Upper Ocean Dye Studies Using a Scanning, Depth-Resolving Airborne LIDAR

Dye release experiments have been used for decades to study both vertical and horizontal mixing in the ocean. The naturally integrating properties of dye tracers provide a means of separating the sometimes weak, yet persistent motions associated with certain mixing processes from other much more energetic processes such as tides and/or wind-driven currents. However, measuring the time and space evolution of a tracer patch poses challenges of its own. In situ sampling from ships is costly and time consuming, and even in the best of circumstances can lack the requisite spatial and temporal resolution to understand all of the underlying physics. Numerical and laboratory studies offer some insight into the possible dynamics of mixing at these scales, and some progress has been made in relating these back to the observations. However, ultimately, improved field approaches are needed to better understand the mechanisms responsible for mixing at these scales.

In the present study we present one possible approach, which relies on airborne remote sensing. Specifically, we present results from a pilot study using fluorescent dye tracer imaged by airborne LIDAR in the ocean surface layer on spatial scales of meters to kilometers and temporal scales of minutes to hours. The LIDAR used employs a scanning, frequency-doubled Nd-Yg laser to emit an infrared (1064 nm) and green (532 nm) pulse 6 ns in duration at a rate of 1 kHz. The received signal is split into infrared, green, and fluorescent (nominally 580-600 nm) channels. The latter two channels are used to compute absolute dye concentration as a function of depth and horizontal position using an approach based in the atmospheric LIDAR literature. Two dye releases performed in the near surface layer off the coast of Florida are examined. Results reveal a complex horizontal and vertical structure of the dye patch, which evolves over a period of 10s of minutes to hours, and on spatial scales of 10s of meters horizontally and a few meters vertically. Concurrent in situ measurements confirm the dye results obtained from the LIDAR, and provide additional context for understanding the underlying physics. The results demonstrate the ability of airborne LIDAR to capture high resolution three-dimensional “snapshots” of the distribution of tracer as it evolves over very short time and space scales. Such measurements offer a powerful observational tool for studies of transport and mixing in the upper ocean on these scales.