Automated threshold selection methods for extreme wave analysis

The study of the extreme values of a variable such as wave height is very important in flood risk assessment and coastal design. Often values above a sufficiently large threshold can be modelled using the Generalized Pareto Distribution, the parameters of which are estimated using maximum likelihood. There are several popular empirical techniques for choosing a suitable threshold, but these require the subjective interpretation of plots by the user.In this paper we present a pragmatic automated, simple and computationally inexpensive threshold selection method based on the distribution of the difference of parameter estimates when the threshold is changed, and apply it to a published rainfall and a new wave height data set. We assess the effect of the uncertainty associated with our threshold selection technique on return level estimation by using the bootstrap procedure. We illustrate the effectiveness of our methodology by a simulation study and compare it with the approach used in the JOINSEA software. In addition, we present an extension that allows the threshold selected to depend on the value of a covariate such as the cosine of wave direction.     

Theoretical Analysis of A Buoyant Jet Interacting with Small Amplitude Waves

A new theoretical solution is presented here for the dynamic characteristics of a buoyant jet due to opposing small amplitude waves.The conservation equations of mass,tangential momentum and vertical momentum are solved by the integral method which encompasses the Gaussian profiles of velocity and density.The action of waves is incorporated into the equations of motion as an external force and a new exact solution is obtained to predict the trajectory,velocity distribution and boundary thickness of the buoyant jet over an arbitrary lateral cross section.It is found that the velocity along the centerline is inversely proportional to the ratio of the momentum of the wave to the buoyant jet.The averaged boundary width varies with the fluctuation of the boundary width,the distance from the orifice and the velocity correction function.Owing to the motion d waves,the fluctuation of the boundary width is proportional to the wave steepness.     

渤黄海海平面的变化及其与ENSO的关系

利用1992年12月至2007年5月的高度计资料,研究了渤黄海海平面的变化特征。统计分析表明,近14a间渤海及北黄海、中央黄海海平面的平均上升高度分别为45.9mm和34.7mm,各海域的海平面上升速度不完全相同。研究发现,南方涛动指数(SOI)、纬向风应力距平都与渤海及北黄海、中央黄海的SLA呈负相关性,渤黄海海平面显著受SOI、纬向风应力调制,并且,SOI与渤黄海海域的风场之间有良好相关。将坐标系进行旋转后,获得与当地海平面异常相关最大的风应力方向。对SLA与新坐标系下风应力距平u的低频分量分析发现,渤海及北黄海海区、中央黄海对海平面影响最大的风应力距平u方向分别为东偏南20°方向、东偏南8°方向,风应力距平u分量与SLA、SOI的低频分量呈现更好的相关性。ENSO通过大气环流过程对渤黄海海域的风场产生影响,当地风场通过纬向风应力对渤黄海海平面的年际变化产生调制作用。因此,ENSO可以通过风应力对渤黄海海平面产生影响。     

Global sea level change and thermal contribution

The global long-term sea level trend is obtained from the analysis of tide gauge data and TOPEX/Poseidon data. The linear trend of global mean sea level is highly non-umiform spatially, with an average rate of 2.2 mm year-1 in T/P sea-level rise from October 1992 to September 2002. Sea level change duc to temperature vanation (the thermosteric sea level) is discussed. The results are compared with TOPEX/Poseidon altimeter data in the same temporal span at different spatial scales. It is indicated that the ther-mal effect accounts for 86% and 73% of the observed seasonal variability in the northern and southern hemispheres, respectively. The TOPEX/Poseidon observed sea level lags behind the TSI, by 2 months in the zonal band of 40°-60° in both the northern and southern hemispheres. Systematic differences of about 1-2cm between TOPEX/Poseidon observations and thermosteric sea level data are obtained. The potential causes for these differences include water exchange among the atmosphere, land, and oceans, and some pos-sible deviations in thermosteric contribution estimates and geophysical corrections to the TOPEX/Poseidon data.     

脂肪水平对厚唇弱棘鯻生长及肠道脂肪酶活性的影响

以鱼油为脂肪源,配制成脂肪水平(质量分数,下同)分别为3.1%、5.8%、9.4%、11.9%和15.1%的5种等氮饲料,探讨饲料脂肪水平对初始体重为(4.05±0.11)g的厚唇弱棘鯻(Hephaestus fuliginosus)生长及肠道脂肪酶活性的影响。结果表明,随着饲料脂肪含量提高,实验用鱼脏体比、肥满度及肌肉粗蛋白含量也随着提高,但各组间差异不显著(P>0.05)。饲料脂肪水平从3.1%增加到9.4%,厚唇弱棘鯻的饲料效率呈升高趋势,相对增重率值由76.42%±5.88%升至102.09%±5.31%;而当饲料脂肪水平从9.4%提高至15.1%,饲料效率呈降低趋势(P<0.05),相对增重率值降为70.54%±2.06%。随着饲料脂肪水平提高,厚唇弱棘鯻肝体比、肌肉粗脂肪含量、肠道脂肪酶活性均呈上升趋势,且各组间差异显著(P<0.05)。表明本实验条件下饲料脂肪含量超过9.4%可导致厚唇弱棘鯻生长和饲料利用下降,对其形体特征、肌肉粗脂肪含量以及肠道脂肪酶活性均有显著影响。     

高空急流区飞机颠簸的一种形成机制的探讨

作者应用动力学分析方法，结合观测飞机颠簸的事实，对高空急流区飞机颠簸的一种形成机理进行了理论探讨。结果表明，高空急流区温度平流分布的不均匀性，会在急流附近等温线密集区边缘激发出空气垂直运动和湍流，引起飞机颠簸。     

马尔可夫链模型在地下水水位预测中的应用

作者将鲁南地区一水文观测孔的９２次地下水水位月平均监测数据,划分了１０种状态范围,运用马尔可夫链模型,对未来地下水水位进行状态范围预测。在与实际监测资料进行对比的基础上,验证了马尔可夫链理论在地下水水位预测中的可行性及可靠性,并对运用中的具体问题进行了探讨。     

地下水位对定点形变观测干扰的抽水实验

抽水所造成的地下水位的变化会即时在局部范围内造成地表的形变，干扰定点地壳形变观测。在蓟县地震台进行的抽水实验表明：这种干扰的幅度与抽水水井到形变监测仪器的距离有关，也和抽水水井水位的最大降深有关。根据实验数据得到了抽水对定点形变观测的影响范围的经验公式。     

冠县地下水资源概况及开发利用建议

介绍了山东省冠县的水文地质条件和地下水资源开发利用现状,分析了由于过量开采浅层地下水而引发的水位下降及水质恶化等问题,提出了合理开发利用地下水的建议。     

西安市地面沉降探讨

本文在讨论西安市地面沉降特征的基础上,论述了西安市地下水开采量、承压水位变化与地面沉降三者之间在时间和空间上的对应关系,初步认定大量开采地下水是地面沉降主要原因。典型水准点沉降量与邻近自备井承压水位变化相关分析表明,两者间相关性极强。经综合分析市区地貌、第四系地质、工程地质和水文地质特征并进行工程地质分层后,可确定出市区地面沉降的第一和第二主要压缩层。据地面沉降与承压水位下降值回归方程所预测的地面沉降发展趋势表明,西安地面沉降今后还会继续发展,将对西安城市建设产生严重影响,故应采取相应治理措施。