一次台风暴雨的初步分析

文章对1994年7月12～13日一次登陆台风造成的华北暴雨作了天气动力学诊断分析。分析表明，9406号台风登陆后，台风和太平洋副热带高压间形成的偏东南风低空急流，具有明显超地转特征，它是触发这次台风暴雨的关键系统。Q矢量诊断表明，Q矢量辐合区与暴雨位置较为一致。     

Attenuation differences in layer 2A in intermediate- and slow-spreading oceanic crust

In situ seismic attenuationQ⁻¹logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate- and slow-spreading oceanic crust.Q⁻¹logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in Ocean Drilling Program Holes 504B and 896A and in Hole 395A, located in 5.9–7.3 Ma crust on the Pacific and Atlantic plates, respectively. Our results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity. We interpret a distinct decrease in the Q⁻¹log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The boundary is located at 465 m depth below the sea floor in both Hole 395A and 504B, although this is likely to be a coincidence of the sediment thickness at these sites. Layer 2A is estimated to be approximately 150 m thick in Hole 504B and > 300 m thick in Hole 395A. Cyclic sequences of high-porosity pillows and low-porosity massive units in the uppermost 100 m of volcanics in Hole 395A result in large velocity heterogeneities which cause > 5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from Hole 504B. We conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration. The observed differences inQ⁻¹profiles and layer 2A thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid-ocean spreading centers for these Pacific and Atlantic ridge segments.     

洋岛,海山碳酸盐岩的沉积特征及其古地理意义

高出水面的洋岛和潜伏水下的海山普遍存在于现代地球表面的各大洋中，地质历史中存在的古洋岛和古海山也逐渐为人们所认识。笔者从地形特征，沉积学特征，成岩作用和生物学特征几个方面对洋岛，海山进行了分析，洋岛，海山通常具有洋岛型火山岩基底和碳酸盐盖层的双层式地层结构，具有低分异度，探讨了古洋岛，古海山的鉴别对于再造古海洋，古地理格局的现实意义。     

ANALYSIS OF COMPOSITE DIAGNOSIS OF OCEANIC EXPLOSIVE CYCLONES*

Using the data of ECMWF (European Center for Medium-range Weather Forecasts) to undertake composite diagnoses of 16 explosive cyclones occurring at the Atlantic and the Pacific Oceans,it is found that there are a lot of obvious discrepancies on the basic fields between these strong and weak explosive cyclones.The major reasons why the explosive cyclones over the Atlantic are stronger than those over the Pacific Ocean are that the non-zonal upper jet and the low-level warm moist flow over the Atlantic are stronger.The non-zonal upper jet offers stronger divergence,baroclinicity and baroclinic instability fields for explosive cyclones.Anticyclonic curvature at the high level of strong explosive cyclones is easy to make the inertia-gravitational wave developing at the moment of northward transfer of energy and stimulate the cyclones deepening quickly.Warm advection and diabatic heating can cause the upper isobaric surface lifting,as a result,the anticyclone curvature of cyclones enlarges,and wave energy develops easily as well.The most powerful period of the development of explosive cyclones is just the time when the positive vorticity advection center is located over the low vortex.At the upper level,when the distribution of potential vorticity contours changes suddenly from rareness to denseness,and the large values of the potential vorticity both in the west and north sides of cyclones extend downwards together,then cyclones are easy to explosively develop.The formation of strong explosive cyclones is closely related with the non-zonality of upper jet and the anticyclonic curvature.     

断块开合说--中国大地构造研究的新思维

对板块说应持分析态度，既充分肯定它的创见，又要看到其不足。张文佑倡导的断块说强调地质演化中的陆洋地壳转化，即拉张造洋、挤压造陆。近年来提出的开合说，继承了板块说、断块说和新地槽-地台说的合理内核。笔者认为，断块-开合大地构造说是中国大地构造研究中有代表性的思潮。本文概括了其基本观点，强调了开合中的多模式、多旋回和非封闭性、不可逆性。     

Geochemistry of the subduction-related magmatic rocks in the Dahong Mountains, northern Hubei Province——Constraint on the existence and subduction of the eastern Mianle oceanic basin

Being a composite collisional orogen between North China and South China blocks, the Qinling orogenic belt is the key to understand the composite combination, prolonged evolutionary history and their continental dynamics. The main suture between north and south Qinling, called Shangdan suture zone (SDSZ), had been studied in detail for about twenty years. Recently, another suture zone, called Mianl黣 suture zone (MLSZ), has been identified in the Qinling Mountains. It is characterized b…     

Mafic Pegmatites Intruding Oceanic Plateau Gabbros and Ultramafic Cumulates from Bolivar, Colombia: Evidence for a 'Wet' Mantle Plume?

The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean ⁴⁰Ar–³⁹Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained     

2003年广西最大一次锋面暴雨天气过程特征分析

本文利用常规观测资料和物理量资料,对2003年6月26～28日广西全区性范围暴雨天气过程进行诊断分析,指出副热带高压逐渐加强西伸与弱冷空气在广西维持对峙是暴雨产生的主要原因;物理量和云图分析表明,暴雨发生前广西上空有深厚的不稳定层结存在,造成本次的强降水是对流层中低层暖湿空气被地面弱冷空气抬升的结果。