Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: George Z. Forristall x
  • Refine by Access: All Content x
Clear All Modify Search
George Z. Forristall

Abstract

Although most offshore structures are open space frames, they still disturb the currents that flow through them. This flow blockage has important consequences both for calculating forces on the structures and for choosing sites for current meters. Full-scale measurements of current blockage were made from the Bullwinkle platform in the Gulf of Mexico using two acoustic Doppler current profilers (ADCPs) that were modified so that all eight beams were in one horizontal plane. The current field was mapped using an objective analysis scheme that was based on the radial current components given by the ADCP beams. This objective analysis technique may also be useful in the interpretation of current measurements made by high-frequency radar systems, which also measure current components along radial lines from their antennas. The analysis of the ADCP measurements at Bullwinkle showed that the average current speed inside the outline of the platform was about 0.8 times the upstream speed. This result agrees with the predictions of actuator disk theory. The wake of the structure extended at least 100 m behind the structure past the end of the ADCP beams. Great care must be taken in measuring currents near offshore structures.

Full access
George Z. Forristall

Abstract

Many empirical and heuristic distribution functions for wave crest heights have been proposed, but their predictions differ considerably. Part of the lack of agreement is due to the difficulty of making measurements that accurately record the true height of the wave crests. Surface following buoys effectively cancel out the second-order nonlinearity by making a Lagrangian measurement. Pressure transducers filter the nonlinear components of the signal in complicated ways. Wave staffs have varying degrees of sensitivity to spray. The location of the instruments also plays an important role. There is clear evidence from measurements in the North Sea that spurious crests due to spray are a problem downwind even from mounting supports that appear transparent.

Much of the theoretical nonlinearity can be captured by calculations correct to second order. Explicit calculation of the interactions of each pair of components in a directional spectrum is straightforward although computationally intensive. This technique has the advantage that the effects of wave steepness, water depth, and directional spreading are included with no approximation other than the truncation of the expansion at second order. Comparisons with measurements that are believed to be of the best quality show good agreement with these second-order calculations. Simulations for a set of JONSWAP spectra then lead to parametric crest distributions, which can be used easily in applications.

Full access
George Z. Forristall and Kevin C. Ewans

Abstract

The directional spreading of waves is important for both theoretical and practical reasons. Enough measurements have now been made to draw conclusions about the behavior of wave spreading at sites in different climatic regimes. The measurements presented here of the directional spreading of fetch-limited waves agree in general with those of M. A. Donelan et al., but additional evidence is also found to support the conclusion of I. R. Young et al. that the spreading function is bimodal at high frequencies. The spreading factor ϕ is defined to be the square root of the in-line variance ratio defined by R. E. Haring and J. C. Heideman. This spreading factor gives an integrated measure of the degree of directional spreading in the wave spectrum and predicts the reduction in the in-line particle velocities under waves due to direction spreading. The value of ϕ is 1 for unidirectional waves and 0.707 for omnidirectional waves. For fetch-limited conditions, ϕ is essentially constant at 0.906. Results from the Exact-NL wave model agree reasonably well with this value, but an operational third-generation model produces directional spreading that is broader than the observations. The statistics of ϕ are calculated from thousands of hours of measurements from many sites around the world. The median value of ϕ is 0.880 for low latitude monsoon conditions and 0.867 for tropical cyclones. For extratropical storms, ϕ decreases with latitude. Regressions on the statistics for high waves give ϕ = 0.944 for latitude 36° and ϕ = 0.869 for latitude 72°. The latitudinal dependence of ϕ is caused by the facts that waves are generally more broadly spread at sites close to the center of a storm and that storm tracks are concentrated at high latitudes.

Full access
Cortis K. Cooper and George Z. Forristall

Abstract

Since 1986, nine years of wave data derived from satellites have been accumulated, and this database will expand dramatically in the next two years as two more satellites are added. Several researchers have begun using this data to estimate extreme value statistics for waves. However, one potential problem with satellite data is space–time resolution, which is a poor match for the scales of storms. Satellites only revisit a site once every 10–35 days, and their tracks are separated by 100–200 km. With this coarse sampling, the satellite may miss storms since they have characteristic length and time scales as short as a few hours and tens of kilometers. The purpose of this paper is to explore the impact of this undersampling on the calculated 100-yr wave height. This is accomplished by running Monte Carlo simulations of simplified but realistic storms sampled by a simulated satellite and site. The authors study the sensitivity of the calculated 100-yr wave to variations in storm type, radius, and forward speed; number of satellites; satellite track; and satellite sampling region. The uncertainty, as measured by the coefficient of variation (cov), of the 100-yr wave based on 10 years of satellite data is 10% in regions like the North Sea that are dominated by extratropical storms, provided the satellite data is sampled over a 200–300-km region. This is about the level accepted by present offshore standards like the American Petroleum Institute. For regions dominated by tropical storms like the Gulf of Mexico, the cov for satellite- or site-derived extremes is much greater than 10% using 10 years of data. The situation improves with increased sample period, storm frequency, or the number of satellites. However, even in these cases some caution must still be exercised near the coast where the satellite data itself may be less reliable and sampling over large regions may remove real spatial gradients. Our conclusions apply to all existing satellite tracks including Geosat, Topex/Poseidon, and ERS.

Full access
George Z. Forristall, Robert C. Hamilton, and Vincent J. Cardone

Abstract

Storm currents are a significant part of the design hydrodynamic flow field in areas subject to tropical storms. In September 1973, Tropical Storm Delia passed over the instrumented Buccaneer platform located in 20 m of water 50 kin south of Galveston, Tex. Current meter records from three depths show the storm produced currents on the order of 2 m s−1 which persisted to near the bottom. A mathematical model of wind-driven current generation was successful in hindcasting the observed current development after a linear slip condition bottom was incorporated in the model.

Full access
James F. Price, Thomas B. Sanford, and George Z. Forristall

Abstract

The upper ocean's response to three hurricanes [Norbert (1984), Josephine (1984) and Gloria (1985)] is examined using field observations and a numerical ocean model. Our goal is to describe the physical processes that determine the structure and amplitude of hurricane-driven upper-ocean currents.

All three of these Northern Hemisphere hurricanes produced a rightward-biased response of the mixed-layer current and transport. This asymmetry arises because the wind stress vector rotates clockwise on the right side of the track and remains nearly parallel with the inertially rotating mixed-layer current during most of the hurricane passage. The maximum observed mixed-layer current varied from 0.8 m s−1 in response to Josephine, which was a large but comparatively weak hurricane, to 1.7 m s−1 in response to Gloria, which was very large and also intense.

These cases have been simulated with a three-dimensional numerical model that includes a treatment of wind-driven vertical mixing within the primitive equations. The simulations give a fairly good representation of the horizontal pattern and amplitude of the mixed-layer current, accounting for over 80% of the variance of the observed current. Model skill varies considerably with the amplitude of the mixed-layer current, being much higher for stronger currents than it is for weaker currents. This and other evidence suggest that a major contributor to the difference between the observed and simulated currents may be a noise component of the observed current that arises from measurement and analysis error and from prehurricane currents.

The Norbert case was distinguished by a large Burger number, ∼1/2, which is a measure of pressure coupling between the forced stage mixed-layer currents and the relaxation stage thermocline currents. The observations and the simulation show upwelling of up to 25 m and strong thermocline-depth currents up to 0.3 m s−1 under the rear half of Norbert. Thermocline currents have a very simple vertical structure, a monotonic decay with increasing depth, and nearly constant direction. Their horizontal structure is more complex but appears to be due to an acceleration toward a low pressure anomaly associated with the first upwelling peak about 100 km behind the eye of Norbert.

Full access
Thomas B. Sanford, Peter G. Black, James R. Haustein, James W. Feeney, George Z. Forristall, and James F. Price

Abstract

The response of the ocean to hurricanes was investigated using aircraft-deployable expendable current profilers (AXCP). The goals were to observe and separate the surface wave and surface mixed layer (SML) velocities under the storms and to map the across-track and along-track velocity and temperature response in the mixed layer and thermocline. Custom instrumentation was prepared, including slower failing AXCPs, and the AXCP equipment was installed on NOAA WP-3D aircraft. Research flights were made into two 1984 hurricanes: Norbert, in the eastern Pacific off Baja California (19°N, 109°W), and Josephine, off the east coast of the United States (29°N, 72°W). Thirty-one probes were deployed in each hurricane, and about half the AXCPs provided temperature and velocity profiles. Most velocity profiles exhibited strong surface wave contributions, slablike velocities in the SML, strong shears beneath the SML, and only weak flows in the upper thermocline. Separation of the surface gravity wave velocities from the steady and inertial motions was obtained by fitting the profiles to steady flows and shears in three layers and to a single surface wave at all levels. The velocity profiles displayed large divergences to the horizontal SML velocities in the wake of the hurricanes. The observations show a strong enhancement of SML velocities to the right of the storm as expected from numerical simulations. The largest SML velocities were 1.1 m s−1 in Norbert and 0.73 m s−1in Josephine. Numerical simulations will be compared with the observations in Part II.

Full access