Influence of non-linear effects upon surge elevations along the west coast of Britain

A two-dimensional vertically integrated hydrodynamic finite-element model of the west coast of Britain is used to examine the response of the region to extreme meteorological forcing. The extent to which tide–surge interaction modifies the computed surge elevation and current distributions is examined in detail. The nature of the finite-element model with its ability to refine the mesh in nearshore regions is ideal for examining the influence of non-linear effects upon surges in these regions. Calculations using spatially uniform orthogonal wind stresses show that the surge elevation and current in shallow water are particularly sensitive to the method used to remove the tide as a result of the highly non-linear nature of the tide–surge interaction in these regions. The most accurate means of de-tiding the solution is by subtracting a tide derived by harmonic analysis of the tide and surge time series at the time of the surge. Subtracting a tide-only solution (the usual approach) leads to tidal energy leaking into the surge solution. Calculations show that this arises because the surge modifies the tidal amplitude and phase in shallow-water regions to such an extent that they are appreciably different to those found in the tide-only calculation. Results suggest that this problem becomes more important, as nearshore meshes are refined in an attempt to improve surge prediction. This suggests that in the future, highly accurate fine-mesh models will be required to compute total water levels without the present linear separation into tidal and surge signal used in operational surge prediction.     

Influence of the modified global ocean tide model with local tides of East and South China Seas on load gravity in China and its neighbor area

Introduction With the development of science and technology, the accuracy of gravity measurement is im-proved. The gravity observation with FG5 gravimeter has reached accuracy of μ magnitude. And the high accurate superconducting gravimeter can detect the tiny signal of 10?2 μ magnitude in frequency domain. With the high-accuracy gravity observation on Earth′s surface, the Earth′s tidal parameters can be determined precisely. And the observations can also be used to invert the struc-ture…     

A numerical experiment on the general circulation of the middle atmosphere with a three-dimensional model explicitly representing internal gravity waves and their breaking

The general circulation of the middle atmosphere is simulated by means of a three-dimensional primitive equation model which covers from the south pole to the north pole but is limited to a ten-degree sector in the latitudinal direction; cyclic conditions are imposed at the east—west lateral boundaries. The model is capable of explicitly representing internal gravity waves of zonal wavelength greater than a few hundred kilometers with the use of a one-degree mesh, but planetary-scale waves were excluded. No parameterization is employed for subgrid-scale eddy viscosity (or diffusivity).With the assumption of a simple external-heating function corresponding to solstice conditions, a time integration was performed for about thirty days from the motionless state. During the whole period, random forcings were imposed on each grid of the lowest level in order to generate small-scale upwardly propagating internal gravity waves.The experiment has shown that small-scale waves were indeed excited, propagated upward, broke up near the mesopause, and greatly changed the thermally induced zonal mean motion and temperature fields in the upper mesosphere and lower thermosphere. As a result, important features of the general circulation at those levels, such as reversals of the zonal motion and the latitudinal gradient of zonal mean temperature were reproduced.     

Monitoring red tide with satellite imagery and numerical models: A case study in the Arabian Gulf

A red tide event that occurred in August 2008 in the Arabian Gulf was monitored and assessed using satellite observations and numerical models. Satellite observations revealed the bloom extent and evolution from August 2008 to August 2009. Flow patterns of the bloom patch were confirmed by results from a HYCOM model. HYCOM data and satellite-derived sea surface temperature data further suggested that the bloom could have been initiated offshore and advected onshore by bottom Ekman layer. Analysis indicated that nutrient sources supporting the bloom included upwelling, Trichodesmium, and dust deposition while other potential sources of nutrient supply should also be considered. In order to monitor and detect red tide effectively and provide insights into its initiation and maintenance mechanisms, the integration of multiple platforms is required. The case study presented here demonstrated the benefit of combing satellite observations and numerical models for studying red tide outbreaks and dynamics.     

A three-dimensional model study of near-inertial motion: generation and influence of an along-shelf flow

A three-dimensional baroclinic model of the Balearic Sea region is used to examine the processes influencing the distribution of near-inertial currents and waves in the region. Motion is induced by a spatially uniform wind impulse. By using a uniform wind, Ekman pumping due to spatial variability in the wind is removed with the associated generation of internal waves. However, internal waves can still be produced where stratification intersects topography. The generation and propagation of these waves, together with the spatial distribution of wind-forced inertial oscillations, are examined in detail. Diagnostic calculations show that in the near-coastal region inertial oscillations are inhibited by the coastal boundary. Away from this boundary the magnitude of the inertial oscillations increases, with currents showing a 180° phase shift in the vertical. The inclusion of an along-shelf flow modifies the inertial currents due to non-linear interaction between vorticity in the flow and the inertial oscillations. Prognostic calculations show that besides inertial oscillations internal waves are generated. In a linear model the addition of an along-shelf flow produces a slight reduction in the energy at the near-inertial frequency due to enhanced viscosity associated with the flow and changes in density field. The inclusion of non-linear effects modifies the currents due to inertial oscillations in a manner similar to that found in the diagnostic model. A change in the effective inertial frequency also influences the propagation of the internal waves. However, this does not appear to be the main reason for the enhanced damping of inertial energy, which is due to the along-shelf advection of water of a different density into a region and increased viscosity and mixing associated with the along-shelf flow.Responsible Editor: Phil Dyke     

Theoretic analysis on phase delay phenomena of well water level tide in the double medium aquifer model

Introduction The phenomenon of water level tide was discovered at Duchort diggings, Czech in 1879. By 1939, Theis, an America hydrogeologist, confirmed that periodical wave of the well water level is caused by the solid tide. In 1964, Melchior, a Belgium geophysist, began to make research on this phenomenon. Then Cooper (1965), Bredehoeft (1967) and WANG, et al (1988) followed. In China the study on water level tide began with 1970s, and the study on well water level phase lagging began …     

西沙群岛西北深水海域的内潮特征分析

基于海南岛至西沙群岛之间深水海域一长达5年的锚定潜标测流资料,采用谱分析、调和分析和动力模态分解等方法主要分析了局部海域内潮的基本特征.结果表明:研究海域的正压和斜压潮均以全日振荡为主,500~900 m的海洋中层全日等密度线垂向振幅可达40 m;全日内潮主要沿垂直于陆坡方向传播,与天文潮锁相的全日内潮可占全日内潮总能量的41%;海洋上层,O1内潮垂向平均振幅与局地海面高度呈显著正相关关系,K1内潮则表现为夏、冬季增强的半年循环特征;超过70%的O1分潮能量集中于第一、二斜压模态上,K1分潮在第三斜压模态上亦有相当能量.     

A three-dimensional model of ionospheric F-region

The three-dimensional stationary in solar-terrestrial system numerical model of the global ionosphere at F-region altitudes is developed. The input parameters are: the structural parameters of empirical thermospheric models (temperature, composition), electric fields, solar UV-radiation spectrum, corpuscular flows at high latitudes. The model includes the calculations of thermospheric circulation, electron density, electron and ion temperatures. The model reproduces the main morphological peculiarities of the distribution of thermospheric circulation and ionospheric plasma parameters. A comparison analysis of the results of the model calculation corresponding to different thermospheric models (DTM, MSIS, Jacchia-77, MSIS-83) is carried out. It is shown, that thermospheric circulation systems are the effective indicator of faithfulness of thermospheric models.     

A simulation model of blowout development

Blowouts are the main feature of wind erosion in the dunes along the Dutch coast. A number of characteristics, such as their growth against the prevailing wind and the spontaneous stabilization when certain dimensions are reached, are believed to be caused by a prominent quality of the southwest wind, viz. its gustiness. This hypothesis is tested in a two-dimensional computer model which simulates the effect of wind gusts on a sandy surface along the main axis of the blowout. A number of constraints based on field observations are built into the model. The results closely resemble the longitudinal section of the blowout.     

武汉超导重力仪观测最新结果和海潮模型研究

利用武汉台站GWR_C032超导重力仪观测资料,在对原始数据进行有效预处理的基础上作调和分析,获得反映地球内部介质特征的重力潮汐参数.基于卫星测高技术和有限元方法同时考虑验潮站数据作约束条件获得的多个全球海潮模型,利用负荷理论和数值褶积积分技术计算了重力负荷,对周日和半日频段内的重力潮汐参数实施负荷改正,提出了“负荷改正有效性”概念,研究了全球海潮模型适应性.数值结果说明,海潮改正的有效性高达91%（O1,NAO99）和92%（M2,ORI96）.基于11个海潮模型对主波（O1,K1,M2和S2）的负荷改正说明平均有效性为（86%,70%,73%和84%）,振幅因子与理论模型间的差异分别从（212%,155%,116%和080%）降到（031%,039%,034%和008%）,同时还说明利用NAO99和ORI96全球海潮模型能获得比其他模型更佳的负荷改正效果.文章还利用国际地球动力学计划网络其他7个台站的超导重力仪观测研究了全球海潮模型的适定性问题,结果说明不同模型中不同潮波具有明显的区域特点,早期构制的SCW80全球海潮模型仍可作为大地测量研究中的重要参考模型.     

On the sensitivity of computed higher tidal harmonics to mesh size in a finite element model

A finite element model of the Irish and Celtic Sea regions with a range of grid resolutions is used to examine the influence of resolution upon the higher harmonics of the tide in the region. Comparisons are also made with published results from finite difference models of the area, and observations. Calculations using fine near-shore elements with non-zero water depths in coastal regions were found to be more accurate and less time consuming than those using a zero coastal water depth. A detailed examination of the spatial variability of the higher harmonics in near-shore regions of the eastern Irish Sea particularly the Solway and Morecambe Bay showed significant small-scale variability. This together with the variation in higher harmonics in the eastern Irish Sea and adjacent estuaries, clearly shows the need for an unstructured grid model of the region that can include the estuaries. To match the high resolution of the model in near-shore regions accurate high-resolution topography is required.     

A field experiment on the role of small scale wind gustiness in aeolian sand transport

Estimates of aeolian sand transport generally use an average wind speed to assess the wind energy. To determine if variation in small-scale wind gustiness (on the order of several seconds duration) has a significant effect on sand movement, detailed field measurements were made of wind speed and transport rate. Average wind speed and several gust parameters were derived from the wind data. The results indicate that small-scale gust information does not improve prediction of transport rates.     

On the effects of wind and tides on the hydrodynamics of a shallow mediterranean estuary

A study is made of the effect of wind and tides on the hydrodynamics of the shallow inner basins of mediterranean estuaries. The paper includes a case study of Harvey Estuary in southwestern Australia where salinity and temperature data exist for 11 years during the 1980s and 1990s when that estuary experienced massive annual blue-green algal blooms. An analysis is made of salt exchange through the channels that join estuarine basins of this class to either the ocean or, as in the case of Harvey Estuary, to another shallow estuarine basin. A detailed three-dimensional numerical model is also implemented for the basin of Harvey Estuary. It is concluded that exchange through the channel is dominated by the (mainly diurnal) tides, despite the general micro-tidal nature of this class of estuary, although the efficiency of this process is found to be controlled by the length of the channel. Wind set-up in the basin also produces channel exchange and for Harvey Estuary this is about 20% of the exchange due to tides. Baroclinic flow through the channel is also capable of producing significant exchange but this is suppressed by the tidal currents in the channel except immediately after riverflow. Salt transport along the basins of this class of estuary is mainly driven by the longitudinal density gradient and the strength of this process is controlled by vertical mixing from the wind. However, there is also significant salt transport from wind-induced advection, the effect of which changes seasonally with the direction of the salt gradient.     

风的垂直切变对滞弹性大气中静力适应过程的影响

本文基于描写滞弹性大气静力适应过程的线性方程组,从波动频散关系、气团运动规律和能量转换的角度出发,研究了水平基流及其垂直切变对该模式大气静力适应过程的影响.构造四种水平基流垂直分布模型进行比较,分别为常数型、线性切变型、反气旋切变型和气旋切变型,得到结论：（1）具有重力波性质的波动是滞弹性大气静力适应过程中扰动能量传播的方式,当垂直折射指数大于零时,基本气流及其垂直切变的存在,不仅改变了波动频率的大小,而且改变波动传播的方向;（2）在静力适应过程中气块的运动轨迹呈椭圆形,水平基流及其垂直切变影响椭圆的扁率,同时也影响扰动物理量之间的偏振关系;（3）水平基流的垂直切变是扰动能量和水平基流能量发生转换的媒介,当存在垂直向上的动量输送时,正的垂直风切变对应扰动能量的衰减,水平基流能量的增加,负的垂直风切变对应扰动能量的增加,水平基流能量的衰减;（4）不同的风的垂直切变模型,对静力适应过程的影响不同;对于水平基流呈反气旋切变型和气旋切变型,扰动发展的波动垂直结构为,急流轴上方波动等相位线自下而上向西倾斜,急流轴下方波动等相位线自下而上向东倾斜,反之亦然.

     

Orographic effects on the southwest monsoon: A review

An overview of the problem of orographic effects on the southwest monsoon using the contributions of all the available analytical and numerical models is attempted. A quasi-geostrophic model is applied to deduce the effect of the topographic complex on the Indian peninsula. This model suggests that the southward bending of the low-level isobars on the peninsula can be ascribed to the topographically-induced southward velocity. This southward velocity triggers a Rossby wave to the east of the peninsula which is manifested as a trough on the southern Bay of Bengal.     

A stochastic model for initial movement of sand grains by wind

A stochastic model for entrainment of sand grains by wind is presented through analysis of the forces exerted on a single spherical grain, coupled with fluctuations of wind velocity and the change in grain position on the surface. The structure of the stochastic model is consistent with experimental data in the literature. The probability of initial motion increases first, and then decreases, with grain size. It reaches a maximum at diameters of about 0·9 mm. Some sand grains are still in motion at less than the conventional threshold velocity, even at very low velocities. The probability of sand grain movement reaches unity at twice the conventional threshold velocity. Considerable discrepancies amongst conventional threshold formulae may result from the different probabilities of initial movement implied in these formulae. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of a littoral Baltic Sea resuspension model in a eutrophic lake-factors behind differences in the model performance

The performance of a linear resuspension model developed in the Baltic Sea was studied in the conditions of a eutrophic Lake Kirkkoj(a|¨)rvi(southern Finland).The model predicts sediment resuspension rate using data on vegetation cover,wind and sediment quality as an input.When the original model coefficients were used,the model resulted on average 1.8 fold overestimation of the resuspension rate in Kirkkoj(a|¨)rvi.This was due to lower fetch and water depth,and less consolidated sediment of Kirkkoj(a|¨)rvi compared with the Baltic Sea study site.When coefficients were adjusted for Kirkkoj(a|¨)rvi,the model predictions were 1.1 times the measured values.Due to the continuous resuspension,the effect of the wind term in the model was so low that it could be excluded without affecting the accuracy of model predictions.The study demonstrated that in a shallow eutrophic lake accurate predictions on resuspension rate can be made using only data on sediment quality and on factors inhibiting resuspension(macrophytes).The model residuals increased with increasing resuspension rate and high rates of resuspension were underestimated by the model.Due to the fluffy sediment in Kirkkoj(a|¨)rvi,erosion of sediment increases more than linear with increasing shear stress.Thus in such conditions,even better predictions could be achieved by a non-linear resuspension model.     

The effects of wind and rainfall on suspended sediment concentration related to the 2004 Indian Ocean tsunami

The effects of rainfall and wind speed on the dynamics of suspended sediment concentration (SSC), during the 2004 Indian Ocean tsunami, were analyzed using spatial statistical models. The results showed a positive effect of wind speed on SSC, and inconsistent effects (positive and negative) of rainfall on SSC. The effects of wind speed and rainfall on SSC weakened immediately around the tsunami, indicating tsunami-caused floods and earthquake-induced shaking may have suddenly disturbed the ocean–atmosphere interaction processes, and thus weakened the effects of wind speed and rainfall on SSC. Wind speed and rainfall increased markedly, and reached their maximum values immediately after the tsunami week. Rainfall at this particular week exceeded twice the average for the same period over the previous 4 years. The tsunami-affected air–sea interactions may have increased both wind speed and rainfall immediately after the tsunami week, which directly lead to the variations in SSC.     

A bi-directional gap model for simulating the directional thermal radiance of row crops

Row crops are a kind of typical vegetation canopy between discrete canopy and continuous canopy. Kimes et al. studied the directional thermal radiation of row crops using the geometrical optical model, which simplified row structure as “box” and neglected the gap among foliage and did not consider the emissivity effects. In this work we take account of the gaps along illumination and viewing directions and propose a bi-direction gap model on the basis of the idea of gap probability of discrete vegetation canopy introduced by “Li-Strahler” and inter-correlation of continuous vegetation developed by Kuusk. It can be used to explain “hot spot” effects in thermal infrared region. The gap model has been validated by field experiment on winter wheat planted in shape of rows and results show that the gap model is better than Kimes’ model in describing the directionality of thermal infrared emission for row crops.     

北京城市化进程对边界层风场结构影响的研究

随着近10年来北京城市化步伐的加快,城市规模迅速扩大,北京三环以外的地区已由20世纪七、八十年代的城郊非均匀下垫面发展成现在粗糙复杂的城区下垫面,市区建筑物相应增多、增密和增高,导致城区地面动力学粗糙度明显增大.本文统计分析了北京325 m气象塔1994年和1997～2003年夏季平均场观测资料,结果表明:(1)在受下垫面影响最为剧烈的近地层,风向逐年趋于紊乱,现在气象塔周围近地面的流场已经具备了典型城市粗糙下垫面的流场特征;(2)近地面夏季平均风速呈现非常明显的逐年递减的趋势,而且距离地表越近,平均风速逐年递减的趋势也越为显著,这种风速逐年递减的趋势直到63 m左右才不明显,说明现在气象塔47 m以下的观测资料反映的是城市冠层的流场特征,城市冠层厚度约在47～63 m之间;(3)随着北京城市化的发展,城区近地面的平均风速存在逐年递减的趋势,但阵风并不存在相似的递减趋势,表明城市冠层的抬升对阵风的影响并不显著.