: M. A. Sundermeyer, E. A. Terray and J. R. Ledwell
Grant Title: Airborne LIDAR Dye Mapping for Upper Ocean Mixing and Dispersion Studies
Funding Agency: The Cecil H. and Ida M. Green Technology Innovation Fund
Award: $34,964
Grant Title: A Pilot Study Using Airborne LIDAR to Survey Dye-Release Experiments:
Supplement to ONR Grant Number: N00014-01-1-0984
Funding Agency: Office of Naval Research
Award: $37,705

Objectives

We conducted a pilot study in summer 2004 using airborne LIght Detection And Ranging (LIDAR) to survey dye release experiments in the upper ocean. Two releases of 5 kg Rhodamine dye were performed off the east coast of Florida under fair weather conditions, and surveyed for 0.5-1.5 hr after release. Airborne surveys used the SHOALS bathymetric LIDAR, a scanning pulsed laser system, manufactured by Optech International, and operated by the U.S. Army Corps of Engineers (USACE) Joint Airborne LIDAR Technical Center of Expertise (JALBTCX). Ship-based in situ fluorometer observations were used as ground truth for the airborne surveys. Results are used to examine the feasibility of using airborne LIDAR to study horizontal and vertical dispersion in the upper ocean on space scales of 2-100 m horizontally and 1-10 m vertically, and time scales of minutes to hours.

 
Sea surface chlorophyll at the 2004 study site. Inset shows the ship tract during the June 3 experiment, with different line colors indicating successive transects through the dye patch.

Description of Field Experiments

Two dye releases were conducted in the near surface waters approximately 5 km due east of Ft. Lauderdale, Florida, one on June 2, and the other on June 3, 2004. On each of the two days, 5 kg of Rhodamine WT were injected in a single streak, and subsequently mapped over a period of 1-2 hrs using a fluorometer/CTD system towed from the ship, as well as airborne LIDAR. We focus here on results from the second of these two releases.

The dye was injected in a stair-step profile approximately 1.5 km in length, starting at a depth of 2.5 m followed by a deeper segment at 5 m, followed by an even deep segment at 10 m, then returning to the surface, and repeating. This resulted in a series of surface segments of the patch, interspersed with progressively deeper segments at discrete depths. Ship-based sampling of the dye patch commenced immediately after injection, and continued for 1-2 hrs thereafter. Surveys of the patch consisted of a single line transect along the major axis of the dye streak, followed by a zig-zag survey, followed by another line transect.

 
Dye streak and boat as viewed from plane. View of fantail from lab during dye injection.
 
SHOALS-1000T LIDAR System Specifications
Laser Nd:YAG (532, 1064 nm)
Power 5-6 mJ
Pulse Duration 6 nsec
Pulse Rate 1 kHz
Depth Penetration 1-50 m
Horizontal Accuracy 2.5 m
Aircraft Speed 125-175 kts
Operating Altitude 200-400 m
Swath Width Variable, up to 0.58 x altitude
Description of the SHOALS Airborne LIDAR

Overflights of the dye patches were conducted using a SHOALS-1000T LIDAR manufactured by Optech Incorporated, and operated by JALBTCX. Specifications of the SHOALS-1000T are listed in the table to the right. For the present experiments, a slight modification of the SHOALS-1000T bathymetric configuration was required. Namely, we replaced the receiver optics of the existing Raman channel with a narrow-band filter centered around the peak emission wavelength of the dye, i.e., approximately 580 nm. This receive channel was then routed to the high gain electronics of the shallow green channel. The latter spanned a wider range of depths than the Raman channel, typically down to about 20 m, compared to only 5-8 m in the Raman. The reconfiguration took about 20 minutes on the ground.

We use the measured profiles of backscatter at 532 nm and fluorescence at 580 nm to estimate dye concentration as a function of depth. Details of the inversion approach are given in Terray et al. (2005). Briefly, we reduce the problem to two dimensions using an exact inversion of our fluorescence signal. This allows us to parameterize the concentration profile in terms of an unknown constant of proportionality, P, which is related to the incident intensity just below the surface, and the dye concentration Cr = C(zr) at some reference depth zr. These parameters can then be determined by a joint least-squares fit to the fluorescence intensity profile, and to the backscatter-derived concentration profile over a limited region where the signal-to-noise ratio is high.

 

LIDAR Results

Raw infrared, green and fluorescence backscatter signals observed from the LIDAR during a pass over the near-surface segment of the dye patch, showing a mix of background (without dye) and profiles within the dye patch.

A total of 21 flight lines were conducted over two hours, 12 north-south (along the axis of the streak) and 9 east-west (across the axis of the streak). As expected, raw LIDAR returns (right) for the green channel showed strong returns at the ocean surface, then decaying with depth. Enhanced attenuation was clearly evident in profiles within the dye patch. In addition, clear peaks in the dye channel were also evident, distinct from profiles in which no dye was detected.

Plan views of the raw lidar signal for each of the north-south flight lines are shown below. The most striking feature of the image is the appearance of distinct bands in the near-surface dye patch on time scales of order tens of minutes. These features have wavelengths in the range 30-50 m. Although their dominant orientation (south-east to north-west) is roughly in the direction of the wind (and the current in the surface stratified layer), the mechanism for their formation is not clear. Langmuir circulation can be ruled out based on the observed wind speed, wave height and stratification. Given the strong shear in the stratified surface layer, we suspect that the observed features may be associated with shear instability in the upper portion of the water column. However, this is still under investigation.

  Plan views of the raw LIDAR signal from the north-south flight lines, with successive overflights are offset in the x direction for clarity. Times of the overflights (hh:mm:ss) are shown below each flight line. The three surface segments yielded the strongest returns, followed by the 5 m segments. A faint signal from the 10 m segments could also be seen at early times. Note, here the weaker signal at depth is not necessarily due to a decrease in dye concentration, but rather to natural attenuation of the laser light in seawater.

Dye concentration from a ship-based transect taken along the major axis of the dye streak (left), as well as results from a corresponding LIDAR overflight (right) are shown in the figure below. The results show a clear signal in both channels, although minor differences between the two channels and the in situ results are also evident. The surface segments of the dye streak are clearly visible in both the green and fluorescence channel inversions. Also visible in the green channel inversion are the two southern-most 5 m segments of the dye patch. The northern-most 5 m segment and the two 10 m segments do not appear, possibly because they had already been advected westward out of the field of view of the LIDAR.


(Click on image to enlarge)
(Left) Dye concentration as measured by ship during a tow-yo transect along the major axis of the dye streak. (Right) Horizontal slices of dye concentration through the tracer patch as measured by airborne LIDAR - (top) concentration inferred from the green channel and (bottom) dye channels, respectively. Vertical slices are approximately every 2 m, from the surface to 12 m.
 
(Click on image to enlarge)

Conclusions

We conducted a pilot study using airborne LIDAR to survey dye release experiments in the upper ocean on spatial scales of meters to kilometers, and temporal scales of minutes to days. Results show qualitative as well as quantitative agreement between dye distributions inferred from airborne LIDAR and ship-based observations using a towed fluorometer. While the LIDAR observations are subject to greater noise than in situ measurements, the very large number of observations made by the LIDAR (of order 30,000 profiles during a single overflight compared to 200 in situ profiles for the entire experiment) provide a wealth of information about overall distribution of the dye patch, which would otherwise not be obtainable from in situ measurements alone. Given the very high resolution, both temporally and spatially, provided by airborne LIDAR, we believe measurements such as those presented here hold great promise for our understanding of small-scale dispersion in the upper ocean.

 

Publications

Sundermeyer, M. A., E. A. Terray, J. R. Ledwell, A. G. Cunningham, P. E. LaRoque, J. Banic, and W. J. Lillycrop. Three-Dimensional Mapping of Fluorescent Dye Using a Scanning, Depth-Resolving Airborne Lidar. J. Atmos. Ocean. Technol. , 24, 1,050-1,065, 2007.
Abstract   PDF  
Terray, E. A., J. R. Ledwell, M. A. Sundermeyer, T. Donoghue, S. Bohra, A. G. Cunningham, P. E. LaRoque, J. Banic, W. J. Lillycrop, and C. E. Wiggins. Airborne Fluorescence Imaging of the Ocean Mixed Layer. In: Proc. of the IEEE/OES Eighth Working Conference on Current Measurement Technology , June, 2005, Southampton, U.K.
Abstract   PDF  
 

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