Executive Summary

The Panel on Boundary Layer Dynamics has critically examined the opportunities that exist to make substantial advances in experimental probing, and in theoretical studies and modeling, of the marine atmospheric boundary layer (MABL) and the oceanic boundary layer (OBL). These two boundary layers are the primary operating environment of the Navy. Quality-controlled observational data from various sensors provide an essential foundation for naval operations. Understanding the variability of boundary layer structure and fluxes, particularly in coastal regions, is essential to the Navy's capability for evaluating the level of uncertainty in weather and undersea forecasts, and in detection capabilities.

Wave properties at the interfacial surface between the MABL and OBL have a direct impact on at-sea operations. Knowledge of signal propagation in all marine environments is absolutely critical for assessing and predicting the performance of diverse defensive and tactical weapons systems as well as for detecting and localizing targets. Properties (physical, chemical, and biological) of the OBL often determine the nature of the working environment and the margin of safety afforded for underwater naval operations.

In this report, the panel describes research opportunities in the following areas: (1) geophysical processes in the MABL and OBL, specifically MABL fluxes and coupled turbulent flows; (2) the important topic of assimilation of data obtained from probing these marine environments; (3) the fundamental techniques for acquiring necessary data and reaching a new level of understanding of surface wave dynamics and electromagnetic propagation and signature physics; and (4) boundary layer dynamics (BLD)-related acoustical and optical oceanography. The order in which these topics are addressed reflects the panel's judgment of their relative priority for NRL research.

Chapter 1 outlines the relationship of the topics to the panel's charge; the topics for research are discussed in detail and related recommendations are made in Chapter 2.

The panel' s chief conclusions and recommendations are as follows:

  1. The spatial and temporal variability of marine atmospheric boundary layer fluxes is large, particularly in coastal regions. This variability greatly affects the reliability and stability of forecasts. The panel recommends that NRL support the development, evaluation, and use of a diverse set of sensing and measurement systems for the marine environment, steered by a small group of scientists, to study marine atmospheric boundary layer fluxes for their fundamental geophysical importance. An additional benefit of such work would be an improved understanding of the effects of these fluxes on aspects of system performance, e.g., on propagating signals. In this NRL effort a visiting scientist and postdoctoral study program is particularly encouraged.

  2. Coupling of the Navy's atmosphere and ocean predictive models has a natural place in the Navy's research mission, and the coupling of the atmospheric and oceanic turbulent flows through computational fluid dynamics techniques, such as large eddy simulation, is an important research area. The panel notes that coupling of these models might lead to little immediate improvement in atmospheric forecasts but should have a real impact



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Boundary Layer Dynamics Executive Summary The Panel on Boundary Layer Dynamics has critically examined the opportunities that exist to make substantial advances in experimental probing, and in theoretical studies and modeling, of the marine atmospheric boundary layer (MABL) and the oceanic boundary layer (OBL). These two boundary layers are the primary operating environment of the Navy. Quality-controlled observational data from various sensors provide an essential foundation for naval operations. Understanding the variability of boundary layer structure and fluxes, particularly in coastal regions, is essential to the Navy's capability for evaluating the level of uncertainty in weather and undersea forecasts, and in detection capabilities. Wave properties at the interfacial surface between the MABL and OBL have a direct impact on at-sea operations. Knowledge of signal propagation in all marine environments is absolutely critical for assessing and predicting the performance of diverse defensive and tactical weapons systems as well as for detecting and localizing targets. Properties (physical, chemical, and biological) of the OBL often determine the nature of the working environment and the margin of safety afforded for underwater naval operations. In this report, the panel describes research opportunities in the following areas: (1) geophysical processes in the MABL and OBL, specifically MABL fluxes and coupled turbulent flows; (2) the important topic of assimilation of data obtained from probing these marine environments; (3) the fundamental techniques for acquiring necessary data and reaching a new level of understanding of surface wave dynamics and electromagnetic propagation and signature physics; and (4) boundary layer dynamics (BLD)-related acoustical and optical oceanography. The order in which these topics are addressed reflects the panel's judgment of their relative priority for NRL research. Chapter 1 outlines the relationship of the topics to the panel's charge; the topics for research are discussed in detail and related recommendations are made in Chapter 2. The panel' s chief conclusions and recommendations are as follows: The spatial and temporal variability of marine atmospheric boundary layer fluxes is large, particularly in coastal regions. This variability greatly affects the reliability and stability of forecasts. The panel recommends that NRL support the development, evaluation, and use of a diverse set of sensing and measurement systems for the marine environment, steered by a small group of scientists, to study marine atmospheric boundary layer fluxes for their fundamental geophysical importance. An additional benefit of such work would be an improved understanding of the effects of these fluxes on aspects of system performance, e.g., on propagating signals. In this NRL effort a visiting scientist and postdoctoral study program is particularly encouraged. Coupling of the Navy's atmosphere and ocean predictive models has a natural place in the Navy's research mission, and the coupling of the atmospheric and oceanic turbulent flows through computational fluid dynamics techniques, such as large eddy simulation, is an important research area. The panel notes that coupling of these models might lead to little immediate improvement in atmospheric forecasts but should have a real impact

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Boundary Layer Dynamics on understanding and prediction of flows in the oceanic mixed layer. NRL has many capabilities and significant needs for application of results of research in this area. The panel recommends that NRL significantly expand the scope of its theoretical expertise in the areas of computational fluid dynamics related to boundary layer dynamics and that it guide and be guided by ongoing experimental efforts in air-sea physics and the development of remote sensing technology, with coupling of the atmosphere and ocean predictive models as a major goal. The goal of a focused, advanced research program in BLD at NRL-East, -Stennis, and -West should be to enhance the Navy forecasts produced at the Fleet Numerical Meteorological and Oceanographic Center (FNMOC)/NRL-West facility. An unusual synergistic opportunity exists at NRL-W that could improve the cohesiveness of efforts made at the three NRL sites; establish a research orientation at NRL-W extending beyond highly applied operational support; and build intersite teams to tackle important initiatives. The panel recommends an integrated multisite program in the area of ocean data assimilation and, in particular, the creation of a new center located at NRL-W to take advantage of the special resources at the Fleet Numerical Meteorological and Oceanographic Center. The European Centre for Medium-range Weather Forecasting may serve as a model. This program will require new observational data, obtained from a sparse set of distributed sensors, or from new satellite-borne sensors. The aim is to ensure that all available environmental data that have the potential for improving forecast products be identified, evaluated, and assimilated. Although progress has been and continues to be made in understanding nonlinear interactions between wave modes, between waves and shear layers, and between waves and variable currents, this work has had little influence thus far on practical ocean wind-wave modeling. OBL and MABL modelers may need better parameterization of the sea state. Better algorithms are needed to interpret and analyze data from remote sensing, and experimentalists need help to improve techniques for measuring air-sea interaction, surface fluxes, and gas transfer in the presence of waves. There is a real opportunity to make progress in BLD by exploiting progress in surface wave dynamics to develop improved deterministic and statistical models that can guide advanced experiments on ocean wind-waves. An active group with a strong theoretical and experimental background in wave physics could interact with many research groups already in existence at NRL, or could provide linkage between disciplines and sites. The panel recommends that NRL initiate a new program in the field of surface wave dynamics that has well-balanced theoretical, numerical, and experimental components. For NRL to achieve this balanced program will require new leadership, particularly in the theoretical components of the research. Scientific talent drawn from present in-house areas such as plasma dynamics may well provide the necessary human resources. The panel is keenly aware that ocean sensing active microwave instruments all make highly indirect measurements of geophysical quantities. The quantitative interpretation of data is hampered by ignorance of both (1) the detailed scattering mechanisms by which the incident radiation is reflected

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Boundary Layer Dynamics back to the receiver from the rough ocean surface and (2) the precise relationship between oceanic and atmospheric processes of interest and the small-scale surface geometry. These issues, while not completely separable, are distinct. Historically, because of the operational (naval or geoscientific) imperative for obtaining data of any sort, the details of the processes have been ignored, and the apparent correlations between backscatter cross sections and the geophysical quantities of interest have been used to generate empirical or semiempirical models. However, the persistent scatter in all extant microwave data sets, the limited range of conditions over which high-quality radar and correlative measurements have been obtained, and the continuing controversies over the accuracies and forms of the empirical models all testify to the fundamental inadequacy of the empirical approach alone. The panel recommends that NRL undertake a strong experimental and theoretical program in electromagnetic propagation and signature physics, particularly in developing and exploring passive microwave techniques such as multispectral and polarimetric methods. In the active microwave sensing area, the panel urges a strong collaborative program with academic (e.g., Office of Naval Research [ONR]-sponsored) groups. The panel believes that the Navy has unique resources to contribute to the passive multispectral microwave sensing area. Spaceborne or airborne passive microwave instruments sense not only signals generated at the ocean surface, but some integrated atmospheric properties as well. The problem requires the resources and teamwork that only a large integrated laboratory complex such as NRL can supply. The panel's suggestion of a new theoretical approach is based on this observation. In recommending a collaborative program with ONR-sponsored groups in active microwave sensing, the panel notes that there have been highly successful models (e.g., the High Resolution Remote Sensing [HRRS] Advanced Research Initiative) from which to draw. On the small scale, the marine microlayer—the thin interfacial boundary between the atmosphere and the ocean—plays a critical role in air-sea interactions. The presence of surface slicks attributable to man-made alteration of this surface can be recognized by the effects of changes in the visco-elastic modulus. There is strong evidence that coastal areas are zones of naturally high levels of surface compounds that alter capillary wave propagation. Understanding of this natural background is essential for improving signal detection. The panel recommends that NRL undertake a strong, coordinated research program that includes simultaneous underwater acoustic, electromagnetic, and optical measurements of the oceanic and marine atmospheric boundary layers in carefully selected coastal zone and continental shelf environments, in which different conditions of atmospheric forcing and the influence of surface slicks can also be quantitatively documented. This effort should involve active and passive electromagnetic and optical (visible and infrared) measurements from above the sea, as well as active and passive undersea probing of the ocean surface and mixed layers. NRL should pay immediate attention to the opportunities identified in this report and to the recommendations of the panel. During the time that this report has been in preparation, key

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Boundary Layer Dynamics personnel have retired and major wind-wave facilities at NRL-E have been torn down. The balanced approach of experimental and theoretical work recommended here, combined with the potential of a new center to exploit the gains in BLD understanding expected from this work, presents a very special opportunity to produce improved forecasts for the Navy. In all endeavors a commitment to ship time is essential; the Navy's working environment is not the laboratory bench top or a computer, but rather the open ocean and coastal waters of the real world.