Clustering spatial–temporal precipitation data using wavelet transform and self-organizing map neural network

A data analysis method is proposed to cluster and explore spatio-temporal characteristics of the 22 years of precipitation data (1982–2003) for Taiwan. The wavelet transform self-organizing map (WTSOM) framework combines the wavelet transform (WT) and a self-organizing map (SOM) neural network. WT is used to extract dynamic and multiscale features of the non-stationary precipitation time-series, and SOM is applied to objectively identify spatially homogeneous clusters on the high-dimensional wavelet-transformed feature space. Haar and Morlet wavelets are applied in the data preprocessing stage to preserve the desired characteristics of the precipitation data. A two-level SOM neural network is applied to identify clusters in the wavelet space in the clustering stage. The performance of clustering is evaluated using silhouette coefficients. The results indicate that singularities or sharp transitions are more significant than changes in the periodicity or data structure in the spatial–temporal precipitation data. The WTSOM results show that six clusters are optimal for both Haar and Morlet wavelet functions, but their corresponding geographic locations are different. The geographic locations of clusters based on the Haar wavelet, which captures the occurrence of extreme hydrological events, appear in blocks while those classified by the Morlet wavelet, which indicates periodicity changes and describes fine structures, appear in strips that cross the island of Taiwan. Principal component analysis is applied to the precipitation data of each cluster. The first principal components explain 62–90% of the total variation of data. Characteristics of precipitation data for each cluster are explored using scalogram analysis. The results show that both extreme hydrological events and periodicity changes appear in the spatial and temporal precipitation data but with different characteristics for each cluster. Recognizing homogeneous hydrologic regions and identifying the associated precipitation characteristics improves the efficiency of water resources management in adapting to climate change, preventing the degradation of the water environment, and reducing the impact of climate-induced disasters. Measures for countering the stress of precipitation variation for water resources management are provided.     

Relation of auroral substorm onset to local AL index and dispersionless particle injections

In this study, we investigate the relation of auroral substorm onset to the sharp decrease in the local AL index (IL) during substorms. With a database of over 4200 onsets determined from auroral images, we have statistically examined the timing between the auroral substorm onset and the sharp decrease in the IL index, as determined with data from the IMAGE magnetometer network. From the database of onsets, 54 substorms were determined to be within 6° of the central meridian of the IMAGE ground array. Our superposed epoch median curve shows that the IL index begins to sharply decrease 3 min before the auroral onset, which is twice the 2 min resolution of the auroral imager. However, the mean difference determined by measuring the time between the start of the IL decrease and the auroral substorm onset is about 1.1±0.6 min. An analysis of the superposed epoch median curves of the SOPA particle data for the LANL spacecraft closest to the auroral onset meridian indicates that both the electron and proton injections begin about 3 min before the auroral onset. However, the mean time of the difference between the minimum of the particle dispersionless injection and the auroral onset is simultaneous within the uncertainty of the auroral onset and the error of the mean for the injection. The location of the electron injection relative to the IMAGE ground array seems to be 01–04 MLT, while the proton injection appears to be in the 22–01 MLT sector. These statistical results support the idea that the field aligned and ionospheric currents of the substorm current wedge begin to flow before the auroral onset.     

The hydrodynamic coefficients for an oscillating rectangular structure on a free surface with sidewall

Based on a two dimensional linear water wave theory, the boundary element method (BEM) is developed and applied to study the heave and the sway problem of a floating rectangular structure in water to finite depth with one side of the boundary is a vertical sidewall and the other boundary is an open boundary. Numerical results for the added mass and radiation damping coefficients are presented. These coefficients are not only depend on the submergence and the width of the structure, but also depend on the clearance between structure and sidewall. Negative added mass and sharp peaks in the damping and added mass coefficients have been found when the clearance with a value close to integral times of half wave length of wave generated by oscillation structure. The important effect of the clearance on the added mass and radiation damping coefficients are discussed in detail. An analytical solution method is also presented. The BEM solution is compared with the analytical solution, and the comparison shows good agreement.     

Observations and modeling of flat subduction and its geological effects

Flat subduction refers to low-angle(10°) or sub-horizontal subduction of oceanic slabs. Flat subduction is only recognized in ～10% of present-day subduction zones, but its impact on the behavior of the overriding plate is particularly strong.For example, flat subduction zones are typically associated with stronger earthquakes. The deformation caused by typical flat subduction will transfer from the trench to the overriding continental interior and form a broad magma belt. The formation mechanism of flat subduction has been linked to the relative buoyancy of subducted oceanic plateaus, overthrusting of the overriding plate, hydrodynamic suction, and trench retreat. However, these mechanisms remain debated. This paper systematically analyzes and summarizes previous studies on flat subduction, and outlines the possible geological effects of flat subduction, such as intracontinental orogeny and magmatism. Using examples from numerical modeling, we discuss the possible formation mechanisms. The most important factors that control the formation of flat subduction are associated with overthrusting of the overriding plate and the arrival of an oceanic plateau at the subduction zone. In addition, trench retreat is necessary to enable flat subduction. Hydrodynamic suction contributes to the reduction of the slab dip angle, but is insufficient to form flat subduction. Future numerical modeling of flat subduction should carry out three-dimensional high-resolution thermo-mechanical simulation, considering the influence of crustal eclogitization(negative buoyancy) and mantle serpentinization(positive buoyancy) of oceanic lithosphere, in combination with geological and geophysical data.     

An integrated earthquake early warning system and its performance at schools in Taiwan

An earthquake early warning (EEW) system with integration of regional and onsite approaches was installed at nine demonstration stations in several districts of Taiwan for taking advantages of both approaches. The system performance was evaluated by a 3-year experiment at schools, which experienced five major earthquakes during this period. The blind zone of warning was effectively reduced by the integrated EEW system. The predicted intensities from EEW demonstration stations showed acceptable accuracy compared to field observations. The operation experience from an earthquake event proved that students could calmly carry out correct action before the seismic wave arrived using some warning time provided by the EEW system. Through successful operation in practice, the integrated EEW system was verified as an effective tool for disaster prevention at schools.     

FORMOSAT-3/COSMIC observations of the ionospheric auroral oval development

The ionospheric radiance and electron density observed by the tiny ionospheric photometer (TIP) and GPS occultation experiment (GOX) payloads on FORMOSAT-3/COSMIC satellites are applied to determine the boundaries of the auroral oval and its width in the winter nighttime ionosphere for both hemispheres. The TIP collects ionospheric emission at 135.6 nm due to electron impact excitation, while the GOX offers ionospheric electron density profiles with radio occultation (RO) technique. Comparison between them shows similar patterns of the plasma structure in the polar caps. The mean width of the auroral bands ranges between about 2 and 11° latitude in the winter nighttime and it varies with longitudes. The comparison by month suggests that the mean radius of the auroral ovals varies with the intensity of the auroral radiance.     

A laboratory plasma experiment for studying magnetic dynamics of accretion discs and jets

This work describes a laboratory plasma experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. A high-speed multiple-frame CCD camera reveals images of the formation and helical instability of a collimated plasma, similar to MHD models of disc jets, and also plasma detachment associated with spheromak formation, which may have relevance to disc winds and flares. The plasmas are produced by a planar magnetized coaxial gun. The resulting magnetic topology is dependent on the details of magnetic helicity injection, namely the force-free state eigenvalue α _gun imposed by the coaxial gun.     

Distinct influential mechanisms of the warm pool Madden–Julian Oscillation on persistent extreme cold events in Northeast China

This study investigates whether and how the Madden–Julian Oscillation (MJO) influences persistent extreme cold events (PECEs), a major type of natural disaster in boreal winter, over Northeast China. Significantly increased occurrence probabilities of PECEs over Northeast China are observed in phases 3 and 5 of the MJO, when MJO-related convection is located over the eastern Indian Ocean and the western Pacific, respectively. Using the temperature tendency equation, it is found that the physical processes resulting in the cooling effects required for the occurrence of PECEs are distinct in the two phases of the MJO when MJO-related convection is consistently located over the warm pool area. The PECEs in phase 3 of the MJO mainly occur as a result of adiabatic cooling associated with ascending motion of the low-pressure anomaly over Northeast Asia. The cooling effect associated with phase 5 is stronger and longer than that in phase 3. The PECEs associated with phase 5 of the MJO are linked with the northwesterly cold advection of a cyclonic anomaly, which is part of the subtropical Rossby wave train induced by MJO-related convection in the tropical western Pacific.摘要 本文利用高分辨率气温数据和热带季节内振荡 (MJO) 实时指数, 研究了1979–2015年冬季MJO活动对中国东北持续性极端低温事件 (PECE) 的影响特征和机理.结果表明:当MJO对流分别位于暖池地区的东印度洋 (位相3) 和西太平洋 (位相5) 时, 中国东北PECE的发生频率显著增加.利用温度方程诊断分析发现MJO两个位相所导致的冷却过程不同: 当 MJO处于位相3时, 中国东北地区为低压异常, 上升运动引起绝热冷却作用; 而位相5所形成的气旋性环流为中国东北地区带来西北风冷平流, 降温过程更强且持续更长时间.     

Dynamic response of cylindrical shell structures subjected to underwater explosion

This study investigated the linear and nonlinear dynamic responses of three cylindrical shell structures subjected to underwater small charge explosions in a 4 m×4 m×4 m water tank. The dimensions of the cylindrical shell structures were 90 cm×30 cm×1 mm (length×diameter×thickness). Both ends of the cylindrical shell were mounted with thick plates to provide support and create an enclosed space. The three cylindrical shell structures were un-stiffened, internally stiffened and externally stiffened, respectively. The experiments involving the dynamic response of cylinders subjected to underwater explosion (UNDEX) were performed under different standoff distances, varying from 210 to 35 cm. A small quantity of explosives was used to generate the shock loading. The plastic deformation of the cylindrical shell was observed at a standoff distance of less than 50 cm. Other conditions were tested to examine cylinder linear response. Dynamic analyses were performed for the experimental model using FEM and compared with the test results. The accelerations and dynamic strains of cylindrical shells obtained from the experiment were compared with those obtained by FE analysis. Finally, problems related to small-scale UNDEX experiments performed in small water tanks were analyzed.