On ensemble prediction of ocean waves

The numerical ensemble prediction is a well accepted method for improving the performance of atmospheric models. In the context of ocean wave modeling little has been researched or documented about this technique. An essential study of the method of ensemble prediction applied to deep water waves has been carried out. A framework is defined for obtaining perturbations of the directional wave spectra and for employing an ensemble of wind fields generated by an atmospheric model. The third-generation global wave model WAM is used with real atmospheric conditions to investigate the effect on wave predictions of perturbed initial conditions and atmospheric forcing. Due to spectral shape stabilisation, perturbing wave initial conditions has limited utility in ensemble prediction. However, the members could be used in wave data assimilation schemes in an interactive way. Using ensembles of the atmospheric condition can generate diverging solutions, justifying the ensemble procedure by itself. In the cases studied, it is observed that the ensemble mean outperformed the other members. The solution behaviour suggests using a lower-order approximation of the model to generate ensemble members with less computational cost.     

Flow-dependent versus flow-independent initial perturbations for ensemble prediction

Ensemble prediction relies on a faithful representation of initial uncertainties in a forecasting system. Early research on initial perturbation methods tested random perturbations by adding 'white noise' to the analysis. Here, an alternative kind of random perturbations is introduced by using the difference between two randomly chosen atmospheric states (i.e. analyses). It yields perturbations (random field, RF, perturbations) in approximate flow balance.
The RF method is compared with the operational singular vector based ensemble at European Centre for Medium Range Weather Forecasts (ECMWF) and the ensemble transform (ET) method. All three methods have been implemented on the ECMWF IFS-model with resolution T _L 255L40. The properties of the different perturbation methods have been investigated both by comparing the dynamical properties and the quality of the ensembles in terms of different skill scores. The results show that the RF perturbations initially have the same dynamical properties as the natural variability of the atmosphere. After a day of integration, the perturbations from all three methods converge. The skill scores indicate a statistically significant advantage for the RF method for the first 2–3 d for the most of the evaluated parameters. For the medium range (3–8 d), the differences are very small.     

影响黄渤海的温带气旋数值集合预报试验

为了改进温带气旋数值预报的精度,基于WRF(Weather Research and Forecasting)模式,利用GSI(Gridpoint Statistical Interpolation)-EnKF(Ensemble Kalman Filter)系统,设计了一套温带气旋集合预报方法,其具有的2种选择方案通过滤掉质量较差的集合成员从而将集合成员数目控制在10以内,达到了大幅降低集合预报计算量的目的。针对2020年7月一次影响黄海的温带气旋个例,开展了一系列决定性预报与集合预报的数值对比试验。分析结果如下:1)不采取任何择优方案的集合预报效果就已经明显优于决定性预报,而采取择优方案使得预报效果进一步得到提升;2)预报初始时刻择优(直接择优方案)的集合预报效果远不如短时积分3 h后才进行择优(积分择优方案)的预报效果; 3)积分择优方案优于直接择优方案的原因是,初始场集合体中的成员经过短时积分后其误差得以放大而使得择优更加准确。多个例的应用结果进一步表明,本文提出的积分择优方案温带气旋集合预报方法具有较好的业务预报应用前景。     

印度洋偶极子预报技巧在多模式中的对比研究

本文采用北美多模式集合产品数据,分析了印度洋偶极子指数在不同模式中实际预报技巧和潜在可预报性的差异,并进一步探究其可能的原因。结果表明,印度洋偶极子的有效预报时效在不同模式中差别较大,从2个月到4个月不等。其中东极子海温异常在不同模式中预报技巧的差别较西极子海表面温度异常更明显,表明模式误差和初始误差对东极子海表面温度异常演变的影响更为显著。另外,印度洋偶极子的实际预报技巧和潜在预报技巧存在明显的线性关系,潜在预报技巧高的模式,其实际预报技巧也高。最后,本文诊断、分析了厄尔尼诺对印度洋偶极子预报技巧的影响,发现在厄尔尼诺和印度洋偶极子相关性较高的气候模式中,印度洋偶极子实际预报技巧也较高。     

Beach memory and ensemble prediction of shoreline evolution near a groyne

In this paper we address the question of estimating the average position of a beach and its inherent variability about this mean. It is demonstrated how, even in a much simplified situation, the ensemble average of beach plan shape involves cross-correlation of the beach position and wave conditions. This renders the governing equations inimical to analytical treatment. A new analytical expression for the mean beach plan shape and its variation are derived for the case of a single groyne exposed to waves varying in direction only. This demonstrates that ‘beach memory’ is directly related to the autocorrelation of wave direction. For more general conditions a semi-analytical expression for the ensemble average of the shoreline position is derived. This solution is estimated with site specific wave conditions using Monte Carlo simulations. The characteristics of the solution are investigated and it is demonstrated that, for this case at least, the terms involving the wave direction are virtually uncorrelated with the terms that do not. It is concluded that, in an ensemble sense, the morphodynamic impact of wave direction is decoupled from that due to wave height and period.     

浒苔漂移轨迹预报系统

2008年6月下旬,青岛海域出现大量浒苔,并迅速蔓延至近岸,对第29届奥运会帆船赛事构成严重威胁。为给浒苔清理和打捞提供决策支持,国家海洋环境预报中心及时作出应急响应,在短时间内建立了浒苔漂移路径预测系统,向有关部门发布浒苔漂移轨迹预报和海洋环境预报,成功协助前线应急指挥中心完成了浒苔的控制和治理。本文对国家海洋环境预报中心建立的浒苔漂移轨迹预报模型进行了详细描述,并开展了浒苔漂移预报误差分析。     

The impact of open boundary forcing on forecasting the East Australian Current using ensemble data assimilation

We investigate the performance of an eddy resolving regional ocean forecasting system of the East Australian Current (EAC) for both ensemble optimal interpolation (EnOI) and ensemble Kalman filter (EnKF) with a focus on open boundary model nesting solutions. The performance of nesting into a global re-analysis; nesting into the system’s own analysis; and nesting into a free model is quantified in terms of forecast innovation error. Nesting in the global reanalysis is found to yield the best results. This is closely followed by the system that nests inside its own analysis, which seems to represent a viable practical option in the absence of a suitable analysis to nest within. Nesting into a global reanalysis without data assimilation and nesting into an unconstrained model were both found to be unable to constrain the mesoscale circulation at all times. We also find that for a specific interior area of the domain where the EAC separation takes place, there is a mixture of results for all the systems investigated here and that, whilst the application of EnKF generates the best results overall, there are still times when not even this method is able to constrain the circulation in this region with the available observations.     

Verification of an operational ocean circulation-surface wave coupled forecasting system for the China's seas

An operational ocean circulation-surface wave coupled forecasting system for the seas off China and adjacent areas(OCFS-C) is developed based on parallelized circulation and wave models. It has been in operation since November 1, 2007. In this paper we comprehensively present the simulation and verification of the system, whose distinguishing feature is that the wave-induced mixing is coupled in the circulation model. In particular, with nested technique the resolution in the China's seas has been updated to(1/24)° from the global model with(1/2)°resolution. Besides, daily remote sensing sea surface temperature(SST) data have been assimilated into the model to generate a hot restart field for OCFS-C. Moreover, inter-comparisons between forecasting and independent observational data are performed to evaluate the effectiveness of OCFS-C in upper-ocean quantities predictions, including SST, mixed layer depth(MLD) and subsurface temperature. Except in conventional statistical metrics, non-dimensional skill scores(SS) is also used to evaluate forecast skill. Observations from buoys and Argo profiles are used for lead time and real time validations, which give a large SS value(more than 0.90). Besides, prediction skill for the seasonal variation of SST is confirmed. Comparisons of subsurface temperatures with Argo profiles data indicate that OCFS-C has low skill in predicting subsurface temperatures between 100 m and 150 m. Nevertheless, inter-comparisons of MLD reveal that the MLD from model is shallower than that from Argo profiles by about 12 m, i.e., OCFS-C is successful and steady in MLD predictions. Validation of 1-d, 2-d and 3-d forecasting SST shows that our operational ocean circulation-surface wave coupled forecasting model has reasonable accuracy in the upper ocean.     

Targeted ensemble prediction for northern Europe and parts of the north Atlantic Ocean

The targeting procedure developed at ECMWF is used to make ensembles specially designed for northern Europe and parts of the north Atlantic Ocean. A total of 35 ensembles are integrated, consisting of 20 winter cases and 15 summer cases in 1997, each consisting of 20 members plus one control forecast. The ensembles are run up to day 10, and the ensemble spread inside the target area continues to increase all through the 10 days. Two distinct regimes of increase can be found, the first increase is consistent with the perturbations moving in and through the target area, it is hypothesised that the latter increase in ensemble spread around forecast day 5‐7 is connected with increasing non‐linearity. The performance of the experimental ensembles is compared to the operational ensemble prediction system (EPS) at ECMWF, both with all 50 members and with only 20 members. The spread increases when the number of members in the ensemble prediction system is increased, and the spread increases inside the target area when targeting is applied. We find that the increase in spread when going from EPS with 50 members to the targeted ensembles is larger than when going from 20 to 50 ensemble members of the operational sets. Clearly targeting must be an option when predicting for a sub‐domain of the hemisphere. Looking at other measures, such as the Brier skill score (BSS), relative operating characteristic (ROC) curves and cost/loss analyses, the impact of the targeting is modest for the winter cases, but the impact for the summer cases is evident. For the winter cases a large part of the operational perturbations were located in the same area as the targeted perturbations, and the differences for the two sets are small. For the summer cases the operational perturbations were mostly split between two locations and hence the targeting will give results differing more from the operational.     

Observation bias correction with an ensemble Kalman filter

This paper considers the use of an ensemble Kalman filter to correct satellite radiance observations for state dependent biases. Our approach is to use state-space augmentation to estimate satellite biases as part of the ensemble data assimilation procedure. We illustrate our approach by applying it to a particular ensemble scheme—the local ensemble transform Kalman filter (LETKF)—to assimilate simulated biased atmospheric infrared sounder brightness temperature observations from 15 channels on the simplified parameterizations, primitive-equation dynamics (SPEEDY) model. The scheme we present successfully reduces both the observation bias and analysis error in perfect-model simulations.     

海上失事目标搜救应急预报系统

详细介绍了国家海洋环境预报中心发展的海上失事目标搜救应急预报系统.该系统基于四阶精度的拉格朗日漂移路径模型,以预报中心发展的高分辨率风场和流场数值预报的结果作为环境动力强迫条件,预报落水人员或失事船舶(渔船)的漂移轨迹,预报结果可基于Googleearth平台显示.该系统的应用有助于提高海上搜救行动的及时性和有效性,减...     

Validation of an operational forecasting system of sea dike risk in the southern Zhejiang Province,South China

In this study,an operational forecasting system of sea dike risk in the southern Zhejiang Province,South China was developed based on a coupled storm-surge and wave model.This forecasting system is important because of the high cost of storm-surge damage and the need for rapid emergency planning.A comparison with astronomical tides in 2016 and the validation of storm surges and high water marks of 20 typhoons verified that the forecast system has a good simulation ability.The system can forecast relatively realistic water levels and wave heights as shown under the parametric atmospheric forces simulated in a case study;the sea dikes in credible high risk were mainly located in the estuaries,rivers,and around the islands in the southern Zhejiang.Therefore,the forecast system is applicable in the southern Zhejiang with a support to the effective prevention from typhoon storm-surge damage.     

海上溢油应急预报业务系统介绍

介绍了国家海洋环境预报中心正在研制开发的海上溢油应急预报业务系统的结构、流程、功能和溢油模式。应用该系统对渤海和黄海的几次溢油事件进行了预报试验,试验结果表明该系统能在一个小时内完成渤海三维模拟预报、中国海二维模拟预报,并动态显示溢油分布范围、油膜面积、位置、溢油的抵岸时间、地点、油量、影响范围、沉降海底的位置、海上残油量等信息。     

美国海军海洋业务预报纵览

回顾了过去几十年美国海军海洋学的发展历程,介绍了海军海洋学业务保障体系,对其科研机构和研发机制以及业务系统发展特点进行总结,同时指出美国海军海洋预报业务系统和总体科研发展规划。通过深入分析国外海洋气象科技发展的最新动态和海洋气象预报业务科研建设的最新成果,对我国海洋气象事业的业务发展和科研规划提供有价值的参考。     

Deterministic and ensemble storm surge prediction for Atlantic Canada with lead times of hours to ten days

Regional deterministic and ensemble surge prediction systems (RDSPS and RESPS respectively) are used to forecast sea levels off the east of Canada and northeast US. The surge models for the RDSPS and RESPS have grid spacings of 1/30° and 1/12° respectively. The models are driven by surface air pressure and 10 m winds generated by operational global deterministic and ensemble prediction systems that are run operationally by the Canadian Meteorological Centre. Surge forecasts are evaluated for the period 1 March, 2013 to 31 March 2014. Based on traditional statistics (e.g., standard deviation of the difference between observations and predictions) both systems are shown to have skill in forecasting surges six days into the future. It is shown however that skill exists beyond six days if allowance is made for errors in the timing of large surges. The usefulness of the RESPS is demonstrated for two positive surges (important for coastal flooding and erosion) and a negative surge (important for safe navigation in shallow water). It is shown that the RESPS can identify events not forecast by the RDSPS, and can also add useful additional information on the timing of the surge, an important consideration in tidally dominated waters. Several new types of display are used to illustrate the sort of information that can be generated by the RESPS to support the issuers of warnings of unusually high and low total water levels.     

国际全球海洋环流预报系统的现状与展望

回顾近年来国外主要海洋预报业务机构全球海洋环流预报技术的发展现状,特别是最近5～10 a来在全球海洋动力数值预报模式和多源资料同化技术方面的主要进展。结合未来相关学科发展的趋势,围绕观测体系建设、数据共享机制、大数据分析等新技术应用等方面提出国内相关研究工作的发展建议。