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Timothy R. Keen and Scott M. Glenn

Abstract

The effects of increased friction and tides on circulation in the Middle Atlantic Bight (MAB) during the SWADE storm of 25–28 October 1990 have been investigated using a three-dimensional hydrodynamic model coupled to a bottom boundary layer model that calculates combined wave–current bottom drag coefficients. Winds were initially parallel to the coast (downwelling favorable) throughout the MAB, first shifting to offshore within the central MAB and then in the northern MAB, while remaining parallel to the coast within the southern MAB. The wind-driven circulation was approximately alongshore, with an onshore component at the surface and an offshore component at depth associated with downwelling. Compared to model runs with a pure current bottom friction formulation, the additional bottom friction in the coupled model decreased currents uniformly in shallow water and caused slight offshore rotation during downwelling circulation, but the effects were limited because of the persistent stratification and the variable wind field during the storm. The effect of tides was much more pronounced, since across-shelf tidal currents were of similar or greater magnitude than the wind-driven currents. The combination of downwelling offshore flow and tidal flow during the storm resulted in weaker bottom currents directed nearly alongshore during flood and stronger currents directed nearly offshore during ebb. Bottom shear stresses were initially highest when storm currents were largest and again later during ebb tides when tidal and storm bottom flows were in the same direction. These results suggest that sedimentation during the storm was directly related to the tidal flow.

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Timothy R. Keen and Scott M. Glenn

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Hurricane Andrew made landfall in the Gulf of Mexico after crossing directly over several moored current meter arrays deployed on the Louisiana–Texas shelf. The resulting three-dimensional current, temperature, and salinity time series are used in a quantitative analysis of the factors affecting the hindcast skill of ocean circulation models. This paper describes parameters for quantifying a model’s skill at matching both maximum currents and time series at specific locations and depths. It then briefly discusses the following factors with respect to currents hindcast with the Princeton Ocean Model: 1) model domain size; 2) horizontal resolution, including the bathymetry and coastline; 3) vertical resolution (i.e., number of model levels); 4) the surface drag formulation;5) the bottom drag coefficient; 6) turbulent mixing parameters and sources of turbulence; and 7) the initial temperature field. Model performance is found to be most dependent on parameters within the turbulent energy closure scheme and the initial temperature and salinity distributions. The best overall model performance is gained by adjusting one of the closure scheme coefficients (B 1) that decreases turbulence dissipation (and increases mixing where a density gradient exists). Results incorporating wave breaking and a depth-dependent initial temperature field, however, are also reasonable, and differences between the model skill parameters are insufficient to determine which approach is preferable.

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Timothy R. Keen and Scott M. Glenn

Abstract

This paper describes a hydrodynamic model with turbulent energy closure that uses a simplified wave-current interaction model of the bottom boundary layer to compute bed drag coefficients. The coupled model is used to investigate the interaction of the upper and lower boundary layers with the geostrophic core flow for simple shelf geometry and forcing, and to evaluate the effects of increased bottom friction on coastal hydrodynamics for summer and winter stratification. The thickness of the bottom boundary layer predicted by the model ranges from 10 to 35 m and is consistent with observations from the California shelf. The increased bottom friction calculated by the coupled model in intermediate water depths increases bottom Ekman veering (leftward in the Northern Hemisphere) by as much as 10° if stratification is strong, thus enhancing downwelling and upwelling. Currents along isobaths in shallow water are uniformly decreased by as much as 25% in the coupled model for both summer and winter initial stratification.

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Scott M. Glenn and Michael F. Crowley

Abstract

A series of Gulf Stream forecast model test cases were developed for the Data Assimilation and Model Evaluation Experiment (DAMEE). The model initialization and verification procedure relies heavily on a series of accurate synoptic snapshots of the Gulf Stream north wall and ring locations. Satellite infrared imagery, Geosat altimetry, and numerous in situ temperature profiles were combined using a geographic information system to construct Gulf Stream and ring location analyses at approximately weekly intervals during a 6-week, data-rich time period. To improve the accuracy of the feature analyses, a new image compositing technique called patching was developed to decrease the spatial smearing experienced with standard warmest pixel composites. During the 6-week test period, three ring formations, two ring absorptions, and one ring merger event were observed. The average difference between the weekly north wall positions ranged from 20 to 34 km (average 27 km). When the DAMEE GSR north wall positions were compared to two independent analyses for the same time period, the average offsets were found to vary from 14 to 53 km (average 29 km) for the first comparison set and 14 to 30 km (average 22 km) for the second. These differences, which are similar in magnitude to the observed weekly evolution, are attributed to differences in the data treatment for cloudy regions and the subjectivity of the image analysts when dealing with incomplete data.

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Josh T. Kohut and Scott M. Glenn

Abstract

A high-frequency (HF) radar system is deployed on the New Jersey continental shelf as part of a coastal ocean observatory. The system includes two remote transmit–receive sites in Brant Beach and Brigantine, New Jersey, and a central processing site in Tuckerton, New Jersey. The system uses radio waves scattered off the ocean to measure the radial velocity, range, and bearing of the scattering surface. Calculation of the bearing for HF radar systems depends on the actual beam pattern of the receive antennas. A series of antenna beam pattern measurements conducted on the New Jersey system shows that these patterns are often distorted when an antenna is deployed in the field. Tests indicate that the local environment, not system hardware, causes the most significant distortion of the pattern from the theoretical shape. Correlation with an in situ acoustic Doppler current profiler (ADCP) indicates that the beam pattern distortion can bias the bearing estimate. It is shown that this bias can be removed if the measured beam patterns are used to estimate the bearing.

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Chung-Chieng A. Lai, Wen Qian, and Scott M. Glenn

The Institute for Naval Oceanography, in cooperation with Naval Research Laboratories and universities, executed the Data Assimilation and Model Evaluation Experiment (DAMÉE) for the Gulf Stream region during fiscal years 1991–1993. Enormous effort has gone into the preparation of several high-quality and consistent datasets for model initialization and verification. This paper describes the preparation process, the temporal and spatial scopes, the contents, the structure, etc., of these datasets.

The goal of DAMÉE and the need of data for the four phases of experiment are briefly stated. The preparation of DAMÉE datasets consisted of a series of processes: 1) collection of observational data; 2) analysis and interpretation; 3) interpolation using the Optimum Thermal Interpolation System package; 4) quality control and reanalysis; and 5) data archiving and software documentation.

The data products from these processes included a time series of 3D fields of temperature and salinity, 2D fields of surface dynamic height and mixed-layer depth, analysis of the Gulf Stream and rings system, and bathythermograph profiles. To date, these are the most detailed and high-quality data for mesoscale ocean modeling, data assimilation, and forecasting research. Feedback from ocean modeling groups who tested this data was incorporated into its refinement.

Suggestions for DAMÉE data usages include 1) ocean modeling and data assimilation studies, 2) diagnosis and theorectical studies, and 3) comparisons with locally detailed observations.

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William D. Grant, Albert J. Williams III, and Scott M. Glenn

Abstract

High quality near-bottom boundary layer measurements obtained at a midshelf location (90 m water depth) in the CODE region off Northern California are described. Bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions (June 1981 during CODE-1) are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates. During the time period described, the mean near-bottom (<2 m) velocity profile are highly logarithmic (R>0.997) approximately 30 percent of the time. Effects induced by unsteadiness from internal waves result in some degradation of the profiles (0.96≤R≤0.997) the rest of the time. Mean stress profiles indicate the logarithmic layer is approximately a constant-stress layer. The near-bottom flow field is Composed of mean currents and oscillatory currents due to well. Typing mean u * values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0.5–1.0 cm s−1. Mean stress values are three to seven times larger than expected from predictions using a typical smooth-bottom drag coefficient and one-and-one-half to three-and-one-half times larger than expected for predictions using a drag coefficient based on the observed rough bottom. Corresponding z 0 values have magnitude of approximately 1 cm, an order of magnitude larger than the observed physical bottom roughness. These values are demonstrated to he consistent with those expected from theoretical models for combined wave and current flows. The u * values estimated from the CODE-1 data and predicted by the Grant and Madsen model typically agree within 10–15 percent.

The waves influencing the midshelf bottom-stress estimates are 12–20 second swell associated with distant Pacific storms. Them waves are present over most of the year. The results demonstrate that waves must be taken into account in predicting bottom stress over the Northern California Shelf and that these predictions can be made using existing theory.

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David L. Porter, Scott M. Glenn, Ella B. Dobson, and Michael F. Crowley

Abstract

An extended synthetic geoid for the western North Atlantic Ocean was constructed by employing Geosat altimeter data, concurrent dynamic model forecasts, and climatology. Estimates of the absolute dynamic topography from the altimeter were compared to estimates of the dynamic topography computed from independent in situ temperature measurements. The rms difference between the two topographic estimates was 0.104 m in the Gulf Stream meander and ring region, 0.081 m in the Sargasso Sea, and 0.098 m overall in this portion of the western North Atlantic ocean. The position of the mean Gulf Stream axis, the 1-σ width of the meander envelope, and the extremes of the meander envelope were determined from the altimetric data. The yearly mean AVHRR- (Advanced Very High Resolution Radiometer) derived surface north walls for 1987 and 1988 were approximately 30 km north of the yearly mean surface maximum velocity axes for the same years. This 30-km offset, however, is affected by the different satellite sampling schemes and the AVHRR data processing techniques. Separation distances derived from individual comparisons of nearly concurrent Geosat, AVHRR, and AXBT (air expendable bathythermograph) datasets result in an average (rms) offset of 17 (±12) km between the axis and the surface north wall, and an 11- (±8) km offset between the axis and the subsurface north wall. The individual axis/surface north wall offsets and their variability were found to increase with increasing anti-cyclonic curvature, consistent with the theoretical effect of centripetal acceleration on a meandering constant potential vorticity jet. This synthetic geoid, validated in portions of the western North Atlantic within a vertical accuracy of about 0.10 m, can be used with the planned Geosat Follow-On mission in the late 1990s to calculate the absolute dynamic topography in near real time.

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Kevin Barjenbruch, Carol M. Werner, Randall Graham, Cody Oppermann, Glenn Blackwelder, Jeff Williams, Glen Merrill, Scott Jensen, and Justin Connolly

Abstract

Over the past several decades, Utah has experienced rapid population growth, resulting in increased demand on Utah’s existing interstate and arterial infrastructure. In the Salt Lake City, Utah, metropolitan area, recurring traffic congestion (i.e., peak commute times) and nonrecurring congestion (weather related) result in an estimated average annual cost of $449 million. Recent Utah Department of Transportation (UDOT) studies have confirmed that inclement weather plays a significant role in nonrecurring congestion and associated negative impacts. In an effort to measure and potentially mitigate weather-related traffic congestion, a cooperative research study between academic (University of Utah), state [Utah Department of Transportation (UDOT)], federal (National Weather Service), and private sector (Weathernet) entities was undertaken.

Driver awareness surveys were conducted for two significant winter storms along the Wasatch Front urban corridor. Participants typically used media and personal sources for gathering weather and road information, with government sources (UDOT and NWS) used less frequently. Use of government and personal sources were significant predictors of behavior change. Satisfaction with all information sources was high. The most frequent commuting changes reported were route changes and shifts in travel schedule, especially leaving early to avoid the storm. Self-reported actions from interviewees were supported by measured changes in speed, flow, and travel time from the Performance Measurement System (PeMS) utilized by UDOT. The long-term goal is to use these results to provide insight into how the weather enterprise might more effectively communicate hazard information to the public in a manner that leads to improved response (change travel times, modes, etc.).

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Jose-Henrique G. M. Alves, Paul Wittmann, Michael Sestak, Jessica Schauer, Scott Stripling, Natacha B. Bernier, Jamie McLean, Yung Chao, Arun Chawla, Hendrik Tolman, Glenn Nelson, and Stephen Klotz

The U.S. National Centers for Environmental Prediction (NCEP) and the Fleet Numerical Meteorology and Oceanography Center (FNMOC) have joined forces to establish a first global multicenter ensemble system dedicated to probabilistic forecasts of windwave heights. Both centers run independent wave ensemble systems (WES), which are combined onto a multicenter system with 41 members. A performance assessment of the multicenter wave-height product is made relative to altimeter data. Computed estimates of mean errors, ability to represent uncertainty, and reliability of probabilistic forecasts indicate that the multicenter ensemble product outperforms individual WES and deterministic wave models alike. The investigation includes an evaluation made at NCEP's National Hurricane Center (NHC) of the multicenter WES product, including severe sea-state events. The interagency collaboration has provided an opportunity to investigate in more depth the properties of wave ensembles, which has led to planned improvements that are expected to increase the accuracy of probabilistic forecasts within the oceanic environment. These outcomes are expected to be of great benefit to the society, the economy, and the environment. The successful operational implementation of the multicenter product has brought new opportunities for further collaboration with operational centers in North America, and a planned upgrade to the current interagency system is the inclusion of 20 additional members from a WES under development at Environment Canada.

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