The middle atmosphere     

Theodore G. Shepherd   《Journal of Atmospheric and Solar》2000,62(17-18)

The last 50 years have seen enormous advances in our knowledge and understanding of the stratosphere and mesosphere, which together comprise the middle atmosphere. Beginning from a phase of basic discovery, we have now reached the stage where most observed phenomena can be modelled from first principles with a reasonable degree of fidelity, and where there is an overall theoretical framework which can be tested against measurements and models. This review surveys a number of major surprises in middle atmosphere science over the past 50 years. A phenomenological and historical approach is adopted in each case, leading up to the current literature. Along the way, a common thread emerges: the central role of waves, of various types, in redistributing angular momentum within the atmosphere, and the global nature of the atmospheric response to such redistribution.  相似文献   

雷州半岛第四纪火山岩激光⁴⁰Ar/³⁹Ar等时线定年研究   总被引：2，自引：1，他引：1  

李蔚然  季建清  桑海清  周晶  涂继耀  洒骁  叶姮 《岩石学报》2013,29(8):2775-2788

雷州半岛是我国新生代火山岩最重要的分布地区之一,火山活动主要集中在中晚更新世。前人对雷州火山岩的年代学研究以K-Ar法为主。研究表明,雷州火山岩测年结果大致分布在0.38～3.04Ma范围内。根据地层和火山岩层的叠置关系,雷州第四纪火山岩由于覆盖在被确定是1.87Ma和0.76Ma沉积的地层之上,故火山岩年龄应小于该地层年龄。K-Ar法定年结果与雷州地区地层叠置关系存在矛盾。本文通过对雷州半岛第四纪火山岩进行野外考察及采样,利用激光40Ar/39Ar年代学方法进行了精细定年。结果表明,雷州火山岩的喷发主要集中18万年前后。定年结果还表明,对于年轻样品,基于尼尔值计算的K-Ar年龄及40Ar/39Ar表观年龄偏老,等时线年龄相对较为可靠。对同一样品的斑晶、基质作斑晶-基质等时线计算,只有在斑晶基质满足同源条件时才有意义。本文首次提出,通过对比未照射样品的初始36Ar/38Ar值的均一性,以检验样品是否同源,确认斑晶-基质等时线年龄的可信度。据此,等时线的处理方法可以推广应用于特定区域内全部同源同时样品。  相似文献   

Notes on fluorine     

E. S. Shepherd 《Bulletin of Volcanology》1940,6(1):189-198

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Observations of horizontal transport effects on high latitude metastable O(¹D), N(²D) auroral emissions     

M.L. Duboin  K. Lassen  G.G. Shepherd 《Planetary and Space Science》1984,32(11):1407-1421

An analysis is presented of photometric measurements of the NI (λ = 520nm),OI(λ = 630nm)and other emissions made at Nord, where the invariant latitude is Λ = 80°4. The time variations of the intensities are interpreted in the following way by comparison with simultaneous ground based or satellite measurements.The N(²D) atoms formed in the dayside cleft are carried by the neutral wind in a plume across the polar cap, so that the ratio of λ(630 nm) to λ(520 nm) intensities decreases along the plume with increasing distance from the source region.In the polar cap, but outside the plume region, 630 nm emission is produced by electron impact of polar rain and by substorms that reach high latitudes. Ionization produced at the same time, especially by the substorms, will produce further 630 nm emission through dissociative recombination. In any case, the region outside the plume may be regarded as a source region, with a high value of the ratio $\text{I(630)}\text{I(520)}$. This explains in part the diurnal variations, since this ratio is depressed as Nord crosses the dayside plume.The electron energy along the oval increases progressively from the dayside to the nightside. The intensity ratio increases with increasing electron energy because N(²D) is quenched more rapidly than O(¹D). Thus the ratio rises progressively from noon to midnight.An effect of the interplanetary magnetic field is superimposed on this pattern : as its North-South component B_z increases, the oval contracts so that Nord becomes nearer from the cleft source and the intensity ratio increases on the dayside. The inverse effect is also observed. On the nightside, negative B_z is associated with substorms that produce poleward expansions of the poleward oval boundary, that brings more energetic precipitation to Nord. This causes the intensity ratio to increase with decreasing B_z in a way that is opposite to that for the dayside.  相似文献   

Photometric observations of the dayside cleft emissions from Cambridge Bay,December 1976     

R.N. Peterson  R.A. Koehler  G.J. Gotshalks  J.F. Pieau  G.G. Shepherd 《Planetary and Space Science》1980,28(2):149-158

Dayside cleft emissions were observed from Cambridge Bay, N.W.T. during quiet, very quiet and disturbed periods from 10–17 December 1976. Good agreement is found in comparisons of the ground based photometric data with ISIS satellite data for the cleft region. A marked asymmetry around noon was found. Dynamical changes, observed especially in the poleward emission boundary, appear to have no counterpart in the nightside magnetometer data.  相似文献   

Multiplicative modelling of catch-at-age data, and its application to catch forecasts     

Shepherd  J. G.; Nicholson  M. D. 《ICES Journal of Marine Science》1991,47(3):284-294

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A mechanism for density-dependent survival of larval fish as the basis of a stock-recruitment relationship   总被引：4，自引：2，他引：2  

Shepherd  J. G.; Cushing  D. H. 《ICES Journal of Marine Science》1980,39(2):160-167

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Inter-hemispheric mesospheric coupling in a comprehensive middle atmosphere model     

Bodil Karlsson  Charles McLandress  Theodore G. Shepherd 《Journal of Atmospheric and Solar》2009,71(3-4):518-530

Observations of noctilucent clouds have revealed a surprising coupling between the winter stratosphere and the summer polar mesopause region. In spite of the great distance involved, this inter-hemispheric link has been suggested to be the principal reason for both the year-to-year variability and the hemispheric differences in the frequency of occurrence of these high-altitude clouds. In this study, we investigate the dynamical influence of the winter stratosphere on the summer mesosphere using simulations from the vertically extended version of the Canadian Middle Atmosphere Model (CMAM). We find that for both Northern and Southern Hemispheres, variability in the summer polar mesopause region from one year to another can be traced back to the planetary-wave flux entering the winter stratosphere. The teleconnection pattern is the same for both positive and negative wave-flux anomalies. Using a composite analysis to isolate the events, it is argued that the mechanism for inter-hemispheric coupling is a feedback between summer mesosphere gravity-wave drag (GWD) and zonal wind, which is induced by an anomaly in mesospheric cross-equatorial flow, the latter arising from the anomaly in winter hemisphere GWD induced by the anomaly in stratospheric conditions.  相似文献   

断裂引起的应变量计算方法     

许顺山  Nieto-Samaniego  AF  Alaniz-Alvarez  SA 《地质力学学报》2008,14(4):320-327

本文介绍了断裂引起的应变量计算方法。断裂作用可导致连续应变和非连续应变。连续应变与断裂位移,断裂长度比值及断裂面上有效应力成正相关关系。影响非连续应变的因素有：断裂几何形态、断裂的旋转性、断裂规模。已经提出三种断裂旋转机制：刚性旋转,垂直剪切和斜向剪切。对于这三种机制,我们分别建立了断裂非连续应变的计算公式。这些公式与断裂的旋转角度和位移大小相关。刚性旋转时,断块内部没有任何塑性变形,因此地层的长度没有变化。它引起的非连续应变最小。垂直剪切作用使断块内地层变形,但水平方向的地层长度不变。推算的公式表明,对于相同的原始数据,它引起的非连续应变比刚性旋转机制引起的非连续应变大。斜向剪切也使断块内地层变形,但水平方向的长度也不变。在同等条件下,它引起的非连续应变比垂直剪切机制引起的非连续应变大。  相似文献   

Validation of a region-wide model of landslide susceptibility in the Manawatu–Wanganui region of New Zealand     

John R. Dymond  Anne-Gaelle Ausseil  James D. Shepherd  Lars Buettner 《Geomorphology》2006,74(1-4):70-79

Since European settlement 160 years ago, much of the indigenous forest in New Zealand hill country has been cleared for pastoral agriculture, resulting in increased erosion and sedimentation. To prioritise soil conservation work in the Manawatu–Wanganui region, we developed a model of landslide susceptibility. It assigns high susceptibility to steep land not protected by woody vegetation and low susceptibility everywhere else, following the commonly used approach for identifying inappropriate land use. A major storm on 15–16 February 2004 that produced many landslides was used to validate the model. The model predicted hills at risk to landsliding with moderate accuracy: 58% of erosion scars in the February storm occurred on hillsides considered to be susceptible. The model concept of slope thresholds, above which the probability of landsliding is high and below which the probability is low, is not adequate because below 30° the probability of landsliding is approximately linearly related to slope. Thus, reforestation of steep slopes will need to be combined with improved vegetation management for soil conservation on moderate slopes to significantly reduce future landsliding.  相似文献