Kazuo Nadaoka

Multiangular and hyperspectral reflectance modeling of seagrass beds for remote sensing studies

A method to process high-resolution multispectral satellite data for seagrass mapping has been implemented based on an inversion of a bidirectional reflectance distribution function (BRDF) developed particularly for seagrass beds. The BRDF simulates radiative transfer from seagrass canopies approximated by assuming certain principles of geometric-optics and photon transport. Reflectance from seagrass canopies are considered to be linear mixtures of leaves and background signatures, which are moreover influenced by parameters such as seagrass leaf architecture and transmittance, seawater column inherent optical properties and substrate reflectance. To validate results of model estimates, transect line surveys along seagrass meadows were conducted to obtain actual seagrass percentage cover, species distribution, shoot density and abundance. Likewise, spectral profiles of the bottom cover, together with pigmentation and turbidity through the water column were measured using field underwater spectrometer and in-situ instruments. High-resolution satellite (IkonosTM) data taken at multiple angles were processed according to the inversion model protocol. Results from model simulations show reasonable agreement between values of modeled and observed spectral reflectance from in-situ and space sensors. Reasonable outcome of modeled against insitu reflectance data reveals that a relatively parsimonious set of variables is enough to assess seagrass biophysical properties with reasonable accuracy. Keywords-Seagrass canopy, reflectance model, benthic coastal environment, Ikonos, shallow water, radiative transfer.

Solution of Rayleigh's instability equation for arbitrary wind profiles

A different approach to the solution of the singular Rayleigh equation is presented in the context of the water wave growth problem as modelled by wind-induced shear instabilities. The approach is based on the analytical solution of a Bessel equation in the vicinity of the singular point, which is obtained from Rayleighs equation with an arbitrary wind profile. Wave growth rates are computed using an integral expression derived from the dispersion relation of the air–sea interface. Computations of the present approach agree well with those of Conte & Miles (1959) for the special case of a logarithmic wind profile. Effects of the shape of the wind profile on the wave growth rate are investigated by using the 1/7-power law to represent the wind profile. Comparisons of the growth rates for the logarithmic wind profile and for the 1/7 profile reveal appreciable differences which must be investigated further, possibly using measured wind profiles within 10 m above the sea surface.

AN APPLICATION OF A NESTING PROCEDURE TO A HIGHLY-RESOLVED CURRENT SIMULATION IN A MANGROVE AREA

To efficiently perform the numerical simulation in a R-type mangal, in which the horizontal scales of the creek and the swamp are quite different, we attempt to apply a nesting procedure to the highly resolved current simulation in the mangrove area at Fukido River of the Ishigaki Island, Okinawa, Japan. In the computation, we introduce three computational domains with the different grid resolutions. The results indicate that the numerical performance of the nesting procedure used here is confirmed for the tidal current simulation in the mangrove estuary through the comparison of the observed and computed results. It should be also found that the present numerical model can simulate complicated flow structures around the creeks.

Boundary-Fitted Nonlinear Dispersive Wave Model for Applications in Geometrically Complicated Regions

A nonlinear dispersive wave model in boundary-fitted coordinates is introduced for practical applications involving complicated lateral geometries. The model is formulated in terms of the contravariant velocity components so that the wall condition on irregular lateral boundaries is satisfied easily and accurately. Test cases involving converging, diverging, and circular channels are considered to check the reliability of the model predictions. Finally, nonlinear wave transformations in an arbitrarily shaped region are simulated as an example to practical applications.

Reflectance sensitivity of coral reef areas to optically-active constituents in coastal waters: computer modeling and field observations

One of the key concerns in coral reef monitoring by remote sensing is to determine the effects of various optically-active constituents (i.e. suspended sediments, chlorophyll-a and yellow substances) present in the coastal waters to the observed reflectance produced by remote sensors. Quantification of these constituents is important since they serve as primary indicators of coral health and their probable waterborne environmental stressors. This paper describes the coupling of coral canopy bidirectional reflectance models previously developed by the authors with a two-flow radiative transfer solution specific to coastal waters to study the sensitivity of spectral reflectance of coral reef areas with combinations solid suspended matter and chlorophyll-a concentrations at various viewing and illumination conditions. Spectral field measurements including surface and subsurface reflectance profiles of coral-bottomed waters were obtained. Chlorophyll-a concentration and turbidity data were gathered from the field by deployment of field instruments. Results indicate comparable reflectance profiles produced by the computational model relative to field data, especially at shallower depths with weaker influence of chlorophyll-a concentration, more so at high concentration of suspended sediments. Inversion methods applied to coral reef reflectance modeling is likewise discussed, particularly its performance in the estimation of coral morphological parameters given variations in suspended sediment concentration and chlorophyll-a values.