Filip Neele

Production of sea spray aerosol in the surf zone

quantitative source function for sea spray aerosol produced by waves breaking in the surf zone was determined from data collected with optical particle counters at both sides of the surf zone at two locations on the Californian coast. Three optical particle counters were used to measure profiles at the base of a pier; a fourth instrument was used at the end of the pier. Careful calibration and intercomparisons of the instruments were made to avoid systematic errors. Aerosol concentrations measured downwind from the surf, in wind speeds of up to 9 m s-1, were 1-2 orders of magnitude higher than those upwind. Surf aerosol concentration gradients and plume heights vary with particle size and with wind speed. The derived surf aerosol source functions are compared with current estimates for the open ocean, taking into account the different proportions of the ocean surface covered by whitecaps. Application of a simple transport model indicates that surf-produced sea spray contributes significantly to the aerosol concentrations at fetches up to at least 25 km. This has implications for, for example, heterogeneous chemistry and electro-optical propagation. Copyright 2000 by the American Geophysical Union.

The effect of small‐scale structure on normal mode frequencies and global inversions

Presently very little is known about the characteristic length scales of the structures in the upper mantle. In generating synthetic seismograms and in large-scale inversions, one usually assumes that the upper mantle is smooth, so that ray theory can be used. However, it is known that structures in the mantle exist on a scale of a few hundred kilometers, notably near subduction zones and other tectonic features. In this paper, a model of all the subduction zones and spreading ridges is used to investigate the effect of small-scale structures on normal mode frequency shifts and on global inversions. The normal mode frequency shifts for this model have the same characteristics as observed normal mode frequency shifts. Furthermore, the global models obtained from inversions of synthetic data for the model of subduction zones and spreading ridges are similar in character to the models obtained from global inversions of real data. For a realistic set of wave paths, the model obtained from the synthetic inversions differs appreciably from a smoothed version of the true model. This is due to an uneven path coverage and to a lesser extent due to neglecting scattering effects. This means that the models obtained from global inversions of seismological data may contain appreciable artifacts.

The use of P wave amplitude data in a joint inversion with travel times for upper mantle velocity structure

A joint inversion of P wave travel time and amplitude data is performed to test whether the amplitudes can be used to increase the resolution of inversions using travel times alone. Geometrical spreading amplitudes depend on the curvature of the slowness field and may thus help to resolve sharp gradients. Travel times and amplitudes of short-period P waves observed at the Washington Regional Seismograph Network are jointly inverted for upper mantle velocity structure below the array. The processing of the observed P wave amplitudes is discussed in detail, and it is shown that it is important to correct the raw data for station statics, as these dominate the observed amplitude signal. A robust averaging procedure is used to identify and remove outliers from the data set, which is comprised of 8697 travel time and 4255 amplitude measurements. The travel time data alone are used to obtain a reference model for the amplitude data and in this way the nonlinear behavior of the amplitude equations is minimized. The results show that the amplitude data induce small, short-scale slowness perturbations to the starting model, producing an amplitude misfit reduction to an arbitrary degree, depending on the regularization. Inversions with synthetic data are performed to explain this result. An idealized model of a subducting slab is used to generate synthetic data sets. The model resulting from a joint inversion produces an amplitude misfit reduction of 77%, while the travel time misfit is unaffected. In agreement with the results for the real data sets, this is achieved by making small adjustments throughout the model, which do not alter the overall slowness variations. The amplitude data therefore do not improve the resolution of the sharp gradients present in the synthetic slab model. These results can be explained by both the strong sensitivity of geometrical spreading amplitude to slowness perturbations along the ray path and to the distribution of the amplitude data set, which is not complete enough to induce more than incoherent changes to the starting model. The strong sensitivity is due to the combination of long teleseismic ray paths and small-scale (about 30 km) slowness variations allowed by the model parameterization. With the present amplitude data set the curvature is changed on the scale of the node spacing in the model, whereas the overall slowness variations on a scale length of several times the node spacing are unaffected. The regularization has little effect on these small-scale changes, as these changes increase the total model roughness by only 1%. It is concluded that with the amplitude data set available, amplitudes do not improve the resolving power of travel time inversions for upper mantle velocity structure. This is due to the number of data required to both decrease the observed variance in the raw amplitudes to the variance in travel time data and to obtain a good coverage of the model. The application of P wave amplitude data as a validation tool is suggested for models obtained with, for example, travel time inversion. Applications to both synthetic and observed data sets are shown. Anelastic damping is ignored in the inversions. Using linear relations between velocity, temperature, and Q, it is shown that the effect of anelastic attenuation is of the order of 10–15% of that of geometrical spreading through the upper mantle.

Sharp 400-km discontinuity from short-period P reflections

P-wave data from the short-period (around 1 Hz) Southern California Seismic Network reveal pre-critical reflections from the 410-km discontinuity below the Gulf of California, appearing as a clear and continuous feature in the data to epicentral distances as small as 10°. Deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arriving P waves. The reflections are used to estimate the width of the gradient zone near 410 km depth, constraining the velocity jump across the discontinuity using critically refracted waves. The velocity jump across the 410-km discontinuity is 6±2.5 %, which is in agreement with existing estimates of the velocity jump for this region. The clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km. Synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km, with most of the velocity increase occurring over about 4 km or less, for the reflections to be visible in short-period data. Applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading centre continue down to at least 410 km depth.

Quantitative assessment of surf-produced sea spray aerosol

The first results are presented from a quantitative model describing the aerosol production in the surf zone. A comparison is made with aerosol produced in the surf zone as measured during EOPACE experiments in La Jolla and Monterey. The surf aerosol production was derived from aerosol concentration gradients measured downwind from the surf zone, after correction for the background size distribution that was measured upwind from the wave breaking zone. The aerosol production model was originally developed from measurements performed along the Baltic coast. The model predicts the aerosol production from the total energy dissipated in the wave breaking zone, calculated from the coastal bathymetry and deep-water surface wave field. In the present work, the parameterization of the aerosol production in the wave breaking zone is maintained, but the energy dissipation in the wave breaking zone is calculated using a different model that produces more realistic surf zone widths. Wave data were obtained from buoys off the Californian coast, while bathymetry data were supplied by the Scripps Institute of Oceanography. Observed and predicted aerosol production in the surf zone are in good agreement, for both sites. The predicted aerosol flux reproduces the day-to-day variations and even some of the observed variations on a time scale of several hours.

Aerosol production in the surf zone and effects on IR extinction

Aerosol concentrations over the surf were measured during the EOPACE (Electro-Optical Propagation Assessment in Coastal Environment) Surf-i experiment in La Jolla, California. Particle size distributions were measured on the beach (at three levels) and across the surf (one level). Concentrations of droplets smaller than i im in diameter are little affected by the surf, while those with diameters in the 1-10 im range increase by up to two orders of magnitude. Clear vertical gradients were observed, which vary with particle size. No relation could be established between the surf production and wind speed or wave properties. Extinction coefficients at visible and infrared wavelengths calculated from the particle size distributions show that these are enhanced by a factor of 30 to 100, depending on the wavelength. Using the measured concentrations as boundary condition, calculations with a simple dispersion model show the gradual decrease in the concentration with fetch in off-shore winds. In on-shore winds the surf-enhanced aerosol concentration is effective over only a short range, but nevertheless significant transmission losses may occur. Obviously, these conclusions apply only to the surf encountered during this specific experiment. The effects of the surf in other areas and other ambient conditions will be assessed from the analysis of data collected at a different location and in different conditions.