Optical influence of ship wakes

Parallel Monte Carlo simulation algorithm for the spectral reflectance and transmittance of the wind-generated bubble layers in the upper ocean using CUDA

The parallel Monte Carlo software CUDAMCML used in the bio-optics field was developed by Erik Alerstam et al. (J. Biomed. Opt., 13, 060504, 2008) based on the Compute Unified Device Architecture (CUDA) and can simulate light transport in multilayered media. In the present study, CUDAMCML is extended to form the new program CUDAMCML-OCEAN using the average sampling method. This new program can handle multiple types of particle seawater containing elements such as colored dissolved organic matter (CDOM) and bubbles. The accuracy and speedup of the new program are analyzed. The results show that when the parameters are set appropriately, the speedup of CUDAMCML-OCEAN is more than 200 times compared with serial code. And the accuracies of the spectral reflectance and transmittance all reached a satisfactory level for different wind speeds and chlorophyll concentrations.

Modeling the ship white water wake in the midwave infrared

We have modeled the white water wake of a ship as a single layer of bubbles packed on the sea surface within the perimeter of the trailing turbulent wake. The size of the bubbles is considered greater than the midwave infrared wavelengths such that the optical geometrical approximation remains valid. The upper half bubble hemisphere is meshed into facets, and we calculate the probability density function of their slopes and constrain that distribution by the geometrical limits imposed by the position of the receiver through the shadowing of facets by other bubbles and of facets that are facing away from the receiver. For the facets that are visible, we compute the midwave infrared emitted and reflected radiance for the white water wake for atmospheric, solar, and sea conditions that prevailed during a ship wake measurement trial using a homegrown simulation code, the Sea Surface Radiance Simulator. The range of slopes that are visible to the receiver for the white water wake greatly exceeds those that are present in the turbulent wake and in the sea background. Consequently, the variability in the white water wake radiance is substantial. As a function of the downstream distance astern of the ship, we have ad hoc assumed that the white water wake fraction decays linearly or proportionally to the turbulent intensity in the wake. Comparing to measurements, we find an agreement in trend behavior of the midwave radiance contrast of the white water wake with downstream distance for a white water wake fraction that decays proportionally to the square of the turbulence intensity.

Monte Carlo simulation of spectral reflectance and BRDF of the bubble layer in the upper ocean

The presence of bubbles can significantly change the radiative properties of seawater and these changes will affect remote sensing and underwater target detection. In this work, the spectral reflectance and bidirectional reflectance characteristics of the bubble layer in the upper ocean are investigated using the Monte Carlo method. The Hall-Novarini (HN) bubble population model, which considers the effect of wind speed and depth on the bubble size distribution, is used. The scattering coefficients and the scattering phase functions of bubbles in seawater are calculated using Mie theory, and the inherent optical properties of seawater for wavelengths between 300 nm and 800 nm are related to chlorophyll concentration (Chl). The effects of bubble coating, Chl, and bubble number density on the spectral reflectance of the bubble layer are studied. The bidirectional reflectance distribution function (BRDF) of the bubble layer for both normal and oblique incidence is also investigated. The results show that bubble populations in clear waters under high wind speed conditions significantly influence the reflection characteristics of the bubble layer. Furthermore, the contribution of bubble populations to the reflection characteristics is mainly due to the strong backscattering of bubbles that are coated with an organic film.

Lidar signature from bubbles in the sea

The lidar signature from a collection of bubbles is proportional to the volume backscatter coefficient at a scattering angle of 180 degrees . This quantity, calculated using a combination of geometric optics and diffraction, is proportional to the void fraction of the bubbles in the water for any bubble size distribution. The constant of proportionality is 233 m(-1) sr(-1)for clean bubbles, slightly less for bubbles coated with a thin layer of organic material, and as large as 1445 m(-1) sr(-1) for a thick coating of protein.

Influence of characteristics of micro-bubble clouds on backscatter lidar signal

Marine micro-bubbles are one of those important constituents that influence scattering characteristics of water column. Monte Carlo Based simulations show that a water entrained bubble cloud generate a characteristic backscatter of incident laser light [M. Xia, J. Opt. A: Pure Appl. Opt. 8, 350 (2006)]. This characteristic can be used to detect and localize bubble clouds, leading to wide ranging applications, especially in optical remote sensing. This paper describes tests of an underwater lidar system applied to detecting cloud of micro-bubbles. Laboratory experiments demonstrate that the system is capable of detecting bubbles ranging from diameter 10 microm approximately 200 microm, over a distance of 7-12 m from the detector. The dependence of the lidar return signal on size distribution of bubbles, concentration, thickness and location of bubble clouds is studied and compared with simulation results.

Spectral variation of the volume scattering function measured over the full range of scattering angles in a coastal environment

The spectral volume scattering function (VSF) was measured in a coastal environment from 0.6 degrees to 177.3 degrees by use of a recently developed device. The spectral variations of the particulate VSF and phase function (i.e., ratio of the VSF to the scattering coefficient) were examined as a function of the scattering angle. The angular dependency of both VSF and phase- function spectra was highly sensitive to the absorption and to the size distribution of the particles. As a result, the use of spectrally neutral phase functions in radiative-transfer modeling is questioned.