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许多研究表明当把分级预报的得分分解为单个级别时,N 级(“接近正常”级)的得分即使不为负值或零,往往也低。我们看到许多对接近正常天气预报困难的讨论,但文献中还没有令人满意的解释。在此给出几个新的实例,通过一些统计学分析和得分的定义,试图解释此种现象。这里引进均方根误差(rms)和得分(skill)的定义,专指给定的距平的。3级预报中 N 级得分低,原因是(a)当具有准高斯分布的预报量的分级宽度在中心附近很窄时,我们预报方法的 rms 误差与预报振幅关系不大。(b)检验观测值偏离 N 级的机会(其两侧均有偏离机会)比其它级的机会大(半闭合级只有单侧偏离机会)。从另外一层意义上解释是:由于在得分的定义中,所需检验方法以随机预报为参考,因而在均值附近得分低。从 rms 的意义上来说,随机预报在均值附近效果最好。均值附近预报得分低并不只出现在分级预报或任何特定时间。本文不是给出一个解决方法,而是提醒人们不能过分解释等级评分的概念。似乎均值附近预报得分低,主要是定义所造成,因而不需要物理学和动力学方面的解释。当取持续性预报为参考代替随机预报时,所有问题就都迎刃而解了。最后,我们注意到,远在我们对评分作预报之前,在实际中均值附近得分低已应用了预测预报得分的概念。分析表明只要预报距平相对于预报 rms 误差来说比较小,则可以预测距平相关随预报幅度线性增加。这已被 Tracton 等(1989)从经验上发现。 相似文献
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Moriarty-Schieven GH Powers JA Butner HM Wannier PG Keene J 《The Astrophysical journal》2000,533(2):L143-L145
L1551NE is a very young (class 0 or I) low-mass protostar located close to the well-studied L1551 IRS 5. Here we present evidence, from 1.3 mm continuum interferometric observations at approximately 1&arcsec; resolution, for a binary companion to L1551NE. The companion, whose 1.3 mm flux density is approximately 13 that of the primary component, is located 1&farcs;43 ( approximately 230 AU at 160 pc) to the southeast. The millimeter-wave emission from the primary component may have been just barely resolved, with a deconvolved size of approximately 0&farcm;82x0&farcm;70 ( approximately 131x112 AU). The companion emission was unresolved (<100 AU). The pair is embedded within a flattened circumbinary envelope of size approximately 5&farcm;4x2&farcm;3 ( approximately 860x370 AU). The masses of the three components (i.e., from the circumstellar material of the primary star and its companion and the envelope) are approximately 0.044, 0.014, and 0.023 M middle dot in circle, respectively. 相似文献
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Haitjema HM 《Ground water》2006,44(1):102-105
The analytic element method, like the boundary integral equation method, gives rise to a system of equations with a fully populated coefficient matrix. For simple problems, these systems of equations are linear, and a direct solution method, such as Gauss elimination, offers the most efficient solution strategy. However, more realistic models of regional ground water flow involve nonlinear equations, particularly when including surface water and ground water interactions. The problem may still be solved by use of Gauss elimination, but it requires an iterative procedure with a reconstruction and decomposition of the coefficient matrix at every iteration step. The nonlinearities manifest themselves as changes in individual matrix coefficients and the elimination (or reintroduction) of several equations between one iteration and the other. The repeated matrix reconstruction and decomposition is computationally intense and may be avoided by use of the Sherman-Morrison formula, which can be used to modify the original solution in accordance with (small) changes in the coefficient matrix. The computational efficiency of the Sherman-Morrison formula decreases with increasing numbers of equations to be modified. In view of this, the Sherman-Morrison formula is only used to remove equations from the original set of equations, while treating all other nonlinearities by use of an iterative refinement procedure. 相似文献
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Determination of HBCD, PBDEs and MeO-BDEs in California sea lions (Zalophus californianus) stranded between 1993 and 2003 总被引：1，自引：0，他引：1
Stapleton HM Dodder NG Kucklick JR Reddy CM Schantz MM Becker PR Gulland F Porter BJ Wise SA 《Marine pollution bulletin》2006,52(5):522-531
Blubber samples from male California sea lions (Zalphophus californianus) stranded between 1993 and 2003 were analyzed for 27 polybrominated diphenyl ether (PBDE) congeners, three isomers of hexabromocyclododecane (HBCD) and 14 methoxylated polybrominated diphenyl ether (MeO-BDE) congeners. Total PBDEs ranged from 450 ng/g to 4740 ng/g wet mass and total HBCD ranged from < 0.3 ng/g to 12 ng/g wet mass. The concentration of HBCD increased from 0.7 ng/g to12.0 ng/g wet mass in sea lion blubber between 1993 and 2003. However, no significant temporal trend was observed for any of the other brominated compounds over this 10 year period. Only one of the 14 MeO-BDE congeners was detected in the blubber samples, 6-methoxy-2,2',4,4'-tetrabromodiphenyl ether (6-MeO-BDE 47), and concentrations ranged from < 0.2 ng/g to 12 ng/g wet mass. A bromo-, chloro-heterocyclic compound, 1,1'-dimethyl-tetrabromo-dichloro-2,2'-bipyrrole (DBP-Br4Cl2), previously reported in marine species along the Pacific coast, was also identified in the sea lion blubber. DBP-Br4Cl2 ranged from 44 ng/g wet mass to 660 ng/g wet mass and was present at concentrations rivaling the dominant PBDE congener, BDE 47 (2,2',4,4'-tetrabromodiphenyl ether). Concentrations of DBP-Br4Cl2 were positively correlated with 6-MeO-BDE 47 (r = 0.7; p < 0.05). Both of these compounds have been identified in marine algae and sponges, and studies suggest they are both produced from natural sources. This study demonstrates that brominated compounds from both anthropogenic and biogenic sources can accumulate to similar levels in marine mammals. In addition, HBCD concentrations appear to be increasing in California sea lion populations, whereas PBDE concentrations, between 1993 and 2003, were highly variable. 相似文献
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Distribution and source apportionments of polychlorinated biphenyls (PCBs) in mariculture sediments from the Pearl River Delta, South China 总被引：1，自引：0，他引：1
Wang HS Du J Leung HM Leung AO Liang P Giesy JP Wong CK Wong MH 《Marine pollution bulletin》2011,63(5-12):516-522
Surface and core sediments collected from six mariculture farms in the Pearl River Delta (PRD) were analyzed to evaluate contamination levels of polychlorinated biphenyls (PCBs). The ∑PCBs (37 congeners) concentrations ranged from 5.10 to 11.0 ng g(-1) (mean 7.96 ng g(-1)) in surface and 3.19 to 22.1 ng g(-1) (mean 7.75 ng g(-1)) in core sediments, respectively. The concentrations were significantly higher than that measured in the sediments of their corresponding reference sites, whereby the average enrichment percentages were 62.0% and 42.7% in surface and core sediments, respectively. Significant correlations (R2=0.77, p<0.05) of PCB homologue group proportions between fish feeds and surface mariculture sediments suggested that fish feed input was probably the main source for the enrichment of PCBs. Due to the fact that PCBs could be transferred along food chains, PCB contamination in fish feeds and mariculture sediments should not be overlooked. 相似文献
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The Mar Menor lagoon is one of the most important ecological singularities in the Mediterranean area. At the same time, it is an area where many economic and industrial activities meet. The sum of the impacts of mining, agriculture and urban development in the surroundings to the lagoon during the last decades has affected its ecosystem. In this paper, we have reviewed the studies done by researchers over two decades regarding the impacts of human activities in the lagoon, reporting data of contamination levels and the effects generated in the ecosystem of the lagoon. The discharge of enriched nutrient waters seems to be the most important input in the lagoon. Changes to more friendly agricultural techniques in the surrounding areas are therefore necessary. After our review, we can conclude that it is necessary to have higher efforts from the public administrations and stricter environmental regulations in order to preserve, on one hand, the important ecological values of the lagoon, and on other hand, public health. 相似文献