Bottom friction and its effects on periodic long wave propagation     

A. Orfila  G. Simarro  P.L.F. Liu 《Coastal Engineering》2007

A new set of Boussinesq-type equations describing the free surface evolution and the corresponding depth-integrated horizontal velocity is derived with the bottom boundary layer effects included. Inside the boundary layer the eddy viscosity gradient model is employed to characterize Reynolds stresses and the eddy viscosity is further approximated as a linear function of the distance measured from the seafloor. Boundary-layer velocities are coupled with the irrotational velocity in the core region through boundary conditions. The leading order boundary layer effects on wave propagation appear in the depth-integrated continuity equation to account for the velocity deficit inside the boundary layer. This formulation is different from the conventional approach in which a bottom stress term is inserted in the momentum equation. An iterative scheme is developed to solve the new model equations for the free surface elevation, depth-integrated velocity, the bottom stress, the boundary layer thickness and the magnitude of the turbulent eddy viscosity. A numerical example for the evolution of periodic waves propagating in one-dimensional channel is discussed to illustrate the numerical procedure and physics involved. The differences between the conventional approach and the present formulation are discussed in terms of the bottom frictional stress and the free surface profiles.  相似文献   

Optimisation of an idealised ocean model,stochastic parameterisation of sub-grid eddies     

《Ocean Modelling》2015

An optimisation scheme is developed to accurately represent the sub-grid scale forcing of a high dimensional chaotic ocean system. Using a simple parameterisation scheme, the velocity components of a 30 km resolution shallow water ocean model are optimised to have the same climatological mean and variance as that of a less viscous 7.5 km resolution model. The 5 day lag-covariance is also optimised, leading to a more accurate estimate of the high resolution response to forcing using the low resolution model.The system considered is an idealised barotropic double gyre that is chaotic at both resolutions. Using the optimisation scheme, we find and apply the constant in time, but spatially varying, forcing term that is equal to the time integrated forcing of the sub-grid scale eddies. A linear stochastic term, independent of the large-scale flow, with no spatial correlation but a spatially varying amplitude and time scale is used to represent the transient eddies. The climatological mean, variance and 5 day lag-covariance of the velocity from a single high resolution integration is used to provide an optimisation target. No other high resolution statistics are required. Additional programming effort, for example to build a tangent linear or adjoint model, is not required either.The focus of this paper is on the optimisation scheme and the accuracy of the optimised flow. However the forcing can provide insights in the design of deterministic and stochastic parameterisations. In the present study, we found that the stochastic parameterisation correcting the model variance is associated with the spatial pattern of eddy-decorrelation timescales rather than the spatial pattern of the amplitude of the variance. The method can be applied in future investigations into the physical processes that govern barotropic turbulence and it can perhaps be applied to help understand and correct biases in the mean and variance of a more realistic coarse or eddy-permitting ocean model. The method is complementary to current parameterisations and can be applied at the same time without modification.  相似文献   

中尺度涡旋混合参数空间变化对物理场以及CFC-11模拟的影响     

刘波  李阳春  徐永福  范广洲 《大气科学》2015,39(6):1149-1164

本文选用中国科学院大气物理研究所全球海洋模式(LICOM),对中尺度涡旋参数化方案(GM90方案)中等密度扩散系数和等密度面厚度扩散系数(统称为涡旋扩散系数A_ρ)对物理场及CFC-11(一氟三氯甲烷)分布的影响进行了研究。本文做了两个试验,即涡旋扩散系数采用常系数方式(对照试验)和采用在非绝热层以下A_ρ随海洋浮力频率垂直变化的参数化方案(浮力试验)。模拟结果表明,依浮力频率垂直变化的方案对模式物理场的模拟能力有一定程度的提升,如南极绕极流的输送强度比常系数方案增大了约20%~30%,与观测事实更接近;浮力试验对对照试验中过强的南极中层水有一定的削弱作用,使得模式对南大洋高纬次表层位密度的模拟有一定的改善。稍有不足的是,浮力试验对南极底层水也有一定的削弱,使得2000~3000 m深度位密度模拟较常系数方案偏低。通过对CFC-11分布、存储以及输送的研究发现,次网格参数取值的不同对南大洋CFC-11模拟情况有较大影响。浮力试验加大了南北高纬CFC-11海表的吸收通量,对南极大陆周边海域向南大洋主储藏区(34°S~60°S)的CFC-11输送能力有一定的增强,使得南大洋对CFC-11储藏量增大,大部分海区与观测资料更接近。通过CFC-11断面分析,浮力试验对南大洋上层海洋位密度模拟的改善使得CFC-11垂直结构与观测更接近。  相似文献   

Another Simple Method of Spectral Correction to Obtain Robust Eddy-Covariance Results   总被引：3，自引：0，他引：3  

Uwe Spank  Christian Bernhofer 《Boundary-Layer Meteorology》2008,128(3):403-422

The eddy-covariance method is the state-of-the-art tool to measure mass and energy fluxes, though many measuring systems (particularly closed-path systems) show strong spectral attenuation that causes significant underestimates of actual mass fluxes. The standard way to correct these underestimates is to use theoretically derived transfer functions. Practical implementation has shown that the attenuation, especially of closed-path systems, is underestimated by this method, even when the additional tube damping is considered. This paper introduces and tests three alternative site-specific and hardware specific correction methods based on spectral analysis, which typically enhance—additionally to the classical correction—the calculated mass fluxes based on closed-path analysers, as their inevitable tubing and the filters used cause additional flow attenuation. Two of the three methods are based on a site-specific and hardware specific transfer functions, the third uses direct comparison of cospectra. Primarily the methods based on transfer functions proved to be easy-to-handle once established for the specific set-up and measurement site. They represent practical and robust methods to correct for spectral attenuation.  相似文献   

A Modelling Study of Flux Imbalance and the Influence of Entrainment in the Convective Boundary Layer   总被引：3，自引：2，他引：1  

Jianping Huang  Xuhui Lee  Edward G. Patton 《Boundary-Layer Meteorology》2008,127(2):273-292

It is frequently observed in field experiments that the eddy covariance heat fluxes are systematically underestimated as compared to the available energy. The flux imbalance problem is investigated using the NCAR’s large-eddy simulation (LES) model imbedded with an online scheme to calculate Reynolds-averaged fluxes. A top–down and a bottom–up tracer are implemented into the LES model to quantify the influence of entrainment and bottom–up diffusion processes on flux imbalance. The results show that the flux imbalance follows a set of universal functions that capture the exponential decreasing dependence on u _*/w _*, where u _* and w _* are friction velocity and the convective velocity scale, respectively, and an elliptic relationship to z/z _i , where z _i is the mixing-layer height. The source location in the boundary layer is an important factor controlling the imbalance magnitude and its horizontal and vertical distributions. The flux imbalance of heat and the bottom–up tracer is tightly related to turbulent coherent structures, whereas for the top–down diffusion, such relations are weak to nonexistent. Our results are broadly consistent with previous studies on the flux imbalance problem, suggesting that the published results are robust and are not artefacts of numerical schemes.  相似文献   

山地复杂下垫面湍流特征观测分析   总被引：1，自引：0，他引：1  

杨胜朋  吕世华  陈玉春  奥银焕  蒋熹 《高原气象》2008,27(2):272-278

利用谱方法和能量闭合度分析等方法,对兰州市皋兰山2006年夏季和2005年冬季涡度相关系统测定的湍流数据质量进行了分析,结果表明:4个观测点测定的三维风速、空气温度的功率谱在惯性副区都符合"-2/3"定律,垂直风速和温度的协谱在惯性副区也基本符合"-4/3"定律.4个观测点都存在能量平衡不闭合现象,夏季观测点绿化地和裸地能量平衡比率(EBR)分别为66%和94%,冬季绿化地和裸地的EBR分别为62%和84%;裸地观测点的闭合程度明显好于绿化地.本文还讨论了能量闭合度的日变化特征及各个观测点的地表能量收支状况.  相似文献   

A Comparison Between Modelled and Measured Mixing-Layer Height Over Munich   总被引：1，自引：0，他引：1  

Aggeliki Dandou  Maria Tombrou  Klaus Schäfer  Stefan Emeis  Anna P. Protonotariou  Elissavet Bossioli  Nikolaos Soulakellis  Peter Suppan 《Boundary-Layer Meteorology》2009,131(3):425-440

An attempt is made to correlate the mixing heights, derived from ceilometer and Sodar measurements, to those simulated by different atmospheric boundary-layer parameterization schemes. The comparison is performed at two sites (one suburban and one rural) close to Munich, Germany for two spring and two winter days. It is found that, under convective conditions, the mixing height determined, by both Sodar and ceilometer, corresponds to the middle or the top of the entrainment zone, respectively, as calculated from the eddy-viscosity profiles. Under stable conditions, the measured mixing height is related to the height where eddy viscosities attain their minimum values (Sodar) or to the height of residual mechanical turbulence (ceilometer). During a foehn case with weak turbulence, the measured mixing height from both Sodar and ceilometer is better inferred by considering the eddy-viscosity profiles during daytime and the height of the low-level jet during nighttime.  相似文献   

Adaptation of the optimal fingerprint method for climate change detection using a well-conditioned covariance matrix estimate   总被引：3，自引：3，他引：0  

Aurélien Ribes  Jean-Marc Azaïs  Serge Planton 《Climate Dynamics》2009,33(5):707-722

The “optimal fingerprint” method, usually used for detection and attribution studies, requires to know, or, in practice, to estimate the covariance matrix of the internal climate variability. In this work, a new adaptation of the “optimal fingerprints” method is presented. The main goal is to allow the use of a covariance matrix estimate based on an observation dataset in which the number of years used for covariance estimation is close to the number of observed time series. Our adaptation is based on the use of a regularized estimate of the covariance matrix, that is well-conditioned, and asymptotically more precise, in the sense of the mean square error. This method is shown to be more powerful than the basic “guess pattern fingerprint”, and than the classical use of a pseudo-inverted truncation of the empirical covariance matrix. The construction of the detection test is achieved by using a bootstrap technique particularly well-suited to estimate the internal climate variability in real world observations. In order to validate the efficiency of the detection algorithm with climate data, the methodology presented here is first applied with pseudo-observations derived from transient regional climate change scenarios covering the 1960–2099 period. It is then used to perform a formal detection study of climate change over France, analyzing homogenized observed temperature series from 1900 to 2006. In this case, the estimation of the covariance matrix is only based on a part of the observation dataset. This new approach allows the confirmation and extension of previous results regarding the detection of an anthropogenic climate change signal over the country.  相似文献   

Preliminary Research on the Center's Location and Interseasonal variabilities of the Cold Eddy in East China Sea     

Wang Gang  Lan Jian  Sun Shuangwen 《地球科学进展》2010,25(2):184-192

To determine the center location of the Cold Eddy in East China Sea(CEECS)′s center, we analyzed typical sections of temperature fields in the sea area around the eddy and improved the proper method to acquire the eddy′s center based on the temperature data of SODA from 1958 to 2001. Results show that the CEECS′s mean center was located at 32.0°N, 125.7°E at the depth of 35 meters during 44 years, while the center was not fixed in one place. From summer to winter, the center of the eddy moves from north to south gradually while it moves from south to north gradually from winter to summer in the longitudinal direction. In the latitudinal direction, the center is obviously in the west only in winter, while in the other seasons it is in the east and the latitude of it changes lightly. The main factor should be Tsushima Current and the Yellow Sea Coastal Current which control the variance of the center′s location. The eddy′s center is not obvious in early fall, late winter and early spring, which should be associated with the seasonality of the current fields.

  相似文献   

大孔径闪烁仪和涡动相关仪观测显热通量之间的尺度关系   总被引：1，自引：0，他引：1  

卢俐  刘绍民  徐自为  白洁  王介民 《地球科学进展》2010,25(11):1273-1282

利用北京昌平小汤山2002年、2004年涡动相关仪和大孔径闪烁仪的观测数据,借助解析足迹模型计算的源区,分析均匀、非均匀下垫面上涡动相关仪和大孔径闪烁仪观测显热通量之间的差异和关系.结果表明:地表的均匀与非均匀性质,直接影响涡动相关仪和大孔径闪烁仪观测的显热通量的差异大小.在分析均匀、非均匀地表上2种仪器源区重叠面积及重叠区域内足迹值大小与观测通量值之间的关系后,构建一个非均匀地表上2台涡动相关仪与1台闪烁仪观测通量之间的尺度关系式.  相似文献