Application of the method of artificial neural networks to the downscaling of precipitation forecasts in the coastal region of the Black Sea

We correct the diurnal data on precipitation obtained from the output of the global system of reanalysis as applied to the observed daily amounts of precipitation at certain geographic points of the coastal region of the Black Sea. The estimations of the actual amounts of precipitation are taken from the ECAD (European Climate Assessment and Dataset) database. We analyze the amounts of precipitation for three winter months. As a working tool for the investigation of correlations between the regular meteorological variables (predictors) taken from the reanalysis and the local amounts of precipitation at certain geographic points (predictants), we use the method of artificial neural networks (ANN). A numerical criterion of adequacy of the estimates of the daily amounts of precipitation performed according to reanalysis and the ANN method is proposed. By using this criterion, we show that the efficiency of the ANN method in simulating precipitation is higher as compared with the procedure of reanalysis. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 3, pp. 23–35, May–June, 2006.     

基于CESM预测系统对全球关键海区海温主要模态后报能力评估

基于HadISST、ERSST和OISST 3种再分析海表面温度数据集,对CESM1全球气候预测系统模拟的重点海域海温的主要模态进行了评估。结果表明,模式能够基本再现再分析资料表征的海表面温度异常时空特征,5个关键海区海表面温度异常EOF分析前两个模态,超前1个月的后报结果均有较高的可预报性。特别是热带太平洋第一模态（ENSO模态）,超前3个月的后报与再分析相关系数能够达到0.79,能够为ENSO的预报提供宝贵参考。模式不可避免的存在较多误差,空间上表现为海表面温度异常（SSTA）显著区域的范围以及位置的差异,特别值得注意的是热带印度洋SSTA第一模态存在一个伪印度洋偶极子模态,同时此海区也是3种再分析资料SSTA第一模态空间差异最大的区域;时间序列上均表现出异常高频信号和异常波动情况,误差的大小通常是由同化、超前1月后报、超前3月后报逐渐增大,体现出初始误差随着积分逐渐积累的特征。热带大西洋SSTA第一模态时间序列前5年的位相存在明显偏差,将其剔除后,其时间序列与再分析资料的相关性有实质性改善。     

ERA-Interim和NCEP/NCAR再分析资料在我国东南近海适用性分析

利用我国东南近海5个浮标站观测资料,对2012—2016年ERA-Interim和NCEP/NCAR再分析资料10 m风、2 m气温、海平面气压的适用性进行了评估。结果表明:NCEP/NCAR的再分析10 m风适用性更好,ERA-Interim的2 m气温适用性更好,海平面气压两者差异不大。风速再分析值与观测值具有较好的一致性,相关系数达0.8～0.9,但再分析风速总体上有偏小的趋势,平均偏差在-1.3～0 m/s之间,均方根误差在1.5～3 m/s。再分析资料的平均风向有顺时针偏差的趋势,温州浮标偏右达14°以上,均方根误差大多在40°～50°。不管风速还是风向,5个浮标站中均以舟山浮标的再分析值与观测值最为接近;分析还表明,再分析资料的冬季风代表性相对较差,这是造成风速和风向系统性偏差的主要原因。再分析资料与观测2 m气温相关系数均在0.95以上,且有偏高的趋势,NCEP偏高更为明显,有4个浮标站平均偏差达1～2℃,而ERA-I仅1个浮标站偏差1～2℃,4个在1℃以内。春季和冬季气温偏高最为明显,春季升温过程存在异常偏高的可能,秋季气温与观测值最为接近。海平面气压适用性较好,总体优于10 m风和2 m气温,且季节间差异也不大。     

Operational Data Assimilation System for the Kuroshio South of Japan: Reanalysis and Validation

We describe an operational ocean data assimilation system for the Kuroshio and its validation using a nine-year reanalysis (historical run from 1993 to 2001) dataset of upper-ocean state estimation in the North Pacific. The horizontal structure of volume transport of the Ryukyu Current System (RCS) is shown from the reanalysis: The RCS is connected to the flow of the subtropical gyre, and its volume transport gradually increases from south-east of Okinawa (5–10 Sv) to the east of Amami-Ohshima Island (20 Sv). Comparing the reanalysis with independent observations on the southeast slope of the Amami-Ohshima Island indicates that the root mean square differences (RMSDs) are 0.076 (0.037) m/s in the period of December 1998 to November 1999 (November 1999 to November 2000) respectively. The reanalysis field has a bias (3.1 Sv) of the volume transport of the RCS and the RMSD (3.5 Sv) which is larger than the observed variability (2.81 Sv). Surface velocity and the Kuroshio axis south of Japan are also examined. Comparison of the reanalysis and ADCP data gave maximum RMSD of 0.749 (0.271) m/s in the strong (weak) current regions, respectively. The annual mean value of the axis error is 19 km in 1998. The RMSD of the error is at most 50 km, in 294 cases in the observation period, which is smaller than the observed root mean square variability of the axis (64 km). This revised version was published online in July 2006 with corrections to the Cover Date.     

基于四种再分析资料的1988-2015年中国近海月均风速特征评估

This study investigates the long-term changes of monthly sea surface wind speeds over the China seas from 1988 to 2015. The 10-meter wind speeds products from four major global reanalysis datasets with high resolution are used: Cross-Calibrated Multi-Platform data set(CCMP), NCEP climate forecast system reanalysis data set(CFSR),ERA-interim reanalysis data set(ERA-int) and Japanese 55-year reanalysis data set(JRA55). The monthly sea surface wind speeds of four major reanalysis data sets have been investigated through comparisons with the longterm and homogeneous observation wind speeds data recorded at ten stations. The results reveal that(1) the wind speeds bias of CCMP, CFSR, ERA-int and JRA55 are 0.91 m/s, 1.22 m/s, 0.62 m/s and 0.22 m/s, respectively.The wind speeds RMSE of CCMP, CFSR, ERA-int and JRA55 are 1.38 m/s, 1.59 m/s, 1.01 m/s and 0.96 m/s,respectively;(2) JRA55 and ERA-int provides a realistic representation of monthly wind speeds, while CCMP and CFSR tend to overestimate observed wind speeds. And all the four data sets tend to underestimate observed wind speeds in Bohai Sea and Yellow Sea;(3) Comparing the annual wind speeds trends between observation and the four data sets at ten stations for 1988-1997, 1988–2007 and 1988–2015, the result show that ERA-int is superior to represent homogeneity monthly wind speeds over the China seaes.     

ERA5海面气压和风速再分析资料在渤海和北黄海适用性分析

利用渤海和北黄海9个站位浮标的海面10 m气压和风速观测资料对ERA5再分析资料的适用性进行了初步评估。结果表明：海面气压和风速再分析资料与观测资料具有良好的相关性,不同时间尺度的统计结果具有一定差异,累年逐月的相关性优于日均和日极值,极端天气下的相关系数最低。不同时间尺度的统计结果显示,气压再分析资料与观测值的偏差总体为负值,即高压再分析资料较观测值偏弱、低压再分析资料较观测值偏强,二者存在一定的系统性偏差;风速统计结果显示,偏差与风力级别有关,大风速时,风速再分析资料普遍小于观测值。累年逐月再分析资料反映了研究区气压和风速的季节变化特征,偏差也呈现出季节性变化,且冬季的适用性优于夏季;极端天气情况下,气压和风速再分析资料的适用性较差,需要开展进一步的订正处理。     

Probability distributions for cyclones and anticyclones from the NCEP/NCAR reanalysis data and the INM RAS climate model

Analysis of statistical characteristics of cyclones and anticyclones in the latitudinal belt between 20° and 80°N has been performed with the NCEP/NCAR reanalysis data and simulations with the general circulation climate model of the Institute of Numerical Mathematics of the Russian Academy of Sciences (INM RAS GCCM). The model results have been analyzed for the second half of the 20th century against the NCEP/NCAR reanalysis data and for the 21st century with the SRES-A2 anthropogenic scenario. Overall for the 20th century, no statistically significant changes in the number of cyclones and anticyclones are obtained from either the NCEP/NCAR reanalysis data [1] or from simulations with the INM RAS GCCM [2]. It is found that the total number of cyclones and anticyclones decreased in the 20th century as compared to the 21st century. It is shown that cumulative distributions of the number of cyclones and anticyclones by their intensities and areas have an exponential form from both the reanalysis data and the model simulations, although the corresponding exponents are different.     

基于观测和再分析资料的气温、海表温度在江苏海域的比较分析

文章利用海洋台站观测资料,对3类再分析资料的气温、海表温度数据在江苏海域进行比较分析。结果表明：总体而言,气温、海表温度的再分析数据与观测数据均具有很高的一致性,且海表温度再分析数据与观测数据的一致性高于气温再分析数据;在3种再分析资料中,ERA5的可靠性优于JRA-55和NCEP;在离岸较远的外磕脚测点,再分析数据与观测数据的一致性高于另2个测点;在台风影响期间,吕四测点、外磕脚测点的再分析数据与观测数据更为接近,在更近岸的连云港测点,再分析数据与观测数据的差异更大;在冷空气造成的气温下降期间,再分析数据与观测数据的差异较小,但在气温缓步回升期间,吕四测点、外磕脚测点的再分析数据明显高于观测数据;在冷空气影响过程中,再分析数据反映的海表温度下降幅度明显低于观测数据。本研究可为再分析资料在江苏海域的适用性和可信度提供评估依据。     

Sensitivity of wave model predictions to wind fields in the Western Mediterranean sea

The paper compares the wave hindcast in the Western Mediterranean sea using the reanalysis wind fields from HIPOCAS and ERA-40 from ECMWF for November 2001. The study has concentrated on the Mediterranean coast of Spain where there are known difficulties with the wind and wave modelling. Two winter storms have been compared. The main differences between the significant wave heights using the ERA-40 reanalysis (ECMWF) and HIPOCAS reanalysis winds were observed to increase when moving southwards in the geographical domain at the offshore locations. Systematic negative biases of Hs were obtained with the ERA-40 data at all the coastal locations analyzed, whereas positive biases are typical for the HIPOCAS reanalysis. For offshore and coastal locations when using the ERA-40 data the Hs biases increased moving to South, while this pattern was not so clear for the HIPOCAS data. The inconsistencies in the comparisons of modelled waves against measurements seem to be associated with the quality of the wind fields.     

中国近海ASCAT和ERA-Interim风场资料的评估

利用中国近海23个浮标对2015年一整年的ASCAT轨道风场和ERA-Interim再分析风场进行质量评估,并对比了ERA-Interim和CFSV2两种再分析风场资料在中国近海的适用性。结果表明:ASCAT在中国近海与浮标风速的一致性优于ERA-Interim,而二者与浮标风向的一致性则相差不大。同时,对比CFSV2与ERA-Interim的误差统计结果发现,CFSV2的风速误差较ERAInterim略小,风向误差相差不大。     

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.     

西太平洋海面风场数据集的建立及其应用

再分析风场数据至今已发展了约30年,目前人们使用区域乃至全球网格化再分析数据集愈加频繁,但风场再分析数据在强天气过程中的强度明显不足,将风场再分析数据同台风观测数据相融合是解决问题的重要途径,本文通过"构建经验台风圆形风场-叠加台风移行风场-流入角修正-叠加背景再分析风场"的基本思路,融合了西太平洋联合警报中心的所有台...     

Wave climate variability in the North Atlantic in recent decades in the winter period using numerical modeling

The study focuses on investigating significant wave height, including both mean and extreme values, in the North Atlantic in winter during the period from 1979 to 2010. We perform a 32-year wind wave hindcast for the North Atlantic using a spectral ocean wave model (WaveWatch III) and a high-resolution nonhydrostatic atmospheric model (WRF-ARW), which provides the wind forcing function. Analysis of the 32-year hindcast of wave characteristics in the North Atlantic reveals stronger mean and extreme waves simulated with high resolution modeling systems and identifies significant downward trends in the mean significant wave height in the subpolar North Atlantic. Such trends were not found in the wave characteristics from ERA-Interim reanalysis. At the same time, the 32-year hindcast did not confirm the statistically significance of strong positive trends in the central Atlantic diagnosed by ERA-Interim reanalysis; differences between the reanalysis and hindcast are discussed.     

Estimates of the sensitivity of cyclonic activity in the troposphere of extratropical latitudes to changes in the temperature regime

Quantitative estimates of the sensitivity of the number and size of extratropical cyclones in the Northern Hemisphere to changes in the surface temperature are obtained with the use of NCEP/NCAR reanalysis data over a 60-year period and are compared with estimates on the basis of a relatively simple model of the cyclonic and anticyclonic activities in the atmosphere of extratropical latitudes associated with characteristics of atmospheric temperature stratification (MMPKh model). The model estimates are also obtained for a dry and moist atmosphere. With the use of the reanalysis data, extratropical latitudes are, on the whole, characterized by a general decrease in the number of cyclones and the density of their packing in extratropical latitudes as the surface temperature increases. However, in the MMPKh model for moist atmosphere, estimates of the parameter of sensitivity of the number of cyclones at midlatitudes and at extratropical latitudes in the Northern Hemisphere as a whole are close to those based on the reanalysis data. The influences of the meridional gradient of the surface temperature and the vertical temperature gradient in the troposphere on changes in the number and size of extratropical cyclones are estimated from the reanalysis data and model calculations. It is noted that the most significant changes in annual mean variations in the number and size of extratropical cyclones are associated with the vertical temperature gradient in the troposphere. In this case, an increase in the vertical temperature gradient in the troposphere decreases the size of cyclones. The relative influences of the vertical and meridional temperature gradients are different for different latitudinal zones.     

遥感海表盐度分析产品的有效分辨率研究

针对SMOS和Aquarius海表盐度误差分析没有区分不同空间频谱信噪特征的问题,基于6种主要的遥感盐度分析产品,根据定性图像、纬向波数谱、均方根误差等指标,分析产品的有效分辨率并探讨其原因机制。研究表明:CATDS-0.25°分析产品所描述的盐度场中小尺度结构失真,其较高谱能量密度在热带海域以噪音为主,而在西边界流等海域以信号为主;BEC-L3-0.25°有着较小的均方根误差、清晰的盐度图像、显著的中尺度能量,最适于描绘中尺度(25~100 km)盐度特征;BEC-L4-0.25°被奇异谱分析方法过度平滑了盐度场;Aquarius-V2-1.00°通过局部平滑处理,在描述大尺度(＞100 km)盐度现象方面表现较好;Aquarius-CAP-1.00°通过主动-被动联合算法(CAP)减小了均方根误差,但图像中卫星轨道形态明显;CATDS-1.00°的图像形态、能量分布和误差特征与Aquarius-V2-1.00°相当。这些结论可为用户正确使用产品进行地球物理学研究提供参考。     

Evaluation of reanalysis surface wind products with quality-assured buoy wind measurements along the north coast of the South China Sea

Three archived reanalysis wind vectors at 10 m height in the wind speed range of 2–15 m/s, namely, the second version of the National Centres for Environmental Prediction(NCEP) Climate Forecast System Reanalysis(CFSv2), European Centre for Medium-Range Weather Forecasting Interim Reanalysis(ERA-I) and NCEPDepartment of Energy(DOE) Reanalysis 2(NCEP-2) products, are evaluated by a comparison with the winds measured by moored buoys in coastal regions of the South China Sea(SCS). The buoy data are first quality controlled by extensive techniques that help eliminate degraded measurements. The evaluation results reveal that the CFSv2 wind vectors are most consistent with the buoy winds(with average biases of 0.01 m/s and 1.76°). The ERA-I winds significantly underestimate the buoy wind speed(with an average bias of –1.57 m/s), while the statistical errors in the NCEP-2 wind direction have the largest magnitude. The diagnosis of the reanalysis wind errors shows the residuals of all three reanalysis wind speeds(reanalysis-buoy) decrease with increasing buoy wind speed, suggesting a narrower wind speed range than that of the observations. Moreover, wind direction errors are examined to depend on the magnitude of the wind speed and the wind speed biases. In general, the evaluation of three reanalysis wind products demonstrates that CFSv2 wind vectors are the closest to the winds along the north coast of the SCS and are sufficiently accurate to be used in numerical models.     

A Comparison of Sea Surface Fluxes over Southern Indian Ocean Cruise Expedition Observation and NCEP Reanalysis

Surface layer atmospheric and ocean observations have been collected along the cruise track from a special scientific expedition to Antarctica. Bulk estimates of surface momentum flux, sensible heat flux and latent heat flux have been computed applying bulk algorithms from the data collected along cruise track during the time period January 27 to March 31, 2006, and compared the results with National Centre for Environmental Prediction (NCEP) reanalysis. Underestimation of surface momentum flux in roaring forties (40°S–50°S) area of Indian Ocean is seen from NCEP reanalysis. Systemic differences in sensible and latent heat fluxes between observed and NCEP reanalysis have been found. Along the cruise track, the average sensible (latent) heat flux was 9.45 Wm^?2 (67.46 Wm^?2) and 3.75 Wm^?2 (64.45 Wm^?2) from the direct measurement and NCEP reanalysis, respectively. The NCEP reanalysis is being widely used in numerical modeling studies, and the discrepancies shown in the NCEP reanalysis in present study will be of immense use to the modeling community of the Indian Ocean in general and Southern Indian Ocean in particular.     

Comparison of seasonal and interannual variability of snow cover in Russian watersheds according to observations and reanalyses

Seasonal and interannual variability of the water equivalent of snow over the period 1966–1996 is analyzed with the use of the ERA/ECMWF reanalysis data, two versions of the NCEP reanalysis, and ground-based instrumental observations. Regions have been determined where snow characteristics from various sources are most consistent. In general, the snow water equivalent data from the ERA reanalysis are closer to the snow survey data than the NCEP reanalysis data. The best consistency of the water equivalent of snow in the reanalyses with measured data is observed in the period when the snow cover begins to form, in November and December, and the largest discrepancies are found for the snowmelt period, April and May. The interannual variability of the water equivalent in Asian Russia in the 1990s is better reproduced by the NCEP/DOE-II reanalysis. The snow cover data from satellite observations are compared with ground-based instrumental observations. It is shown that large discrepancies between the snow surveys and satellite observations may be observed at the snow-cover boundary during periods of snow formation and snowmelt.     

Short-Range Prediction Experiments with Operational Data Assimilation System for the Kuroshio South of Japan

The short-range (one month) variability of the Kuroshio path was predicted in 84 experiments (90-day predictions) using a model in an operational data assimilation system based on data from 1993 to 1999. The predictions started from an initial condition or members of a set of initial conditions, obtained in a reanalysis experiment. The predictions represent the transition from straight to meander of the Kuroshio path, and the results have been analyzed according to previously proposed mechanisms of the transition with eddy propagation and interaction acting as a trigger of the meander and self-sustained oscillation. The reanalysis shows that the meander evolves due to eddy activity. Simulation (no assimilation) shows no meander state, even with the same atmospheric forcing as the prediction. It is suggested therefore that the initial condition contains information on the meander and the system can represent the evolution. Mean (standard deviation) values of the axis error for all 84 cases are 13, 17, and 20 (10, 10, and 12) km, in 138.5°E, in the 30-, 60-, and 90-day predictions respectively. The observed mean deviation from seasonal variation is 30 km. The predictive limit of the system is thus about 80 days. The time scale of the limit depends on which stage in the transition is adopted as the initial condition. The gradual decrease of the amplitude in a stage from meander to straight paths is also predicted. The predictive limit is about 20 days, which is shorter than the prediction of the opposite transition. This revised version was published online in July 2006 with corrections to the Cover Date.