NCOM forecasts, 50 m resolution, Gulf of Mexico, during the SCOPE experiment, December 2013

The numerical model is the Navy Coastal Ocean Model (NCOM) (Barron et. al, 2006). A single-nested domain is constructed for the experiment covering the entire Gulf of Mexico. The domain is forced by boundary conditions from the operational global NCOM (Barron et al., 2006). The vertical setup is uses with 34 sigma levels >and 16 Z levels beneath (50 total levels). Sigma levels cover the surface to 550m depth, and the Z levels cover the lower water column. The thinnest layer at the surface has a thickness of 0.5m, and deeper layers telescope to the thickest sigma layer of 85m at a depth of 510m. The high resolution in the surface is intended to properly represent submesoscale physics. Both model experiments are forced by the same atmospheric conditions from the Navy Operational Global Atmospheric Prediction System (NOGAPS, Rosmond et. al, 2002; Goerss, 2009). The surface wind stress is determined from the atmospheric model wind velocity. Surface heat fluxes are computed using bulk flux formulations that use the 10-m air-temperature and humidity along with the ocean model SST. Tidal potential forcing is applied to the inner domain, and tidal boundary conditions for water level and barotropic velocity are provided by the Oregon State University global Ocean Tide Inverse Solution (OTIS) (Egbert and Erofeeva, 2002). Thus, locally generated internal tides are present in the model. Data assimilation is used to produce similar mesoscale structure in the experiment. In the analysis cycle each day, all data over the 24 hours prior to 00Z for the present day are used in the analysis. The analysis is accomplished through a 3D variational (3DVar) approach (Cummings, 2005). Observation increments are computed by differencing observation values and model forecasts at the same time. The analysis increment is inserted into a 24 hour hindcast by rerunning the model over the prior 24 hours and adding the analysis divided by the number of time steps to the state variables throughout the 24 hour hindcast. This represents a correction to the slowly evolving state field rather than resetting the initial condition at 00Z. Direct insertion of the corrections and resetting the initial conditions can generate spurious internal and inertial waves that, in ocean models, require several days to damp out. The 24 hour forecast then provides the background for the next assimilation cycle. The horizontal covariance length scales used are based on latitudinally varying Rossby radius of deformation and vertical scales are based vertical gradients. The Rossby radius varies from 80 km at the southern extent of the domain to 31 km at the northern extent. A factor of 0.82 is used to scale the Rossby radius to provide the decorrelation length scales in the MVOI resulting in an average decorrelation scale of 45 km. Satellite SSH and SST observations are used to construct synthetic profiles through subsurface covariances (Fox et al., 2002) which are used in the 3DVar. Barron, C.N., A.B. Kara, P.J. Martin, and R.C. Rhodes (2006), Formulation, implementation and examination of the vertical coordinate choices in the Global Navy Coastal Ocean Model (NCOM), Ocean Modelling, 11, 347-375. Egbert, G. D., and S. Y. Erofeeva (2002), Efficient inverse modeling of barotropic ocean tides. J. Atmos. Oceanic Tec., 19, 183-204. Fox, D. N., C. N. Barron, M. R. Carnes, M. Booda, G. Peggion and J. V. Gurley (2002), The Modular Ocean Data Assimilation System, Oceanography, 15, 22-28. Rosmond, T. E., J. Teixeira, M. Peng, T. F. Hogan and R. Pauley (2002), Navy Operational Global Atmospheric Prediction System (NOGAPS): Forcing for ocean models, Oceanography, 15, 99–108. Barron, C.N., A.B. Kara, R.C. Rhodes, C. Rowley and L.F. Smedstad (2007), Validation Test Report for the 1/8 Global Navy Coastal Ocean Model Nowcast/Forecast System, NRL Tech Report NRL/MR/7320--07-9019, Naval Research Laboratory, Washington, DC. This is NCOM run B06. This dataset was created by the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE). This research was made possible by a grant from BP/The Gulf of Mexico Research Initiative.本数值模型采用美国海军近岸海洋模型（Navy Coastal Ocean Model, NCOM）（Barron等人，2006）。本实验构建了单嵌套区域，覆盖整个墨西哥湾海域，其边界条件由业务化全球NCOM模型提供（Barron等人，2006）。 垂直坐标设置采用34层σ坐标与16层底部Z坐标，总计50层。其中σ坐标层覆盖从海表至550米水深范围，Z坐标层则覆盖水体下部区域。海表处的最薄一层厚度为0.5米，随水深增加σ层逐渐增厚，在510米水深处达到最厚的85米。海表区域的高分辨率设置旨在准确表征亚中尺度物理过程。 两组模型实验均采用美国海军业务化全球大气预报系统（Navy Operational Global Atmospheric Prediction System, NOGAPS，Rosmond等人，2002；Goerss，2009）提供的相同大气强迫场。海表风应力由大气模式的风速场计算得到；海表热通量采用整体通量参数化方案计算，该方案使用10米高度的气温、相对湿度以及海洋模式输出的海表温度（Sea Surface Temperature, SST）作为输入。 内区域施加潮汐势能强迫，水位和正压流速的潮汐边界条件由俄勒冈州立大学全球海洋潮汐反演解（Oregon State University global Ocean Tide Inverse Solution, OTIS，Egbert与Erofeeva，2002）提供，因此模型中可模拟局地生成的内潮现象。 本实验采用数据同化方法以重现相似的中尺度结构特征。每日同化循环中，分析过程将使用当日世界时00点前24小时内的所有观测数据，分析过程采用三维变分（3D variational, 3DVar）方法实现（Cummings，2005）。观测增量通过同一时刻的观测值与模式预报值求差计算得到。将分析增量融入24小时后报流程的具体方式为：重新运行前24小时的模式，并将分析增量除以总时间步数后，逐次添加至该时段内所有状态变量中。该方法仅对缓慢演变的状态场进行订正，而非在世界时00点重置初始条件——若直接插入订正结果并重置初始条件，则会产生虚假的内波与惯性波，在海洋模式中这类虚假波需耗时数日才能消散。完成的24小时预报场将作为下一次同化循环的背景场。 所用的水平协方差长度尺度基于随纬度变化的罗斯贝变形半径，垂直尺度则基于垂直梯度。该区域的罗斯贝变形半径从南部边界的80千米变化至北部边界的31千米。采用0.82的缩放系数对罗斯贝变形半径进行缩放，以得到MVOI中的去相关长度尺度，最终得到平均去相关尺度为45千米。 卫星观测的海表高度（Sea Surface Height, SSH）与海表温度数据通过次表层协方差关系构建合成剖面（Fox等人，2002），并将其应用于三维变分同化过程中。 参考文献： 1. Barron, C.N.、A.B. Kara、P.J. Martin与R.C. Rhodes（2006），《全球海军近岸海洋模型（NCOM）的垂直坐标选择方案构建、实现与验证》，《海洋建模》，第11卷，347-375页。 2. Egbert, G.D.与S.Y. Erofeeva（2002），《正压海洋潮汐的高效反演建模》，《大气与海洋技术期刊》，第19卷，183-204页。 3. Fox, D.N.、C.N. Barron、M.R. Carnes、M. Booda、G. Peggion与J.V. Gurley（2002），《模块化海洋数据同化系统》，《海洋学》，第15卷，22-28页。 4. Rosmond, T.E.、J. Teixeira、M. Peng、T.F. Hogan与R. Pauley（2002），《海军业务化全球大气预报系统（NOGAPS）：海洋模型强迫场》，《海洋学》，第15卷，99-108页。 5. Barron, C.N.、A.B. Kara、R.C. Rhodes、C. Rowley与L.F. Smedstad（2007），《1/8度全球海军近岸海洋模型临近预报/预报系统的验证测试报告》，美国海军研究实验室技术报告NRL/MR/7320--07-9019，华盛顿特区海军研究实验室。 本数据集采用NCOM模式B06运行方案，由环境碳氢化合物输送高级研究联盟（Consortium for Advanced Research on Transport of Hydrocarbon in the Environment, CARTHE）构建。本研究得到BP公司/墨西哥湾研究计划的资助。