Application of Two Turbulence Closure Schemes in the Modelling of Tidal Currents and Salinity in the Hooghly Estuary

This paper has two purposes. The first is to study the circulation and salinity in Hooghly Estuary, along the east coast of India and the second is to compare the performance of two turbulence closure schemes by modelling it. A breadth averaged numerical model using a sigma co-ordinate system in the vertical is briefly described. Vertical diffusion of momentum and salt are parameterized by a simple first-order turbulent closure or by a one equation model for turbulent kinetic energy (TKE) which uses a specified mixing length. The results are compared with the available neap and spring tide observations along the estuary for both low and high discharge periods.The computed elevations and currents are in reasonable agreement with the observations showing no major differences in vertical current profiles by both the turbulent schemes. However, there is a slight under-prediction of bottom currents. The salinity profiles predicted by TKE model show better matching with observations. Statistical tests are also conducted to study the comparative performance of the turbulent closure schemes. The maintenance of two layer structure in residual currents and salt variability are also studied by using the model.     

Modeling sea dynamics and turbulent zones on high spatial resolution nested grids

The problem of simulating sea dynamics in areas comprising near-shore zones and zones of high turbulence is considered. A mathematical model and the numerical algorithm of its solving are formulated. The model is based on the equations for nonhydrostatic dynamics and includes (k-ε) and (k-ω) parameterization of turbulent processes. The equations of the model are written in a σ-coordinate system. The numerical algorithm for solving the problem is based on the use of implicit schemes owing to the splitting with respect to the physical processes and space coordinates. The model calculations were performed for four nested sea basins with different spatial resolution: the Baltic Sea (3.7-km space resolution), the Gulf of Finland (1.85-km resolution), the Tallinn-Helsinki area (560-m resolution), and Tallinn Bay (93-m resolution). The results of the experiment show that the model well simulates the processes of enhanced turbulent activity in the near-shore zones that affect the local features of the sea characteristics.     

Interannual variability and seasonal contribution of thermal expansion to sea level in the Persian Gulf

In an attempt to understand the causes of the sea-level seasonal cycle in the Persian Gulf, we investigated the relationships of sea-level data from 11 stations with atmospheric pressure and thermosteric level. Sea level is significantly correlated among all stations. The mean trend in sea level for the Persian Gulf is about 2.34 mm/year. The thermosteric sea-level variability is estimated from temperature profiles at one-degree grid points. Contour maps of thermosteric level show that the height due to thermal expansion is high in summer and autumn, and low during winter and spring. The monthly mean thermostric height ranges from +2.2 cm in July to −2.1 cm in February. The major change in sea level due to the thermosteric level seems to be associated with the large change of the thermohaline circulation in the Persian Gulf. The maximum expansion occurs in summer, and the maximum contraction occurs in winter.Results of the regression analysis demonstrate that from 62% to 90.2% of the variance in the seasonal cycle is due to atmospheric pressure. The inclusion of the thermosteric sea level as a secondary forcing in the regression model improves the variance explained to 78.1–90.7%. The remaining change should be due to the halosteric effect and upwelling. Tide-gauge stations located at the Gulf's head show high correlation with Ekman vertical velocity. There are two distinct tide gauge stations in the Persian Gulf. One is found in the first cyclonic gyre and the other in the second gyre. The inclusion of Ekman upwelling to the model, improves significantly the variations explained as well, from 82.3% to 91.9%.     

2D basin modeling study of the Binak Trough,northwestern Persian Gulf,Iran

The northwestern part of the Persian Gulf is one of the most prominent hydrocarbon exploration and production areas. Oilfields are located in structural highs formed around the Cenomanian depression known as Binak Trough. To evaluate the highly variable source rock maturity, timing of hydrocarbon generation as well as migration pattern and the remaining hydrocarbon potential of the early Cretaceous source rocks, burial and thermal histories were constructed for four production wells and one pseudo well. In addition two cross sections covering the depression and the structural highs around the trough were investigated by 2D basin modeling to provide a better regional overview on basin evolution.The modeling results indicate that whereas the Cretaceous source rocks are immature or early mature at the location of oilfields, they reached sufficient maturity to generate and expel considerable amounts of hydrocarbons in the Binak depression. The main phase of oil generation and expulsion from the Cretaceous source rocks is relatively recent and thus highly favorable for the conservation of hydrocarbon accumulations. Trap charging occurred through the late Miocene to Pliocene after the Zagros folding. 2D models predict that the Albian source rock still has significant hydrocarbon generation potential whereas the lower Neocomian source rock has reached already a high transformation ratio within the deep kitchen area. Oil migration occurs in both lateral and vertical directions. This migration pattern could explain the distribution of identified oil families in the northwestern part of the Persian Gulf.     

Assessment of turbulence closure models for resonant inertial response in the oceanic mixed layer using a large eddy simulation model

Large eddy simulation (LES) of the resonant inertial response of the upper ocean to strong wind forcing is carried out; the results are used to evaluate the performance of each of the two second-order turbulence closure models presented by Mellor and Yamada (Rev Geophys Space Phys 20:851–875, 1982) (MY) and by Nakanishi and Niino (J Meteorol Soc Jpn 87:895–912, 2009) (NN). The major difference between MY and NN is in the formulation of the stability functions and the turbulent length scale, both strongly linked with turbulent fluxes; in particular, the turbulent length scale in NN, unlike that in MY, is allowed to decrease with increasing density stratification. We find that MY underestimates and NN overestimates the development of mixed layer features, for example, the strong entrainment at the base of the oceanic mixed layer and the accompanying decrease of sea surface temperature. Considering that the stability functions in NN perform better than those in MY in reproducing the vertical structure of turbulent heat flux, we slightly modify NN to find that the discrepancy between LES and NN can be reduced by more strongly restricting the turbulent length scale with increasing density stratification.     

Modeling of the eddy viscosity by breaking waves

Breaking wave induced nearsurface turbulence has important consequences for many physical and biochemical processes including water column and nutrients mixing,heat and gases exchange across air-sea interface.The energy loss from wave breaking and the bubble plume penetration depth are estimated.As a consequence,the vertical distribution of the turbulent kinetic energy(TKE),the TKE dissipation rate and the eddy viscosity induced by wave breaking are also provided.It is indicated that model results are found to be consistent with the observational evidence that most TKE generated by wave breaking is lost within a depth of a few meters near the sea surface.High turbulence level with intensities of eddy viscosity induced by breaking is nearly four orders larger than υwl(=κuwz),the value predicted for the wall layer scaling close to the surface,where uw is the friction velocity in water,κ with 0.4 is the von Kármán constant,and z is the water depth,and the strength of the eddy viscosity depends both on wind speed and sea state,and decays rapidly through the depth.This leads to the conclusion that the breaking wave induced vertical mixing is mainly limited to the near surface layer,well above the classical values expected from the similarity theory.Deeper down,however,the effects of wave breaking on the vertical mixing become less important.     

Coastal management in the Persian Gulf region within the framework of the ROPME programme of action

Persian Gulf is a semi-enclosed sea located in the Middle East and is connected to oceans through the narrow 55-km Strait of Hormuz. The Persian Gulf holds an estimated 57–66% of the world's known reserves of oil. The occurrence of three major battles in the Gulf region during the past three decades has created an atmosphere of commotion and uncertainty. Because of its marine geology, geographical location, and geopolitical sensitivity, coastal management in the Gulf region cannot be considered independently of its vast oil and natural gas reserves and environmentally related matters. The Regional Organization for Protection of Marine Environment (ROPME) forum was established in Kuwait in 1979 and quickly ratified by seven new member states (Bahrain, Iran, Iraq, Oman, Qatar, Saudi Arabia and the United Arab Emirates). Rapid growth of ROPME and shared coastal and marine environmental issues among littoral States have resulted in numerous successful plans laying the basis for future coastal management and development in the Persian Gulf region.Different plans were investigated to reach sustainable coastal management and environmental pollution prevention programmes in the Persian Gulf region and it was concluded that such plans could only be implemented when littoral states prioritize the management schemes in the Persian Gulf region and incorporate them into their own national legislation.Similar to many Regional Sea Programmes, ROPME has faced many drawbacks since its inception. Military conflicts, poor enforcement of protocols, lack of adequate coordination, disharmony among littoral states and lack of sufficient funding have put many coastal management programmes on hold.Demilitarization, enforcement of ROPME resolutions, and implementation of long-term economical growth planning are all part of an integrated coastal management programme that can bring about significant changes in the Persian Gulf area. Despite all existing differences and difficulties, many important tasks have been accomplished in the past two decades. Coastal management issues have been analyzed and well documented by ROPME. With the existing situation in the Gulf region, ROPME can effectively coordinate and implement the following tasks: monitor water quality and coastal habitat, develop and implement a comprehensive pollution prevention scheme, educate the public in terms of coastal preservation, train technical staff, put in place an effective pollution prevention and waste management programme, and establish the basis for an integrated regional coastal zone management plan.     

The improvement of the one-dimensional Mellor-Yamada and K-profile parameterization turbulence schemes with the non-breaking surface wave-induced verticalmixing

Both the level 2.5 Mellor-Yamada turbulence closure scheme(MY) and K-profile parameterization(KPP) are popularly used by the ocean modeling community.The MY and the KPP are improved through including the non-breaking surface wave-induced vertical mixing(Bv),and the improved schemes were tested by using continuous data at the Papa ocean weather station(OWS) during 1961–1965.The numerical results showed that the Bv can make the temperature simulations fit much better with the continuous data from Papa Station.The two improved schemes overcame the shortcomings of predicting too shallow upper mixed layer depth and consequently overheated sea surface temperature during summertime,which are in fact common problems for all turbulence closure models.Statistical analysis showed that the Bv effectively reduced the mean absolute error and root mean square error of the upper layer temperature and increased the correlation coefficient between simulation and the observation.Furthermore,the performance of vertical mixing induced by shear instability and the Bv is also compared.Both the temperature structure and its seasonal cycle significantly improved by including the Bv,regardless of whether shear instability was included or not,especially for the KPP mixing scheme,which suggested that Bv played a dominant role in the upper ocean where the mean current was relatively weak,such as at Papa Station.These results may provide a clue to improve ocean circulation models.     

风暴潮预报模式在渤海海域中的应用研究

首先运用COHEREN S三维多功能大陆架水动力学模型,对渤海四个主要分潮进行了调和分析,建立了渤海天文潮预报模型。结合9712号热带风暴的路径记录资料,运用风场、气压场估算公式,估算了风暴所经地带风场、气压场的变化过程。以此为输入条件,运用COHEREN S模型模拟了9712号热带风暴作用下渤海的增水过程,还模拟讨论了底摩阻系数对水位的影响。     

A natural limit to the containment and removal of oil spills at sea

A simple statistical model is developed to investigate the effect of turbulence in removing oil from the sea surface. The relationship and importance of vertical turbulent transport to oil spill containment and clean-up operations is emphasized. In its final form, the model allows a tentative conclusion to be made concerning the probable succedd of such operations, with wind speed and oil type being the governing parameters.     

Matrilineal evidence for genetic structure and Late Pleistocene demographic expansion of the Ornate goby Istigobius ornatus (Teleostei: Gobiidae) in the Persian Gulf and Oman Sea

The Persian Gulf and Oman Sea are characterized by an interesting paleoclimatic history and different ecological settings, and offer a unique study area to investigate the genetic structure of marine organisms including fishes. The Ornate goby Istigobius ornatus is widely distributed throughout the tropical Indo‐West Pacific including the Persian Gulf and Oman Sea. Here, we present the population structure, genetic diversity, and demographic history of four populations of I. ornatus from the latter two regions using the D‐loop marker of mitochondrial DNA. The results reveal a shallow genealogy, a star‐like haplotype network, significance of neutrality tests, and unimodal mismatch distribution. This is concordant with a recent demographic expansion of I. ornatus in the Persian Gulf and Oman Sea at about 63,000–14,000 years ago, which appears to be related to Late Pleistocene sea level fall and rise. The results of the pairwise Fst estimates imply high gene flow along the coast of the Persian Gulf, which is probably due to larval dispersion, whereas the Oman Sea population clearly differs from all Persian Gulf populations. The AMOVA result indicates that 7.74% of the variation is related to differences among ecoregions, while inter‐ and intra‐population differences explained ?3.20% and 95.47% of the variation, respectively. The haplotype network depicts two groups of haplotypes, most of them were specific to the Persian Gulf. No further evidence for geographic lineage substructuring was evident. The Mantel test result indicates that isolation by distance is not the main mechanism that promoted the genetic differentiation among the studied populations of I. ornatus. We suggest that cumulative effects of ecological and geographic barriers such as salinity, oceanographic conditions, and the presence of the Strait of Hormuz have shaped the genetic structure of I. ornatus in the Persian Gulf and Oman Sea.     

Large-scale turbulence under a solitary wave

The structure of large-scale turbulence under a broken solitary wave on a 1 in 50 plane slope was studied. Three-component velocity measurements were taken at different heights above a smooth bed in the middle surf zone using an acoustic Doppler velocimeter. The measured data showed that turbulent velocity components were well correlated in the middle part of the water column. The velocity correlations could be produced by an oblique vortex similar to the obliquely descending eddy observed previously by other investigators. The vertical distributions of the relative values of the components of the Reynolds stress tensor showed that the structure of turbulence evolved continuously between the free surface and the bottom. The evolution was related to transition from two-dimensional to three-dimensional flow structures and the effect of the solid bottom on flow structures. Time histories of measured turbulent kinetic energy and turbulence stresses showed episodic turbulent events near the free surface but more sporadic turbulence in the lower layer. Large or intense turbulent events were found to have short duration and time lag relative to the wave crest point. These events also maintained good correlations between the turbulence velocity components close to the bottom.Instantaneous turbulent velocity fields were measured near the bottom at the same cross-shore location by using a stereoscopic particle image velocimetry system. These measurements showed that the near-bed flow field was characterized by large-scale, coherent flow structures that were the sources of most of the turbulent kinetic energy and turbulence stresses. The types of organized flow structures observed included vortices and downbursts of turbulence descending directly from above, lateral spreading of turbulent fluid along the bed, and formation of vortices in shear layers between fluid streams. A common feature of the organized flow structures near the bed was the large turbulence velocities in the longitudinal and transverse directions, which reflected the influence of a solid bottom on the breaking-wave-generated turbulence arriving at the bed.     

参数化浅海流体动力学模型中的耗散与频散

对几个参数化浅海流体动力学模型的耗散性和频散关系进行了理论分析,提出三点结论:1.深度平均(或积分)的浅海流体动力学模式不能充分表达湍耗散;2.浅海中的强耗散将对长波振荡具有本质的影响;3.几种不同的湍参数化动力模型将对长波产生不同的频散效应。     

波浪和太阳短波辐射对渤、黄海夏季温度垂直结构的影响

基于普林斯顿(POM)模式,采用不同的垂直混合方案并考虑太阳短波辐射的作用,对渤海、黄海夏季垂直热结构进行了数值试验。试验结果表明,夏季波浪混合控制着渤海、黄海上混合层的形成,加入波浪混合能明显改善陆架浅海的夏季温度垂直结构。太阳短波辐射对渤海、黄海夏季上层垂直热结构有一定的作用,研究夏季海洋上层垂直热结构应该包括太阳短波辐射的影响,特别是对于水深相对较深的黄海。     

A generalized coordinate ocean model and a comparison of the bottom boundary layer dynamics in terrain-following and in z-level grids

Sensitivity studies with a new generalized coordinate ocean model are performed in order to compare the behavior of bottom boundary layers (BBLs) when terrain-following (sigma or combined sigma and z-level) or z-level vertical grids are used, but most other numerical aspects remain unchanged. The model uses a second-order turbulence closure scheme that provides surface and BBL mixing and results in a quite realistic climatology and deep water masses after 100 year simulations with a coarse resolution (1° × 1°) basin-scale terrain-following grid. However, with the same turbulence scheme but using a z-level grid, the model was unable to produce dense water masses in the deep ocean. The latter is a known problem for coarse resolution z-level models, unless they include highly empirical BBL schemes.A set of dense water overflow experiments with high-resolution grids (10 and 2.5 km) are used to investigate the influence of model parameters such as horizontal diffusivity, vertical mixing, horizontal resolution, and vertical resolution on the simulation of bottom layers for the different coordinate systems. Increasing horizontal diffusivity causes a thinner BBL and a bottom plume that extends further downslope in a sigma grid, but causes a thicker BBL and limited downslope plume extension in a z-level grid. A major difference in the behavior of the BBL in the two grids is due to the larger vertical mixing generated by the turbulence scheme over the step-like topography in the z-level grid, compared to a smaller vertical mixing and a more stably stratified BBL in the sigma grid. Therefore, the dense plume is able to maintain its water mass better and penetrates farther downslope in the sigma grid than in the z-level grid. Increasing horizontal and vertical resolution in the z-level grid converges the results toward those obtained by a much coarser resolution sigma coordinate grid, but some differences remain due to the basic differences in the mixing process in the BBL.     

Floc size variability under strong turbulence: Observations and artificial neural network modeling

The flocculation of cohesive sediment in the presence of waves is investigated using high-resolution field observations and a newly-developed flocculation model based on artificial neural networks. Vertical profiles of suspended sediment concentration and turbulent intensity are estimated using measurements of current profile and acoustic backscatter. The vertical distribution of floc size is estimated using an artificial neural network (ANN) that is trained and validated using floc size measurements at one vertical level. Data analysis suggests a linear correlation between suspended sediment concentration and turbulence intensity. Observations and numerical simulations show that floc size is inversely related to sediment concentration, turbulence intensity and water temperature. The numerical results indicate that floc growth is supported by low concentration and low turbulence. In the vertical direction, mean size of flocs decreases toward the bottom, suggesting floc breakage due to increasing turbulence intensity toward the bed. A significant decrease in turbulent shear could occur within the bottom few-cm, related to increased damping of turbulence by sediment induced density stratification. The results of the numerical simulations presented here are consistent with the concept of a cohesive sediment particle undergoing aggregation-fragmentation processes, and suggest that the ANN can be a precise tool to study flocculation processes.     

海面对风应力的非定常响应

针对常垂直湍流系数和变垂直湍流系数两种情况，给出了开阔静止海面对风应力的非定常响应的解析表达式，并讨论了水深，风应力强弱对响应过程的影响，尽管没有考虑风场的非定常性，但本文的结果对理解海洋对风应力的非定常响应过程具有一定的帮助，对分析海面停风后的消衰过程也具有实际意义。通过与实测资料比较，认为垂直湍流系数模式要比常垂直湍流系数模式更为合理。     

Influence of the dynamics of currents on the hydrophysical structure of the waters and the vertical exchange in the active layer of the Black Sea

An analysis of the data of measurements of the fine structure and microstructure fluctuations of hydrophysical fields in the upper 200-m layer of the Black Sea carried out using CTD profilers and a Baklan free falling microstructure and turbulence profiler revealed the existence of a positive correlation between the intensity of the fine structure and microstructure fluctuations and the dynamics of the currents. On the other hand, the level of the fine structure and microstructure fluctuations reflects the rate of the vertical turbulent exchange. It was shown that, in the case of the absence of the Black Sea Rim Current (BSRC) jet or clearly manifested mesoscale eddy structures, the vertical turbulent exchange in the pycnocline is weak, while, in the opposite case, it is stronger. The results obtained support the supposition that the interbasin dynamics play an important role in the maintenance of the rate of small-scale mixing in the pycnocline and halocline and provide the vertical transport of dissolved oxygen from the cold intermediate layer into the deeper layers of the sea.     

Investigating the seasonal vertical structure of phytoplankton in shelf seas

A 1-dimensional (vertical) physical-biological coupled model is presented. The model is designed for investigations into the link between the vertical turbulent structure of a coastal or shelf sea water column, and the primary production. The physical model employs a turbulence closure scheme to provide the link between local vertical stability (driven by seasonal solar heating) and the vertical turbulent mixing (driven by tidal currents and surface wind stress). The biological component of the model is a simple cell quota, threshold limitation model, with either 1 or 2 taxa/species of phytoplankton growing in response to light and dissolved inorganic nitrogen. The user has control over all model physical and biological driving parameters. Graphical screen output, suitable for basic visualisation and teaching purposes, is generated as the model operates, and more detailed data are written to files for later analysis.In order to demonstrate the model's use in simple hypothesis investigation, an example of the model operation is illustrated. This focuses on the effect that the springs-neaps tidal cycle has on the production within the summer sub-surface biomass maximum, illustrating the fortnightly input of new nitrogen into the thermocline and the subsequent new production.