Validation of ECMWF and NCEP–NCAR Reanalysis Data in Antarctica

The European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis (ERA-40) and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) ECMWF (ERA-40) and NCEP-NCAR reanalysis data were compared with Antarctic station observations, including surface-layer and upper-layer atmospheric observations, on intraseasonal and interannual timescales. At the interannual timescale, atmospheric pressure at different height levels in the ERA-40 data are in better agre...     

NCEP-FNL与T213L31同化资料对比 分析中的地形差异影响

分析了2005年5月11日至6月10日的NCEP—FNL与T213L31分析资料在位势高度、温度和风场上的整体差异，揭示了低层差异最小，差异极值主要分布在3个关键区的特征。进一步通过地形高度差异与位势高度差异的相关性分析，认为青藏高原附近的地形高度差异的极值是引起两种资料位势高度差异的主要原因，并计算分析了这种差异对高原周围中低层大气环流的动力和热力影响。结果显示，在NCEP—FNL中的西南涡强度和热源作用较强，中层风速切变较弱。     

NCEP-FNL与T213L31同化资料对比分析中的地形差异影响

分析了2005年5月11日至6月10日的NCEP-FNL与T213L31分析资料在位势高度、温度和风场上的整体差异,揭示了低层差异最小,差异极值主要分布在3个关键区的特征。进一步通过地形高度差异与位势高度差异的相关性分析,认为青藏高原附近的地形高度差异的极值是引起两种资料位势高度差异的主要原因,并计算分析了这种差异对高原周围中低层大气环流的动力和热力影响。结果显示,在NCEP-FNL中的西南涡强度和热源作用较强,中层风速切变较弱。     

A 10-Yr Global Land Surface Reanalysis Interim Dataset (CRA-Interim/Land): Implementation and Preliminary Evaluation

A land surface reanalysis dataset covering the most recent decades is able to provide temporally consistent initial conditions for weather and climate models, and thus is crucial to verifying/improving numerical weather/climate forecasts/predictions. In this paper, we report the development of a 10-yr China Meteorological Administration (CMA) global Land surface ReAnalysis Interim dataset (CRA-Interim/Land; 2007–2016, 6-h intervals, approximately 34-km horizontal resolution). The dataset was produced and evaluated by using the Global Land Data Assimilation System (GLDAS) and NCEP Climate Forecast System Reanalysis (CFSR) global land surface reanalysis datasets, as well as in situ observations in China. The results show that the global spatial patterns and monthly variations of the CRA-Interim/Land, GLDAS, and CFSR climatology are highly consistent, while the soil moisture and temperature values of the CRA-Interim/Land dataset are in between those of the GLDAS and CFSR datasets. Compared with ground observations in China, CRA-Interim/Land soil moisture is comparable to or better than that of GLDAS and CFSR datasets for the 0-10-cm soil layer and has higher correlations and slightly lower root mean square errors (RMSE) for the 10-40-cm soil layer. However, CRA-Interim/Land shows negative biases in 10-40-cm soil moisture in Northeast China and north of central China. For ground temperature and the soil temperature in different layers, CRA-Interim/Land behaves better than the CFSR, especially in East and central China. CRA-Interim/Land has added value over the land components of CRA-Interim due to the introduction of global precipitation observations and improved soil/vegetation parameters. Therefore, this dataset is potentially a critical supplement to the CRA-Interim. Further evaluation of the CRA-Interim/Land, assimilation of near-surface atmospheric forcing variables, and extension of the current dataset to 40 yr (1979–2018) are in progress.     

Analysis of the surface temperature and wind forecast errors of the NCAR-AirDat operational CONUS 4-km WRF forecasting system

Investigating the characteristics of model-forecast errors using various statistical and object-oriented methods is necessary for providing useful guidance to end-users and model developers as well. To this end, the random and systematic errors (i.e., biases) of the 2-m temperature and 10-m wind predictions of the NCAR-AirDat weather research and forecasting (WRF)-based real-time four-dimensional data assimilation (RTFDDA) and forecasting system are analyzed. This system has been running operationally over a contiguous United States (CONUS) domain at a 4-km grid spacing with four forecast cycles daily from June 2009 to September 2010. In the result an exceptionally useful forecast dataset was generated and used for studying the error properties of the model forecasts, in terms of both a longer time period and a broader coverage of geographic regions than previously studied. Spatiotemporal characteristics of the errors are investigated based on the 24-h forecasts between June 2009 and April 2010, and the 72-h forecasts between May and September 2010. It was found that the biases of both wind and temperature forecasts vary greatly seasonally and diurnally, with dependency on the forecast length, station elevation, geographical location, and meteorological conditions. The temperature showed systematic cold biases during the daytime at all station elevations and warm biases during the nighttime above 1,000 m above sea level (ASL), while below 600 m ASL cold biases occurred during the nighttime. The forecasts of surface wind speed exhibited strong positive biases during the nighttime, while the negative biases were observed in the spring and summer afternoons. The surface wind speed was mostly over-predicted except for the stations located between 1,000 and 2,100 m ASL, for which negative biases were identified for most forecast cycles. The highest wind-speed errors were found over the high terrain and near sea-level stations. The wind-direction errors were relatively large at the high-terrain elevation in the Rocky and Appalachian mountain ranges and the western coastal areas and the error structure exhibited notable diurnal variability.     

3种再分析资料的高空温度与中国探空温度资料的对比:年平均特征

利用中国105站的探空资料以及NCEP/NCAR、ERA和JRA 3种再分析资料,采用线性趋势、标准差、相关系数、EOF分析等多种统计分析方法,对再分析资料年平均的高空温度在中国区域的可信度进行了分析.研究表明:在数值上,3种再分析资料的高空温度均小于探空资料的高空温度,NCEP/NCAR资料在对流层上层更接近于探空资料,ERA和JRA资料则在对流层中下层与探空资料更为接近;在描述年际变化和长期变化趋势方面,ERA资料在我国北方的对流层上层的再现能力较好,NCEP/NCAR资料在我国南方的对流层上层的再现能力较好,而3种再分析资料在对流层中下层的再现能力相当;在时空变化特征方面,NCEP/NCAR和ERA资料能较好地表现高空温度的年代际变化特征,而ERA和JRA资料则能较好地表现年际变化特征.     

ANALYSIS OF SEA-SURFACE DRAG PARAMETERIZATIONS IN OPEN OCEAN CONDITIONS

Data from the Surface Waves and Processes Program (SWAPP) are employed to test current sea-surface drag parameterizations in open ocean conditions. General trends in the data indicate that drag increases with increasing wind speed and wave height, and decreases with wave age. However, scatter in the data limits the use of these parameters and other wave dependent parameterizations for modelling efforts. Upon close inspection, it is found that during the onset of three wind events analyzed separately, each of these parameters correlate well with the drag coefficient. However, the dependence of the drag coefficient on each of these parameters varies markedly from event to event. The disparity appears most closely linked to the turning rate of the wind, indicating that temporal and directional effects may play an important role. A temporal lag of O(4) hours between the rise of the wind and subsequent rise in the drag coefficient is also noticed, further pointing out the complexity of the wind-stress system.     

Intercomparison of Multiple Hydroclimatic Datasets across the Lower Nelson River Basin,Manitoba, Canada

ABSTRACT

This study evaluates the 1981–2010 spatiotemporal differences in six available climate datasets (daily total precipitation and mean air temperature) over the Lower Nelson River Basin (LNRB) in ten of its sub-watersheds at seasonal and annual time scales. We find that the Australian National University spline interpolation (ANUSPLIN), and inverse distance weighted (IDW) interpolated observations from 14 Environment and Climate Change Canada (ECCC) meteorological stations show dry biases, whereas reanalysis products tend to overestimate precipitation across most of the basin. All datasets exhibit prominent disagreement in precipitation trends whereby the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) and European Union Water and Global Change (WATCH) Forcing Data ERA-Interim (WFDEI) show exceptional wetting trends, while the IDW and ANUSPLIN data manifest drying trends. Mean air temperature trends generally agree across most of the datasets; however, the North American Regional Reanalysis (NARR) and IDW show stronger warming relative to other datasets. Overall, analyses of the different climate datasets and their ensemble reveal that the choice of input dataset plays a crucial role in the accurate estimation of historical climatic conditions, particularly when assessing trends, for the LNRB. Using the ensemble has the distinct advantage of preserving the unique strengths of all datasets and affords the opportunity to estimate the uncertainty for hydrologic modelling and climate change impact studies.     

Variability and Long-Term Trend of Total Cloud Cover in China Derived from ISCCP, ERA-40, CRU3, and Ground Station Datasets

Total Cloud Cover (TCC) over China determined from four climate datasets including the International Satellite Cloud Climatology Project (ISCCP), the 40-year Re-Analysis Project of the European Centre for Medium-Range Weather Forecasts (ERA-40), Climate Research Unit Time Series 3.0 (CRU3), and ground station datasets are used to show spatial and temporal variation of TCC and their differences. It is demonstrated that the four datasets show similar spatial pattern and seasonal variation. The maximum value is derived from ISCCP. TCC value in North China derived from ERA-40 is 50% larger than that from the station dataset; however, the value is 50% less than that in South China. The annual TCC of ISCCP, ERA-40, and ground station datasets shows a decreasing trend during 1984-2002; however, an increasing trend is derived from CRU3. The results of this study imply remarkable differences of TCC derived from surface and satellite observations as well as model simulations. The potential effects of these differences on cloud climatology and associated climatic issues should be carefully considered.     

20CR再分析资料在东亚夏季风区的质量评估

本文利用NCEP1和ERA40再分析资料, 并结合观测资料, 对最新公布的一套再分析资料——20CR再分析资料在东亚夏季风区的质量进行了综合评估。本文主要是从气候态、年际变率、年代际变化三个方面, 来评估20CR再分析资料在东亚夏季风区的质量。结果表明, 在气候平均态上, 20CR再分析资料基本合理再现了东亚夏季风区的高低层环流场(包括南亚高压、副热带西风急流、近地层风场)以及经向环流圈特征。但相较于NCEP1和ERA40, 20CR所刻画的南亚高压偏强, 西风急流偏北, 对流层中上层温度系统性偏高。在年际变率方面, 除了NCEP1在1967年之前存在偏差, 使其结果和ERA40、20CR不同之外, 三套再分析资料刻画的东亚夏季风变率在其它时段高度一致。三套资料在以纬向风为基础的东亚夏季风指数上的一致性, 高于以经向风为基础的东亚夏季风指数, 其中以低层纬向风为基础的东亚夏季风指数的一致性最高。20CR再分析资料可以较好地再现与东亚夏季风相联系的地表气温和降水年际变化特征, 其刻画的地表气温正相关中心位置偏西、强度最强, 且在河套平原地区有一个弱的负相关中心, 而其描述的降水在孟加拉湾和长江流域较之另外两套再分析资料更接近观测结果, 在热带地区和海上却反之。在年代际变化方面, 20CR再分析资料未能合理再现东亚夏季风年代际减弱的现象, 这也体现在不能合理再现青藏高原下游年代际变冷和“南涝北旱”降水型上, 这主要是因为20CR再分析资料所刻画的东亚地区对流层中上层年代际变冷信号偏弱所致。而在百年时间尺度上, 20CR再分析资料所刻画的东亚夏季风变化与观测较为一致;20CR再分析资料可以合理再现出东亚夏季风区1920年代前的显著冷期和1990年代之后的迅速增暖期, 但对1920～1950年代相对暖期和1950～1980年代相对冷期的再现能力较差。     

4种再分析资料在中国区域的适用性研究进展

根据NCEP/NCAR、NCEP/DOE、ERA-40和JRA-25共4种比较常用的再分析资料在中国区域的适用性研究作了概述。结果表明：4种资料既呈现出一定的共性,又在不同区域、不同时间尺度上差异显著;再分析资料的可信度普遍表现为在东部地区好于西部地区,低纬地区好于高纬地区,这可能与中国地面气象站点＂东密西疏＂有关;1979年前的再分析资料存在较大的不确定性,因此在应用该时段NCEP/NCAR资料研究长期气候变化研究时需要谨慎;再分析资料的不同要素在不同时间、不同地域的适用性不同,因此应分区域、分要素和分季节选择合适的资料。从不同角度探讨了再分析资料的优缺点及在中国区域的适用性,旨在为中国区域气候变化研究选用再分析资料提供借鉴和参考。     

An Online Model Correction Method Based on an Inverse Problem:PartⅡ——Systematic Model Error Correction

An online systematic error correction is presented and examined as a technique to improve the accuracy of real-time numerical weather prediction, based on the dataset of model errors(MEs) in past intervals. Given the analyses, the ME in each interval(6 h) between two analyses can be iteratively obtained by introducing an unknown tendency term into the prediction equation, shown in Part I of this two-paper series. In this part, after analyzing the 5-year(2001–2005) GRAPESGFS(Global Forecast System of the Global and Regional Assimilation and Prediction System) error patterns and evolution,a systematic model error correction is given based on the least-squares approach by firstly using the past MEs. To test the correction, we applied the approach in GRAPES-GFS for July 2009 and January 2010. The datasets associated with the initial condition and SST used in this study were based on NCEP(National Centers for Environmental Prediction) FNL(final) data.The results indicated that the Northern Hemispheric systematically underestimated equator-to-pole geopotential gradient and westerly wind of GRAPES-GFS were largely enhanced, and the biases of temperature and wind in the tropics were strongly reduced. Therefore, the correction results in a more skillful forecast with lower mean bias and root-mean-square error and higher anomaly correlation coefficient.     

Wind Waves in the Northwestern Black Sea

The study presents the results of long-term monitoring of wind waves was carried out on the offshore fixed gas production platform in the northwestern part of the Black Sea in 1995–2011. The analysis of more than 31000 wave records provided reliable statistical characteristics of wind waves in the analyzed region. It was found that the maximum wave height reached 4.8 m in summer and 8.76 m in winter. The maximum hourly wave height exceeds significant wave height by 1.9 times in the vast majority of cases. The method of annual maxima revealed that in the Karkinit Bay the maximum wave height with the return period of 50 years is equal to 9.2 m.     

The <Emphasis Type="SmallCaps">int</Emphasis>OA Experiment: A Study of Ocean-Atmosphere Interactions Under Moderate to Strong Offshore Winds and Opposing Swell Conditions in the Gulf of Tehuantepec,Mexico

The Gulf of Tehuantepec air–sea interaction experiment (intOA) took place from February to April 2005, under the Programme for the Study of the Gulf of Tehuantepec (PEGoT, Spanish acronym for Programa para el Estudio del Golfo de Tehuantepec). PEGoT is underway aiming for better knowledge of the effect of strong and persistent offshore winds on coastal waters and their natural resources, as well as performing advanced numerical modelling of the wave and surface current fields. One of the goals of the intOA experiment is to improve our knowledge on air–sea interaction processes with particular emphasis on the effect of surface waves on the momentum flux for the characteristic and unique conditions that occur when strong Tehuano winds blow offshore against the Pacific Ocean long period swell. For the field campaign, an air–sea interaction spar (ASIS) buoy was deployed in the Gulf of Tehuantepec to measure surface waves and the momentum flux between the ocean and the atmosphere. High frequency radar systems (phase array type) were in operation from two coastal sites and three acoustic Doppler current profilers were deployed near-shore. Synthetic aperture radar images were also acquired as part of the remote sensing component of the experiment. The present paper provides the main results on the wave and wind fields, addressing the direct calculation of the momentum flux and the drag coefficient, and gives an overview of the intOA experiment. Although the effect of swell has been described in recent studies, this is the first time for the very specific conditions encountered, such as swell persistently opposing offshore winds and locally generated waves, to show a clear evidence of the influence on the wind stress of the significant steepness of swell waves.     

What shapes mesoscale wind anomalies in coastal upwelling zones?

Observational studies have shown that mesoscale variations in sea surface temperature may induce mesoscale variations in wind. In eastern subtropical upwelling regions such as the California coast, this mechanism could be of great importance for the mean state and variability of the climate system. In coastal regions orography also creates mesoscale variations in wind, and the orographic effect may extend more than 100?km offshore. The respective roles of SST/wind links and coastal orography in shaping mesoscale wind variations in nearshore regions is not clear. We address this question in the context of the California Upwelling System, using a high-resolution regional numerical modeling system coupling the WRF atmospheric model to the ROMS oceanic model, as well as additional uncoupled experiments to quantify and separate the effects of SST/wind links and coastal orography on mesoscale wind variations. After taking into account potential biases in the representation of the strength of SST/wind links by the model, our results suggest that the magnitude of mesoscale wind variations arising from the orographic effects is roughly twice that of wind variations associated with mesoscale SST anomalies. This indicates that even in this region where coastal orography is complex and leaves a strong imprint on coastal winds, the role of SST/winds links in shaping coastal circulation and climate cannot be neglected.     

A Statistical Model for the Prediction of Wind-Speed Probabilities in the Atmospheric Surface Layer

Wind fields in the atmospheric surface layer (ASL) are highly three-dimensional and characterized by strong spatial and temporal variability. For various applications such as wind-comfort assessments and structural design, an understanding of potentially hazardous wind extremes is important. Statistical models are designed to facilitate conclusions about the occurrence probability of wind speeds based on the knowledge of low-order flow statistics. Being particularly interested in the upper tail regions we show that the statistical behaviour of near-surface wind speeds is adequately represented by the Beta distribution. By using the properties of the Beta probability density function in combination with a model for estimating extreme values based on readily available turbulence statistics, it is demonstrated that this novel modelling approach reliably predicts the upper margins of encountered wind speeds. The model’s basic parameter is derived from three substantially different calibrating datasets of flow in the ASL originating from boundary-layer wind-tunnel measurements and direct numerical simulation. Evaluating the model based on independent field observations of near-surface wind speeds shows a high level of agreement between the statistically modelled horizontal wind speeds and measurements. The results show that, based on knowledge of only a few simple flow statistics (mean wind speed, wind-speed fluctuations and integral time scales), the occurrence probability of velocity magnitudes at arbitrary flow locations in the ASL can be estimated with a high degree of confidence.     

High-resolution sea wind hindcasts over the Mediterranean area

The goal of this study is to develop a high-resolution atmospheric hindcast over the Mediterranean area using the WRF-ARW model, focusing on offshore surface wind fields. In order to choose the most adequate model configuration, the study provides details on the calibration of the experimental saet-up through a sensitivity test considering the October–December 2001 period (the 2001 super-storm event in the West Mediterranean). A daily forecast outperforms the spectral technique of previous products and the boundary data from ERA-Interim reanalysis produces the most accurate estimates in terms of wind variability and hour-to-hour correspondence. According to the sensitivity test, two data sets of wind hindcast are produced: the SeaWind I (30-km horizontal resolution for a period of 60 years) and the SeaWind II (15-km horizontal resolution for 20 years). The validation of the resulting surface winds is undertaken considering two offshore observational datasets. On the one hand, hourly surface buoy stations are used to validate wind time series at specific locations; on the other hand, wind altimeter satellite observations are considered for spatial validation in the whole Mediterranean Sea. The results obtained from this validation process show a very good agreement with observations for the southern Europe region. Finally, SeaWind I and II are used to characterize offshore wind fields in the Mediterranean Sea. The statistical structure of sea surface wind is analyzed and the agreement with Weibull probability distribution is discussed. In addition, wind persistence and extreme wind speed (50 year return period) are characterized and relevant areas of wind power generation are described by estimating wind energy quantities.     

黄海典型气旋与反气旋式海洋涡旋特征及影响机制模拟研究

为探讨黄海海洋涡旋的三维结构特征、能量输送与转换及影响机制,对黄海海域典型台风海洋气旋与近海海湾反气旋式涡旋个例进行数值模拟和时空诊断分析。采用FVCOM(Finite Volume Community Ocean Model)区域海洋数值模式精细化描述台风海洋涡旋与近海海洋中小尺度涡旋系统。对涡旋能量传输特征模拟显示,气旋式和反气旋式海洋涡旋中,非对称强流区动能能量下传比涡旋中心部位的强度更强,维持时间更长,下传深度更深。反气旋式海洋涡旋因Ekman流动形成的向中心辐合作用,造成此类差异更显著。气旋涡的动能主要来源于台风的近海面风应力动能和海洋涡旋有效位能的转换,反气旋涡旋区域风动力偏弱,其动能强度维持在低位,其涡旋增强伴随着有效位能的增加。环境因子影响机制从风浪,底摩擦和地形三方面讨论。结果显示:耦合波浪模块后,台风强风应力和风浪的综合作用扩大台风海洋涡旋尺度,并增强涡旋环流强度,同时对相邻的反气旋涡有压缩和减弱作用。风浪效应对台风海洋涡旋有正贡献。强台风过程表层环流响应台风应力而浅水地形和底摩擦强烈影响涡旋下层,造成台风海洋涡旋结构在垂直方向上偏移,并影响到下层环流速度减小,流向与表层相反。在海洋气旋涡和反气旋涡的显著辐散区,其混合层下方有温盐要素的涌升对应,辐合区有温盐要素的下沉对应;同时海底地形的升降也造成温盐强迫上升与下降,其强度与地形起伏尺度成正比,较环流系统作用更强。     

黄海西部海洋工程风、浪设计参数的分析和计算

使用黄海西部站点观测、船舶记录及海洋调查资料，建立计算海面风速的转换关系式。综合转换风速、台风报告或台风风场计算风速，分七个区块建立年最大风速序列；藉助极值Ⅰ型概率分布，推导了各区不同重现期的大风极值。开发使用美国SOWM波浪谱模式产品，推导了二个格点上不同重现期的波高极值，该值与近海波浪计算值一起组成合理的极端波高分布。     

Performance of Steady-State Dispersion Models Under Low Wind-Speed Conditions

We examine the performance of two steady-state models, a numerical solution of the advection-diffusion equation and the Gaussian plume-model-based AERMOD (the American Meteorological Society/Environmental Protection Agency Regulatory Model), to predict dispersion for surface releases under low wind-speed conditions. A comparison of model estimates with observations from two tracer studies, the Prairie Grass experiment and the Idaho Falls experiment indicates that about 50% of the concentration estimates are within a factor of two of the observations, but the scatter is large: the 95% confidence interval of the ratio of the observed to estimated concentrations is about 4. The model based on the numerical solution of the diffusion equation in combination with the model of Eckman (1994, Atmos Environ 28:265–272) for horizontal spread performs better than AERMOD in explaining the observations. Accounting for meandering of the wind reduces some of the overestimation of concentrations at low wind speeds. The results deteriorate when routine one-level observations are used to construct model inputs. An empirical modification to the similarity estimate of the surface friction velocity reduces the underestimation at low wind speeds.