Abstracts of FLIP-related research presented at the American Geophysical Union Conference in December 2002 in San Francisco, California.
Stable Platform Designs for Global DEOS Moorings
John A Orcutt
Frank L Vernon III
Scripps Institution of Oceanography, 9500 Gilman Drive, 0210, La Jolla, CA 92093, United States
Oceanography has been dominated for at least two centuries by an expeditionary approach and examples include the voyage of the Beagle in 1831-1836 and the Challenger Expedition in 1872 - 1876. In the U.S., the capabilities for expeditionary research were greatly amplified during and especially following WWII. Today the U.S. alone has established a research fleet of 28 vessels organized through UNOLS. While experimental oceanography has made enormous contributions over the decades and centuries, this approach has not been well suited to investigating processes in which transients are important. The Dynamics of Earth and Ocean Systems (DEOS) program was developed in 1997 to promote the idea of making long-term observations in the oceans - to establish a long-term presence in the oceans. DEOS, now under the sponsorship of the Consortium for Ocean Research and Education (CORE) with support from the NSF, advocates the collection of long-term time-series data with the recognition that this is the only viable approach to observe transients and changes and to enhance the signal-to-noise ratio of weak signals. Moored ocean buoys are a technically feasible approach for making sustained time series observations in the oceans and will be an important component of any long-term ocean observing system. Scripps and Woods Hole developed the ocean mooring systems, designed for decadal time scales, in an NSF-sponsored design study. One of the designs bears a strong familial resemblance to R/P FLIP and is especially well suited for maximizing system life as well as ensuring robust Internet connectivity. I will review this design and describe feasibility experiments conducted to test communications feasibility. Because of the broad spectrum of scientific needs identified during planning, it is clear that there is no single buoy or mooring design that will meet all of these needs while at the same time minimizing costs. An alternative British design may be particularly well suited for high latitude deployments. Ongoing experiments to demonstrate components of the mooring program will be discussed.
Internal Waves, Reference Frames and the Search for Intrinsic Frequency
Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0213, United States
For the past 40 years, internal wave and fine-scale fields have been studied using a variety of spectral techniques. Frequency spectra of vertical displacement and horizontal velocity appear to be continuous, with the addition of discrete near-inertial and tidal peaks. When the combined space-time variability of the fields is tested against linear internal wave theory, agreement is generally poor. An added "vortical" or "fine-structure" field is often invoked to explain observations. Working from deep-sea stable platforms both in the Arctic (the polar ice cap) and the open ocean (FLIP), recent data suggest that much of the continuous nature of the internal wave frequency spectrum results from simple Doppler spearing of a few principal spectral constituents. The apparent role of the vortical field is dependent on the reference frame in which observations are made. Such observations encourage revision of our view of the spectral cascade of energy from large to small scales.
Observations of Langmuir Circulation from FLIP
Jerome A Smith
Scripps Institution of Oceanography, 0213 U.C.S.D, La Jolla, CA 92093-0213, United States
Langmuir circulation has significance across the marine disciplines. Enhanced deepening and inhibited re-stratification can alter the surface temperature and hence net air-sea exchanges. Organization of bubbles into windrows introduces dramatic sound speed variability and also affects air/sea gas fluxes. Organization of seaweed and plankton affects marine life, including pelagic fisheries. Finally, dispersal by Langmuir circulation is a major component in models for oil-spill tracking and for search-and-rescue operations. To get an adequate picture of the forcing and response of Langmuir circulation (and the wind-mixed layer in general), the observations needed include windstress, directional waves, wave breaking, heat and moisture fluxes, stratification (temperature and salinity profiles), velocity profiles across the mixed layer and thermocline, spacing and orientation of windrows, and a measure of the strength of the circulation (e.g., surface rms velocities). These measurements span both the air/sea interface and the thermocline, and must be maintained continuously for many days to span storms and daily, tidal, and inertial cycles. In addition, the total power requirements exceed that comfortably supplied by batteries or local generation by wind or solar energy. It appears that FLIP is uniquely qualified as a platform from which the required range of measurements may all be made. Findings concerning the evolution and dynamics of Langmuir circulation that were facilitated by FLIP are reviewed and summarized, with emphasis on observations from 1990, 1995, and 2002.
Spar Buoy Laboratories - Origins and Early Realizations
Fred N. Spiess
Scripps Institution of Oceanography, Marine Physical Laboratory,
8635 Discovery Way, La Jolla, CA 92037, United States
At least as early as the 1950's there was a realization in the ocean research community of a need for stable platforms that could remain on station in the deep ocean for protracted periods. The 1959 report (Oceanography 1960-1970) of the NAS/NRC Committee on Oceanography includes the recommendation that a manned spar buoy laboratory should be among the new types of research platforms that should be built. By the late 1960s there were at least four craft of this type in operation: Cousteau's Bouee-Laboratoire, US Naval Ordnance Laboratory's SPAR, General Motors Defense Laboratory's POP, and the Marine Physical Laboratory's FLIP. All of these achieved their stability by using relatively deep draft spar buoy configurations. They differed, however, in their design philosophies and thus in their overall dimensions, general configurations, ultimate uses and longevity. FLIP has had the longest life of any of the four, for a variety of reasons, but primarily due to its versatility, as attested to in other papers in this session. This paper will discuss the origins, design considerations and careers of these and other similar craft.
Langmuir Cells, Mixed Layer Evolution, and the Search for the Ekman Layer
Robert A Weller
Albert J Plueddemann
Woods Hole Oceanographic Institution, Clark 204A MS29, Woods Hole, MA 02543, United States
Making the accurate near-surface velocity measurements needed to both describe and understand the structure and variability of the oceanic mixed layer has challenged oceanographers for many years. Deployment of prototype Vector Measuring Current Meters (VMCMs) from the Research Platform FLIP in the 1970s produced some of the first velocity observations that resolved the vertical structure of upper ocean currents. When the relation between the time series of surface stress and upper ocean currents was examined, the phase angle between the wind-driven flow and the surface wind stress was found to depend on the frequency of the variability as predicted by Ekman theory, though the vertical structure of the mean flow did not in detail match an Ekman spiral. Subsequent cruises on FLIP identified the role of the surface buoyancy forcing in driving diurnal variations in the velocity and density structure of the upper ocean which, when averaged, modified the mean vertical structure of the wind-driven flow near the surface. The relationship of upper ocean structure and the evolution of the mixed layer to the combination wind stress and buoyancy forcing was analyzed and the resulting understanding used as the basis for developing the Price-Weller-Pinkel (PWP) one-dimensional mixed layer model. The model was found to often work well, replicating the temporal evolution of the upper ocean velocity and density structure. Built into the model physics is rapid vertical mixing within the homogenous part of the surface layer. It was hypothesized that the presence of Langmuir cells could provide such rapid vertical mixing; and further work from R/P FLIP turned to efforts to first determine if Langmuir cells could be observed, and later to study the role of Langmuir circulation in mixed layer dynamics. A combination of deployments of computer cards to mark surface flow patterns and in-situ acoustic Doppler measurements within the mixed layer showed that Langmuir cells could be visualized and observed. A VMCM modified to measure vertical (w) as well as horizontal velocities showed the circulation could be strong, with w in excess of 20 cm s-1. As techniques to image Langmuir Cells improved, it was found that the circulation was variable in time, with growth and decay modulated by a combination of wind stress and surface wave Stokes drift. Work remains to be done to better understand the dynamics of Langmuir circulation and other influences of surface waves on mixed layer dynamics and to include these processed in mixed layer models.
Microwave and Electro-optical Transmission Experiments in the Air-sea Boundary Layer
Kenneth D. Anderson
SPAWARSYSCEN, SAN DIEGO, Code 2858 53560 Hull St., San Diego, CA 92152, United States
Microwave and electro-optical signal propagation over a wind-roughened sea is strongly dependent on signal interaction with the sea surface, the mean profiles of pressure (P), humidity (Q), temperature (T), wind (U), and their turbulent fluctuations (p, q, t, u). Yet, within the marine surface layer, these mechanisms are not sufficiently understood nor has satisfactory data been taken to validate propagation models, especially under conditions of high seas, high winds, and large surface gradients of Q and T. To address this deficiency, the Rough Evaporation Duct (RED) experiment was designed to provide first data for validation of meteorological, microwave, and electro-optical models in the marine surface layer for rough surface conditions including the effects of surface waves. The RED experiment was conducted offshore of the Hawaiian Island of Oahu in late summer, mid-August to mid-September, of 2001. R/P FLIP, moored about 10 km off of the NE coast of Oahu, hosted the primary meteorological sensor suites and served as a terminus for the propagation links. There were eleven scientists and engineers aboard R/P FLIP who installed instruments measuring mean and turbulent meteorological quantities, sea wave heights, directions, and kinematics, upward and downward radiance, near surface bubble generation, atmospheric particle size distributions, laser probing of the atmosphere, and sources for both microwave and electro-optic signals. In addition to R/P FLIP, two land sites were instrumented with microwave and electro-optic receivers and meteorological sensors, two buoys were deployed, a small boat was instrumented, and two aircraft flew various tracks to sense both sea and atmospheric conditions. In all, more than 25 people from four countries, six universities, and four government agencies were directly involved with the RED experiment. While the overall outcome of the RED experiment is positive, we had a number of major and minor problems with the outfitting, deployment, operation, and recovery of R/P FLIP. These problems ranged from the U.S.N.S. Sioux cutting a mooring line, which delayed deployment by more than 4 days, nearly loosing Tommy during the first attempt at deployment, inadequate air conditioning in the lab spaces, causing at least one instrument to temporarily fail, and problems associated with too many people and too many sensors on board. These issues will be discussed and recommendations will be made to improve future microwave and electro-optical experiments at sea.
Zooplankton in Langmuir Cells Observed from FLIP
We sought to test the hypothesis of Stommel and others that Langmuir cells (LCs) foster pattern in the distribution and abundance of zooplankton in the open ocean. To this end, one of us (DMC) participated in the 1995 Marine Boundary Layer (MBL) Experiment in which FLIP was deployed in deep water off Monterey in springtime. Atmospheric and wave forcing of LCs was measured by MBL participants. Periodically, we profiled continuously at 1 meter per second for hours to days within the upper ocean with a package containing a CTD and Optical Plankton Counter. Throughout, we deployed a CTD and Acoustic Doppler Velocimeter at a mean depth of 6 then 8 m. On occasion, simultaneous collections were made with a plankton pump and net system and the samples were microscopically enumerated. The resultant data were analyzed for pattern using geostatistics. Conditions progressed over a period of two weeks from benign, with a stratified upper ocean, to strong winds and high waves, with well-developed LCs, followed by abatement. Forcing was quantified by estimating LC convergent velocity from wind stress and wave height time series. LCs were manifest in the temperature distribution of our profiler data. In particular, in the time-depth domain, sections within LCs showed cool water apparently entrained upward from the base of the mixed layer to the surface. These patterns persisted on the scale of hours. Temperature at a single depth within well-developed LCs varied in a periodic fashion over a range of the order 0.02 deg C. Such measurements provide the physical context in which to interpret our biological observations. Variograms were used to assess spatial pattern of temperature and plankton. Significant pattern existed for temperature in LCs in the horizontal but not along profiles in the vertical, within the mixed layer, consistent with our observations of LCs in the time-depth domain. The fixed and profiling CTDs yielded consistent time series, confirming the accuracy of the profiler temperature data and, thus, existence of LCs. No significant pattern existed in LCs in the horizontal for zooplankton-sized particles sensed by the profiling OPC. Similarly, no structure, thus pattern, existed in LCs in the horizontal for zooplankton collected with the plankton pump. Conversely, during benign conditions, pattern was evident in the zooplankton sensed by the profiling OPC and collected by the pump. Residual velocities of LCs we observed in the open ocean appear to exceed the swimming speed of individual zooplankters. Hence, whereas LCs caused pattern in temperature, they mixed the plankton. FLIP remains a unique and excellent platform for interdisciplinary studies of plankton and air-sea interactions. Future work might include the use of video to study the plankton and marine snow (particle aggregates) in a range of forcing conditions. Collaboration between the disciplines is enhanced by FLIP's tight quarters. This work was supported by the Office of Naval Research.
Biogeochemical and Bio-optical Measurements from Stable Platforms and the Coming Ocean Observatories
Tommy D Dickey
University of California, Santa Barbara, Ocean Physics Laboratory
6487 Calle Real, Suite A, Goleta, CA 93117, United States
Problems such as global climate change, carbon and biogeochemical cycling, upper ocean ecology, biomass and bio-optical variability, waning fisheries, population dynamics, and generally ocean prediction are hindered by insufficient time series data. These problems and others require interdisciplinary data that need to be collected simultaneously and effectively span ten orders of magnitude in time. New technologies are enabling interdisciplinary sampling of the ocean at unprecedented time and space scales. Autonomous sampling of interdisciplinary variables using platforms including stable platforms such as R/P FLIP, moorings, drifters, profiling floats, gliders, and autonomous underwater vehicles (AUVs) has become a major emphasis of observational oceanography. Autonomous measurements now include several key chemical, bio-optical, and biological variables. Moorings and R/P FLIP have been used to test sensors and systems, which have been, or likely will be, transitioned to other autonomous sampling platforms. A natural extension of this work is to future stable platforms and observatories. Some examples of interdisciplinary time series results obtained during with suites of sensors are presented. Visions of new sensor technologies and a network of integrated, interdisciplinary, global-scale, three-dimensional time series observations using multiple platform-types including stable platforms and observatories and modeling are presented. Ongoing international efforts and plans for implementation of an array of platforms and observatories equipped with interdisciplinary sensors will be described.
A Decade of Ocean Acoustic Measurements from R/P FLIP
Gerald L. D'Spain
Marine Physical Laboratory Scripps Institution of Oceanography,
291 Rosecrans St., San Diego, CA 92106, United States
Studies of the properties of low frequency acoustic fields in the ocean continue to benefit from the use of manned, stable offshore platforms such as R/P FLIP. A major benefit is providing the at-sea stability required for deployment of extremely large aperture line arrays, line arrays composed of both acoustic motion and acoustic pressure sensors, and arrays that provide measurements in all 3 spatial dimensions. In addition, FLIP provides a high-profile (25 m) observation post with 360 deg coverage for simultaneous visual observations of marine mammals. A few examples of the scientific results that have been achieved over this past decade with ocean acoustic data collected on FLIP are presented. These results include the normal mode decomposition of earthquake T phases to study their generation and water/land coupling characteristics using a 3000 m vertical aperture hydrophone array, simultaneous vertical and horizontal directional information on the underwater sound field from line arrays of hydrophones and geophones, the strange nighttime chorusing behavior of fish measured by 3D array aperture, the mirage effect caused by bathymetry changes in inversions for source location in shallow water, and the diving behavior of blue whales determined from 1D recordings of their vocalizations. Presently, FLIP serves as the central data recording platform in ocean acoustic studies using AUVs.
Vertical Acoustic Arrays in the Deep Ocean
Scripps Institution of Oceanography, 9500 Gilman Drive Mail Code 0701, La Jolla, CA 92093, United States
The R/P FLIP has made possible the deployments of vertical arrays to study sound propagation and ambient noise in the deep ocean in ways never before possible from existing research vessels. Long vertical arrays can be deployed without the flow noise contamination from platform motion, long a bane for making such studies. The vertical stability of FLIP combined with the deep mooring capability developed by Earl D. Bronson made it possible to deploy multi-element arrays beginning with a versatile 20 element array with variable spacing developed by Bill Whitney in Fred Spiess's group. The 20 element array consisted of bungee mounted hydrophones in metal cages at either uniform spacing or variable spacing to meet directivity or other requirements. It was assembled on station in the vertical and deployed to the desired depths for the elements. Gerald Morris at MPL conducted ambient noise studies using variable spacing of the elements to below the critical depth as well as in the water column above. Vic Anderson used it for his DIMUS processing system for detecting low level signals masked by ambient noise. As a 500 meter array, I used it for a series of CONTRACK (Continuous Tracking of signals at long range) experiments to resolve multipaths so they wouldn't interfere with one another. The VEKA vertical array developed by Rick Swenson of NORDA was deployed in very deep (below 3300 m) water by Dan Ramsdale of NORDA using the winch and double lay armored cable on FLIP, the same cable system for the MPL 20 element array. In my group Bruce Williams designed a rapidly deployable array to study vertical anisotropy of ambient noise as a function of range from near shore shipping via downslope conversion in a series of 48 hours FLIP stations 350, 1000 and 1500 miles from the Pacific coast. A short 120 element array, 1000 meters long, was built by John Hildebrands's group for a test of matched field processing and the SLICE experiment in acoustic tomography research of Peter Worcester and Walter Munk in 1987. Later a different 200 element array over 3000 meters long was also built by John Hildebrand's group for deployment in the VAST experiment in 1987. This array included acoustic navigation to measure element location for several different experiments including matched field processing at 1000 km, normal mode studies and down-slope conversion of shipping noise and by Stan Flatte of UCSC for looking at long range barotropic wave reflections from Alaska. In a separate talk, Gerald D'Spain will discuss a trifar (3D) vertical array developed at MPL.
Air-Sea Interaction Measurements from R/P FLIP
Carl A Friehe
University of California, Irvine, Departments of Mechanical Engineering and Earth System Science, Irvine, CA 92697-3975, United States
Soon after its inception, R/P FLIP was used to study the interaction of the atmosphere and ocean due to its unique stability and low flow distortion. A number of campaigns have been conducted to measure the surface fluxes of heat, water vapor and horizontal momentum of the wind with instrumentation as used over land, supported by the Office of Naval Research and the National Science Foundation. The size of FLIP allows for simultaneous ocean wave and mixed-layer measurements as well. Air-sea interaction was a prime component of BOMEX in 1968, where FLIP transited the Panama Canal. The methods used were similar to the over-land "Kansas" experiment of AFCRL in 1968. BOMEX was followed by many experiments in the north Pacific off San Diego, northern California, and Hawaii. Diverse results from FLIP include identification of the mechanism that causes erroneous fluctuating temperature measurements in the salt-aerosol-laden marine atmosphere, the role of humidity on optical refractive index fluctuations, and identification of Miles' critical layer in the air flow over waves.
Air-Sea Interaction and Remote Sensing Experiments Using R/P FLIP
Andrew T. Jessup
Applied Physics Laboratory, University of Washington, 1013 NE 40th St., Seattle, WA 98105-6698, United States
Although the Research Platform FLIP was originally designed for sonar studies, its unique characteristics have made it an ideal platform for experiments using remote sensing techniques to study air-sea interaction. The combination of stability and access to the air-sea interface provides the capability to make a variety of remote sensing measurements simultaneously with direct measurements of the relevant atmospheric and oceanic parameters. When FLIP is freely drifting, the hull rotates so that it is in the same orientation relative to the wind. Judicious use of the variety of booms available for instrument mounting makes it possible to view the sea surface without platform interference. This ability to make continuous measurements regardless of changes in wind direction is a major advantage of FLIP over fixed platforms. A survey of remote sensing measurements made from FLIP will be presented, including a variety of active microwave sensors (radars and scatterometers), passive microwave sensors (radiometers), infrared sensors (radiometers and imagers), and visible sensors (video cameras).
FLIP II - Concept Designs to Meet Future Scientific Mission Requirements
Duane H. Laible
The Glosten Associates, Inc., 605 First Avenue Suite 600,
Seattle,WA 98104, United States
R/P FLIP has successfully operated for 40 years in support of important oceanographic research missions. The simple platform, which has the unique ability to provide a heave-stable operating location in open ocean environments, has over time been modified and upgraded. Its capability has been extended to the physical limits imposed by buoyancy and stability constraints. Nonetheless, there are oceanographic research operations that can use FLIP's unique characteristics, but which exceed its capabilities. Over the years researchers at the Marine Physical Laboratory of Scripps Institution of Oceanography have led investigations into second generation heave-stable ocean platforms with capabilities substantially exceeding those of R/P FLIP. This paper discusses several design concepts that have been developed. The designs are presented in terms of the ability to meet current and future scientific mission requirements.
Challenges in Measuring Air-Sea Interaction: Platforms and Sensors
W. Kendall Melville
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92037-0213, United States
Air-sea fluxes of heat, mass (gas and aerosols), momentum and energy are important in constraining the role of the oceans in weather, climate and the major biogeochemical cycles. However, the direct measurement of these fluxes is very difficult, especially in the energetic environment of the air-sea interface during high wind and wave events. The fact that the important fluxes typically scale as some significant power of the wind speed means that very short periods of high winds can contribute as much to the fluxes as very long periods of low winds. While remote sensing of air-sea fluxes has developed significantly over the last two decades, it is still the case that remote sensing algorithms are only reliable in the parameter ranges for which there is good "ground truth". It is especially in the high wind-speed regimes that the algorithms need to be carefully tested against in situ measurements. The development of platforms and instruments that can withstand the rigors of operating successfully in this environment is an important component of air-sea interaction research. No one platform is universally useful in providing a base for the measurements, and the judicious use of a variety of techniques is required to address the issues of both spatial and temporal coverage in a range of environments. While the development of small autonomous platforms has been very successful, they are not yet at the stage where their computational capabilities and communication bandwidths are sufficient to fully exploit the data that can be collected. In many cases, manned platforms are required, especially during the early stages of the development of new techniques when data acquisition and analysis are exploratory rather than operational. This is particularly the case for modern imaging techniques that generate large amounts of data. In this paper I will discuss these issues, presenting past efforts and potential future work in the use of platforms and sensors for the measurement of air-sea fluxes.