Comparisons of Two Ensemble Mean Methods in Measuring the Average Error Growth and the Predictability

In this paper, taking the Lorenz system as an example, we compare the influences of the arithmetic mean and the geometric mean on measuring the global and local average error growth. The results show that the geometric mean error (GME) has a smoother growth than the arithmetic mean error (AME) for the global average error growth, and the GME is directly related to the maximal Lyapunov exponent, but the AME is not, as already noted by Krishnamurthy in 1993. Besides these, the GME is shown to be more appropriate than the AME in measuring the mean error growth in terms of the probability distribution of errors. The physical meanings of the saturation levels of the AME and the GME are also shown to be different. However, there is no obvious difference between the local average error growth with the arithmetic mean and the geometric mean, indicating that the choices of the AME or the GME have no influence on the measure of local average predictability.     

风向的统计方法研究

在核安全分析和环境影响评价中,必须使用到的是小时气象数据。对自动气象站小时风向的计算方法和小时数据如何统计,地面气象规范中和核安全导则中还没有定论,因此本文对小时风向统计方法展开讨论。目前小时风向值的计算方法有算术平均法、滑动平均法、矢量平均法和频率最高法,针对算术平均法和滑动平均法对经过0°的风向统计容易出现误差,本文提出对此的修正方法并对4种计算方法进行了比较。结果表明：文中过零风向修正方法简便准确,小时数据统计方法为正点前10min数据时,修正算术平均法更可靠,但该方法对风速为零时的判断容易出现误差,因此在小风、静风频率高的地方推荐矢量平均法。关于小时数据的划分方法,美国核管会RG1.23与我国核安全导则及地面气象规范中的规定不同,因此文中利用实测资料对不同小时数据统计方法所得结果比较,分析表明,取整点前或其他时段的10min和15min的数据进行平均的风向相关矩阵一致性为97.87%;取4个15min平均值的平均或6个10min数据平均值的平均作为小时值的风向相关矩阵一致性为99.96%,这两种统计方法与取10min和15min的一致性为86.00%,相对较差;取60min时段的平均值作为小时值则与其余方法一致性最差。     

遥感图像几何校正的实现

描述了遥感图和何畸变的原因,阐述了几何精校正原理,并在此基础上给出了一个简单衫的精校正方案及其算法。理论上,此校正方法具有小于一个像素点的误差,满足业务上所需的订正精度。     

误差非线性的增长理论及可预报性研究

对非线性系统的误差发展方程不作线性化近似,直接用原始的误差发展方程来研究初始误差的发展,提出了误差非线性的增长理论。首先,在相空间中定义一个非线性误差传播算子,初始误差在这个算子的作用下,可以非线性发展成任意时刻的误差;然后,在此基础上,引入了非线性局部Lyapunov指数的概念。由平均非线性局部Lyapunov指数可以得到误差平均相对增长随时间的演变情况;对于一个混沌系统,误差平均相对增长被证明将趋于一个饱和值,利用这个饱和值,混沌系统的可预报期限可以被定量地确定。误差非线性的增长理论可以应用于有限尺度大小初始扰动的可预报性研究,较误差的线性增长理论有明显的优越性。     

基于人工引雷的粤港澳闪电定位系统性能评估

利用2014—2019年中国气象局雷电野外科学试验基地广州从化人工触发闪电试验所获资料,评估粤港澳闪电定位系统(Guangdong-Hongkong-Macau Lightning Location System,GHMLLS)性能,结果表明:GHMLLS对人工触发闪电和回击的探测效率分别为96%(48/50)和88%(233/265),回击位置定位误差的算术平均值、几何平均值和中值分别为279 m,193 m和202 m。对于触发闪电的回击过程,GHMLLS探测的回击电流峰值(I_LLS)全部偏低,与通道底部雷电流峰值的直接测量结果(I_DM)相比,I_LLS的相对偏差平均值(中值)为-37%(-36%),但I_LLS和I_DM相关系数为0.93,存在显著正相关关系(达到0.01显著性水平);截距为0的线性拟合结果表明I_LLS与I_DM存在65%的比例关系,利用该系数校正I_LLS,结果的相对偏差绝对值的平均值(中值)为15%(12%)。GHMLLS有对应定位记录的233次触发闪电回击中,16次定位结果为云闪,判别正确率为93%。被误判为云闪的回击的I_DM更低,可用于定位的站点数量更少,定位误差更大,I_LLS的精度更低。     

统计降尺度法对华北地区未来区域气温变化情景的预估

迄今为止,大部分海气耦合气候模式（AOGCM）的空间分辨率还较低,很难对区域尺度的气候变化情景做合理的预测。降尺度法已广泛用于弥补AOGCM在这方面的不足。作者采用统计降尺度方法对1月和7月华北地区49个气象观测站的未来月平均温度变化情景进行预估。采用的统计降尺度方法是主分量分析与逐步回归分析相结合的多元线性回归模型。首先,采用1961～2000年的 NCEP再分析资料和49个台站的观测资料建立月平均温度的统计降尺度模型,然后把建立的统计降尺度模型应用于HadCM3 SRES A2 和 B2 两种排放情景, 从而生成各个台站1950～2099年1月份和7月份温度变化情景。结果表明:在当前气候条件下,无论1月还是7月,统计降尺度方法模拟的温度与观测的温度有很好的一致性,而且在大多数台站,统计降尺度模拟气温与观测值相比略微偏低。对于未来气候情景的预估方面,无论1月还是7月,也无论是HadCM3 SRES A2 还是B2排放情景驱动统计模型,结果表明大多数的站点都存在温度的明显上升趋势,同时7月的上升趋势与1月相比偏低。     

Ensemble mean forecast skill and applications with the T213 ensemble prediction system

Ensemble forecasting has become the prevailing method in current operational weather forecasting. Although ensemble mean forecast skill has been studied for many ensemble prediction systems(EPSs) and different cases, theoretical analysis regarding ensemble mean forecast skill has rarely been investigated, especially quantitative analysis without any assumptions of ensemble members. This paper investigates fundamental questions about the ensemble mean, such as the advantage of the ensemble mean over individual members, the potential skill of the ensemble mean, and the skill gain of the ensemble mean with increasing ensemble size. The average error coefficient between each pair of ensemble members is the most important factor in ensemble mean forecast skill, which determines the mean-square error of ensemble mean forecasts and the skill gain with increasing ensemble size. More members are useful if the errors of the members have lower correlations with each other, and vice versa. The theoretical investigation in this study is verified by application with the T213 EPS. A typical EPS has an average error coefficient of between 0.5 and 0.8; the 15-member T213 EPS used here reaches a saturation degree of 95%(i.e., maximum 5% skill gain by adding new members with similar skill to the existing members) for 1–10-day lead time predictions, as far as the mean-square error is concerned.     

Evaluation of mid-depth currents of NCEP reanalysis data in the tropical Pacific using ARGO float position information

1. Introduction The reanalysis dataset (Ji et al., 1995; Kalnay et al., 1996) offered by the National Centers for Environ- mental Prediction (NCEP) has been applied widely in relevant multidisciplinary research. Along with in- creasingly extensive observa…     

BCC_CSM1.1(m) 模式对热带太平洋潜热通量的评估

利用1979—2005年OAFlux (Objectively Analyzed air-sea Fluxes) 观测资料以及CMIP5的15个耦合模式的模拟结果,评估了BCC_CSM1.1(m) 模式对热带太平洋年平均潜热通量气候态和变化趋势的模拟能力,并分析造成趋势偏差的可能原因。结果表明:BCC_CSM1.1(m) 模式模拟热带太平洋年平均潜热通量气候态在各纬度上差异较大, 其中在赤道的模拟能力较佳,而在10°N和8°S附近模拟偏差较大;BCC_CSM1.1(m) 模式对热带太平洋年平均潜热通量趋势的模拟能力一般,造成趋势偏差的主要原因是该模式低估了风速对潜热通量的局地贡献以及它对风速的非局地贡献的模拟存在较大偏差。此外,该模式未能较好地模拟出风速对全球变暖响应。因此,BCC_CSM1.1(m) 模式对热带太平洋年平均潜热通量趋势模拟的改进需加强其对风速模拟的改进。     

由最高最低气温求算的平均气温对我国年平均气温序列影响

针对研究全国近百年平均气温长期变化的实际需要，利用603个测站1961—2002年气温观测资料，比较分析了最高最低平均气温距平序列和4次观测记录平均气温距平序列的差异，讨论了最高、最低气温变化趋势。结果表明:两种统计方法得到的平均气温距平序列及增温速率的差异均不明显，在一定条件下两者可以互相替换。此外，最高、最低气温变化普遍存在不对称现象，且可分为4种类型，这种不对称性对平均气温变化速率并没有明确一致的影响。     

不完整气象资料下基于作物模型的产量预报方法

针对基于作物模型开展产量实时预报后期气象资料的获取问题,提出通过相似类比,从历史气象资料库中获取替代资料的方案,基于CERES-Rice模型系统评估了平均值处理方案和历史相似类比方案的可预报性和误差分布特征。结果表明:水稻产量对成熟前2个月内的气象条件较为敏感,基于气象资料和作物模型开展产量预测,在5%误差范围内可获得60%以上的预测概率;以多年气候平均值替代起报日后期气象资料,在成熟前2个月起报预测概率约为60%,成熟前1个月约为70%,但预报误差系统性偏高;采用气候相似类比方法,从历史气象资料中获取起报日后期替代资料,可有效降低预报误差的系统偏差,若引入后期气候趋势信息,成熟前2个月起报预测概率可达80%以上,较采用历史平均值有显著提高。研究结果为基于作物模型和气象观测及气候预测信息开展产量预报提供了技术方案。     

集合平均方法减小混沌系统计算误差的效果研究

研究了Lorenz非线性系统中使用的集合平均方法来减小计算误差的效果,通过检查5组数值试验(每组20个样本)的结果发现:集合平均对计算误差的减小和消除不如高精度算法有效,这主要体现在以下几方面:1)普通的算法和双精度的计算环境中,若截断误差是主导误差(当初值误差很小时),各集合的平均结果并不收敛于真值,而是收敛于含截断误差的数值解;2)若初值误差为主导时,系统受到初值误差增长规律的影响,数值解收敛于由初值误差主导的误差解;3)这两种误差量级接近的时候,两种误差都无法消除掉。对解的统计特征进行研究表明,可信的数值解与含计算误差的数值解有许多相似的地方,但是与集合平均的数值解有很大不同,同样说明了集合平均不适用于减小计算误差这样的问题。此外,试验结果表明即使数值解的概率分布形式基本正确,也不能保证数值解是正确的。     

The Relationship between Deterministic and Ensemble Mean Forecast Errors Revealed by Global and Local Attractor Radii

It has been demonstrated that ensemble mean forecasts, in the context of the sample mean, have higher forecasting skill than deterministic(or single) forecasts. However, few studies have focused on quantifying the relationship between their forecast errors, especially in individual prediction cases. Clarification of the characteristics of deterministic and ensemble mean forecasts from the perspective of attractors of dynamical systems has also rarely been involved. In this paper, two attractor statistics—namely, the global and local attractor radii(GAR and LAR, respectively)—are applied to reveal the relationship between deterministic and ensemble mean forecast errors. The practical forecast experiments are implemented in a perfect model scenario with the Lorenz96 model as the numerical results for verification. The sample mean errors of deterministic and ensemble mean forecasts can be expressed by GAR and LAR, respectively, and their ratio is found to approach2~(1/2) with lead time. Meanwhile, the LAR can provide the expected ratio of the ensemble mean and deterministic forecast errors in individual cases.     

Study on ETKF-Based Initial Perturbation Scheme for GRAPES Global Ensemble Prediction

Initial perturbation scheme is one of the important problems for ensemble prediction. In this paper, ensemble initial perturbation scheme for Global/Regional Assimilation and PrEdiction System (GRAPES) global ensemble prediction is developed in terms of the ensemble transform Kalman filter (ETKF) method.A new GRAPES global ensemble prediction system (GEPS) is also constructed. The spherical simplex 14-member ensemble prediction experiments, using the simulated observation network and error characteristics of simulated observations and innovation-based in ation, are carried out for about two months. The structure characters and perturbation amplitudes of the ETKF initial perturbations and the perturbation growth characters are analyzed, and their qualities and abilities for the ensemble initial perturbations are given. The preliminary experimental results indicate that the ETKF-based GRAPES ensemble initial perturbations could identify main normal structures of analysis error variance and reflect the perturbation amplitudes.The initial perturbations and the spread are reasonable. The initial perturbation variance, which is approximately equal to the forecast error variance, is found to respond to changes in the observational spatial variations with simulated observational network density. The perturbations generated through the simplex method are also shown to exhibit a very high degree of consistency between initial analysis and short-range forecast perturbations. The appropriate growth and spread of ensemble perturbations can be maintained up to 96-h lead time. The statistical results for 52-day ensemble forecasts show that the forecast scores ofensemble average for the Northern Hemisphere are higher than that of the control forecast. Provided that using more ensemble members, a real-time observational network and a more appropriate inflation factor,better effects of the ETKF-based initial scheme should be shown.     

一种新的集合预报权重平均方法

提出了一种新的考虑权重的集合预报成员平均方法。使用气候等概率区间来对集合成员进行分组, 并根据气候等概率区间的大小及其中的成员数, 对集合成员的权重进行调整, 得到了一种改进的集合平均预报结果。检验表明, 它可以进一步提高集合平均预报的效果。相对于提高模式分辨率或发展庞大的集合预报系统, 这种方法的效果是显著的。     

轨线不稳定与误差增长

从一般的谱展开方程出发，详细推导了误差增长方程。结果表明误差增长率主要由准确解的切线性方程所决定，扰动非线性平流作用不产生方差意义下的误差增长，而只起分配误差的作用。轨线不稳定是产生误差增长的根本原因。文中提出了计算轨线不稳定增长率的方法。这一方法也适合于时间演变状态不稳定问题的讨论，对Ｌｏｒｅｎｚ系统的轨线不稳定计算表明了理论分析的正确及其意义。     

An analysis of six simultaneous wind records taken near Vancouver

Abstract

Hourly wind speeds and directions taken at six sites near Vancouver, Canada over a period of one year during 1971–1972 have been analyzed. Mean magnitude ratios obtained for different wind directions under high wind conditions vary from about 0.3 to 1.4 and can be explained qualitatively by local shelter effects, local terrain effects or anemometer heights. The mean magnitude ratio is in every case shown to be essentially identical to the ratio of mean wind magnitudes, for high wind conditions; this is justified by simple analysis. The standard deviation of the magnitude ratio lies between 0.24 and 0.69 of the mean ratio. Uncertainty in this mean ratio, which is an important link in wind engineering design, may have to be incorporated into statistical design procedures to avoid taking unknown risks in the estimation of design wind speeds.     

Determining the Spectrum of the Nonlinear Local Lyapunov Exponents in a Multidimensional Chaotic System

For an n-dimensional chaotic system, we extend the definition of the nonlinear local Lyapunov exponent(NLLE) from one-to n-dimensional spectra, and present a method for computing the NLLE spectrum. The method is tested on three chaotic systems with different complexity. The results indicate that the NLLE spectrum realistically characterizes the growth rates of initial error vectors along different directions from the linear to nonlinear phases of error growth. This represents an improvement over the traditional Lyapunov exponent spectrum, which only characterizes the error growth rates during the linear phase of error growth. In addition, because the NLLE spectrum can effectively separate the slowly and rapidly growing perturbations, it is shown to be more suitable for estimating the predictability of chaotic systems, as compared to the traditional Lyapunov exponent spectrum.     

中国内陆高原地闪特征的统计分析

利用微秒级时间分辨率的宽带慢天线电场变化仪首次在中国内陆高原地区对雷暴过程中的正、负地闪特征进行了测量和系统分析 ,发现每次雷暴过程中正闪的比例有随总闪频数增大而减少的趋势 ,弱雷暴过程更有利于正地闪的产生。平均来讲 ,正地闪占闪电总数的 16 % ,介于美国夏季雷暴和日本冬季雷暴之间。负地闪闪击间隔的算术平均值和几何平均值分别为 6 4.3ms和 46 .6ms。 5 4%的负地闪有至少一次继后回击强度大于首次回击 ,而且有 2 0 %的继后回击其强度大于首次回击强度。继后回击强度与首次回击强度的比例几何平均值为 0 .46 ,算术平均值为 0 .70 ,平均回击数为 3.76 ,39.8%为单次回击地闪。正地闪的多次回击只占 13.0 % ,且闪击之间的时间间隔也较大 ,算术平均值为 91.7ms。     

观测误差对GRAPES区域集合预报影响的敏感性试验

通过设计3组不同的观测误差均方差,对2012年8月1日—29日进行了基于GRAPES-M EPS(Global/Regional Assimilation and Prediction System-Mesoscale Ensemble Prediction System)的集合预报敏感性试验,研究观测误差均方差对集合预报初始扰动场结构、扰动量及垂直扰动总能量发展的影响,评估集合预报结果的差异,并分析了一次典型的江淮流域强降水个例。结果显示,模式变量扰动结构和扰动振幅对观测误差均方差较敏感,较小的观测误差均方差使得温度和风等模式变量的初始扰动量增大,扰动总能量增长更快,降水集合预报效果更优。因此在GRAPES-MEPS中,可以考虑对观测误差均方差进行适当的扰动,以体现观测误差均方差的不确定性对集合预报的影响,提高GRAPES-MEPS的集合预报技巧。