Walton E. McBride

Improvements of satellite SST retrievals at full swath

The ultimate goal of the prediction of Sea Surface Temperature (SST) from satellite data is to attain an accuracy of 0.3°K or better when compared to floating or drifting buoys located around the globe. Current daytime SST algorithms are able to routinely achieve an accuracy of 0.5°K for satellite zenith angles up to 53°. The full scan swath of VIIRS (Visible Infrared Imaging Radiometer Suite) results in satellite zenith angles up to 70°, so that successful retrieval of SST from VIIRS at these higher angles would greatly increase global coverage. However, the accuracy of present SST algorithms steadily degrades to nearly 0.7°K as the satellite zenith angle reaches 70°, due mostly to the effects of increased atmospheric path length. We investigated the use of Tfield, a gap-free first guess temperature field used in NLSST, as a separate predictor to the MCSST algorithm in order to clearly evaluate its effects. Results of this new algorithm, TfieldSST, showed how its rms error is heavily dependent on the aggressiveness of the pre-filtering of buoy matchup data with respect to Tfield. It also illustrated the importance of fully exploiting the a priori satellite-only information contained in Tfield, presently tamed in the NLSST algorithm due to the fact that it shows up as a multiplier to another predictor. Preliminary results show that SST retrievals using TfieldSST could be obtained using the full satellite swath with a 30% improvement in accuracy at large satellite zenith angles and that a fairly aggressive pre-filtering scheme could help attain the desired accuracy of 0.3°K or better using over 75% of the buoy matchup data.

Fresnel reflection by wavy sea surface

In studying light and image transfer in sea waters, the influence of Fresnel surface reflection is as significant as scattering and absorption phenomena. In these cases knowledge of the reflective properties of the sea surface at different wind speeds is very important. At present, little is published about these properties. The authors present results of numerical modeling of angular reflection coefficients of sea water as a function of zenith angle of illumination and wind speed.

Optical spectral signatures of marine sediments

In studying light and image transfer in coastal waters the influence of bottom reflection is as significant as scattering and absorption phenomena. In these cases a knowledge of the reflective properties of different types of bottoms is very important. At present, little is known about these properties. We present results of experimental spectral measurements of different kinds of sedimental material such as sands and clays, both major components of coastal water bottoms. We have determined the spectral index of refraction from measurements of the optical spectral signatures of various clays and sands, as the represent the most common bottom components found in coastal waters. The measured optical spectral signatures and the associated complex indexes of refraction are presented. This preliminary study should provide insight on improving the inverse problem algorithm for extracting the spectral index of refraction. This spectral information can then be used as input into radiative transfer models which include the ocean bottom.

Scattered photons as useable signal for underwater imaging

The orthodox approach to designing an underwater imaging system with artificial illumination has been to consider only the unscattered target photons as useable signal while looking at scattered photons as a nuisance to be mitigated. Photons scattered from the target towards the receiver cause blurring of fine target details in the collected imagery, while photons backscattered by the water column as the artificial source illuminates the target act as a veiling luminance that reduces overall image contrast. Typical performance for the Laser Line Scanner and Pulsed Range-Gated imagers can reach up to 6 attenuation lengths, which can still represent very short ranges in the turbid waters of coastal regions. In the early 1970s, with the goal of extending these performance ranges, the Visibility Laboratory explored an unconventional concept that was called imagery by means of Time Varying Intensity (TVI). TVI uses both scattered and unscattered photons from the laser-scanned target as useable signal. This novel approach enabled high-quality imagery to be collected over 20 attenuation lengths between the target and receiver. Although this system was eventually shelved, it has been resurrected by using a modulated laser illuminator to communicate critical information about the laser scan to a distant receiver via both the scattered and unscattered photons. With this knowledge, a high-fidelity image of target detail can then be recreated. In this paper, a real-time interactive simulation of TVIs expected imaging performance is presented and model predictions are compared with experimental imagery acquired when laser and receiver are both located underwater.

A Simple Method of Deriving Three-Dimensional Temperature Fields Using Remotely Sensed and In Situ Data for Application to Numerical Hydrodynamic Models of Estuaries and Bays

Abstract This paper describes the subjective interpolation method (SIM) for generating three-dimensional temperature distributions from remotely sensed sea surface temperature (SST) fields. SIM incorporates MATLAB-based cloud removal software and a method of generating synthetic temperature profiles based on observations. This approach depends on the human facility for recognizing patterns in complex images. Three-dimensional temperature fields produced by SIM are compared to analogous fields based on optimal interpolation (OI) methods by using temperature fields interpolated by the two methods to initialize a baroclinic coastal ocean circulation model. The initial SST surface fields from both methods have a bias of less than −0.5°C and rms errors of less than 1.5°C. After running for 48 h, the bias and rms errors for the OI simulations are 0.3° and 1.2°C, respectively, whereas the same errors for the SIM run are 0.7° and 0.9°C. The OI and SIM approaches can be combined to allow preprocessing of SST data ...

Irradiance calculation model of CT-based underwater visibility

Contrast transmittance (CT) theory has been used to derive a simple two-parameter visibility model that has been used by the civilian and military sectors for over seventy years. We review this classical theory with special attention to its application to problems in estimating liminal visibility range for in-water divers. Previous work by Duntley of the MIT/SIO Visibility Lab states that calculations based on this simple model give reasonable agreement with observation provided all embedded assumptions hold. Recent dat form divers conducting shallow water observations indicate that these assumptions cause the model to give poor results in this domain. We investigate an enhanced method that eliminates some of these assumptions to provide better estimate of underwater diver visibility.

Volume scattering function of light for a mixture of polydisperse small particles with various optical properties

Scattering of electromagnetic waves by a polydisperse system of small spherical hydrosol particles in the scalar approximation is considered. The equation for the scattering cross-section which generalizes the well-known Rayleigh-Gans-Rocar formula for a scattering medium containing particles characterized by continuous distributions in their dielectric permittivities at any given size is derived. For a natural situation, such as a coastal environment, the difference between volume scattering functions proposed in this paper and conventionally derived scattering functions can be as large as 50% for the particle size distributions with the maximum size of particles less than ten wavelengths of light.