Forecasting and hindcasting waves with the SWAN model in the Southern California Bight

The Naval Research Laboratory created a wave forecasting system in support of the Nearshore Canyon Experiment (NCEX) field program. The outer nest of this prediction system encompassed the Southern California Bight. This forecasting system is described in this paper, with analysis of results via comparison to the extensive buoy network in the region. There are a number of potential errors, two of which are poor resolution of islands in the Bight—which have a strong impact on nearshore wave climate—and the use of the stationary assumption for computations. These two problems have straightforward solutions, but the solutions are computationally expensive, so an operational user must carefully consider their cost. The authors study the impact of these two types of error (relative to other errors, such as error in boundary forcing) using several hindcasts performed after the completion of NCEX. It is found that, with buoy observations as ground truth, the stationary assumption leads to a modest increase in root-mean-square error; this is due to relatively poor prediction of the timing of swell arrivals and local sea growth/decay. The model results are found to be sensitive to the resolution of islands; however, coarse resolution does not incur an appreciable penalty in terms of error statistics computed via comparison to buoy observations, suggesting that other errors dominate. Inaccuracy in representation of the local atmospheric forcing likely has a significant impact on wave model error. Perhaps most importantly, the accuracy of directional distribution of wave energy at the open ocean boundaries appears to be a critical limitation on the accuracy of the model-data comparisons inside the Bight.     

Modified McCormicks model for equilibrium shorelines behind a detached breakwater

Experimental data of equilibrium shorelines behind a detached breakwater obtained by previous investigators were collected and re-reviewed to investigate the major parameters affecting the shoreline shapes. The result shows that the equilibrium shorelines depend not only on the breakwater length B and the distance of the breakwater from the initial shoreline S, but also on the incident wave steepness H₀/L₀, beach slope S_b and the sand size D₅₀. Most of equilibrium shorelines behind a detached breakwater could be approximately described by a couple of elliptic curves as proposed by McCormick (Ports, Coastal & Ocean Engineering ASCE 119, 1993, p. 657). However, after re-examination, this paper shows that the dimensionless semiminor axis b/S and the dimensionless distance G/b in the McCormick’s elliptic-curve model should be modified, as compared with the available experimental data. The modified expressions of b/S and G/b were proposed, and the performances of the modified expressions were also detaily examined in this paper.     

Measurements of the height of the convective surface boundary layer over an arid coast on the Red Sea

The height of the convective boundary layer over an arid coast on the Red Sea was measured by high-resolution radiosondes. These measurements can be used to compute sensible heat flux by the method devised by Danard (1981). The average heat flux computed is in good agreement with results obtained independently by both the energy balance method and the free-convection equation.     

An operational forecasting model for the variation of mean maximum mixing heights across the coastal zone

Variation exists in the maximum mixing height (MMH) across the coastal zone because of major differences in heat capacity between land and sea. By assuming that synoptic and microscale effects are all small as compared to the contribution from mesoscale temperature differences, a model is proposed to explain this variation. The model states that the variation of the MMH across the coastal zone is due primarily and is linearly proportional to the difference in maximum temperature between land and sea. Since the MMH is one of the most important parameters in the computation of distribution and concentration of aerosols, water vapor, and pollutants, a simple equation is also provided for the operational forecasting of the MMH or the mixing height over the sea. All inputs into the equation are routinely available. The model has been verified by available data and a relevant field experiment.     

A random process for earthquake simulation

Some existing models for the simulation of earthquake acceleration show the difficulties in determining the involved parameters and in describing frequency content. A non-stationary modulated random process obtained as the product of a time envelope function and a stationary random function is used to simulate earthquake acceleration. The parameters and the distribution of frequencies of the simulation process are obtainable from past earthquake records. This simple and realistic model is suggested for use as the input process in aseismic design of structures.     

Torsional spectrum for earthquake motions

A computational scheme is presented to construct torsional spectra due to the rotational component of seismic ground motions. The rotational component of ground motion is estimated from the measured earthquake acceleration records. In contrast to previous studies, no differentiation of acceleration records is involved in the present scheme. The torsional spectrum of the 1940 El Centro earthquake is computed and compared with previous results. An average and a mean plus one standard deviation torsional spectrum is presented for design purposes. These spectra are results based on four historical records (1934 El Centre, 1940 El Centro, 1949 Olympia and 1952 Taft) normalized to the 1940 El Centro intensity.     

A verification of an analytical formula for estimating the height of the stable internal boundary layer

A stable thermal internal boundary layer (IBL) develops when warm air is advected from warmer land upstream to a cooler sea downstream. It is shown that the analytical model for estimating the height (h) of this stable IBL as formulated by Garratt (1987) is verified. It is also demonstrated that a simpler equation, % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWGObGaeyisIS% RaaGymaiaaiAdacaWGybWaaWbaaSqabeaadaWcgaqaaiaaigdaaeaa% caaIYaaaaaaaaaa!390B!\[h \approx 16X^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \] (where h is in meters and X, the fetch downwind, is in kilometers), is useful operationally as a first approximation.     

Wave-induced velocities and turbulent reynolds stresses above an air-water interface

The progressive, sinusoidal (1.0 Hz) water-wave-induced perturbations in a turbulent air flow above an air-water interface were studied in a large laboratory facility. The velocity field was measured in both fixed and wave-following reference frames with a cross hot-film probe. Both the magnitude and phase of the wave-induced velocities and turbulent Reynolds stresses were obtained across the turbulent boundary layer for different free stream velocities.     

Positron annihilation lifetime spectral changes on dehydration of natural beryls and cordierites

The effect on positron annihilation lifetime spectra, measured at room temperature, of the dehydration of single crystals of beryl and cordierite was studied. In each case, the spectra were satisfactorily fitted to three lifetime components. For the beryls, the dehydration considerably enhanced the intensity of the intermediate-lifetime component (I ₂) and reduced the intensity of the longest-lived component (I ₃). I ₂ is attributed to positron annihilation in the empty cages in the channels of the beryl structure and I ₃ to annihilation by a pick-off process via unknown foreign molecules. However, for the cordierites, the main effect of the dehydration was a slight (～10%) increase in the lifetime of the intermediate component, τ₂. Here I ₂ was relatively high both before and after dehydration and the increase in τ₂ is attributed to Si-Al ordering. No changes in the lifetime spectra were produced by γ irradiation.