El Ni?o事件大气-洋流异常与暖海温传播机理研究

利用1971～2000年逐月SODA (Simple Ocean Data Assimilation) 同化资料（Carton等，2004）、1980～2000年逐月NCEP/NCAR再分析资料（Kalnay等，1996）探讨中部型（暖海水首先出现在120°W以西）和东部型（暖海水首先出现在120°W以东）El Ni?o事件赤道纬向风应力及洋流的异常变化与暖海水信号的传播特征。研究指出:（1）中部型和东部型El Ni?o事件发生时，太平洋上赤道海表最大西风应力距平在西太平洋地区都有显著的东传现象，但中部型El Ni?o事件西风应力距平强度强，造成西太平洋赤道表层的东向流可达东太平洋地区，这类El Ni?o事件强度偏强。（2）中部型El Ni?o事件，赤道表层洋流辐合区及其下沉运动由西太平洋向东太平洋传播，辐合下沉运动抑制了深层冷海水上翻，西太平洋暖水能够传到东太平洋与西太平洋赤道表层洋流辐合区及其下沉运动的东移有关。（3）东部型El Ni?o事件西太平洋赤道表层洋流辐合区及其下沉运动没能直接传到东太平洋地区，东太平洋暖水形成与局地（120°W以东）辐合下沉运动抑制深层冷海水上翻有关；东部型El Ni?o事件暖池次表层的暖水，不是沿着西太平洋赤道次表层向东传播到东太平洋地区，而是由南太平洋西边界流将暖池海水带到40°S左右的西风漂流区， 再由西风漂流平流到东太平洋。     

西太平洋副热带高压影响下北京区域性暴雨的个例分析

北京地区大降水(≥25 mm/d)天气有72%出现在7~8月份,其中直接受西太平洋副热带高压(副高)影响的大降水占46%,它们主要产生在西风槽与副高相互作用的天气形势下。通过对该类暴雨的典型个例分析表明,由于受副高的影响,对流层低层副高南面的偏东气流将海上充沛的水汽输送到大陆,并折向北与偏北气流相遇,汇集在北京附近。同时,副高西北边缘与西风槽之间形成较强的能量锋区,该锋区南面的高温高湿气团蕴藏了较深厚的湿有效位能,在湿位涡的斜压项作用下转化为动能,倾斜垂直涡度发展激发了强烈的上升运动。在斜压扰动作用下,对流层中层差动涡度平流和副高西侧的暖平流破坏了北京及邻近区域的准地转平衡,动力强迫和热力强迫共同作用激发了次级环流,该处的上升运动得到加强。暴雨就是产生在强的能量锋附近低层气旋性涡度发展、高层辐散显著的地方。此外,副高西侧低层的暖湿气流北上,较强的差动假相当位温平流促使大气层结向对流不稳定发展,这对于暴雨局地的再增强是不可忽视的因素。由此,形成对该类暴雨天气预报的着眼点。     

大地测量反演模型准外部检验的进一步探讨

给出了大地测量反演模型准外部检验的基本理论,提出了观测数据的划分方法和反演模型准外部质量控制的标准,实现了在进行反演之前对反演模型的事先检验。     

Change of Gas Hydrate Reservoir and Its Effect on the Environment in Xisha Trough since the Last Glacial Maximum

In this article, Milkov and Sassen’s model is selected to calculate the thickness of the gas hydrate stable zone (GHSZ) and the amount of gas hydrate in the Xisha (西沙) Trough at present and at the last glacial maximum (LGM), respectively, and the effects of the changes in the bottom water temperature and the sea level on these were also discussed. The average thickness of the GHSZ in Xisha Trough is estimated to be 287 m and 299 m based on the relationship between the GHSZ thickness and the water depth established in this study at present and at LGM, respectively. Then, by assuming that the distributed area of gas hydrates is 8 000 km2 and that the gas hydrate saturation is 1.2% of the sediment volume, the amounts of gas hydrate are estimated to be ~2.76×1010 m3 and ~2.87×1010 m3, and the volumes of hydrate-bound gases are ~4.52×1012 m3 and ~4.71×1012 m3 at present and at LGM, re- spectively. The above results show that the thickness of GHSZ decreases with the bottom water tem- perature increase and increases with the sea level increase, wherein the effect of the former is larger than that of the latter, that the average thickness of GHSZ in Xisha Trough had been reduced by ~12 m, and that 1.9×1011 m3 of methane is released from approximately 1.1×109 m3 of gas hydrate since LGM. The released methane should have greatly affected the environment.     

Numerical analysis for random temperature fields of embankment in cold regions

The stochastic finite element equations for random temperature are obtained using the first-order per-turbation technique taking into account the random thermal properties and boundary condition, based on heat transfer variational principle. The local average method for 2-D is used to discretize random fields. Then, the random temperature fields of embankment in cold regions are investigated on condi-tion that the thermal properties and boundary condition are taken as random fields, respectively, by using the program, which is written by the methods. The expected value of temperature field and the standard deviation of the temperature field of embankment in cold regions are obtained and analyzed.     

2005年10月8日巴基斯坦7.8级地震热红外异常

收集了2005年巴基斯坦7.8级地震区Noaa17/AVHRR的2004-2005年的热红外遥感资料,并进行地表温度反演,得到了以震中为中心7°×7°范围的地表温度值。经热红外图像解译与构造关系对比分析,表明热红外解译图像与构造分布具有一致性。震源区地表温度时间序列显示:震前大约3个月内震源区存在明显的热红外异常。     

Analysis and modeling of the seasonal South China Sea temperature cycle using remote sensing

The present paper describes the analysis and modeling of the South China Sea (SCS) temperature cycle on a seasonal scale. It investigates the possibility to model this cycle in a consistent way while not taking into account tidal forcing and associated tidal mixing and exchange. This is motivated by the possibility to significantly increase the model’s computational efficiency when neglecting tides. The goal is to develop a flexible and efficient tool for seasonal scenario analysis and to generate transport boundary forcing for local models. Given the significant spatial extent of the SCS basin and the focus on seasonal time scales, synoptic remote sensing is an ideal tool in this analysis. Remote sensing is used to assess the seasonal temperature cycle to identify the relevant driving forces and is a valuable source of input data for modeling. Model simulations are performed using a three-dimensional baroclinic-reduced depth model, driven by monthly mean sea surface anomaly boundary forcing, monthly mean lateral temperature, and salinity forcing obtained from the World Ocean Atlas 2001 climatology, six hourly meteorological forcing from the European Center for Medium range Weather Forecasting ERA-40 dataset, and remotely sensed sea surface temperature (SST) data. A sensitivity analysis of model forcing and coefficients is performed. The model results are quantitatively assessed against climatological temperature profiles using a goodness-of-fit norm. In the deep regions, the model results are in good agreement with this validation data. In the shallow regions, discrepancies are found. To improve the agreement there, we apply a SST nudging method at the free water surface. This considerably improves the model’s vertical temperature representation in the shallow regions. Based on the model validation against climatological in situ and SST data, we conclude that the seasonal temperature cycle for the deep SCS basin can be represented to a good degree. For shallow regions, the absence of tidal mixing and exchange has a clear impact on the model’s temperature representation. This effect on the large-scale temperature cycle can be compensated to a good degree by SST nudging for diagnostic applications.     

Imaging Offsets in the Moho: Synthetic Tests using Gaussian Beams with Teleseismic Waves

We carry out a sequence of numerical tests to understand conditions under which rapid changes in crustal thickness can be reliably imaged by teleseismic body waves. Using the finite-difference method over a 2-D grid, we compute synthetic seismograms resulting from a planar P-wavefield incident below the grid. We then image the Moho using a migration scheme based on the Gaussian beam representation of the wavefield. The use of Gaussian beams for the downward propagation of the wavefield is particularly advantageous in certain geologically critical cases such as overthrusting of continental lithosphere, resulting in the juxtaposition of high-velocity mantle material over crustal rocks. In contrast to ray-based methods, Gaussian beam migration requires no special treatment to handle such heterogeneities. Our results suggest that with adequate station spacing and signal-to-noise ratios, offsets of the Moho, on the order of 10 km in height, can be reliably imaged beneath thickened crust at depths of about 50 km. Furthermore, even sharp corners and edges are faithfully imaged when precise values of seismic wave speeds are available. Our tests also demonstrate that flexibility in choices of different types of seismic phases is important, because any single phase has trade-offs in issues such as spatial resolution, array aperture, and amplitude of signals.     

Wavefield continuation datuming using a near surface model

When topography and low velocity zone differences vary greatly, conventional vertical static time shifts will cause wavefield distortion and influence wave equation seismic imaging for seismic data acquired on a complex near surface. In this paper, we propose an approach to datum correction that combines a joint tomography inversion with wavefield continuation to solve the static problem for seismic data on rugged acquisition topography. First, the near surface model is obtained by refracted wave tomography inversion. Second, the wavefield of sources and receivers are continued downward and upward to accomplish datum correction starting from a flat surface and locating the datum above topography. Based on the reciprocal theorem, Huygens＇ and Fresnel principles, the location of sources and receivers, and regarding the recorded data on the surface as a secondary emission, the sources and receivers are upward-continued to the datum above topography respectively. Thus, the datum correction using joint tomography inversion and wavefield continuation with the condition of a complex near surface is accomplished.     

BP、Hopfield神经网络在位场反演中的应用比较

比较了BP、Hopfield二种神经网络模型的特性及其运行机制,分别用于位场反演,还比较了各自在位场反演中的应用效果。结果表明:这二种神经网络模型虽然都可用于位场反演,但由于Hopfield网络缺乏学习能力,不能较好地利用已知地质、地球物理信息而受到限制。而BP神经网络具有较强的学习能力,能从已知的信息中得到有利于解决最优化问题的结论,比Hopfield神经网络更加适合于位场的反演问题。