Remote Measurements of Horizontal Eddy Diffusivity

Darek J. Bogucki Division of Applied Marine Physics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Search for other papers by Darek J. Bogucki in
Current site
Google Scholar
PubMed
Close
,
Burton H. Jones Department of Biological Sciences, University of Southern California, Los Angeles, California

Search for other papers by Burton H. Jones in
Current site
Google Scholar
PubMed
Close
, and
Mary-Elena Carr Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

Search for other papers by Mary-Elena Carr in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

The rate of horizontal diffusivity or lateral dispersion is key to understanding the dispersion of tracers and contaminants in the ocean, and it is an elusive, yet crucial, parameter in numerical models of circulation. However, the difficulty of parameterizing horizontal mixing is exacerbated in the shallow coastal ocean, which points to the need for more direct measurements. Here, a novel and inexpensive approach to remotely measure the rate of horizontal diffusivity is proposed. Current shipboard measurement techniques require repeated surveys and are thus time consuming and labor intensive. Furthermore, intensive in situ sampling is generally impractical for routine coastal management or for rapid assessment in the case of emergencies. A remote approach is particularly useful in shallow coastal regions or those with complex bathymetry.

A time series of images from a dye-release experiment was obtained with a standard three-megapixel digital camera from a helicopter that hovered over the study area. The red–green–blue (RGB) images were then 1) analyzed to distinguish the dye from the ambient color of the water and adjacent land features, 2) orthorectified, and 3) analyzed to obtain advection and diffusion rates of the thin subsurface dye layer. A horizontal current of the order of 6 cm s−1 was found. The estimated horizontal eddy diffusivity rate for scales of O(10 m) in the harbor was 0.1 m2 s−1. The dye diffusivity and advection rate that are calculated from the images are consistent with independent calculations based on in situ measurements of current speed fluctuations.

Corresponding author address: Dr. Darek J. Bogucki, Division of Applied Marine Physics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy. Miami, FL 33149-1098. Email: dbogucki@rsmas.miami.edu

Abstract

The rate of horizontal diffusivity or lateral dispersion is key to understanding the dispersion of tracers and contaminants in the ocean, and it is an elusive, yet crucial, parameter in numerical models of circulation. However, the difficulty of parameterizing horizontal mixing is exacerbated in the shallow coastal ocean, which points to the need for more direct measurements. Here, a novel and inexpensive approach to remotely measure the rate of horizontal diffusivity is proposed. Current shipboard measurement techniques require repeated surveys and are thus time consuming and labor intensive. Furthermore, intensive in situ sampling is generally impractical for routine coastal management or for rapid assessment in the case of emergencies. A remote approach is particularly useful in shallow coastal regions or those with complex bathymetry.

A time series of images from a dye-release experiment was obtained with a standard three-megapixel digital camera from a helicopter that hovered over the study area. The red–green–blue (RGB) images were then 1) analyzed to distinguish the dye from the ambient color of the water and adjacent land features, 2) orthorectified, and 3) analyzed to obtain advection and diffusion rates of the thin subsurface dye layer. A horizontal current of the order of 6 cm s−1 was found. The estimated horizontal eddy diffusivity rate for scales of O(10 m) in the harbor was 0.1 m2 s−1. The dye diffusivity and advection rate that are calculated from the images are consistent with independent calculations based on in situ measurements of current speed fluctuations.

Corresponding author address: Dr. Darek J. Bogucki, Division of Applied Marine Physics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy. Miami, FL 33149-1098. Email: dbogucki@rsmas.miami.edu

Save
  • Barnard, A. H., Zaneveld J. V. , and Pegau W. S. , 1999: In situ determination of the remotely sensed reflectance and the absorption coefficient: Closure and inversion. Appl. Opt., 38 , 51065117.

    • Search Google Scholar
    • Export Citation
  • Csanady, G. T., 1980: Turbulent Diffusion in the Environment. D. Reidel Publishing, 250 pp.

  • Ledwell, J. R., and Watson A. J. , 1991: The Santa Monica tracer experiment: A study of diapycnal and isopycnal mixing. J. Geophys. Res., 96 , 1670916719.

    • Search Google Scholar
    • Export Citation
  • Mobley, C. D., 1994: Light and Water: Radiative Transfer in Natural Waters. Academic Press, 592 pp.

  • Okubo, A., 1971: Oceanic diffusion diagrams. Deep-Sea Res., 18 , 789802.

  • Okubo, A., and Karweit M. J. , 1969: Diffusion from a continous source in a uniform shear flow. Limnol. Oceanogr., 14 , 514520.

  • Rehmann, C. R., and Duda T. F. , 2000: Diapycnal diffusivity inferred from scalar microstructure measurements near the New England shelf/slope front. J. Phys. Oceanogr., 30 , 13541371.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Richardson, L. F., 1926: Atmospheric diffusion shown on a distance-neighbour diagrams. Proc. Roy. Soc. London, 110A , 709737.

  • Suijlen, J., and Buyse J. , 1994: Potentials of photolytic rhodamine WT as a large-scale water tracer assessed in a long-term experiment in the Loosdrecht lakes. Limnol. Oceanogr., 39 , 14111423.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sundermeyer, M. A., and Ledwell J. R. , 2001: Lateral dispersion over the continental shelf: Analysis of dye release experiments. J. Geophys. Res., 106C , 96039621.

    • Search Google Scholar
    • Export Citation
  • Tennekes, H., and Lumley J. L. , 1972: A First Course in Turbulence. MIT Press, 301 pp.

  • Tsai, R. Y., and Huang T. S. , 1984: Multiframe image restoration and registration. Trans. J. ACM, 31 , 317339.

  • Walker, R. E., 1994: Marine Light Field Statistics. Wiley Interscience, 660 pp.

  • Watson, A. J., and Ledwell J. R. , 2000: Oceanographic tracer release experiments using sulphur hexafluoride. J. Geophys. Res., 105 , C6,. 1432514337.

    • Crossref
    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 1003 400 113
PDF Downloads 556 103 5