James K. Lewis

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

Higher frequency ambient noise in the Arctic Ocean

Higher frequency (1000 Hz) arctic ambient noise episodes during nonsummer months were used to study generating mechanisms. In most cases in which noise amplitudes were relatively high (>103 μPa), thermal fracturing of sea ice was responsible. Numerical simulations with a daily heating cycle and no snow cover implied that maximum noise occurred at 1900 h local. Radiational heat balances were more important than sensible heat flux in producing fracturing of sea ice. With snow cover, a daily heating cycle produces maximum fracturing at 0300–0800 h local, a common feature seen in noise data. Fracturing estimated from numerical simulations using observed arctic heat flux parameters was in good agreement with the observed 1000‐Hz noise signal. However, results indicate that blowing snow and ice fog may be additional factors in the heat flux balance of sea ice and the generation of arctic ambient noise. The short space scales of higher frequency noise are likely a result of spatial variations in snow cover. The ...

Arctic ambient noise in the Beaufort Sea: Seasonal relationships to sea ice kinematics

Various modes of sea ice motion in the Beaufort Sea are correlated with under‐ice noise at 10, 32, and 1000 Hz. Seasonal variations are considered, and a parameterization of ice microfracturing caused by sensible heat flux is included in the correlations. During the summer, the correlations indicate that all frequencies are primarily a response to the ice rushing through the water. During the fall, 10‐ and 32‐Hz noise correlate best with a linear combination of the speed of the ice parcel plus the total rate of change in the shape of the ice parcel. This latter factor indicates noise generation due to the individual ice floes moving past one another as they rearrange into new shapes. The correlations indicate differential motions of other forms (primarily ice convergence) become important in generating lower frequency noise only during winter. As for 1000 Hz during fall and winter, the correlations are low and the results are ambiguous. Several factors are discussed that emphasize our lack of knowledge ab...

Model-oriented ocean tomography using higher frequency, bottom-mounted hydrophones.

A tomographic scheme is presented that ingests ocean acoustic measurements into an ocean model using data from bottom-mounted hydrophones. The short distances between source-receiver pairs (1-10 km) means arrival times at frequencies of 8-11 kHz are readily detectable and often distinguishable. The influence of ocean surface motion causes considerable variability in acoustic travel times. Techniques are presented for measuring travel times and removing the variability due to surface waves. An assimilation technique is investigated that uses differences in measured and modeled acoustic travel times to impose corrections on the oceanographic model. Equations relating travel time differences to oceanographic variables are derived, and techniques are presented for estimating the acoustic and ocean model error covariance matrices. One test case using a single source-receiver pair shows that the tomographic information can have an impact on constraining the solution of the ocean circulation model but can also introduce biases in the predictions. A second test case utilizes knowledge of a bias in a model-predicted variable to limit grid cells that are impacted by the tomographic data. In this case, using the tomographic data results in significant improvements in the model predictions without introducing any biases.

Mesoscale air‐sea interactions related to tropical and extratropical storms in the Gulf of Mexico

Data indicate that the climatological vorticity of the lower atmosphere of the northwestern Gulf of Mexico is relatively strong, with monthly means exceeding a Rossby number of 3 from October through March, with a maximum of 4.6 in December. It has been hypothesized that wintertime cyclogenesis over the northwestern gulf is enhanced by this lower level atmospheric vorticity field when it occurs simultaneously with appropriate upper level venting. Observations from November 1982 to mid-February 1983 were studied in which seven significant cyclones were generated in the northwestern gulf. It was found that all seven storms occurred when the vorticity correlate of horizontal air temperature difference was ∼3–5°C above the climatological mean difference. Although these results corroborate the postulate that such lower level vorticity is related to cyclogenesis, the data from mid-February through March 1983 were less supportive. During those months, only two of the six significant storms developed when the horizontal air temperature difference was above the climatological mean. Surprisingly, the location of the most unstable air within the atmospheric boundary layer is not found in the northwestern Gulf of Mexico but instead along the shelf break. Relatively strong free convection in the atmospheric boundary layer is found over the Loop Current and the deep western gulf as expected. The unstable regions of the deep gulf can enhance the potential for frontogenesis and cyclogenesis, but it is likely that the linear character of the strong shelf break free convection does not enhance cyclogenesis. It is shown that a maximum in the frequency of tropical storms within the Gulf of Mexico exists some 275 km south of the Mississippi delta at 27°N, 90°W. This maximum is a result of only those storms which originate within the gulf. Two plausible effects of the Loop Current and its rings on tropical storms are discussed. One is that these ocean features are large and consolidated heat and moisture sources from which a nearby, slowly moving atmospheric disturbance can extract energy. The second is that of the cyclonic vorticity that can be generated in the lower atmosphere by such oceanographic features.

Arctic ambient noise in the Beaufort Sea: seasonal space and time scales

Data from the Beaufort Sea are used to quantify some of the seasonal characteristics of under‐ice ambient noise. Noise at 10, 32, and 1000 Hz was studied and seasonal space and time scales were calculated. An interesting result is that summer 10‐Hz noise apparently can often be contaminated by cable strum. The summer period is also the only time during which 1000‐Hz noise is almost totally a product of ice motion. Within the central Beaufort Sea, lower frequency noise tends to have e‐folding space scales of the order of 1000 km. This is true for 1000‐Hz noise only during summer, with space scales for the rest of the year being ∼300 km. The time scale analysis also shows this trend. During summer, all frequencies have e‐folding time scales of the order of 10–30 h. During fall and winter, the time scales for lower frequencies increase (up to 72 h or longer), but the 1000‐Hz time scales drop to as low as 3 h.

Observations and modeling of thermally induced stresses in first-year sea ice

During spring 1992, ice property, geophone, meteorological, and stress data were collected on first-year ice southwest of Cornwallis Island within the Canadian archipelago. One of the goals of the study was to specify the average characteristics of the ice, use these characteristics in a model of thermally induced stresses in the ice, and examine the fracturing associated with the occurrence of those stresses. The results of simulations with a thermal stress model indicate that stress variations within the ice can be reasonably approximated by a viscoelastic rheology. The rheology takes into consideration thermally induced strains generated locally as well as strains generated elsewhere and then mechanically transmitted through the ice. The geophone data showed both ice-borne and water-borne propagation paths for individual fracturing events. The data imply a detection radius out to 500–600 m for the ice-borne signatures of fractures. An investigation of a region after fracturing showed that (1) fracturing occurred in an area with a 10- to 15-cm snow cover, (2) the snow cover had been scored down to the surface of the ice, and (3) cracks in the ice were found under each location where the snow had been scored. The cracks were 5–6 m long and at least 15 cm deep. A review of these and other experimental results draw us to the conclusion that the forces required to produce fractures in response to natural forcing is greater for first-year floes than for multiyear floes.

Motion-induced stresses in pack ice

We consider motion-induced stresses in pack ice through the analyses of a variety of observations collected during the Sea Ice Mechanics Initiative study conducted in the Beaufort Sea during 1993. Motion-induced components of in situ stress from stress gauge data are compared to stresses calculated as residuals based on a force balance argument using observed wind, current, and ice motion data. There is reasonable qualitative and quantitative agreement between the observed and calculated motion-induced stresses in the north-south direction if the residual stress is assumed to be distributed over a horizontal distance of ∼10 m. To obtain a general agreement with the magnitudes of the observed and calculated stresses in the east-west direction, the residual stress must be considered to be distributed over a horizontal distance of ∼50 m. There are three significant stress events determined by the force balance calculations, but only the one event in the north-south direction has a strong corresponding signal in the stress gauge data. There is very little indication of the two events in the east-west direction in the gauge data. Numerical simulations of the distribution of motion-induced stresses within a floe show that significant variations in the character of the stresses can occur over short horizontal distances throughout the floe. Hence a seeming lack of a clear correspondence between the observed and calculated stress may be due to our inability to properly recognize the modified signature of the event at the specific locations of the stress gauges. The results suggest that to effectively develop an understanding of the role that point stress measurements can play in developing our understanding of the process of ice deformation, it may be necessary to couple the stress measurements with models of the patterns of motion-induced stresses within a floe. Finally, we consider the relationship between the residual stress and the differential motion of the ice pack as seen by a cluster of drifters on various floes. The three main stress events seen in the residual stresses all occurred during periods of convergence of the floes. However, we have tested various relationships between stress and strain, and they indicate that there should have been additional stress events as a result of other periods of substantial convergences of the ice pack. This suggests the possibility that the residual stresses were not locally generated.

RELATING ARCTIC AMBIENT NOISE TO THERMALLY INDUCED FRACTURING OF THE ICE PACK

Thermally induced stresses within pack ice floes are simulated using a viscoelastic model that accounts for the time variation of the atmospheric forcing parameters and the temporal and spatial variations of snow cover. To relate the model‐predicted stresses to the observed 500 Hz under‐ice noise variations, a fracturing paradigm was developed. The fracturing paradigm includes factors related to ice tensile strength given its salinity and temperature and stress relief as a result of fracturing. A fracture count time history was produced using the thermal stress model and the fracturing paradigm for the fall of 1988 in the eastern Arctic Ocean. The results were compared with the observed 500‐Hz noise. The fracture count shows increases for 11 of 13 under‐ice noise episodes. Most of the observed under‐ice noise episodes were of the type identified in previous studies; i.e., those associated with nightly cooling. In addition, this study identifies under‐ice noise episodes related to longer‐term occurrences a...

Local data assimilation in the estimation of barotropic and baroclinic open boundary conditions

The problem of data assimilation in the specification of open boundary conditions for limited area models is addressed in this paper. Optimization approaches are detailed, which are based on combining available data on an open boundary with the physics of the hydrodynamical model. In our case the physics is in terms of the flux of energy through the open boundary. These optimized boundary conditions, for both barotropic and baroclinic situations, interpreted physically as special linearizations of the Bernoulli equation for each normal mode. Because of the complexity of decomposing variables into normal modes for open boundaries with varying bathymetry, we present two alternative approaches. The first is a simplification of the optimized baroclinic boundary condition based on normal modes. The second makes use of empirical orthogonal functions instead of normal modes. The results of testing and comparisons of these approaches are presented for coupling coarse- and fine-resolution models. In this case our approach is in assimilating values and variables from a large-scale model (along the open boundaries of a limited area model). In the proposed coupling schemes the energy fluxes are estimated either from coarse or from fine-grid model results. With the progress of oceanographic observing systems we would like to explore ways of combining model outputs with the oceanographic measurements in order to estimate energy fluxes used in optimized open boundary conditions.