A Comparative Study of Radiation Fog and Quasi-Fog Formation Processes During the ParisFog Field Experiment 2007

Fog is an atmospheric phenomenon that has important environmental consequences related to visibility, air quality and climate change on local and regional scales. The formation of radiation fog results from a complex balance between surface radiative cooling, turbulent mixing in the surface layer, aerosol growth by deliquescence and activation of fog droplets. During the ParisFog field experiment, out of 16 events forecasted for radiation fog, activated fog materialized in seven events, while in five other events the visibility dropped to 1–2 km but haze particle size remained below the critical size of activation. To better understand the conditions that lead to or do not lead to sustained fog droplet activation, we performed a comparative study of dynamic, thermal, radiative and microphysical processes occurring between sunset and fog (or quasi-fog) onset. We selected two radiation fog events and two quasi-radiation fog events that occurred under similar large-scale conditions for this comparative study. We identified that aerosol growth by deliquescence and droplet activation actually occurred in both quasi-fog events, but only during <1 h. Based on ParisFog measurements, we found that the main factors limiting sustained activation of droplets at fog onset in the Paris metropolitan area are (1) lack of mixing in the surface layer (typically wind speed <0.5 ms^?1), (2) relative humidity exceeding 90 % throughout the residual layer, (3) low cooling rate in the surface layer (typically less than ?1 °C per hour on average) due to weak radiative cooling (0 to ?30 Wm^?2) and near zero sensible heat fluxes, and (4) a combination of the three factors listed above during the critical phase of droplet activation preventing the transfer of cooling from the surface to the liquid layer. In addition, we found some evidence of contrasted aerosol growth by deliquescence under high relative humidity conditions in the four events, possibly associated with the chemical nature of the aerosols, which could be another factor impacting droplet activation.     

A new probabilistic approach to estimating marine gastropod densities from baited traps

A new probabilistic approach is proposed to assess muricid species population abundances at scales relevant to both Ancient and Modern coastal fisheries. Motivated by the long‐term goal of reconstructing the dynamics of exploited murex populations during Antiquity, the objective was to estimate the population density of the banded dye‐murex, Hexaplex trunculus (Linnaeus, 1758) from successive captures with baited traps, using a method similar to the technique employed in the Mediterranean purple dye industry. The stochastic model developed simulates cumulative captures while accounting for high variability. It was calibrated with data acquired during a field trapping experiment (Crete Island, Greece). Traps’ catchability and Effective Area of Attraction (EAA) were estimated using the individual speed and behavioural response towards bait from laboratory experiments. Average density of H. trunculus was estimated as 2.2 ± 1.4 SE individuals per square metre, with no significant differences between seagrass and rocky habitats. The clearing time of successive capture experiments averaged 84 ± 6 SE hr. Clearing ca. 0.4 ha of subtidal area would be necessary to produce ca. 1.0 g of pure Tyrian purple pigment. The method described is generalizable to making population abundance estimates for similar groups, such as whelks, in modern fisheries.     

Variations in the pallial organ sizes of the invasive oyster, Crassostrea gigas, along an extreme turbidity gradient

Spatial size variations of labial palps, gills and the adductor muscle of the invasive feral oyster, Crassostrea gigas, were studied along two gradients of suspended particulate matter (SPM) concentrations in the temperate macrotidal Bourgneuf Bay, (annual mean SPM concentration gradient of 21.0–154.0 mg l⁻¹) and the adjacent Loire Estuary (annual mean SPM concentration gradient of 24.1–630.4 mg l⁻¹) on the French Atlantic Coast. The gill-to-palp (G:P) ratios decreased with increasing turbidity, both in the bay and the estuary. Changes in G:P ratios were chiefly related to palp area variations, increasing gradually from low turbidity to very high-turbidity sites, while gill area showed no clear relationship with turbidity conditions. The G:P ratio, showing a significant relationship (r² = 0.97) with SPM concentrations, is proposed as a pallial organ index of oyster acclimation to turbidity conditions. The area of the striated part of the adductor muscle was always greater than that of the smooth one, and adductor muscle area tended to decrease when SPM concentration increased. These observations show the morphological capacity of the oyster C. gigas to tolerate SPM concentrations above the feeding cessation thresholds previously determined experimentally. They also suggest that pallial organ size variations could help explain the success of recent feral oyster invasions in temperate turbid ecosystems.     

Flow separation and vertical motions in a tidal flow interacting with a shallow-water island

This paper reports on the case study of Rattray Island (Great Barrier Reef, northeast Australia), lying perpendicular to tidal flow in shallow waters. At ebb and flood, attached (stable) eddies develop in the wake where swirls of turbidity suggest that sediment-laden waters are brought to the surface as a result of vertical transport. Both eddy and tip upwellings are encountered in the tidal flow around Rattray Island but there is currently no clear-cut answer as to which secondary flow generates upwelling with the largest intensity. This paper addresses this specific issue through idealized and realistic high-resolution numerical experiments. The analysis is supported by physical arguments based on the theory of flow separation. Given Rattray's geometry and surrounding bathymetry, the mechanism of flow separation in shallow waters helps explain the asymmetry in size of the eddies and their intensity. The results of idealized numerical experiments also suggest that eddy and tip upwellings may be of similar intensity at Rattray Island.     

Northern marine transportation corridors: Creation and analysis of northern marine traffic routes in Canadian waters

The concept of the Northern Marine Transportation Corridors (NMTC) initiative was developed under the Government of Canada World‐Class Tanker Safety System Initiative (WCTSS). The NMTC is an interdepartmental Arctic initiative within the Department of Fisheries and Oceans Canada (DFO), through the Canadian Coast Guard (CCG) and the Canadian Hydrographic Service (CHS), in collaboration with Transport Canada (TC). The NMTC initiative was developed to strengthen the safety of marine navigation in the Arctic, and to offer an efficient planning guide for present and future Arctic investments. Transportation corridors identified through this initiative will provide the Government of Canada the framework needed to better prioritize and deliver on its programs and services, including: nautical charts and products; aids to navigation; icebreaking services; and marine safety regulations. The corridors were generated and analyzed with a Geographic Information System (GIS) using two main data sources: the Automated Identification System (AIS) and CHS's nautical charts and publications. The geographic extent of the NMTC is defined as the Northern Canada Vessel Traffic Service Zone (NORDREG Zone) and the Mackenzie River. With close to 4 million km² of water in the Arctic and 162,000 km of coastline, surveying the Arctic to modern standards represents an enormous challenge to CHS. By adopting a corridor‐based approach, CHS and other government programs can prioritize their efforts on 12% of the Canadian Arctic waters. CHS currently has 32% of the NMTC adequately surveyed, with an additional 3% surveyed to modern standards.     

Information infrastructure for Australia’s Integrated Marine Observing System

Australia’s Integrated Marine Observing System (IMOS, imos.org.au) is research infrastructure to establish an enduring observing program for Australian oceanic waters and shelf seas. The observations cover physical, biological, and chemical variables to address themes of multi-decadal ocean change, climate variability and weather extremes, boundary currents and inter-basin flows, continental shelf processes and ecosystem responses.IMOS observations are collected by national facilities based on various platform types and operated by partner institutions around the country. In this paper we describe the infrastructure and workflows developed to manage and distribute the data to the public. We highlight the existing standards and open-source software we have adopted, and the contributions we have made. To demonstrate the value of this infrastructure we provide some illustrations of use and uptake.All IMOS data are freely and openly available to the public via the Ocean Portal (https://imos.aodn.org.au). All IMOS-developed software is open-source and accessible at https://github.com/aodn.     

Effects of the Upstream-Flow Regime and Canyon Aspect Ratio on Non-linear Interactions Between a Street-Canyon Flow and the Overlying Boundary Layer

Large-scale structures within a rough-wall boundary layer generated over a cube array have recently been linked to small-scale fluctuations close to the roughness through a dynamical mechanism similar to amplitude modulation. Demonstrating the existence of this mechanism for different roughness types is a crucial step towards the development of a generic model for wind fluctuations in the urban canopy. Here the influence of the upstream roughness geometry (two-dimensional (2D) and three-dimensional (3D)) and planform packing density (\( \lambda_{p} \)) and street-canyon aspect ratio on the non-linear interactions between large-scale momentum regions and the small scales induced by the presence of the roughness is studied within a wind tunnel using combined particle-image velocimetry and hot-wire anemometry. A multi-time delay linear stochastic estimation is used to decompose the flow into large scales that participate in modulation and the remaining small scales. Using three different upstream roughness configurations composed of either 3D cubes or 2D rectangular blocks it is shown that the upstream roughness configuration has an influence on the non-linear interactions in the rough-wall boundary layer. Analysis of the turbulence skewness decomposition shows a change in the location of the maximum of the term \( \overline{{u_{L}^{\prime} u_{S}^{\prime 2}}} \), which represents the influence of the large-scale momentum regions on the small scales, whilst the temporal correlation shows a modification of the interaction located closer to the roughness with a change from 3D to 2D roughness. Furthermore, a two-point spatio–temporal correlation demonstrates that the non-linear relationship is significantly modified in the wake-interference-flow regime compared to the skimming-flow regime. Through skewness decomposition and temporal correlations the canyon aspect ratio is shown to have no influence on the non-linear interactions, indicating that the mechanism depends only on the flow developing upstream. Finally, although the upstream roughness configuration is shown to influence the non-linear interactions, the nature of the mechanism remains the same in all configurations.     

A sampler-based technique for the study of near-bed sediment dynamics: application in the eastern English Channel

A simple technique for studying near-bed hydrodynamics and sediment dynamics is presented. The method combines the use of (1) a benthic tripod hosting a series of electromagnetic current meters, and (2) a newly developed near-bed multi-level water-sediment mixture sampler. The instrument package was deployed successfully at a shallow water station in the eastern English Channel. The currents at elevation 0.9 m above the bottom were asymmetrical, the flood current peak being slightly stronger than the ebb current peak. At elevation 0.3 m above the bottom, the ebb/flood current peak asymmetry vanished. The observed SSCs (suspended sediment concentrations) were tidally modulated, with a contrasting vertical distribution over the ebb and flood phases of the tidal current: the profile was uniform in the ebb phase whereas a stratification appeared in the flood phase. The depth dependence and time evolution of the SSCs are attributable to a combination of local resuspension and advection-dispersion of remotely suspended fine sediment by ebb currents. Suspended sediment fluxes were uniform during the ebb phase and increased with elevation above the bottom during the flood phase.