Application of a sigma coordinate sea model to the calculation of wind-induced currents

A three-dimensional numerical sea model is formulated in terms of sigma coordinates in the vertical. The vertical grid spacing in the model is arbitrary and can be refined to give enhanced resolution in high shear regions (e.g., close to the sea surface in wind-driven flows, and/or across the thermocline in stratified flows). A method of accurately determining surface currents and indicating how fine a grid is required in the surface layer is described.The problem of determining a suitable formulation of vertical eddy viscosity to use in a model of wind-induced flow in a tidal sea is considered in detail. A formulation in which surface eddy viscosity depends upon the roughness of the sea surface and the transfer of momentum to depth by surface waves appears reasonable. Below the surface layer turbulence is related to the current at depth.Idealized calculations are performed to demonstrate the accuracy and stability of the sigma coordinate model. Results of these calculations indicate that the formulation of eddy viscosity developed in this paper can explain the high surface shears reported in lake measurements of wind-induced surface currents, and the lack of shear under strong wind conditions in the open sea (GORDON, 1982, Journal of Geophysical Research, 87, 1939–1951).Surface current to surface wind ratio are also computed.     

The temperature minimum at tidal fronts

This paper presents a mechanism to explain the observed formation of a surface temperature minimum at tidal fronts in shelf seas. Tidal fronts mark the boundary between water which is kept vertically mixed by fast tidal currents and water which stratifies in summer. The fronts are associated with strong horizontal surface gradients of several water properties, including temperature. In the early studies of tidal fronts, a minimum in surface temperature was occasionally observed between the cool surface waters on the mixed side of the front and the warm surface waters on the stratified side. It was suggested that this was caused by upwelling of deep water at the front. In this paper we describe an alternative and simpler explanation based on the local balance of heating and stirring. The net heat flux into the sea in spring and early summer is greater on the mixed side of the front than on the stratified side. This happens because the heat loss mechanism is dependent on sea surface temperature and stratified waters, having a higher surface temperature, lose more heat. The stratified water near the front therefore has lower heat content (and lower depth-mean temperature) than the mixed water. If some of the stratified water becomes mixed, for example with increased tidal stirring at spring tides, a zone of minimum surface temperature will be formed at the front. A numerical model for the study of this mechanism shows that the temperature minimum at tidal fronts can be explained by the process described above. The minimum appears most clearly at spring tides, but can still be present in a weaker form at neap tides. A further prediction of the model is an increase of the horizontal temperature gradient at spring tides, which is in agreement with observations. An unexpected outcome of the modelling is the prediction of the formation of a marked sea surface temperature minimum, not yet observed, occurring in the autumn and located at the summer position of the tidal front.     

Analysis of the influence of Taihu Lake and the urban heat islands on the local circulation in the Yangtze Delta

The influences of human activities on regional climate and weather are tremendous. The re-gional temperature and the distributing of wind field are influenced, whereas the precipitation in-tensity and the spatial and temporal distribution of the precipita…     

Linkage of deep sea rapid acidification process and extinction of benthic foraminifera in the deep sea at the Paleocene/Eocene transition

Ocean Drilling Program Leg 199 Site 1220 provides a continuous sedimentary section across the Paleocene/Eocene (P/E) transition in the carbonate‐bearing sediments on 56–57 Ma oceanic crust. The large negative δ¹³C shift in seawater is likely due to the disintegration of methane hydrate, which is expected to be rapidly changed to carbon dioxide in the atmosphere and well‐oxygenated seawater, leading to a reduction in deep‐sea pH. A pH decrease was very likely responsible for the emergence of agglutinated foraminiferal fauna as calcareous fauna was eliminated by acidification at the P/E transition at Site 1220. The absence of the more resistant calcareous benthic foraminifera and the presence of the planktonic foraminifera at Site 1220 is interesting and unique, which indicates that calcareous benthic foraminifera suffered greatly from living on the seafloor. Box model calculation demonstrates that, assuming the same mean alkalinity as today, pCO₂ must increase from 280 ppm to about 410 ppm for the calcite undersaturation in the deep ocean and for the oversaturation in the surface ocean during the P/E transition. The calculated increased pCO₂ coincides with paleo‐botanical evidence. The current global emission rate (～7.3 peta (10¹⁵) gC/y) of anthropogenic carbon input is approximately 30 times of the estimate at the P/E transition. The results at the P/E transition give an implication that the deep sea benthic fauna will be threatened in future in combination with ocean acidification, increased sea surface temperature and more stratified surface water.     

Reduced-Order Salinity Modeling of the Urmia Lake Using MIKE3 and Proper Orthogonal Decomposition Models

Increasing salinity in Urmia Lake, located in the north-west of Iran, has turned into a critical issue, particularly because the lake is the habitat of a unique multi-cellular organism called Artemia Urmiana. During the past decades, several anthropogenic changes have taken place in the lake, which have resulted in increased salinity. This study introduces a reduced-order framework based on MIKE3 simulation model and proper orthogonal decomposition (POD) to simulate salinity patterns in Urmia Lake. Spatio-temporal variations of salinity in the lake firstly were simulated by MIKE3, and close matches were observed between salinity estimates from MIKE3 and those of the field data. Thereafter, 365 daily snapshots were taken from MIKE3 simulations, and subsequently 365 POD basis modes were computed. Due to high percentage of conserved energy of the lake system (salinity of lake) within the first ten POD basis modes, these modes were considered to develop a reduced-order salinity model (ROSM). Finally, results from MIKE3 were compared with the ROSM. It was shown that the first ten modes (among 365 modes) obtained by the POD conserved approximately more than 99.8% of the energy of the system. Moreover, using the first ten modes resulted in an error in magnitude of less than 0.01. Therefore, the ROSM could successfully capture the variations of salinity in the lake via its first ten modes.     

浅水湖泊水质系统滤波模型研究

本文建立了浅水湖泊水质系统滤波模型和相应算法,根据滇池流场比较稳定等特点,对湖泊进行单元分区,考虑模型误差扰动和观测误差扰动的存在,建立系统的状态方程和观测方程,然后采用吉尔（Gill)算法求解微分方程,进行系统多点连续模拟和实时预报,用此法对1988年滇池BOD,COD时空变化进行连续预报,取得满意结果。     

Hydrological processes of a closed catchment‐lake system in a semi‐arid Mediterranean environment

Data collected in 4 years of field observations were used in conjunction with continuous simulation models to study, at the small‐basin scale, the water balance of a closed catchment‐lake system in a semi‐arid Mediterranean environment. The open water evaporation was computed with the Penman equation, using the data set collected in the middle of the lake. The surface runoff was partly measured at the main tributary and partly simulated using a distributed, catchment, hydrological model, calibrated with the observed discharge. The simplified structure of the developed modelling mainly concerns soil moisture dynamics and bedrock hydraulics, whereas the flow components are physically based. The calibration produced high efficiency coefficients and showed that surface runoff is greatly affected by soil water percolation into fractured bedrock. The bedrock reduces the storm‐flow peaks and the interflow and has important multi‐year effects on the annual runoff coefficients. The net subsurface outflow from the lake was calculated as the residual of the lake water balance. It was almost constant in the dry seasons and increased in the wet seasons, because of the moistening of the unsaturated soil. During the years of observation, rainfall 30% higher than average caused abundant runoff and a continuous rise in the lake water levels. The analysis allows to predict that, in years with lower than the average rainfall, runoff will be drastically reduced and will not be able to compensate for negative balance between precipitation and lake evaporation. Such highly unsteady situations, with great fluctuations in lake levels, are typical of closed catchment‐lake systems in the semi‐arid Mediterranean environment. Copyright © 2012 John Wiley & Sons, Ltd.     

A Modified Soil Moisture Model for Two‐Layer Soil

There can be marked variations in soil hydraulic properties in a soil vertical profile from the soil surface to the base of the root zone. Many existing two‐layer soil moisture (TLSM) models cannot well describe typical stratified soil profiles. A modified TLSM model is presented in this study. The modified model results and those from two existing models are compared with field observations. The modified TLSM model had the best agreement with the field observations. In both the surface layer and the root zone layer, the root mean square errors of soil moisture estimated by the modified model were smaller than those for the other models. The parameters in the modified TLSM model are relatively easy to determine. The modified TLSM model offers clear advantages over current TLSM models.     

Application of a spectral model to the calculation of wind drift currents in an idealized stratified sea

The solution of the hydrodynamic equations describing wind drift currents in an idealized stratified sea is developed using a modal expansion in the vertical. The influence of the magnitude of wind induced surface eddy viscosity, within the surface mixed layer, and reduced eddy viscosity at depth associated with a stable pycnocline, upon wind induced current structure is examined.Changes in modal structure with eddy viscosity, mixed layer depth, pycnocline thickness and total water depth are considered. It is evident from the modal equations that the time variation of the contribution of each mode to current structure is determined by the modal eigenvalue, mean eddy viscosity, water depth and the vertical integral of the mode squared.Calculations show that the magnitude and direction of steady state surface currents under strongly stratified conditions, are significantly influenced by mixed layer depth and pycnocline thickness. Surface eddy viscosity is also important, and in weakly stratified conditions eddy viscosity within and below the pycnocline influences surface current.The presence of a thin near surface ‘wall layer’ is shown to influence significantly the direction of surface current, but to have little effect upon currents below this surface layer.     

GEOSAT卫星的ERM测高数据处理与中国近海海平面

对ＧＥＯＳＡＴ测高卫星在中国近海区域（０°－３５°Ｎ,１０５°－１２７°Ｅ）以及１２７°－１３５°Ｅ内６个ＥＲＭ周期（１９８７年１月１日－４月１２日）的地球物理数据记录（ＧＤＲ）中的数据进行了编辑和预处理．根据卫星弧段的实际长度选取了混合轨道误差模型,并采用最小二乘技术对上升弧段与下降弧段交叠点处的不符值进行了平差计算．处理结果表明,所选用的方法可以大大地消除径向轨道误差的影响,使交叠点处的不符值由原来的５６ｃｍ（ＲＭＳ）降低到现在的２４ｃｍ（ＲＭＳ）在此基础上,构造出６个１°×１°的中国近海海平面及其平均海平面．该平面被称为＂测高大地水准面＂与美国Ｏｈｉｏ州立大学的ＯＳＵ９１Ａ重力场模型的大地水准面相比,两者具有同等量级的精度及一致的形态。     

Geostrophic currents in shelf areas

A method for obtaining geostrophic currents in a stratified ocean is discussed. It combines the use of hydrographic data and numerical modelling. For geostrophic flow in a flatbottom stratified ocean it is shown along the lines of Gill and Clarke (1974, Deep-Sea Research, 21, 325–345) that the surface elevation is determined by the barotropic mode. Experience indicates strongly that this also is the case when the depth varies. Hence the surface elevation, or equivalently the surface velocity, can be obtained from the depth-averaged equations.The variation with depth of the current is determined from hydrographic measurements, assuming that the baroclinic field is in geostrophic balance. When integrating in the vertical, the velocity at the sea surface obtained from the numerical computations is used as an absolute reference velocity.The method has been applied to the flow in the Faroe-Shetland Channel using meteorological data for the atmospheric forcing. The field data are from Foldvik andKvinge (1975), and were collected during ‘Overflow—1973’. The computations were performed for one particular wind event associated with an atmospheric low pressure passage. We find a net flux of 2.9 Sv of Atlantic water into the Norwegian Sea for the considered period of time. At the bottom of the channel there was a flow of 0.7 Sv of Norwegian Sea water into the Atlantic in the same period. On examining our calculated vertical velocity profiles in the channel, we find a level of zero motion which is not far from the position of the 35‰ isohaline. These results are discussed in connection with long-time average fluxes reported elsewhere in the literature.     

RCS analysis in the detection of a generic missile target with sea clutter background

The detection of a missile target in heavy sea clutter is a significantly challenging problem due to the clutter effects. In this paper, the radar cross sections (RCS) of a pre-assumed generic missile model is computed with multilevel fast multi-pole algorithm (MLFMA), while the RCS of ocean surface is computed by a more reduced form of the fractional Weierstrass scattering model proposed here. At last, the computed RCS of missile model is compared with that of sea surface, and then the comparisons of missile-to-ocean RCS ratios of different incident angles, incident frequencies, and polarization patterns are also presented. The discussion and comparisons of RCS of the missile and ocean surface can help us to plan and design a radar system in the application of detection of a missile target or other analogous weaker targets in the strong sea clutter background.     

A stable isotope study of the Garda lake, northern Italy: Its hydrological balance

Discontinuous measurements of the isotopic composition of surface water samples of the Garda lake carried out between 1998 and 2006 showed almost constant δ¹⁸O, δD and d-excess values through time. During 2006 and 2007 monthly vertical profiles of water samples were collected in the northernmost section of the lake, not far from the main inflow (Sarca river) to check whether there was any detectable influence from this inflowing river and whether there was a vertical isotopic stratification of the lake water. The isotopic measurement of water samples from the vertical profiles yielded isotopic values which were almost equal to those obtained from surface waters showing no detectable effect of the inflowing river water and no isotopic vertical stratification. The attempt to evaluate the evaporation rate of lake water by means of current models was totally unsuccessful. Despite the marked summer warming of the surface layer no isotopic fractionation related to evaporation processes could be detected. This anomalous behaviour may be related to the large amount of spring and summer precipitation characteristic of this area. The water balance of the lake calculated according to the amount of the inflowing water (Sarca river water plus rain water on the lake plus 20% of the precipitations on the whole catchment basin) and to the amount of outflowing water (Mincio river) showed a large imbalance, the river outflow alone resulting on average, during the last decade, at least double the inflow. To explain this imbalance of the lake, a large recharge by concealed groundwater is suggested: its isotopic composition should be quite close to the mean isotopic composition of precipitations over that area. This would be in agreement with the almost constant isotopic composition of both surface and deep waters and with the lack of vertical isotopic stratification. A few measurements of the tritium concentration carried out on lake water show values that are considerably higher than modern tritium values either in precipitation or in the Sarca river water: these results are in good agreement with the hypothesis of a recharge of the lake by deep aquifers.     

Air–sea interactions during strong winter extratropical storms

A high-resolution, regional coupled atmosphere–ocean model is used to investigate strong air–sea interactions during a rapidly developing extratropical cyclone (ETC) off the east coast of the USA. In this two-way coupled system, surface momentum and heat fluxes derived from the Weather Research and Forecasting model and sea surface temperature (SST) from the Regional Ocean Modeling System are exchanged via the Model Coupling Toolkit. Comparisons are made between the modeled and observed wind velocity, sea level pressure, 10 m air temperature, and sea surface temperature time series, as well as a comparison between the model and one glider transect. Vertical profiles of modeled air temperature and winds in the marine atmospheric boundary layer and temperature variations in the upper ocean during a 3-day storm period are examined at various cross-shelf transects along the eastern seaboard. It is found that the air–sea interactions near the Gulf Stream are important for generating and sustaining the ETC. In particular, locally enhanced winds over a warm sea (relative to the land temperature) induce large surface heat fluxes which cool the upper ocean by up to 2 °C, mainly during the cold air outbreak period after the storm passage. Detailed heat budget analyses show the ocean-to-atmosphere heat flux dominates the upper ocean heat content variations. Results clearly show that dynamic air–sea interactions affecting momentum and buoyancy flux exchanges in ETCs need to be resolved accurately in a coupled atmosphere–ocean modeling framework.     

Neotectonics and the Kos Plateau Tuff eruption of 161 ka,South Aegean arc

A high-resolution seismic-reflection survey of the area between Kos and Tilos islands is used to constrain the nature of the Kos Plateau Tuff (KPT) eruption and post-eruptive subsidence. A unique acoustically incoherent unit tens of metres thick at a subbottom depth of 15–35 m is recognised throughout the West Kos basin, which lies between Pachia, Yali and Kos. It commonly unconformably overlies folded, stratified sediment, but in places is concordant with stratified sediment more than 100 m thick. In places south of Kos, the acoustically incoherent unit is overlain by an unconformity and irregularly stratified sediment interpreted as terrestrial or shallow marine. Southeast of Nisyros, a correlative acoustically incoherent unit overlies a planar marine transgression erosion surface that extends almost to Tilos. The stratigraphic level of this unit is dated by comparison with the global eustatic sea-level record and the presence of major transgressive erosion surfaces on adjacent continental shelves, constrained by regional sedimentation rates, and indicates that it is of similar age to the Kos Plateau Tuff eruption. The relationship of this unit to coastal erosion surfaces, and its absence in many areas where seismic-reflection profiles show continuous marine sedimentation, suggests that it is a pyroclastic deposit of subaerial, or at most very shallow marine, origin from the Kos Plateau Tuff eruption. This presence of transgressive unconformities implies that a coastal plain or shallow sea extended southeast of Nisyros to Tilos and the Datça peninsula, and thus it is unlikely that pyroclastic flows crossed large stretches of deep water towards Tilos, as proposed by Allen and Cas (Allen, S.R., Cas, R.A.F., 2001. Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece. Bull. Volcanol. 62, 441-456). Late Pleistocene tectonic subsidence has taken place throughout the whole region between Kos and Tilos, in places at an average rate of at least 3 mm/a.The older dacites of Pyrgousa and Pachia and the post-KPT volcanics of Yali, Strongili and Nisyros lie on a NE–SW-trending lineament marked by major faults and abrupt changes in bathymetry. This NE–SW faulting was initiated in the early Pleistocene and parallels prominent lineaments in the east Cretan Sea. ENE–WSW sinistral strike-slip faulting initiated in the middle Pleistocene in the area from Santorini to Kos would have produced extension on this older lineament, playing a major role in channelling magma to the surface and permitting the ingress of water to the magma conduit.     

Simulation of tides in hydrocarbon lakes on Saturn’s moon Titan

Numerous hydrocarbon lakes have recently been detected on Saturn’s largest moon Titan, representing the only known large bodies of liquids on a planetary surface outside the Earth. In the context of comparative oceanography, tides and tidal currents in two representative lakes on Titan (Kraken Mare and Ontario Lacus) are simulated by a three-dimensional baroclinic ocean circulation model. Since the tide-generating force on Titan is an order of magnitude larger than on Earth and the gravitational acceleration is small, tides and currents are substantially larger than in Earth’s lakes and are more comparable with those in Earth’s oceans. The predicted maximum tidal range in Kraken Mare is 4 m. The tidal wave propagates around the basin of Kraken Mare, while a nearly standing tidal wave is excited in Ontario Lacus. Titan’s rotation is too slow to affect the tidal flow in any Titan’s lake. The tidal current velocity in Kraken Mare amounts to a few centimeters per second except in the vicinity of a narrow strait, where it is enhanced by an order of magnitude. In summer, when the lake is stratified, internal tides can develop. Seiches cannot be caused by tide. In the largest lakes, atmospheric tide may cause additional lake surface displacements.     

Level inversion in stratified lowland reservoirs and the intensification of currents within the water body

The effect of the formation of reverse slope of water level in stratified reservoirs and the appearance of current velocity peak within the water body are described. Velocity profiles with a peak within the flow have been recorded in experiments at relatively small vertical density gradients; therefore, the stratification alone is not enough to cause such effect. The wind as a cause of this effect is also ruled out. An important part in this problem is shown to belong to the effect of reservoir morphometry on the interrelated behavior of the free surface and the interfaces between layers. It is shown that, at a constant transit flow, the free surface in a stratified reservoir may be a nonmonotonic function decreasing in the general direction of transit flow and inversions in the level curve can appear, i.e., an inverse level course with an inverse slope of the free surface may form. Velocity profiles with an inflection point and a peak in underlying layers form in such cases.     

Modelling hydrodynamics in Yachiyo Lake using a non‐hydrostatic general circulation model with spatially and temporally varying meteorological conditions

In this study, a three‐dimensional (3D) non‐hydrostatic circulation model was applied to study the thermal structure, its evolution and water circulation of Yachiyo Lake in Hiroshima, Japan. The simulations were conducted for 1 month during July 2006. The meteorological forcing variables such as wind stress, surface atmospheric pressure and heat flux transfer through the lake surface were provided by an atmospheric mesoscale model run. The vertical mixing process of the lake was calculated using the Mellor‐Yamada turbulence model. The 1‐month numerical simulation revealed the wind‐induced currents of the lake, two gyres in the mid‐layer, and depth‐averaged monthly mean currents. Further numerical experiments studying the mechanism of the two gyres in the lake showed the important role of topography in gyre formation. The thermal structure of the lake and its evolution both in space and in time as predicted by the model showed very good agreement with the observed values and characteristics of Yachiyo Lake. The internal gravity waves, which are crucial for mixing in the stratified lake, are depicted by the vertical fluctuation of isotherms. Using the non‐dimensional gradient Richardson number, Yachiyo Lake was determined to be stable under strong stratification during the study period, and therefore very sensitive to wind stress. Copyright © 2009 John Wiley & Sons, Ltd.     

Study on the simulation of transparency of Lake Taihu under different hydrodynamic conditions

It was indicated in this study that there were negative relations between the concentrations of suspended solid (SS) and transparency according to the analysis of measured data of Lake Taihu. Their relations in pervious studies were reviewed, which showed that the changes of transparency in Lake Taihu could be reflected by simulating suspended solid concentration (SSC). Measured data showed that the changes of SSC with wind speed were similar at different water depths. SSC increased with the increasing of wind speed. Both wave and lake current of Lake Taihu had positive relations with SSC. However, wave was the main factor affecting sediment suspension, while flow took the second place. In this study, a numerical model coupling lake current, wave and SSC of Lake Taihu was developed. In the SS model, the combined effects of wave and current were included. The amounts of suspended and deposited sediments near the lake bed surface layer were treated separately. The stochastic characteristics of turbulent flow pulsation near lake beds were also considered, and the start-up conditions of sediment suspension were introduced to the model. The model elucidated the mutual exchange processes between sediment particles in SS and active sediments within and on the bed surface layer. Simulated results showed that lake current had relatively significant effects on the SSC at littoral areas of Lake Taihu, while SSC at the central area of the lake was mainly influenced by wave. The changes of transparency with SSC were simulated for Lake Taihu using this model. Calculated results were validated by measured data with good fitness, which indicated that the model is basically suitable for the simulation and prediction of transparency of Lake Taihu.     

Study on the simulation of transparency of Lake Taihu under different hydrodynamic conditions

It was indicated in this study that there were negative relations between the concentrations of suspended solid (SS) and transparency according to the analysis of measured data of Lake Taihu. Their relations in pervious studies were reviewed, which showed that the changes of transparency in Lake Taihu could be reflected by simulating suspended solid concentration (SSC). Measured data showed that the changes of SSC with wind speed were similar at different water depths. SSC increased with the increasing of wind speed. Both wave and lake current of Lake Taihu had positive relations with SSC. However, wave was the main factor affecting sediment suspension, while flow took the second place. In this study, a numerical model coupling lake current, wave and SSC of Lake Taihu was developed. In the SS model, the combined effects of wave and current were included. The amounts of suspended and deposited sediments near the lake bed surface layer were treated separately. The stochastic characteristics of turbulent flow pulsation near lake beds were also considered, and the start-up conditions of sediment suspension were introduced to the model. The model elucidated the mutual exchange processes between sediment particles in SS and active sediments within and on the bed surface layer. Simulated results showed that lake current had relatively significant effects on the SSC at littoral areas of Lake Taihu, while SSC at the central area of the lake was mainly influenced by wave. The changes of transparency with SSC were simulated for Lake Taihu using this model. Calculated results were validated by measured data with good fitness, which indicated that the model is basically suitable for the simulation and prediction of transparency of Lake Taihu.