Scaling wave impact pressures on vertical walls

It is widely recognized that the use of Froude similarity for scaling up wave impact pressures recorded during physical model tests may lead to over-estimation of impact maxima. Based on reviewing historical work dating back to the 30s and further developments in the 60s and 80s, a general method is presented that is suitable for scaling up impact pressures and rise times measured during small scale physical model tests. The method accounts for the effect of air leakage and is applicable to most wave impact loads. The model is applied to scale wave impact pressures on vertical walls and similar structures, and consistent correction factors for the Froude scaling law are derived.     

Towards end-to-end models for investigating the effects of climate and fishing in marine ecosystems

End-to-end models that represent ecosystem components from primary producers to top predators, linked through trophic interactions and affected by the abiotic environment, are expected to provide valuable tools for assessing the effects of climate change and fishing on ecosystem dynamics. Here, we review the main process-based approaches used for marine ecosystem modelling, focusing on the extent of the food web modelled, the forcing factors considered, the trophic processes represented, as well as the potential use and further development of the models. We consider models of a subset of the food web, models which represent the first attempts to couple low and high trophic levels, integrated models of the whole ecosystem, and size spectrum models. Comparisons within and among these groups of models highlight the preferential use of functional groups at low trophic levels and species at higher trophic levels and the different ways in which the models account for abiotic processes. The model comparisons also highlight the importance of choosing an appropriate spatial dimension for representing organism dynamics. Many of the reviewed models could be extended by adding components and by ensuring that the full life cycles of species components are represented, but end-to-end models should provide full coverage of ecosystem components, the integration of physical and biological processes at different scales and two-way interactions between ecosystem components. We suggest that this is best achieved by coupling models, but there are very few existing cases where the coupling supports true two-way interaction. The advantages of coupling models are that the extent of discretization and representation can be targeted to the part of the food web being considered, making their development time- and cost-effective. Processes such as predation can be coupled to allow the propagation of forcing factors effects up and down the food web. However, there needs to be a stronger focus on enabling two-way interaction, carefully selecting the key functional groups and species, reconciling different time and space scales and the methods of converting between energy, nutrients and mass.     

NPZ Models of Plankton Dynamics: Their Construction, Coupling to Physics, and Application

Nutrient-phytoplankton-zooplankton (NPZ) models have been in use in oceanography for at least three decades, and are still a common research tool. Given the discoveries of the last two decades, particularly concerning the role of bacteria in the plankton, there are questions as to whether NPZ models can still be supported as a useful tool in planktonic research. Here I review the construction of NPZ models, and some of the physical platforms they have been coupled to. I then discuss the applications of NPZ-physical models, and conclude that they still constitute an important and viable research tool, provided that the questions being explored are clearly stated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Wind and wave induced behaviour of offshore guyed tower platforms

Offshore guyed tower platforms belong to the group of compliant offshore platforms which are most suited for deep water exploration. The basic feature of compliant offshore platforms is that they are designed to move with the waves, in at least some degrees-of-freedom. As far as excitation of wave frequencies is concerned, the system opposes wave forces by inertial effects. The offshore guyed tower derives its stability against lateral movement from its mooring system.In this study, the response of offshore guyed towers to random forces generated by wind and wave is investigated. The exposed portion of the tower is subjected to the action of turbulent wind, while the submerged portion is acted upon by random wave forces. The analysis includes the nonlinearities due to the Morison equation of drag force, the variable submergence effect due to waves, the instantaneous position of the tower and force excursion relation of the mooring lines. A parametric study is conducted to investigate the behaviour of the tower under waves, and the combined effect of wind and wave forces.     

Forecasting ocean waves: Comparing a physics-based model with statistical models

The literature on ocean wave forecasting falls into two categories, physics-based models and statistical methods. Since these two approaches have evolved independently, it is of interest to determine which approach can predict more accurately, and over what time horizons. This paper runs a comparative analysis of a well-known physics-based model for simulating waves near shore, SWAN, and two statistical techniques, time-varying parameter regression and a frequency domain algorithm. Forecasts are run for the significant wave height, over horizons ranging from the current period (i.e., the analysis time) to 15 h. Seven data sets, four from the Pacific Ocean and three from the Gulf of Mexico, are used to evaluate the forecasts. The statistical models do extremely well at short horizons, producing more accurate forecasts in the 1–5 hour range. The SWAN model is superior at longer horizons. The crossover point, at which the forecast error from the two methods converges, is in the area of 6 h. Based on these results, the choice of statistical versus physics-based models will depend on the uses to which the forecasts will be put. Utilities operating wave farms, which need to forecast at very short horizons, may prefer statistical techniques. Navies or shipping companies interested in oceanic conditions over longer horizons will prefer physics-based models.     

Is there any hope for fisheries management?

Fisheries is not the only discipline where models have been used in attempts to fine tune an aspect of the economy. Such fine-tuning can prove ineffective because of the uncertainties in the scientific underpinnings of the models and because of the omission of critical elements. In fisheries, the biological goal is to set allowable catches so that the harvest is not so large that it endangers the future health of the fish stock while it is not so low as to waste food, while the economic goal is to maximize the net economic rent generated by the fishery. It has long been recognized that the science underlying the setting of the total allowable catch is often too uncertain to justify such fine tuning, and that attempts to achieve that delicate balance has helped lead to crises in fisheries. One solution is to abandon such marginalism in favor of seriously reducing current catches.     

Weak and strong constraint data assimilation in the inverse Regional Ocean Modeling System (ROMS): Development and application for a baroclinic coastal upwelling system

We describe the development and preliminary application of the inverse Regional Ocean Modeling System (ROMS), a four dimensional variational (4DVAR) data assimilation system for high-resolution basin-wide and coastal oceanic flows. Inverse ROMS makes use of the recently developed perturbation tangent linear (TL), representer tangent linear (RP) and adjoint (AD) models to implement an indirect representer-based generalized inverse modeling system. This modeling framework is modular. The TL, RP and AD models are used as stand-alone sub-models within the Inverse Ocean Modeling (IOM) system described in [Chua, B.S., Bennett, A.F., 2001. An inverse ocean modeling system. Ocean Modell. 3, 137–165.]. The system allows the assimilation of a wide range of observation types and uses an iterative algorithm to solve nonlinear assimilation problems. The assimilation is performed either under the perfect model assumption (strong constraint) or by also allowing for errors in the model dynamics (weak constraints). For the weak constraint case the TL and RP models are modified to include additional forcing terms on the right hand side of the model equations. These terms are needed to account for errors in the model dynamics.Inverse ROMS is tested in a realistic 3D baroclinic upwelling system with complex bottom topography, characterized by strong mesoscale eddy variability. We assimilate synthetic data for upper ocean (0–450 m) temperatures and currents over a period of 10 days using both a high resolution and a spatially and temporally aliased sampling array. During the assimilation period the flow field undergoes substantial changes from the initial state. This allows the inverse solution to extract the dynamically active information from the synthetic observations and improve the trajectory of the model state beyond the assimilation window. Both the strong and weak constraint assimilation experiments show forecast skill greater than persistence and climatology during the 10–20 days after the last observation is assimilated.Further investigation in the functional form of the model error covariance and in the use of the representer tangent linear model may lead to improvement in the forecast skill.     

硒剂量对钝顶螺旋藻的生理生化影响

对钝顶螺旋藻(Spirulina platensis)进行不同剂量水平的硒处理,以单位生物量S.platensis的加硒量为硒剂量(x_Se=C_Se/W(S.platensis)),通过每天向培养液中添加一定量的硒,维持螺旋藻生长期的硒剂量为恒定水平(即每天的x_Se相同),研究不同硒剂量对S.platensis的生理生化影响.结果表明:各种抗氧化酶的活性除了APX之外基本上随x_Se的增大而增大;当x_Se不大于0.1时,硒对S.platensis的生长有促进作用,藻的色素含量与蛋白含量高于对照组,藻体MDA含量小于或接近于对照组;当x_Se不小于0.2时,藻的色素含量与蛋白含量下降,其中,x_Se为0.4的实验组的生物量则比对照组下降了21%;藻体MDA含量随x_Se的增大而上升.实验结果提示,当x_Se不大于0.1时,硒对螺旋藻产生营养作用,而当x_Se不小于0.2时硒对螺旋藻产生毒性作用.     

Forecasting ocean wave energy: The ECMWF wave model and time series methods

Recently, the technology has been developed to make wave farms commercially viable. Since electricity is perishable, utilities will be interested in forecasting ocean wave energy. The horizons involved in short-term management of power grids range from as little as a few hours to as long as several days. In selecting a method, the forecaster has a choice between physics-based models and statistical techniques. A further idea is to combine both types of models. This paper analyzes the forecasting properties of a well-known physics-based model, the European Center for Medium-Range Weather Forecasts (ECMWF) Wave Model, and two statistical techniques, time-varying parameter regressions and neural networks. Thirteen data sets at locations in the Atlantic and Pacific Oceans and the Gulf of Mexico are tested. The quantities to be predicted are the significant wave height, the wave period, and the wave energy flux. In the initial tests, the ECMWF model and the statistical models are compared directly. The statistical models do better at short horizons, producing more accurate forecasts in the 1-5 h range. The ECMWF model is superior at longer horizons. The convergence point, at which the two methods achieve comparable degrees of accuracy, is in the area of 6 h. By implication, the physics-based model captures the underlying signals at lower frequencies, while the statistical models capture relationships over shorter intervals. Further tests are run in which the forecasts from the ECMWF model are used as inputs in regressions and neural networks. The combined models yield more accurate forecasts than either one individually.     

Analysis of body supercavitation in shallow water

Motion of a cavitating body in shallow waters undergoes a blockage effect. There are influences of the rigid boundary (the sea bottom) and the free boundary (sea surface) in shallow waters. As shown by computation carried out with the ideal fluid theory, the combination of these influences leads to an increase of cavitation number for a cavity of a fixed length and to 3D deformations of the cavity cross-sections, with a swelling of the down cavity part.