Selecting stars for determining azimuth     

Leslie Stoch 《Journal of Geodesy》1963,37(3):293-298

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A possible new molecular mechanism of thundercloud electrification   总被引：1，自引：1，他引：1  

Pavel Jungwirth  Daniel Rosenfeld  Victoria Buch 《Atmospheric Research》2005,76(1-4):190

Thunderclouds are electrified when charge is transferred between small and large ice particles colliding in a cloud that contains strong updrafts. The small ice particles rise with one type of charge and the large ice particles fall and carry with them downward the other type of charge, which is most often negative, so that normally lightning lowers negative charge from cloud to the ground. While the collisional mechanism of thundercloud charging is well established, the nature of the charge transfer between the colliding ice particles is not very well understood on the atomic level, and no present theory can explain it in full detail. Here we propose a new charge separation mechanism that is based on molecular dynamics simulations of particle surfaces and collisions, keeping track of the individual charges as they move in the form of salt ions from one ice particle to another. Under normal conditions, when sulfates dominate as cloud condensation nuclei, this ionic mechanism is consistent with the prevailing negative charging of graupels in thunderclouds. Moreover, with dearth of sulfate anions, the present mechanism predicts a shift towards positive charging. This fits well to a large range of observations of enhanced positive lightning, connected with smoke rich in chlorides and nitrates, that could not be explained satisfactorily previously.  相似文献   

近似对称矩阵的直接解法及其应用     

郑宏  谭国焕  刘德富 《岩土力学》2006,27(11):1880-1884

所谓近似对称矩阵是指矩阵中仅有极少一部分元素是非对称的,在将对角线以上的非对称元素用其对角线以下的对称元素替代后,该矩阵就变成了一个对称矩阵。在求解非线性连续介质力学问题时常常会遇到近似非对称矩阵。基于Sherman-Morrison定理,给出了一种新的近似非对称矩阵的分解算法。在确保数值稳定性的前提下,无论在求解效率还是在内存开销方面新算法都优于一般的非对称稀疏矩阵的求解器,且仅需对传统的基于LDLT分解的求解器略做修改,即可开发出适应于对称和非对称稀疏矩阵的求解器。最后用一个摩擦接触算例,显示了新算法的优越性。  相似文献   

Erratum to: Australian east coast rainfall decline related to large scale climate drivers     

Milton Samuel Speer  Lance M. Leslie  Alexandre O. Fierro 《Climate Dynamics》2011,36(7-8):1431-1434

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Modeling the mudslide aftermath of the 2007 Southern California Wildfires     

Diandong Ren  Rong Fu  Lance M. Leslie  Robert E. Dickinson 《Natural Hazards》2011,57(2):327-343

An investigation is made of the influence of fire-caused deforestation on precipitation-triggered, shallow landslide susceptibilities in southern California, using a scalable and extensible geo-fluid model (SEGMENT), that accounts for soil mechanics, root distribution, and relevant hydrologic processes. SEGMENT is applicable to variable regional topography, soil thickness profiles, and vegetation coverage. In this study, for southern California following the 2007 wildfires, three experiments were performed with rainfall recurrence periods chosen to be 2, 10, and 25 years. These intervals correspond to 24-h storm rainfall totals of 17, 25.4, and 33 mm. The model generated landslide stability maps that identified three areas of high instability. These unstable regions are located in the San Fernando Valley, the San Gabriel Mountains, and the Santa Monica Mountains. In each case, the vegetation cover had been severely burned during the preceding 2007 wildfires. The model results showed that burning from wildfires is a major destabilizing factor for southern California. Burn sites are more prone to landslides than vegetated slopes because the soil more readily exceeds its critical moisture content. Severe droughts in a future warming climate are expected to increase the likelihood of more frequent and intense wild fires. Higher temperatures combined with decreased total rainfalls facilitate more intense landslides, including devastating mudslides, following heavy precipitation. Finally, the model is designed to assist in developing timely mitigation measures for post-fire, storm-triggered landslides.  相似文献   

A 3-D time-domain coupled model for nonlinear waves acting on a box-shaped ship fixed in a harbor     

王大国  邹志利 《中国海洋工程》2011,25(3):441-456

A 3-D time-domain numerical coupled model is developed to obtain an efficient method for nonlinear waves acting on a box-shaped ship fixed in a harbor. The domain is divided into the inner domain and the outer domain. The inner domain is the area beneath the ship and the flow is described by the simplified Euler equations. The remaining area is the outer domain and the flow is defined by the higher-order Boussinesq equations in order to consider the nonlinearity of the wave motions. Along the interface boundaries between the inner domain and the outer domain, the volume flux is assumed to be continuous and the wave pressures are equal. Relevant physical experiment is conducted to validate the present model and it is shown that the numerical results agree with the experimental data. Compared the coupled model with the flow in the inner domain governed by the Laplace equation, the present coupled model is more efficient and its solution procedure is simpler, which is particularly useful for the study on the effect of the nonlinear waves acting on a fixed box-shaped ship in a large harbor.  相似文献   

Dust and hydrogen molecules in the extremely metal-poor dwarf galaxy SBS 0335–052     

HiroyukiHirashita †  Leslie K.Hunt  reaFerrara 《Monthly notices of the Royal Astronomical Society》2002,330(1):L19-L23

During the early stages of galaxy evolution, the metallicity is generally low and nearby metal-poor star-forming galaxies may provide templates for primordial star formation. In particular, the dust content of such objects is of great importance, because early molecular formation can take place on grains. To gain insight into primeval galaxies at high redshift, we examine the dust content of the nearby extremely low-metallicity galaxy SBS  0335–052  which hosts a very young starburst (≲10⁷ yr). In young galaxies, the dust formation rate in Type II supernovae governs the amount of dust, and by incorporating recent results on dust production in Type II supernovae we model the evolution of dust content. If the star-forming region is compact (≲100 pc), as suggested by observations of SBS  0335–052  , our models consistently explain the quantity of dust, far-infrared luminosity, and dust temperature in this low-metallicity object. We also discuss the H₂ abundance. The compactness of the region is important to H₂ formation, because the optical depth of dust for UV photons becomes large and H₂ dissociation is suppressed. We finally focus on implications for damped Ly α systems.  相似文献   

Infrared spectral energy distribution model for extremely young galaxies     

Tsutomu T. Takeuchi †  Hiroyuki Hirashita ‡  Takako T. Ishii  Leslie K. Hunt  Andrea Ferrara 《Monthly notices of the Royal Astronomical Society》2003,343(3):839-850

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The Monitor project: rotation of low-mass stars in the open cluster M34     

Jonathan Irwin  Suzanne Aigrain  Simon Hodgkin  Mike Irwin  Jerome Bouvier  Cathie Clarke  Leslie Hebb  Estelle Moraux 《Monthly notices of the Royal Astronomical Society》2006,370(2):954-974

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Evaluating Lower Computational Burden Approaches for Calibration of Large Environmental Models     

Randall J. Hunt  Jeremy T. White  Leslie L. Duncan  Connor J. Haugh  John Doherty 《Ground water》2021,59(6):788-798

Realistic environmental models used for decision making typically require a highly parameterized approach. Calibration of such models is computationally intensive because widely used parameter estimation approaches require individual forward runs for each parameter adjusted. These runs construct a parameter-to-observation sensitivity, or Jacobian, matrix used to develop candidate parameter upgrades. Parameter estimation algorithms are also commonly adversely affected by numerical noise in the calculated sensitivities within the Jacobian matrix, which can result in unnecessary parameter estimation iterations and less model-to-measurement fit. Ideally, approaches to reduce the computational burden of parameter estimation will also increase the signal-to-noise ratio related to observations influential to the parameter estimation even as the number of forward runs decrease. In this work a simultaneous increments, an iterative ensemble smoother (IES), and a randomized Jacobian approach were compared to a traditional approach that uses a full Jacobian matrix. All approaches were applied to the same model developed for decision making in the Mississippi Alluvial Plain, USA. Both the IES and randomized Jacobian approach achieved a desirable fit and similar parameter fields in many fewer forward runs than the traditional approach; in both cases the fit was obtained in fewer runs than the number of adjustable parameters. The simultaneous increments approach did not perform as well as the other methods due to inability to overcome suboptimal dropping of parameter sensitivities. This work indicates that use of highly efficient algorithms can greatly speed parameter estimation, which in turn increases calibration vetting and utility of realistic models used for decision making.  相似文献