Large-Eddy Simulation Of Radiation Fog   总被引：1，自引：0，他引：1  

Mikio Nakanishi 《Boundary-Layer Meteorology》2000,94(3):461-493

In order to study the three-dimensional structure of radiation fogand to obtain a basic understanding of its generation mechanism,a numerical experiment is performed with a large-eddysimulation model and compared with the observation at Cabauw in the Netherlands. After confirming that the results are insatisfactory agreement with the observations, the structure of thefog and its generation mechanism are examined in more detail.Before the fog forms, the atmosphere is stable and an inversionlayer exists almost adjacent to the ground surface. As the fog grows, however, the stratification is destabilized and a mixed layerdevelops gradually. The longwave radiative cooling near thefog top contributes to the destabilization more than thecondensational heating does.The evolution of the fog can be classified into three stagesaccording to the behaviour of turbulent kinetic energy (TKE):formation, development, and dissipation stages.The fog layer has different flow structures at each stage.During the formation stage, longitudinal rolls similar tostreaks in channel flows appear near the ground surface.The development stage is characterized by an initiation oftransverse bands due to Kelvin–Helmholtz instability anda sudden increase of TKE. During the dissipation stage, longitudinalrolls and polygonal cells due to convective instability are organized.  相似文献   

对流边界层的Lyapounov稳定性分析   总被引：5，自引：3，他引：2  

薛具奎  胡隐樵 《高原气象》2000,19(2):235-243

针对一个简单的大气边界层干热对流模式,根据耗散结构理论及Ｌｙａｐｏｕｎｏｖ第二稳定性理论,分析了热对流边界层的热力学和动力学稳定性,并将二者结合构造了一个广义Ｌｙａｐｏｕｎｏｖ函数,进而在小扰动下给出了边界层的非平衡定态（参考态）发生不稳定的临界Ｒａｙｌｅｉｇｈ数和对流得以发展的临界浮力加速度。数值试验结果与理论分析一致。  相似文献   

Thermal convection in a heterogeneous mantle     

Anne Davaille  Michael Le Bars  Catherine Carbonne 《Comptes Rendus Geoscience》2003,335(1):141-156

Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics.  相似文献   

Styles of mantle convection and their influence on planetary evolution     

David J. Stevenson 《Comptes Rendus Geoscience》2003,335(1):99-111

Mantle convection is the method of heat elimination for silicate mantles in terrestrial bodies, provided they are not too small or too hot. Bodies that are small (～Moon or smaller, possibly even Mercury) may rely largely on conduction or melt migration, and bodies that are very hot (Io, very early Earth) may use massive melt migration (magma oceans) and heat pipes. In the standard, simple picture, we can use scaling laws to determine the secular cooling of a planet, likelihood and form of volcanism, and the possibility of a core dynamo. Contrary to popular belief, small planets do not cool faster than larger planets (provided they convect) but they do tend to have a slightly lower internal temperature at all times and thus may cease to be volcanically active at an earlier epoch. On the other hand, a larger volume fraction of a small planet may be involved in melt generation. However, our understanding of heat transfer by mantle convection is limited by three very important, largely unsolved problems: The complexities of rheology, the effects of compositional gradients, and the effects of phase transitions, especially melting. The most striking manifestation of the role of rheology lies in the difference between a mobile lid mode (plate tectonics for Earth) and a stagnant lid mode (other large terrestrial bodies). This difference may arise because of the role of water, but perhaps also because of melting, or size (gravity), or the vagaries of history. It has profound effects for the differences in history of Earth, Venus and Mars, including their surface geology, volatile reservoirs and magnetic fields. Since thermal convection is driven by small density differences, it can also be greatly altered or limited by compositional or phase effects. Melt migration introduces additional complications to the heat transport as well as being a source for the irreversible differentiation that might promote layering. Our limited understanding and ability to model these processes continues to limit the development of a predictive framework for the differences among the terrestrial planets.  相似文献   

A study on the structure of the convective atmosphere over the Bay of Bengal during BOBMEX-99     

U. C. Mohanty  N. V. Sam  S. Das  A. N. V. Satyanarayana 《Journal of Earth System Science》2003,112(2):147-163

Convective activity is one of the major processes in the atmosphere influencing the local and large-scale weather in the tropics. The latent heat released by the cumulus cloud is known to drive monsoon circulation, which on the other hand supplies the moisture that maintains the cumulus clouds. An investigation is carried out on the convective structure of the atmosphere during active and suppressed periods of convection using data sets obtained from the Bay of Bengal and Monsoon Experiment (BOBMEX). The cumulus convection though being a small-scale phenomenon, still influences its embedding environment by interaction through various scales. This study shows the variation in the kinematic and convective parameters during the transition from suppressed to active periods of convection. Convergence in the lower levels and strong upward vertical velocity, significant during active convection are associated with the formation of monsoon depressions. The apparent heat source due to latent heat release and the vertical transport of the eddy heat by cumulus convection, and the apparent moisture sink due to net condensation and vertical divergence of the eddy transport of moisture, are estimated through residuals of the thermodynamic equation and examined in relation to monsoon activity during BOBMEX.  相似文献   

对流和加热对重力惯性波的激发     

王兴宝 《热带气象学报》1997,(4)

用线性模型讨论了对流和加热对重力惯性波的激发，分别讨论了初始垂直速度扰动和初始位温扰动及大气内部加热在静态层结大气中激发的重力惯性波，及重力惯性波过程引起的大气温压场的变化。得到的结果可以帮助理解中小尺度系统的发生发展、对流过程与环境大气的相互作用。  相似文献   

Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain     

Antony?Z.?OwinohEmail author  Julian?C.?R.?Hunt  Andrew?Orr  Peter?Clark  Rupert?Klein  H.?J.?S.?Fernando  Frans?T.?M.?Nieuwstadt 《Boundary-Layer Meteorology》2005,116(2):331-361

We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u _* when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u _*). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h /|L_MO| over the time scale t_MO~|L _MO|/(10u_*) (where L _MO and t _MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h /|L _MO| (>>1), over the time scale t _MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.  相似文献   

Where and why do large shallow intraslab earthquakes occur?     

Tetsuzo Seno  Masaki Yoshida 《Physics of the Earth and Planetary Interiors》2004,141(3):183-206

We try to find how often, and in what regions large earthquakes (M≥7.0) occur within the shallow portion (20-60 km depth) of a subducting slab. Searching for events in published individual studies and the Harvard University centroid moment tensor catalogue, we find twenty such events in E. Hokkaido, Kyushu-SW, Japan, S. Mariana, Manila, Sumatra, Vanuatu, N. Chile, C. Peru, El Salvador, Mexico, N. Cascadia and Alaska. Slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down-dip tensional. Except for E. Hokkaido, Manila, and Sumatra, the upper plate shows horizontal stress gradient in the arc-perpendicular direction. We infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore-arc, balancing the down-dip tensional stress of the slab. This tectonic situation in the subduction zone might be realized as part of the convection system with some conditions, as shown by previous numerical simulations.  相似文献   

青藏高原隆升过程的三阶段模式   总被引：24，自引：8，他引：16       下载免费PDF全文

傅容珊  李力刚  黄建华  徐耀民  常筱华 《地球物理学报》1999,42(5):609-617

综合分析了前人对青藏高原岩石层构造和动力学研究的成果,提出印度板块和欧亚板块会聚、大陆碰撞及大陆形变的基本特征为青藏高原地壳的加厚和地壳缩短,地壳物质的横向流动;青藏高原隆升过程呈现出阶段性、多样性和复杂性;组成青藏高原的各块体可能有不同的主导隆升机制．认识到在板块构造理论所揭示的全球构造格局中,青藏高原不仅仅是印度板块和欧亚板块会聚、碰撞以及大陆形变的结果,它也是青藏高原大陆岩石层和下伏地幔物质运动的相互耦合、相互作用的结果．从地幔动力学的角度出发讨论了青藏高原隆升的断离险升－挤压隆升－对流隆升三阶段模式（ＢＣＣＭ）,结合数值模拟的结果分析了与此模式相对应的该区域岩石层构造、运动的地幔深部物质运移和动力学背景．  相似文献   

影响珠江三角洲地区的两次冷空气涌过程的地面结构和天气   总被引：1，自引：0，他引：1  

韦统健 《热带气象学报》1994,10(3):222-230

利用珠江三角洲地区的地面加密观测资料，揭示了影响该地区的一类中小尺度天气系统－锋后冷空气桶，并详细分析了它的地面结构和天气影响。结果表明，这类对流系统非常相似于热带飑线，往往给三角洲地区带来突发性暴雨，但却不易产生强风。该系统的水平尺度很小，只有３０－６０ｋｍ，持续时间达４小时，移速快，约为每小时６０ｋｍ，移向定常，并且它在进入三角洲地区时发展加强，而在移出时减弱消失。和经典的飑线结构的差异在于它  相似文献