Biot-Savart流体力学理论与索马里低空急流形成机理的研究

首先利用格子Boltzmann模型模拟了越赤道索马里低空急流对我国青藏高原东部大气环流的影响,再借助于Biot-Savart定律和流体力学理论以及美国国家环境预报中心的数据资料,研究分析了下垫面感热条件与索马里低空急流发生和发展的关系.太阳直射从南回归线逐渐向北移动过程中,索马里半岛和阿拉伯半岛地表温度逐渐增高;而在此期间,西北印度洋海表温度却增加缓慢.两半岛地表温度高的区域就会使空气上升,而海表低温区域空气就会下沉.海陆温差的增加有利于Rayleigh-Benard对流环流的生成和发展,也使得陆地和海面上正负垂直相对涡强度Γ增强.根据Biot-Savart定律,涡强度Γ的增强必然诱导出相应强大的水平速度.两半岛和海面上这一对正负相对涡度场耦合成一部高效率的"索马里抽气泵".这一抽气泵将气流从南半球吸入,在索马里沿岸附近排出.索马里半岛和阿拉伯半岛地表增温以及与西北印度洋海表温差是驱动索马里抽气泵运转的主要能源. 关键词： 索马里低空急流 Biot-Savart定律 索马里抽气泵     

Thermal aspect of the diurnal variation of tropical convective and stratiform rainfall

The diurnal variation of radiation plays a key role in determining the diurnal variations of tropical oceanic convective and stratiform rainfall, and the examination of such a relationship requires a direct link between the radiation term in a heat budget and the surface rain rate in a cloud budget. Thus, the thermally related surface rainfall budgets derived from the combination of cloud and heat budgets are analysed with two-dimensional equilibrium cloud-resolving model simulation data to study the effects of sea surface temperature (SST) and cloud radiative, and microphysical processes on the diurnal variations of convective and stratiform rainfall. The results show that the increase in SST, the inclusion of diurnal variation of SST and the exclusion of cloud radiative processes increase negative diurnal anomalies of heat divergence over rainfall-free regions during the nighttime through changing the vertical structures of diurnal anomaly of radiation in the troposphere. The strengthened negative diurnal anomalies of heat divergence over rainfall-free regions enhance positive diurnal anomalies of heat divergence over convective regions, which intensifies the positive diurnal anomaly of convective rainfall. The exclusion of microphysical effects of ice clouds increases the negative diurnal anomaly of heat divergence over rainfall-free regions during the nighttime through reducing latent heat; this appears to enhance the positive diurnal anomaly of heat divergence over raining stratiform regions, and thus stratiform rainfall.     

A phase analysis of vorticity vectors associated with tropical convection

Three new vorticity vectors have been proposed by Gao et al to study the two-dimensional tropical convection. In the present paper, phase relations between surface rain rate and the vorticity vectors are analysed with the calculations of lag correlation coefficients based on hourly zonally-averaged mass-integrated cloud-resolving simulation data. The cloud-resolving model is integrated with the vertical velocity, zonal wind, horizontal thermal and moisture advections, and sea surface temperature observed and derived from tropical ocean global atmosphere - coupled ocean atmosphere response experiment （TOGA-COARE） for 10 days. Maximum local increase of the vertical component of the convective vorticity vector leads maximum surface rain rate by 2 hours mainly due to the interaction between vorticity and zonal gradient of ice heating. While maximum local increase of the vertical component of the moist vorticity vector leads maxfinum surface rain rate by 2 hours mainly because of the interaction between zonal specific humidity gradient and zonal buoyancy gradient. And the maximum local decrease of the zonal component of the dynamic vorticity vector leads maximum surface rain rate by 2 hours mainly due to the interactions between vorticity and vertical pressure gradient as well as vorticity and buoyancy.     

斜压二层模式框架下的准地转理论及其应用

准地转理论是短期天气预报的理论基础.针对中纬度大尺度运动得到的准地转运动方程组是准地转理论的核心.在斜压二层模式框架下,由准地转运动方程组得出的准地转位势倾向方程和准地转垂直运动方程,可以更为明确地解释高空和地面天气系统的移动和发展.结果显示:500 hPa高空槽的移动取决于本层的涡度平流,500 hPa高空槽的发展取决于本层的涡度平流和250与750h Pa层的微差温度平流;地面气旋系统的移动和发展取决于500hPa的正温度平流,250与750 hPa层的微差涡度平流.由实际天气过程对上述应用进行的分析表明,在斜压二层模式框架下得到的准地转位势倾向方程及准地转垂直运动方程,可以更好地确定中纬度天气尺度斜压发展系统的特征,并有助于对天气系统发展客观规律的了解和对数值预报产品内含的物理依据的认识.     

在金属与光折变晶体界面形成的表面波研究

应用质点振荡模型和数值模拟方法研究了在金属与光折变晶体界面形成表面波的条件及其能量变化. 结果表明: 传播常数的正负影响表面波的类型及波能量分布, 当传播常数取负值时在界面处形成非局域表面波, 取正值时在界面处形成振荡表面波和局域表面波, 局域表面波的能量随传播常数的变大而单调递增. 在一给定的物理系统中, 可通过调节决定非线性效应强度的可变参量控制不同阶数局域表面波模及其传播波形.     

Positive ion speed at the plasma–sheath boundary of a negative ion-emitting electrode

One-dimensional fluid model for a planar sheath in front of a negative ion-emitting electrode surface immersed in a collision-less, non-magnetized, electronegative plasma is presented. It was found that the positive ion speed at the plasma–sheath boundary (PSB) increases linearly with negative ion emission from the electrode but attains a saturation value as soon as a virtual cathode is formed near the electrode surface. The effect of negative ion emission on the pre-sheath region shows that the potential drop increases across the pre-sheath in accordance with the rise in positive ion speed at the PSB. The sheath width obtained using the present model shows a similar trend as the Child-Langmuir law, but its magnitude is found to be consistently higher compared with a non-emitting electrode. A plausible explanation has been given to explain these effects.     

利用气象要素估算海洋大气近地层光学湍流

基于近地层相似理论以及极端层结下对该理论的修正,建立了海洋大气近地层光学湍流估算模型,并进行了数值分析。结果表明:当气温与海温相差较大时,温度起伏对光学湍流贡献最大;当气温与海温相差较小时,海面湿度越小,光学湍流越强;在较低湿度条件下,尤其在红外窗区,湿度扰动对光学湍流贡献起支配作用。当气温小于海温时,光学湍流先随海面风速增大而增强,后随风速增大而减弱;当气温大于海温时,光学湍流总体上是伴随海面风速增大而减弱。当气温小于海温或与海温接近时,与可见光和近红外波段相比,红外窗区光学湍流显著偏强;当气温远大于海温时,两波段光学湍流接近。在中性和近中性层结下,大气折射率结构参量C2n依赖于垂直高度的负幂指数接近-2/3,伴随海面大气层结稳定度或不稳定度增强,C2n依赖于垂直高度的负幂指数的绝对值分别逐渐减小或增大,并分别趋近于稳定极限下的0或不稳定极限下的-4/3。     

Anomalous electrical transport properties of a quenched iron film system

An iron film system, deposited on glass surfaces by thermal evaporation method and quenched with a floating oil layer immediately after the deposition, has been fabricated. The temperature dependence of the resistance and the transversal magnetoresistance (MR) of the iron films have been studied. The experiment shows that, as the temperature decreases, the sample resistance increases first and then drops monotonically, finally it increases again. Furthermore, a crossover of MR from positive to negative is observed as the magnetic field increases. It is proposed that these anomalous phenomena originate from the characteristic microstructure of the samples.     

Linear mechanism of surface gravity wave generation in horizontally sheared flow

An analysis is presented of a linear mechanism of surface gravity wave generation in a horizontally sheared flow in a fluid layer with free boundary. A free-surface flow of this type is found to be algebraically unstable. The development of instability leads to the formation of surface gravity waves whose amplitude grows with time according to a power law. Flow stability is analyzed by using a nonmodal approach in which the behavior of a spatial Fourier harmonic of a disturbance is considered in a semi-Lagrangian frame of reference moving with the flow. Shear-flow disturbances are divided into two classes (wave and vortex disturbances) depending on the value of potential vorticity. It is shown that vortex disturbances decay with time while the energy of wave disturbances increases indefinitely. Transformation of vortex disturbances into wave ones under strong shear is described.     

An open magnet utilizing ferro-refraction current magnification

Ferro-refraction is the field magnification that is obtained when a current segment is near a high magnetic permeable boundary. It is shown that ferro-refraction may be used in the design of magnets for NMR or MRI to increase the efficiency of these magnets. The field may be modeled analytically with the Biot--Savart law and the inclusion of mirror image currents. Ferro-refraction is particularly useful in the design of monohedral magnets, magnets producing a remote homogeneous region which have the magnetic sources arranged to one side. These magnets have also been called planar magnets. Two designs for a monohedral magnet which produce good agreement between experimental and analytic results are presented.     

Generation of vortices by gravity waves on a water surface

The generation of a vortex motion on a water surface by gravity waves at frequencies of 3 and 4 Hz and wavelengths of 17 and 9.7 cm, respectively, has been studied experimentally. It has been shown that the results can be described by a model of the formation of a vorticity by nonlinear waves. It has been shown for the first time that the vorticity amplitude on a water surface depends on the phase difference between the waves propagating at an angle of 90° with respect to each other and with a period of 360°. A quadratic dependence of the surface vorticity amplitude on the angular amplitude of the waves has been observed. Transfer of the energy of the vortex motion from the pumping region to a larger scale has been discovered.     

干旱区不同下垫面能量分配机理及对微气候反馈的研究

本文利用位于美国新墨西哥州距离接近并且天气背景相同的四种下垫面(沙漠草原、稀疏灌木林、稀树草原和浓密灌木林)站点的通量观测资料, 探讨了几种典型干旱下垫面的能量分配差异, 并分析了其对微气候的反馈机理. 结果表明, 四种下垫面叶面积指数和粗糙度在由沙漠草原指向浓密灌木林的梯度方向上呈现增加的趋势, 低覆盖度下垫面表现出更强的湍流输送阻力. 总体来看, 高覆盖度下垫面的净辐射、感热和潜热相对更高, 尤其在生长季更明显. 利用Penman-Monteith公式以及净辐射结合波文比两种方法诊断了在不同下垫面更替中湍流通量各影响因子的贡献. 随着植被覆盖程度的提高, 净辐射的变化对感热和潜热的变化起着决定作用, 且为正贡献; 地表阻抗和空气动力学阻抗变化引起的贡献相反. 此外, 沙漠草原和稀疏灌木林的地表温度和气温均高于浓密灌木林, 主要源于稀疏植被增大的空气动力学阻抗和波文比引起的增温贡献, 同时抵消了净辐射减小引起的降温效应, 表明在相同气候和天气背景下, 不同下垫面的陆面过程确实会对近地层微气候有明显的反馈作用.     

读“等效的磁荷观点”有感

文章在假想的“磁荷世界”中修改了麦克斯韦方程组和洛伦兹力公式.由磁的库仑定律出发,并根据磁荷与电荷产生的电磁场的等效性,得到真空中静磁场的高斯定理和环路定理.仿照“电荷世界”中的电流定义了磁流强度,又根据磁场的相对论变换,由毕奥—萨伐尔定律和法拉第电磁感应定律得到稳恒电场D的高斯定理和环路定理.     

部分相干修正贝塞尔高斯光束通过球差透镜的聚焦特性

由部分相干光的传输理论出发,对部分相干修正贝塞尔高斯光束通过球差透镜聚焦后轴上光强分布进行了研究.数值计算表明:当光谱相干度较小时,正、负球差对应的最佳聚焦点位于无球差时对应的最佳聚焦点的两侧;当光谱相干度较大时,正、负球差对应的最佳聚焦点将位于无球差时对应的最佳聚焦点的左侧.研究进一步表明,最佳聚焦点随着透镜菲涅耳数的增大向几何焦点方向靠近,并趋于一个定值.当光束菲涅耳数为1时,无球差时对应的最佳聚焦点逐渐趋于0.908,该值与光谱相干度无关,当透镜具有球差时,该值与光谱相干度有关.     

非线性两能级系统中的Landau-Zener-Coulomb跃迁

非线性跃迁动力学的研究是量子操控中的一个重要课题,在原子分子光物理、量子信息以及固体物理等领域都有着重要的应用.本文研究了具有粒子间相互作用两能级系统中的Landau-Zener-Coulomb跃迁动力学.结果表明,对于能级的斜率正负不同的情况,粒子间相互作用对系统跃迁动力学的影响完全不同.为正时,粒子间相互作用总是抑制能级间的非绝热跃迁,且相互作用强度越大,两能级间的非绝热跃迁越难发生.而为负时,弱相互作用会促进能级间的非绝热跃迁,在强相互作用情况下,跃迁概率会出现振荡.随着相互作用强度的增大,振荡幅度逐渐减小,能级间的非绝热跃迁受到抑制.     

自旋场效应晶体管中隧道磁阻的势垒相关反转效应

考虑自旋场效应晶体管中Rashba自旋轨道相互作用和自旋输运量子相干性,研究了势垒强度对自旋场效应晶体管的自旋相关量子输运的影响. 研究发现,势垒强度较低时,隧道结电导随Rashba自旋轨道相互作用强度的变化呈现明显的振荡现象,势垒强度较高时,电导表现出明显的势垒相关“电导开关”现象. 当势垒强度逐渐增强时,平行结构电导呈现出单调下降趋势,而反平行结构电导产生波动,这种波动导致该隧道磁阻也随势垒强度的变化表现出振荡现象,且在合适的准一维电子气厚度情况下隧道磁阻值可以产生正负反转,这个效应将会在基于自旋的电子器件信息的存储上获得应用. 关键词： 自旋场效应管 开关效应 量子相干 隧道磁阻     

高能电子辐照绝缘厚样品的表面电位动态特性

采用数值计算和实验测量相结合的方法, 阐明了高能电子束照射下绝缘厚样品的表面电位和电子产额动态特性. 结果表明: 由于电子在样品内部的散射和输运, 沿着深度方向, 空间电位先缓慢下降到最小值, 然后逐渐升高并趋近于零; 随着电子束照射, 样品的表面电位逐渐下降, 可至负千伏量级, 电子总产额逐渐增大至一个接近于1的稳定值; 电子束停止照射后, 长时间放置下, 表面电位将逐渐升高, 但带电并不会消除; 表面电位随电子束能量的升高近似线性下降, 随入射角的增大而升高, 而随样品厚度的增大仅略有下降.     

Charge transfer between isomer domains on n(+)-doped Si(111)-2?×?1: energetic stabilization

Domains of different surface reconstruction-negatively or positively buckled isomers-have been previously observed on highly n-doped Si(111)-2?×?1 surfaces by angle-resolved ultraviolet photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. At low temperature, separate domains of the two isomer types are apparent in the data. It was argued in the previous work that the negative isomers have a lower energy of their empty surface states than the positive isomers, providing a driving force for the formation of the negative isomers. In this work we show that the relative abundance of these two isomers shows considerable variation from sample to sample, and it is argued that the size of the isomer domains is likely to be related to this variation. A model is introduced in which the electrostatic effect of charge transfer between the domains is computed, yielding total energy differences between the two types of isomer. It is found that the transfer of electrons from domains of positive isomers to negative ones leads to an energetic stabilization of the negative isomers. The model predicts a dependence of the isomer populations on doping that is in agreement with most experimental results. Furthermore, it accounts, at least qualitatively, for the marked lineshape variation from sample to sample observed in photoemission spectra.     

Thermostimulated electron emission from polar cleavage of a triglycine sulfate crystal

This paper reports the results of experimental investigations into thermostimulated electron emission from the positive and negative cleavage surfaces of a ferroelectric triglycine sulfate crystal. It is shown that the current density j of emission from the negative surface is universally higher than that from the positive surface over the entire range from room temperature to the Curie point. The shape of the j(T) curves substantially depends on the degree of natural unipolarity of the studied samples. The results obtained are discussed in the framework of the mechanism of field electron emission from the surface electronic states.     

速比对混合层湍流参数影响的实验研究

本文使用PIV术对低高速侧速比为0.25、0.33和0.5时竖直通道内的混合层流动进行实验研究,基于速度差和通道水力直径的雷诺数范围15840～132000.研究发现混合层内湍流参数的分布不仅和雷诺数有关,还和速比有关.混合层内同一横截面上平均雷诺应力的最大值随雷诺数的增大而增大,而在同一横截面上相同雷诺数时雷诺应力的最大值则随速比的增大而减小.在同一横截面上平均涡量随雷诺数的增大而增大,雷诺数相同时平均涡量的最大值随速比的增大而增大.无量纲平均涡量的最大值随混合层的发展按指数规律衰减,速比越大衰减速度越快.