三种垂向混合方案对HYCOM模拟北太平洋西边界流系的影响研究

基于HYCOM(Hybrid Coordinate Oceanic Circulation Model),以OFES(OGCM for the Earth Simulator)资料为参考,分析了KPP、MY2.5、KT三种不同混合方案对北太平洋西边界流系的模拟结果的影响。结果表明:三种不同混合方案模拟的上层海洋平均流场与OFES资料相似,但在流向和流幅上略有差异,其中KPP方案模拟的流速与OFES资料最为接近,MY2.5方案次之,KT方案与其差别最大。通过代表性断面上流速的对比分析,对模式就北赤道流、棉兰老流、棉兰老潜流、黑潮的模拟效果进行比较,KPP方案模拟的效果同前人的观测和研究最为接近。分析了北赤道流,棉兰老流,棉兰老潜流,黑潮的流量的季节变化特征,其中KPP方案与OFES资料计算的棉兰老流和棉兰老潜流的季节变化特征与前人描述比较一致,表现为春强秋弱。KPP方案和OFES资料的计算结果表明,北赤道流和棉兰老流大致上是同向变化的,而在冬季棉兰老流同黑潮的变化基本上是一致的。     

Numerical simulations of the Mediterranean sea outflow: impact of the entrainment parameterization in an isopycnic coordinate ocean model

Gravity current entrainment is essential in determining the properties of the interior ocean water masses that result from marginal sea overflows. Although the individual entraining billows will be unresolvable in large-scale ocean models for the foreseeable future, some large-scale simulations are now being carried out that do resolve the intermediate scale environment which may control the rate of entrainment. Hallberg [Mon. Wea. Rev. 128 (2000) 1402] has recently developed an implicit diapycnal mixing scheme for isopycnic coordinate ocean models that includes the Richardson number dependent entrainment parameterization of Turner [J. Fluid Mech. 173 (1986) 431], and which may be capable of representing the gravity current evolution in large-scale ocean models. The present work uses realistic regional simulations with the Miami Isopycnic Coordinate Ocean Model (MICOM) to evaluate ability of this scheme to simulate the entrainment that is observed to occur in the bottom boundary currents downstream of the Mediterranean outflow. These simulations are strikingly similar to the observations, indicating that this scheme does produce realistic mixing between the Mediterranean outflow and the North Atlantic Central Water. Sensitivity studies identify the critical Richardson number below which vigorous entrainment occurs as a particularly important parameter. Some of these experiments also show meddies detaching from the Mediterranean undercurrent at locations that appear to be highly influenced by topographic features.     

Product water mass formation by turbulent density currents from a high-order nonhydrostatic spectral element model

In light of the pressing need for development of parameterizations of gravity current entrainment in ocean general circulation models, the behavior of turbulent gravity currents in the presence of ambient stratification is studied via numerical simulation, for cases in which equilibrated product water masses are formed. The main objective is to explore how the ambient stratification impacts entrainment and the properties of product water masses, which are of ultimate interest to ocean and climate modelers. Numerical experiments are conducted by employing the high-order nonhydrostatic spectral element model Nek5000. It is investigated how the separation level of the current from the bottom, the propagation speed, the salinity of the product water masses, and the transport and entrainment of the gravity current are affected as a function of the strength of the ambient stratification and the slope angle. Results show that, for the case of constant slope angle and linear ambient stratification, the gravity current separates from the bottom such that the entrained mass flux is independent of the slope angle. The entrainment mass transport, product mass transport, and product salinity then depend only on the ambient stratification, and these quantities are approximated as simple algebraic functions of the ambient stratification parameter that modify the source properties.     

垂向混合参数化方案对模拟黄海夏季上层温度结构的影响研究

采用POMgcs(Princeton Ocean Model with generalized coordinate system)和MITgcm(MIT General Circulation Model)两个海洋数值模式,研究了M-Y2.0、基于固壁近似假定的M-Y2.5、基于波浪破碎作用的M-Y2.5和KPP 4种垂向混合参数化方案对模拟黄海夏季上层温度结构的影响。结果表明,M-Y2.0和基于固壁近似假定的M-Y2.5方案低估了黄海上层的湍动能,模拟的黄海夏季温度上混合层的效果与实测相比均偏浅,不能够很好地重构黄海夏季温度的垂直结构。而基于波浪破碎作用的M-Y2.5和KPP方案均可以增加海洋上层湍动能的输入量,模拟的黄海夏季温度上混合层的效果与实测较为一致。故推测黄海夏季的上层结构是受波浪混合和流场剪切等物理机制共同调节的,若通过合理的垂向混合参数化方案将这些物理机制的作用加以体现,将会较真实地模拟和重构出黄海夏季海温上层结构。     

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.     

海气耦合模式中挟卷参数化的比较

利用美国国家气象中心的综合性大气-海洋资料序列(COADs),采用三种海气耦合模式对海洋挟卷速度的表达方法,计算了实测资料影响下模式挟卷速度的分布特征。利用求得的挟卷速度和由资料序列求得的热量通量分布,采用Niiler—Kraus一维混合层SST预报模式,估计了各种模式扶卷作用对SST的影响。通过比较各个模式的计算结果,提出: 1.挟卷作用是影响SST变化分布的一个重要因子。挟卷速度的分市取决于所采用参数化形式中主要影响因素的分布情况。不考虑平流作用时,模式SST的分布也取决于挟卷参数化形式中主要影响因素的分布。2.由海气耦合不稳定模态的分析得出三个模式对不稳定海气耦合波影响的特征。     

A hybrid coordinate ocean model for shelf sea simulation

The general circulation in the North Sea and Skagerrak is simulated using the hybrid coordinate ocean model (HYCOM). Although HYCOM was originally developed for simulations of the open ocean, it has a design which should make it applicable also for coastal and shallow shelf seas. Thus, the objective of this study has been to examine the skills of the present version of HYCOM in a coastal shelf application, and to identify the areas where HYCOM needs to be further developed. To demonstrate the capability of the vertical coordinate in HYCOM, three experiments with different configurations of the vertical coordinate were carried out. In general, the results from these experiments compares quite well with in situ and satellite data, and the water masses and the general circulation in the North Sea and Skagerrak is reproduced in the simulations. Differences between the three experiments are small compared to other errors, which are related to a combined effect of model setup and properties of the vertical mixing scheme. Hence, it is difficult to quantify which vertical coordinate configuration works best for the coastal region. It is concluded that HYCOM can be used for simulations of coastal and shelf seas, and further suggestions for improving the model results are given. Since HYCOM also works well in open ocean and basin scale simulations, it may allow for a realistic modelling of the transition region between the open ocean and coastal shelf seas.     

Coupling winds to ocean surface currents over the global ocean

A Wind stress–Current Coupled System (WCCS) consisting of the HYbrid Coordinate Ocean Model (HYCOM) and an improved wind stress algorithm based on Donelan et al. [Donelan, W.M., Drennan, Katsaros, K.B., 1997. The air–sea momentum flux in mixed wind sea and swell conditions. J. Phys. Oceanogr. 27, 2087–2099] is developed by using the Earth System Modeling Framework (ESMF). The WCCS is applied to the global ocean to study the interactions between the wind stress and the ocean surface currents. In this study, the ocean surface current velocity is taken into consideration in the wind stress calculation and air–sea heat flux calculation. The wind stress that contains the effect of ocean surface current velocity will be used to force the HYCOM. The results indicate that the ocean surface velocity exerts an important influence on the wind stress, which, in turn, significantly affects the global ocean surface currents, air–sea heat fluxes, and the thickness of ocean surface boundary layer. Comparison with the TOGA TAO buoy data, the sea surface temperature from the wind–current coupled simulation showed noticeable improvement over the stand-alone HYCOM simulation.     

Observation and analysis of mixing in a tidal and wind-mixed coastal region

In this article, the authors first present oceanic observations collected in a coastal area in May 2007. The evolution of temperature profiles exhibits a very clear atmospheric heating signal and is used to study mixing. Modelled atmospheric fluxes are evaluated using the oceanic measurements. The K-profile parameterisation (KPP) is chosen to identify the most important mixing processes and its parameters are tuned to minimise differences with respect to the observations.It is found that:• the tuned KPP is able to accurately represent the effect of mixing in this case;• surface and bottom boundary layers, as well as interior shear instability mixing processes all play an important role in the observed evolution of the temperature profile, the bottom boundary being the source of the most intense mixing;• the nonlocal effects in KPP (activated during nocturnal cooling periods) have to be switched off for a better agreement.     

Evaluations of threshold and curvature mixed layer depths by various mixing schemes in the Mediterranean Sea

Predictive ability of five different embedded turbulent mixing models that range from second-order turbulent closure to bulk mixing parameterization is examined in the Mediterranean Sea. Each is embedded in the HYbrid Coordinate Ocean Model (HYCOM). Mixed layer depth (MLD), which is one of the most important upper ocean variables, is used to evaluate the treatment of turbulent processes in each model run. In addition to overall spatial and temporal variability, analyses of MLD are presented using an extensive set (3976) of temperature and salinity profiles from various data sources during 2003–2006. Results obtained from simulations (with no data assimilation and relaxation only to salinity) for the five mixing models are compared with observed MLDs obtained from in situ temperature and salinity profile observations. To ensure the robustness of the validation statistics MLD is computed using both curvature and threshold based methodologies. Results indicate that while all mixing schemes represent the MLD well, the bulk mixing models have substantial accuracy deficiencies relative to the higher order mixing models. The modeled MLDs are slightly deeper than observed MLDs with the mean bias error ∼10 m for the higher order mixing models while the bulk mixing model bias error is 15 m or more. The RMS error for the higher order mixing models is ∼40 m while it is ∼50 m for the bulk mixing models. The bulk mixing models had substantially larger errors particularly for the curvature MLD definition.     

利用卫星测高、GRACE和GOCE资料估计全球海洋表面地转流

重力恢复和气候试验GRACE(gravity recovery and climate experiment)卫星极大地提高了地球重力场的精度和分辨率,特别是中长波分量,联合卫星测高数据可获得全球海洋表面大尺度洋流循环。另外,新一代地球重力和海洋环流探测卫星GOCE(gravity field and steady-state ocean circulation explorer)于2009年3月成功发射,采用卫星重力梯度测量原理,对重力场的高频部分非常敏感,使其高分辨率监测全球海洋循环成为可能。本文利用1~7年GRACE观测数据确定的重力场模型和18个月GOCE观测数据确定的地球重力场模型GO_CONS_GCF_2_TIM_R3,联合卫星测高确定的平均海面高模型MSS_CNES_CLS_11,分别估计全球海洋表面地转流,并且与实测浮标数据结果进行比较。分析表明GOCE重力卫星确定的重力场模型具有更高的空间分辨率,能够确定高精度和高空间分辨率的全球海洋地转流,如墨西哥湾暖流的细节和特征,并且与实测浮标结果基本一致。而基于1~4年GRACE观测资料的模型不能很好估计全球地转流特征,基于7年GRACE观测资料的重力场模型ITG-Grace2010s确定的全球地转流的精度仍低于18个月GOCE观测数据确定的地球重力场模型GO_CONS_GCF_2_TIM_R3的结果,估计的全球地转流仍含有较大的噪声,不能很好地反应中小尺度地转流细节特征。并计算ITG_Grace2010s和GOCE_TIM3的稳态海面地形和全球平均地转流的内符合精度,结果显示,在全球范围内,GOCE_TIM3的稳态海面地形和全球平均地转流的精度都比ITG_Grace2010s结果的精度有着很大的改善,其中ITG_Grace2010s的稳态海面地形的精度为21.6cm,而GOCE_TIM3的结果则为7.45cm,ITG_Grace2010s的全球平均地转流的精度为40.7cm/s,而GOCE_TIM3的结果则为19.6cm/s。     

基于Argo历史观测的南海海盆尺度中层流场研究

针对南海海域海流环境复杂、中层实测数据量少的现状,本文基于2006-2016年布放在南海海域的114个Argo剖面浮标的卫星定位等信息,采用基于背景流和惯性流外推的最小二乘方法,获取南海海域1 200 m深中层流场信息,并采用Divand变分插值的方法形成网格化季节流场。结果表明:（1）针对南海中层（1 200 m）流场,单个Argo浮标可以刻画出具体的中尺度结构,如越南沿岸的反气旋涡,半径约为120 km,最大流速约为9.6 cm/s,平均流速为5.3 cm/s;（2） Argo网格化流场表明海盆尺度中层流场南海南部为反气旋环流结构,北部为气旋式环流,同时在吕宋海峡口存在从南海至太平洋的水体交换;（3）将该流场信息与HYCOM和YoMaHa'07两种资料对比,吻合度较高,与HYCOM再分析资料的偏差分布趋近于正态分布,海流的东西向分量的均方根误差为3.28 cm/s,南北向分量的均方根误差为3.26 cm/s。总体而言,利用Argo轨迹资料能够有效地反演出南海地区海盆尺度的中层环流特征。     

基于ROMS模式的南海SST与SSH四维变分同化研究

卫星遥感观测获得了大量高分辨率的海面实时信息,包括海面温度（SST）和海面高度（SSH）等,同化进入数值模式可有效提升模拟精度。本文基于ROMS模式与四维变分同化方法（4DVAR）,使用AVHRR SST和AVISO SSH数据,开展了南海区域同化实验。为检验同化的效果,分别利用HYCOM再分析资料和Argo温盐实测数据分析了同化结果的海面高度、流场及温盐剖面的精度。对比结果表明,SST和SSH的同化能够改善ROMS的模拟结果:同化后海面高度场能够更为准确地捕捉海洋的中尺度特征,与HYCOM海面高度再分析资料相比,平均绝对偏差和均方根误差分别为0.054 m和0.066 m;与HYCOM 10 m层流场相比,东向与北向流速平均绝对偏差分别为0.12 m/s和0.11 m/s,相比未同化均提升约0.01 m/s;温盐同化结果与Argo温盐实测具有较高的一致性,温度和盐度平均绝对偏差为0.45℃、0.077,均方根误差为0.91℃、0.11,单个的温盐廓线对比说明,同化结果与HYCOM再分析资料精度相当。     

基于HYCOM的海洋环流仿真建模研究

HYCOM海洋模式是垂向混合坐标模式的代表。基于HYCOM海洋模式,采用三层嵌套技术对东海南部海域的海洋环流进行仿真建模研究。通过对仿真结果的验证表明,HYCOM模式的垂向混合坐标方案能够很好的模拟地形复杂海域的环流。     

吕宋海峡不同湍流估算方法的应用对比研究

由于湍流混合直接观测技术难度大、成本高,很大程度限制湍流混合的研究,所以基于温、盐、流资料估算海洋湍流混合的方法应运而生。本文应用在吕宋海峡观测到的23个自由下降微结构湍流剖面仪观测数据和水文观测数据,首次对目前常用的Gregg-Henyey-Polzin(GHP)细尺度参数化、Mackinnon and Gregg(MG)参数化和Thorpe尺度方法进行比较研究,评估它们的适用性。发现GHP参数化方法能够很好地估算吕宋海峡的湍流混合。虽然GHP参数化方法估算的耗散率总体上要偏弱于观测的结果,但估算和观测的差异在2倍以内的结果占71%,与微结构湍流剖面仪观测到的耗散率在水平分布和垂向分布上呈现出相同的分布特征。基于MG参数化方法发现估算的吕宋海峡西侧1200m以浅的耗散率比观测值大,但总体上呈现出相同的分布特征。另外,MG参数化估算与观测差异在2倍以内的结果占58%。表明相比于GHP参数化方法,MG参数化方法的估算值更偏离观测值。Thorpe尺度方法在估算吕宋海峡的耗散率时,估算和观测的差异在2倍以内的结果仅有30%,70%的估算结果与观测结果相差高出1个量级,空间分布上与观测结果差异较大。对比吕宋海峡湍流混合参数化方法的结果表明GHP参数化方法最优,MG参数化方法其次,Thorpe尺度方法相对GHP和MG参数化方法较差。     

HYCOM模式对赤道及北太平洋海表温度的模拟

本文使用HYCOM数值模式,根据两种海气通量数据集(COADS、ECMWF)和两种海气通量块体参数化方案(常数块体参数化方案和非常数块体参数化方案)的不同结合,构成4组数值实验,分别模拟了赤道及北太平洋的气候态海表温度.实验结果表明:1)在本文的实验中,非常数块体参数化方案优于常数块体参数化方案;在太平洋40°N-20°S区域内,采用前者得到的年平均海表温度比Pathfinder卫星资料高约0.21 °C,而采用后者得到的年平均海表温度比Pathfinder卫星资料高约0.63 °C.2)HYCOM数值模式很好的模拟了赤道及北太平洋的气候态海表温度变化及西太平洋暖池空间分布的月变化.特别是实验2(采用COADS数据集和非常数块体参数化方案),在太平洋40°N - 20°S区域内,冬春两季平均SST仅比Pathfinder卫星数据集高0.02 °C.3)不同的海气通量数据会对模式结果产生明显的影响.对比采用COADS数据集的实验2结果与采用ECMWF数据集的实验4结果可以发现,在模拟区域的西北部,实验2比实验4的年平均SST高约1 °C;在模拟区域的东南部,实验4比实验2的年平均SST高约1 °C.两者差的最大值出现在58°N、140°E附近及中国渤海,约为4 °C(实验2比实验4的年平均SST高约4 °C).     

Impact on oceanic dynamics from assimilation of satellite surface height anomaly data into the Hybrid Coordinate Ocean Model Ocean Model (HYCOM) over the Atlantic Ocean

Observed along-track data of sea surface height anomalies (SSHAs) over the Atlantic Ocean from the Jason-1 and Jason-2 satellites were assimilated into the Hybrid-Coordinate Ocean Model (HYCOM) with the Ensemble Optimal Interpolation scheme (EnOI). The impact of assimilation of SSHA with focus on oceanic dynamics was investigated. Time series of analyzed and forecasted values were compared with a model free run with the same forcing but without assimilation. In addition, the results were compared with an independent run, the so-called HYCOM + NCODA analysis from the US Navy. The study shows that the assimilation technique with some modifications allowed substantial improvement in the 24 h ocean prediction by reducing the forecast errors in comparison with the free run. It is also shown that the analyzed sea surface fields contain mesoscale and synoptic variability, which are poorly seen in the free run.     

Determining the Gravitational Gradient Tensor Using Satellite-Altimetry Observations over the Persian Gulf

With the advent of satellite altimetry in 1973, new scientific applications became available in oceanography, climatology, and marine geosciences. Moreover, satellite altimetry provides a significant source of information facilitated in the geoid determination with a high accuracy and spatial resolution. The information from this approach is a sufficient alternate for marine gravity data in the high-frequency modeling of the marine gravity field quantities. The gravity gradient tensor, consisting of the second-order partial derivatives of the gravity potential, provides more localized information than gravity measurements. Marine gravity observations always carry a high noise level due to environmental effects. Moreover, it is not possible to model the high frequencies of the Earth's gravity field in a global scale using these observations. In this article, we introduce a novel approach for a determination of the gravity gradient tensor at sea level using satellite altimetry. Two numerical techniques are applied and compared for this purpose. In particular, we facilitate the radial basis functions (RBFs) and the harmonic splines. As a case study, the gravitational gradient tensor is determined and results presented in the Persian Gulf. Validation of results reveals that the solution of the harmonic spline approach has a better agreement with a theoretical zero-value of the trace of the Marussi gravitational gradient tensor. However, the data-adaptive technique in the RBF approach allows more efficient selection of the parameters and 3-D configuration of RBFs compared to a fixed parameterization by the harmonic splines.     

南海北部中尺度涡流预报及应用

南海北部海域是南海中尺度涡的高发区,该海区的多尺度动力过程及相互作用经常对海上工程安全造成重大影响。针对“FPSO-119”海洋工程施工船在2021年5月8日20时左右遇到“怪流”后瞬时大幅度失位现象,在排除内波等其他海洋现象与外因影响的前提下,利用海表面高度异常(SLA)数据、HYCOM模式数据以及现场实测数据,分析认为“怪流”是施工海域内中尺度涡与潮流正向叠加所导致。在此基础上,结合TPXO潮流预报数据,提出了一种将中尺度涡流与潮流矢量叠加的涡流预报方法,并通过FVCOM数值预报,对施工海域的中尺度涡流进行预报。经过与现场实测数据的后报检验,该方法能够反映施工海域内涡流在未来2 d内的主要运动特征,可作为海上工程应对“怪流”的重要参考,在工程应用中结合内波流、风海流等其他信息综合考虑分析,可更好地为海洋工程和船舶航行等提供安全保障。     

垂直混合方案对全球海洋环流模式模拟结果的影响对比研究

利用Modular Ocean Model version4(MOM4)海洋环流模式设计了2个全球海洋-海冰耦合的数值实验,以分析比较Pacanowski and Philander(PP)和K-Profile Parameterization(KPP)两种不同垂直混合方案在全球海洋上层模拟中的表现。实验结果表明,PP和KPP方案在中纬和高纬海域模拟海温差别较大,后者模拟结果好于前者;在低纬海域差别较小,但赤道断面PP方案模拟结果较好;KPP方案能很好地模拟太平洋赤道潜流,而PP方案模拟的赤道潜流位置偏浅。