Collaborative Research: LIDAR Studies of Lateral Dispersion in the Seasonal Pycnocline

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Intellectual Merit. Lateral stirring and mixing in the stratified ocean at scales of 30 m to 3 km are poorly understood. Yet mixing at these scales is an important aspect of marine ecology. Also, mixing at these scales must usually be parameterized in numerical models of ocean circulation and ocean biogeochemistry, since such small scales are not explicitly resolved in most models. The aim of the proposed work is to visualize and to understand the processes governing lateral stirring and mixing at these small scales. One hypothesis to test is that lateral stirring is driven by the relaxation of patches of weakly stratified water created by vertical mixing events. Another is that it is due to the combined action of vertical mixing and vertical shear in the horizontal velocities. There may also be stirring processes independent of vertical mixing. Drs. Ledwell and Sundermeyer will test these hypotheses and ideas and be prepared to formulate others on the basis of their experiments. The approach is to release patches of fluorescent dye in the stratified upper ocean off Bermuda, where the water is very clear. Rapid surveys of the evolving dye patches will be made with a scanning LIDAR (LIght Detecting and Ranging) system carried by a low-flying aircraft. Profiles made by a small boat in the patches will provide calibration data for the airborne measurements and would sample hydrographic structures of interests within the patches. The small boat will be tended by a research vessel, from which measurements will be made of inherent optical properties of the water, water velocity profiles, ambient radiation and meteorological conditions near the patches. The airborne LIDAR system will be a modification of a topographic mapping system operated by the NAVAIR Flight Facility. It will have a depth resolution of 1 m and a lateral resolution of 2 to 3 meters, and will sense dye to depths of 30 meters. Navigation and mapping of the dye patches will be relative to a set of drifting buoys with drogues set at the depth of the releases, and with GPS receivers and radio transmitters for communication with the ship, the small boat, and the aircraft. The experiments will be conducted during the month of July when the water is usually the clearest and when the wind-mixed layer is the thinnest. The releases will be between 10 and 20 meters below the surface, and each experiment will last on the order of 24 hours. The dye distribution will be inferred from sophisticated inversions of the returned signal in both the transmitted and fluoresced frequency bands. Broader Impacts. Understanding of the physical processes at work in small scale lateral mixing in the ocean will be of direct benefit to those trying to study ocean ecosystems and hence to those working to protect the health of the ocean, as well as those trying to develop numerical models of the ocean for this and a great variety of other practical purposes. We will entrain undergraduate and/or graduate students and a school teacher, through the ARMADA program, into the field work, which will be conducted during the summer break. A post doctoral fellow will be entrained for the field work and analysis. Development of the NASA LIDAR system at Wallops Flight Facility into one that can effectively see into the upper ocean, whether dye is used or not, will be a substantial addition to the research tools available to oceanographers.