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1.
近海岸区域平均海面高在大地测量学、物理海洋学以及地球物理学研究中具有非常重要的意义.受各种条件的制约和限制,目前卫星测高技术主要应用于深海区域,在近海区域尤其是海岸线附近区域的应用几乎为空白.本文根据ERS-1测高卫星回波波形特征,采用五参数线性模型,由最小二乘拟合方法,对近海区域尤其是靠近海岸线附近的ERS-1测高波形数据进行波形重构.比较波形重构前、后解算平均海面高,表明波形重构技术不仅明显改善了解算近海海面高的精度,而且增加了近海测高海平面的分辨率,并使卫星测高有效观测延伸至海岸线附近.随后,本文利用波形重构后海面高数据构造了近海多年平均海平面,并对我国近海海平面特征进行了初步分析. 相似文献
2.
A complex and highly dynamical ocean region, the Agulhas Current System plays an important role in the transfer of energy, nutrients and organic material from the Indian to the Atlantic Ocean. Its dynamics are not only important locally, but affect the global ocean-atmosphere system. In working towards improved ocean reanalysis and forecasting capabilities, it is important that numerical models simulate mesoscale variability accurately—especially given the scarcity of coherent observational platforms in the region. Data assimilation makes use of scarce observations, a dynamical model and their respective error statistics to estimate a new, improved model state that minimises the distance to the observations whilst preserving dynamical consistency. Qualitatively, it is unclear whether this minimisation directly translates to an improved representation of mesoscale dynamics. In this study, the impact of assimilating along-track sea-level anomaly (SLA) data into a regional Hybrid Coordinate Ocean Model (HYCOM) is investigated with regard to the simulation of mesoscale eddy characteristics. We use an eddy-tracking algorithm and compare the derived eddy characteristics of an assimilated (ASSIM) and an unassimilated (FREE) simulation experiment in HYCOM with gridded satellite altimetry-derived SLA data. Using an eddy tracking algorithm, we are able to quantitatively evaluate whether assimilation updates the model state estimate such that simulated mesoscale eddy characteristics are improved. Additionally, the analysis revealed limitations in the dynamical model and the data assimilation scheme, as well as artefacts introduced from the eddy tracking scheme. With some exceptions, ASSIM yields improvements over FREE in eddy density distribution and dynamics. Notably, it was found that FREE significantly underestimates the number of eddies south of Madagascar compared to gridded altimetry, with only slight improvements introduced through assimilation, highlighting the models’ limitation in sustaining mesoscale activity in this region. Interestingly, it was found that the threshold for the maximum eddy propagation velocity in the eddy detection scheme is often exceeded when data assimilation relocates an eddy, causing the algorithm to interpret the discontinuity as eddy genesis, which directly influences the eddy count, lifetime and propagation velocity, and indirectly influences other metrics such as non-linearity. Finally, the analysis allowed us to separate eddy kinetic energy into contributions from detected mesoscale eddies and meandering currents, revealing that the assimilation of SLA has a greater impact on mesoscale eddies than on meandering currents. 相似文献
3.
We report on numerical experiments to test the sensitivity of Lagrangian coherent structures (LCSs), found by identifying ridges of the finite-time Lyapunov exponent (FTLE), to errors in two systems representing the California Current System (CCS). First, we consider a synthetic mesoscale eddy field generated from Fourier filtering satellite altimetry observations of the CCS. Second, we consider the full observational satellite altimetry field in the same region. LCS are found to be relatively insensitive to both sparse spatial and temporal resolution and to the velocity field interpolation method. Strongly attracting and repelling LCS are robust to perturbations of the velocity field of over 20% of the maximum regional velocity. Contours of the Okubo–Weiss (OW) parameter are found to be consistent with LCS in large mature eddies in the unperturbed systems. The OW parameter is unable to identify eddies at the uncertainty level expected for altimetry observations of the CCS. At this expected error level, the FTLE method is reliable for locating boundaries of large eddies and strong jets. Small LCS features such as lobes are not well resolved even at low error levels, suggesting that reliable determination of lobe dynamics from altimetry will be problematic. 相似文献
4.
We have studied, for the first time, variations in absolute surface geostrophic currents (SGC) using satellite data only. The proposed approach combines 18 years’ altimetry data, which provide reliable measurements of absolute sea level (ASL), with a gravity field and steady-state ocean circulation explorer geoid model to obtain dynamic topography, and achieves unprecedented precision and accuracy. Our proposal overcomes the main limitations of existing approaches based solely on altimetry data (which suffer from lack of an independent reference for derivation of ASL maps), and approximations based on in-situ data (which are characterized by a sparse and inhomogeneous coverage in time and space). Features of annual variations of SGC are also addressed. As a result of our study we provide new absolute SGC climatology in the form of a 52-week data set of surface current fields, gridded at quarter degree longitude and latitude resolution and resolving spatial scales as short as 140 km. For presentation, this data set is averaged monthly and the results, presented as monthly climatology, are compared with climatology based on in-situ observations from drifter data. 相似文献
5.
Baptiste Mourre Pierre De Mey Yves Ménard Florent Lyard Christian Le Provost 《Ocean Dynamics》2006,56(5-6):473-486
We evaluate in this paper the ability of several altimeter systems, considered separately as well as together with tide gauges,
to control the time evolution of a barotropic model of the North Sea shelf. This evaluation is performed in the framework
of the particular model errors due to uncertainties in bathymetry. An Ensemble Kalman Filter (EnKF) data assimilation approach
is adopted, and observing-systems simulation experiments (OSSEs) are carried out using ensemble spread statistics. The skill
criterion for the comparison of observing networks is, therefore, not based on the misfit between two simulations, as done
in classic twin experiments, but on the reduction of ensemble variance occurring as a consequence of the assimilation. Future
altimeter systems, such as the Wide Swath Ocean Altimeter (WSOA) and satellite constellations, are considered in this work.
A single WSOA exhibits, for instance, similar performance as two-nadir satellites in terms of sea-level correction, and is
better than three satellites in terms of model velocity control. Generally speaking, the temporal resolution of observations
is shown to be of major importance for controlling the model error in these experiments. This result is clearly related to
the focus adopted in this study on the specific high-frequency response of the ocean to meteorological forcing. Altimeter
systems lack adequate temporal sampling for properly correcting the major part of model error in this context, whereas tide
gauges, which provide a much finer time resolution, lead to better global statistical performance. When looking into further
detail, tide gauges and altimetry are demonstrated to exhibit an interesting complementary character over the whole shelf,
as tide gauge networks make it possible to properly control model error in a ∼100-km coastal band, while high-resolution altimeter
systems are more efficient farther from the coast. 相似文献
6.
在流体静力平衡状态下,海洋Coriolis力和压力梯度平衡就形成地转流,世界上大多数海流都近似为地转流.本文利用卫星测高交叉点方法计算海洋表面地转流速度,分析了利用测高交叉点计算地转流速度的不确定性,上升和下降弧段的海面倾斜在分辨率50 km上可以达到10-7量级,才可能获得优于10 cm/s的地转流速度.在低纬度或者纬度接近卫星轨道倾角的地区,由交叉点方法计算的地转流速度精度低于中纬度地区.以中国台湾东部黑潮为试验区,利用最新的中国台湾周边海域大地水准面模型参考场计算高精度的大地水准面高,利用TOPEX/Poseidon和Jason-1的GDR数据(2002~2005年)计算海面高,然后计算交叉点的动力高,确定交叉点的地转流速度,结果与中国台湾NCOR(National Center for Ocean Research)的流速基本一致. 相似文献
7.
Paolo Cipollini Francisco M. Calafat Svetlana Jevrejeva Angelique Melet Pierre Prandi 《Surveys in Geophysics》2017,38(1):33-57
We examine the issue of sustained measurements of sea level in the coastal zone, first by summarizing the long-term observations from tide gauges, then showing how those are now complemented by improved satellite altimetry products in the coastal ocean. We present some of the progresses in coastal altimetry, both from dedicated reprocessing of the radar waveforms and from the development of improved corrections for the atmospheric effects. This trend towards better altimetric data at the coast comes also from technological innovations such as Ka-band altimetry and SAR altimetry, and we discuss the advantages deriving from the AltiKa Ka-band altimeter and the SIRAL altimeter on CryoSat-2 that can be operated in SAR mode. A case study along the UK coast demonstrates the good agreement between coastal altimetry and tide gauge observations, with root mean square differences as low as 4 cm at many stations, allowing the characterization of the annual cycle of sea level along the UK coasts. Finally, we examine the evolution of the sea level trend from the open to the coastal ocean along the western coast of Africa, comparing standard and coastally improved products. Different products give different sea level trend profiles, so the recommendation is that additional efforts are needed to study sea level trends in the coastal zone from past and present satellite altimeters. Further improvements are expected from more refined processing and screening of data, but in particular from the constant improvements in the geophysical corrections. 相似文献
8.
A global 1/4° resolution product of surface currents has been developed by the Centre de Topographie des Océans et de l’Hydrosphère.
The surface current is calculated from a combination of Ekman currents derived from wind estimates from QuikSCAT satellite,
geostrophic current anomalies derived from altimetry, and a mean geostrophic current derived from climatology. In the equatorial
band, the currents are adjusted following the methodology proposed by Lagerloef et al. (J Geophys Res, 104(C10):22313–22326,
1999). These satellite-derived currents have been compared to different types of in situ current observations. A global validation
is performed using Lagrangian surface drifting buoys and acoustic Doppler current profiler current observations along ship
tracks. The comparison shows a very good agreement in the subtropical and mid-latitude bands. The correlation between the
satellite-derived currents and the drifter currents in zonal mean bands is around 0.7 for most of the world oceans, both for
the zonal and the meridional components. This correlation rises up to 0.8 in the regions of strong boundary currents. In the
equatorial band, the correlation with the surface drifting buoys is reduced. A direct comparison with the TOGA/TAO moored
current meter data at the equator shows that the low frequency currents are captured by the satellite current product, but
there is a substantial high-frequency signal (<20 days), which is not reproduced. This is especially the case for the meridional
component and is mainly related to the tropical instability waves. We also show that using daily QuikSCAT wind forcing improves
the satellite current product, particularly in the high-latitude westerly wind belt and in the tropical Indian Ocean. 相似文献
9.
It is becoming well known that aliasing associated with ocean tides could be a major source of systematic error in altimeter sea-level measurements, due to asynoptic sampling and imperfect tide modelling. However, it has been shown that signals of non-tidal origin may also contribute significantly to the observed aliasing. In this paper, numerical simulations are performed to demonstrate the full aliasing potential associated with altimeter observations of seasonal sea-level variability and annual Rossby waves. Our results indicate that ignorance of non-tidal aliasing may lead to the possibility of underestimating the total aliasing and misinterpreting or overlooking existing geophysical phenomena. Therefore, it is argued that an entire aliasing picture should be kept in mind when satellite altimeter data are analysed. 相似文献
10.
Seafloor pressure records, collected at 11 stations aligned along a single ground track of the Topex/Poseidon and Jason satellites,
are analyzed for their tidal content. With very low background noise levels and approximately 27 months of high-quality records,
tidal constituents can be estimated with unusually high precision. This includes many high-frequency lines up through the
seventh-diurnal band. The station deployment provides a unique opportunity to compare with tides estimated from satellite
altimetry, point by point along the satellite track, in a region of moderately high mesoscale variability. That variability
can significantly corrupt altimeter-based tide estimates, even with 17 years of data. A method to improve the along-track
altimeter estimates by correcting the data for non-tidal variability is found to yield much better agreement with the bottom-pressure
data. The technique should prove useful in certain demanding applications, such as altimetric studies of internal tides. 相似文献
11.
The oceanic response to a typhoon, where mesoscale ocean circulations co-exist, was investigated by analyzing the independent
observations of profiling floats data at three different locations, satellite altimetry data near the eye of Typhoon Man-Yi
(2007) before and after its passage, and synthetic aperture radar data taken during the typhoon’s passage. In spite of the
nearly symmetric wind pattern around the eye, the distribution of mesoscale eddies had a major impact on the surface currents
and mixed layer (ML) depths. As a result, the entrainment of the water below the ML into the ML was affected by the mesoscale
circulation and became asymmetric, which accounted for most of the changes observed in the temperature profiles. Changes in
the isotherms were driven primarily by the westward propagation of the mesoscale pattern rather than by the typhoon-induced
shoaling. The typhoon-induced shoaling could have played a significant role in the generation of high-frequency (e.g., near-inertial)
oscillations and/or sub-mesoscale structures. Although a similar or even greater energy flux was observed at the surface,
the entrainment within the anticyclonic circulation was weaker than that within the cyclonic circulation and at the edge of
the anticyclonic circulation because of the thick pre-existing ML. A strong ocean response to Typhoon Man-Yi (2007) within
a cyclonic circulation or at the edge of an anticyclonic circulation, rather than within an anticyclonic eddy, has implications
for the role of mesoscale ocean circulations in better understanding and forecasting the typhoon intensity. 相似文献
12.
Toward a global ocean data assimilation system based on ensemble optimum interpolation: altimetry data assimilation experiment 总被引：3，自引：0，他引：3
A global ocean data assimilation system based on the ensemble optimum interpolation (EnOI) has been under development as the
Chinese contribution to the Global Ocean Data Assimilation Experiment. The system uses a global ocean general circulation
model, which is eddy permitting, developed by the Institute of Atmospheric Physics of the Chinese Academy of Sciences. In
this paper, the implementation of the system is described in detail. We describe the sampling strategy to generate the stationary
ensembles for EnOI. In addition, technical methods are introduced to deal with the requirement of massive memory space to
hold the stationary ensembles of the global ocean. The system can assimilate observations such as satellite altimetry, sea
surface temperature (SST), in situ temperature and salinity from Argo, XBT, Tropical Atmosphere Ocean (TAO), and other sources
in a straightforward way. As a first step, an assimilation experiment from 1997 to 2001 is carried out by assimilating the
sea level anomaly (SLA) data from TOPEX/Poseidon. We evaluate the performance of the system by comparing the results with
various types of observations. We find that SLA assimilation shows very positive impact on the modeled fields. The SST and
sea surface height fields are clearly improved in terms of both the standard deviation and the root mean square difference.
In addition, the assimilation produces some improvements in regions where mesoscale processes cannot be resolved with the
horizontal resolution of this model. Comparisons with TAO profiles in the Pacific show that the temperature and salinity fields
have been improved to varying degrees in the upper ocean. The biases with respect to the independent TAO profiles are reduced
with a maximum magnitude of about 0.25°C and 0.1 psu for the time-averaged temperature and salinity. The improvements on temperature
and salinity also lead to positive impact on the subsurface currents. The equatorial under current is enhanced in the Pacific
although it is still underestimated after the assimilation. 相似文献
13.
基于频域法,利用最新的GOCE卫星重力场模型和卫星测高数据计算了稳态海面动力地形.结合海洋表层漂流浮标的观测结果,对稳态海面动力地形进行了最优空间滤波尺度分析,给出了区域、纬度带和全球稳态海面动力地形的最优空间滤波尺度因子.在此基础上,给出了全球和区域地转流.结果表明:在中高纬度和全球区域,可以分别获得空间尺度优于102km和127km的稳态海面动力地形信息.与海洋表层漂流浮标对比可知,在强流区域,采用稳态海面动力地形得到的地转流速可以解释观测浮标流速的70%;在中高纬度区域,由GOCE重力场得到的地转流略优于对应的GRACE结果;在近赤道区域,由GOCE重力场得到的地转流精度略低于对应的GRACE结果;在北大西洋和阿古拉斯强流区域,由GOCE得到的地转流场明显优于对应的GRACE结果,其精度分别提高了16%和24%. 相似文献
14.
Benchmarking the mesoscale variability in global ocean eddy-permitting numerical systems 总被引：1，自引：1，他引：0
Andrea Cipollone Simona Masina Andrea Storto Doroteaciro Iovino 《Ocean Dynamics》2017,67(10):1313-1333
The role of data assimilation procedures on representing ocean mesoscale variability is assessed by applying eddy statistics to a state-of-the-art global ocean reanalysis (C-GLORS), a free global ocean simulation (performed with the NEMO system) and an observation-based dataset (ARMOR3D) used as an independent benchmark. Numerical results are computed on a 1/4 ° horizontal grid (ORCA025) and share the same resolution with ARMOR3D dataset. This “eddy-permitting” resolution is sufficient to allow ocean eddies to form. Further to assessing the eddy statistics from three different datasets, a global three-dimensional eddy detection system is implemented in order to bypass the need of regional-dependent definition of thresholds, typical of commonly adopted eddy detection algorithms. It thus provides full three-dimensional eddy statistics segmenting vertical profiles from local rotational velocities. This criterion is crucial for discerning real eddies from transient surface noise that inevitably affects any two-dimensional algorithm. Data assimilation enhances and corrects mesoscale variability on a wide range of features that cannot be well reproduced otherwise. The free simulation fairly reproduces eddies emerging from western boundary currents and deep baroclinic instabilities, while underestimates shallower vortexes that populate the full basin. The ocean reanalysis recovers most of the missing turbulence, shown by satellite products , that is not generated by the model itself and consistently projects surface variability deep into the water column. The comparison with the statistically reconstructed vertical profiles from ARMOR3D show that ocean data assimilation is able to embed variability into the model dynamics, constraining eddies with in situ and altimetry observation and generating them consistently with local environment. 相似文献
15.
Cyclogenesis and long-fetched winds along the southeastern coast of South America may lead to floods in populated areas, as the Buenos Aires Province, with important economic and social impacts. A numerical model (SMARA) has already been implemented in the region to forecast storm surges. The propagation time of the surge in such extensive and shallow area allows the detection of anomalies based on observations from several hours up to the order of a day prior to the event. Here, we investigate the impact and potential benefit of storm surge level data assimilation into the SMARA model, with the objective of improving the forecast. In the experiments, the surface wind stress from an ensemble prediction system drives a storm surge model ensemble, based on the operational 2-D depth-averaged SMARA model. A 4-D Local Ensemble Transform Kalman Filter (4D-LETKF) initializes the ensemble in a 6-h cycle, assimilating the very few tide gauge observations available along the northern coast and satellite altimeter data. The sparse coverage of the altimeters is a challenge to data assimilation; however, the 4D-LETKF evolving covariance of the ensemble perturbations provides realistic cross-track analysis increments. Improvements on the forecast ensemble mean show the potential of an effective use of the sparse satellite altimeter and tidal gauges observations in the data assimilation prototype. Furthermore, the effects of the localization scale and of the observational errors of coastal altimetry and tidal gauges in the data assimilation approach are assessed. 相似文献
16.
本文研究了基于泊松小波径向基函数融合多代卫星测高及多源重力数据精化大地水准面模型的方法.分别以沿轨垂线偏差和大地水准面高高差作为卫星测高观测量,研究了使用不同类型测高数据对于大地水准面建模精度的影响.针对全球潮汐模型在浅水区域及部分开阔海域精度较低的问题,引入局部潮汐模型研究了不同潮汐模型对于大地水准面的影响.数值分析表明:相比于使用沿轨垂线偏差作为测高观测量,基于沿轨大地水准面高高差解算得到的大地水准面模型的精度更高,特别是在海域区域,其精度提高了2.3cm.由于使用沿轨大地水准面高高差作为测高观测量削弱了潮汐模型长波误差的影响,采用不同潮汐模型对大地水准面解算的影响较小.总体而言,船载重力及测高观测数据在海洋重力场的确定中呈现互补性关系,联合两类重力场观测量可以提高局部重力场的建模精度. 相似文献
17.
18.
A method for splitting sea surface height measurements from satellite altimetry into geoid undulations and sea surface topography is presented. The method is based on a combination of the information from altimeter data and a dynamic sea surface height model. The model consists of geoid undulations and a quasi-geostrophic model for expressing the sea surface topography. The goal is the estimation of those values of the parameters of the sea surface height model that provide a least-squares fit of the model to the data. The solution is accomplished by the adjoint method which makes use of the adjoint model for computing the gradient of the cost function of the least-squares adjustment and an optimization algorithm for obtaining improved parameters. The estimation is applied to the North Atlantic. ERS-1 altimeter data of the year 1993 are used. The resulting geoid agrees well with the geoid of the EGM96 gravity model. 相似文献
19.
Besides providing an estimate of the changing ocean state, an important result of the dynamically consistent estimating the circulation and climate of the ocean (ECCO) state estimate approach is the provision of a posterior model–data residuals which contain important information about elements in the assimilated observations that are inconsistent with the model dynamics or with the information present in other ocean data sets that are being used as constraints in the assimilation procedure. Based on decreased GECCO2 model–data residuals, upon using the altimeter data through the ESA climate change initiative (cci) sea-level (SL) project, we show here that the recently reprocessed ESA SL_cci altimeter data set (SL1) has been improved relative to the earlier AVISO altimetry data set and is now more consistent with the GECCO2 estimate and with the information about the changing ocean state embedded in other ocean data sets. The improvement can be shown to exist separately for both TOPEX/POSEIDON and ERS data sets. The study reveals that especially in regions characterized by small sea surface height (SSH) variability and small signal-to-noise ratio in the SSH data, improvements can be on the order of 30% of previously existing model–data residuals. However, in some regions we can find degradations, particulary in those where GECCO2 has little skill in representing the altimeter data and where evaluation of the products with GECCO2 is thus not advisable. Upon the assimilation of the new SL1 data set, the GECCO2 synthesis was further improved. However, adding the sea surface temperature (SST) from the SST_cci project as additional constrain, no further impact can be identified. 相似文献
20.
HSU HouTse 《中国科学D辑(英文版)》2007,50(6):918-926
According to the features of spatial spectrum of the dynamic ocean topography (DOT),wavelet filter is proposed to reduce short-wavelength and noise signals in DOT. The surface geostrophic currents calculated from the DOT models filtered by wavelet filter in global and Kuroshio regions show more detailed information than those from the DOT models filtered by Gaussian filter. Based on a satellite gravity field model (CG01C) and a gravity field model (EGM96),combining an altimetry-derived mean sea surface height model (KMSS04),two mean DOT models are estimated. The short-wavelength and noise signals of these two DOT models are removed by using wavelet filter,and the DOT models asso-ciated global mean surface geostrophic current fields are calculated separately. Comparison of the surface geostrophic currents from CG01C and EGM96 model in global,Kuroshio and equatorial Pacific regions with that from oceanography,and comparison of influences of the two gravity models errors on the precision of the surface geostrophic currents velocity show that the accuracy of CG01C model has been greatly improved over pre-existing models at long wavelengths. At large and middle scale,the surface geostrophic current from satellite gravity and satellite altimetry agrees well with that from oceanography,which indicates that ocean currents detected by satellite measurement have reached relatively high precision. 相似文献