Engineering topics

Bliven, L. F.

Publications and source records attributed to Bliven, L. F..

15 records

NASA NTRS1997

Scattering by Artificial Wind and Rain Roughened Water Surfaces at Oblique Incidences

Rain affects wind retrievals from scatterometric measurements of the sea surface. To depict the additional roughness caused by rain on a wind driven surface, we use a ring-wave spectral model. This enables us to analyse the rain effect on K(u) band scatterometric observations from two laboratory experiments. Calculations based on the small perturbation method provide good simulation of scattering measurements for the rain-only case, whereas for combined wind and rain cases, the boundary perturbation method is appropriate.

Craeye, C.↗

NASA NTRS1993

A laboratory study of friction-velocity estimates from scatterometry - Low and high regimes

Measurements from scatterometers pointing at wind-waves in three large wave tanks are examined to study fetch effects and the correlation with wind friction velocity. Time-series measurements were made at 13, 35, and 95 m with a Ka-band scatterometer aimed upwind at 30 deg incidence angle and vertical polarization. Average normalized radar cross-section (sigma-0) values from all fetches follow a common trend for sigma-0 as a function of wind friction velocity, so the fetch dependence is negligible. An empirical power-law model yields a high correlation between sigma-0 and wind friction velocity, but, because systematic anomalies arise, we reexamine a turbulence approach that delineates low and high regimes with a transition at a wind friction velocity of approximately 25 cm/s. Using this criteria, the data are well represented by a two-section power-law relationship between sigma-0 and wind friction velocity.

Bliven, L. F.↗

NASA NTRS1993

An experimental study of microwave scattering from rain- and wind-roughened seas

This paper investigates radar cross-section (RCS) characteristics of rain- and wind-roughened sea-surfaces. We conducted experiments in laboratory wind-wave tanks using artificial rain. The study includes light rain rates, light wind speeds, and combinations of these. A 36 Ghz scatterometer was operated at 30 deg incidence angle and with vertical polarization. RCS data were obtained not only with the scatterometer pointing up-wind but also as a function of azimuthal angle. We use a scatterometer rain and wind model SRWM-1, which relates the total average RCS in storms to the sum of the average RCS due to rain plus the average RCS due to wind. Implications of the study for operational monitoring of wind in rainy oceanic areas by satellite-borne instruments is discussed.

Bliven, L. F.↗

NASA NTRS1991

Relationship between gas exchange, wind speed, and radar backscatter in a large wind-wave tank

The relationships between the gas exchange, wind speed, friction velocity, and radar backscatter from the water surface was investigated using data obtained in a large water tank in the Delft (Netherlands) wind-wave tunnel, filled with water supersaturated with SF6, N2O, and CH4. Results indicate that the gas-transfer velocities of these substances were related to the wind speed with a power law dependence. Microwave backscatter from water surface was found to be related to gas transfer velocities by a relationship in the form k(gas) = a 10 exp (b A0), where k is the gas transfer velocity for the particular gas, the values of a and b are obtained from a least squares fit of the average backscatter cross section and gas transfer at 80 m, and A0 is the directional (azimuthal) averaged return.

Wanninkhof, Richard H.↗

NASA NTRS1988

Effects of rainfall on scatterometer derived wind speeds

Rainfall modification of scatterometer response from the sea surface was simulated in wind-wave tank experiments. Data show that for a given wind speed, radar cross section increases as rainfall rate increases, but this effect decreases as wind speed increases. An empirical model accounts for these observations.

Bliven, L. F.↗

NASA NTRS1988

Rainfall on microwave return from the sea surface

The long range goal remains unchanged; to conduct experiments and develop/test theoretical models to permit useful algorithms to be constructed for microwave systems that observe oceanic processes. This topic is relevant to altimeters, scatterometers, and rain rate measurements. The current focus is attention to scatterometer wind velocity measurement. One component of the laboratory efforts is an experiment conducted, in the wind wave tank at the GSFC/WFF, to quantify the effect of rain-generated surface wave brightening of radar cross section. Laboratory conditions can be characterized as light wind, functional rain rates, a single drop size, and a 36 GHz radar system at 30 degrees inclination.

Bliven, L. F.↗

NASA NTRS1988

Wind-Wave-Current Tank Research Facility usage and status

This summary is to provide information as to: (1) research activities, and (2) facilities status of the wind-wave-current tank research facility located at the GSFC/WFF. Research Activities include: (1) Wave-Turbulence Interaction; (2) Velocity Structure Below Waves; (3) Short-Wave Modification by Long-Waves; (4) Wind-Wave Generation Time Scale; (5) Wave-Current Interaction; (6) Rain Effects on Microwave Scattering from the Sea-Surface; and (7) Gas Exchange Rates versus Scatterometer Power.

Bliven, L. F.↗

NASA NTRS1986

A study of the relationship among wind speed, sea state, and the drag coefficient for a developing wave field

Controlled laboratory experiments are reported which demonstrate directly and quantitatively the influence of wave conditions in determining the drag law at the air-sea interface under neutral stability conditions. It is concluded that the analytic form first proposed by Kitaigorodskii (1970) models the roughness scale very well when the sea is dominated by the locally generated waves. It is demonstrated that, by using a unified two-parameter wave spectral model by Huang et al. (1981), Kitaigorodskii's result can be shown to contain the formulas of Charnock (1955) and Hsu (1974) as special cases. The results also identify two wind and wave-related parameters as important in determining the drag coefficient for developing wave fields.

Huang, N. E.↗

NASA NTRS1986

Experimental study of the influence of wind on Benjamin-Feir sideband instability

A laboratory investigation of the influence of wind on the evolution of mechanically generated regular (m.g.r.) waves is reported. Surface elevation measurements were made at four fetches for steep (0.1 less than ak(bar) less than 0.2, 2 Hz) m.g.r. waves, moderate (15 less than u(asterisk) less than 25 cm/s, 3-6 Hz) wind waves, and combinations of the m.g.r. and wind waves. The m.g.r. wave spectra exhibit Benjamin-Feir sidebands that grow exponentially with fetch and whose growth rate increases as the initial wave steepness increases. As fetch increases for the wind cases, total energy increases and the frequency of the spectral maximum downshifts, but no spectral lines representing Benjamin-Feir sidebands were detected even though the wave steepness and fetch were similar to the m.g.r. waves whose spectra displayed sidebands. As wind speed increased over the m.g.r. waves, sideband magnitude, sideband growth rate, and low-frequency perturbation components associated with the instability mechanism were reduced.

Bliven, L. F.↗

NASA NTRS1984

The Harp probe - An in situ Bragg scattering sensor

A wave sensor, consisting of parallel, evenly spaced capacitance wires, whose output is the sum of the water surface deflections at the wires, has been built and tested in a wave tank. The probe output simulates Bragg scattering of electromagnetic waves from a water surface with waves; it can be used to simulate electromagnetic probing of the sea surface by radar. The study establishes that the wave probe, called the 'Harp' for short, will simulate Bragg scattering and that it can also be used to study nonlinear wave processes.

Mollo-Christensen, E.↗

NASA NTRS1984

A new type of overshoot phenomenon in wind wave development and its implication in remote sensing of the ocean

Laboratory measurements using both wave probes (contact and optical) and radar confirm the existence of a new type of overshoot phenomenon in the wind-wave development processes. This type of overshoot, which occurs at gravity-capillary wavelengths, is practically a function of local wind stress only. It is named the overshoot phenomenon of the third kind. With the existence of this overshoot it becomes obvious that the assumed simple power-law relationship between the rms value of surface slope at a fixed wavelength and the wind stress is untenable. Since such a relationship is one of the key assumptions invoked in the development of algorithms used in scatterometry, the published scatterometer data are reexamined, pointing out the implications of this overshoot phenomenon. Recommendations for improvement of the performance of the scatterometer are also made in light of this new understanding of the wind-wave development processes.

Huang, N. E.↗

NASA NTRS1984

The non-Gaussian joint probability density function of slope and elevation for a nonlinear gravity wave field

On the basis of the mapping method developed by Huang et al. (1983), an analytic expression for the non-Gaussian joint probability density function of slope and elevation for nonlinear gravity waves is derived. Various conditional and marginal density functions are also obtained through the joint density function. The analytic results are compared with a series of carefully controlled laboratory observations, and good agreement is noted. Furthermore, the laboratory wind wave field observations indicate that the capillary or capillary-gravity waves may not be the dominant components in determining the total roughness of the wave field. Thus, the analytic results, though derived specifically for the gravity waves, may have more general applications.

Huang, N. E.↗

NASA NTRS1983

A study on the spectral models for waves in finite water depth

From an extension of the Wallops Spectrum (Huang et al., 1981) for the deep water waves, spectral models for waves in finite water depths are developed. Stokes wave expansions are found to offer a good approximation for intermediate water depth. The spectral function in this case is controlled by three parameters: the significant slope, the nondimensional depth, and the peak frequency. It is pointed out that solitary and cnoidal wave models must be used for the shallow water waves. The controlling parameters now reduce to the Urell number and the peak frequency. Even though the resulting spectral models place special emphasis on the energy-containing range of the spectrum, they are not limited to this range and they are not limited to any particular sea state. They are seen as offering a possible explanation of the variations in the special slope observed by previous investigators.

Huang, N. E.↗

NASA NTRS1983

A non-Gaussian statistical model for surface elevation of nonlinear random wave fields

Probability density function of the surface elevation of a nonlinear random wave field is obtained. The wave model is based on the Stokes expansion carried to the third order for both deep water waves and waves in finite depth. The amplitude and phase of the first-order component of the Stokes wave are assumed to be Rayleigh and uniformly distributed and slowly varying, respectively. The probability density function for the deep water case was found to depend on two parameters: the root-mean-square surface elevation and the significant slope. For water of finite depth, an additional parameter, the nondimensional depth, is also required. An important difference between the present result and the Gram-Charlier representation is that the present probability density functions are always nonnegative. It is also found that the 'constant' term in the Stokes expansion, usually neglected in deterministic studies, plays an important role in determining the details of the density function. The results compare well with laboratory and field experiment data.

Huang, N. E.↗

NASA NTRS1983

A case study of the energy dissipation of the gravity wave field based on satellite altimeter measurements

Wave breaking is proposed as the primary energy dissipation mechanism for the gravity wave field. The energy dissipation rate is calculated based on the statistical model proposed by Longuet-Higgins (1969) with a modification of the breaking criterion incorporating the surface stress according to Phillips and Banner (1974). From this modified model, an analytic expression is found for the wave attenuation rate and the half-life time of the wave field which depend only on the significant slope of the wave field and the ratio of friction velocity to initial wave phase velocity. These expressions explain why the freshly generated wave field does not last long, but why swells are capable of propagating long distances without substantial change in energy density. It is shown that breaking is many orders of magnitude more effective in dissipating wave energy than the molecular viscosity, if the significant slope is higher than 0.01. Limited observational data from satellite and laboratory are used to compare with the analytic results, and show good agreement.

Huang, N. E.↗