Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-Eddy Simulations. Part II: Forced Convection

Forced convection in a quasi-steady atmospheric boundary layer is investigated based on a large-eddy simulation (LES) model. The performed simulations show that in the upper portion of the mixed layer the dimensionless (in terms of mixed layer scales) vertical gradients of temperature, humidity, and wind velocity depend on the dimensionless height z/z _i and the Reech number Rn. The peak values of variances and covariances at the top of the mixed layer, scaled in terms of the interfacial scales, are functions of the interfacial Richardson number Ri. As a result expressions for the entrainment rates, in the case when the interfacial layer has a finite depth, and a condition for the presence of moistening or drying regimes in the mixed layer, are derived. Profiles of dimensionless scalar moments in the mixed layer are proposed to be expressed in terms of two empirical similarity functions F _m and F _i , dependent on dimensionless height z/z _i , and the interfacial Richardson number Ri. The obtained similarity expressions adequately approximate the LES profiles of scalar statistics, and properly represent the impact of stability, shear, and entrainment. They are also consistent with the parameterization proposed for free convection in the first part of this paper.     

一次罕见冰雹天气的多普勒雷达回波分析

运用多普勒雷达的基本反射率(R)、风暴相对径向速度(SRM)、垂直积分液态含水量(VIL)等产品,分析了2004年6月20日华北南部罕见的冰雹大风天气过程。结果表明:这次冰雹大风天气过程主要是由超级单体回波中的中尺度气旋引起的;该过程持续时间比较长的原因是“指状”回波和超级单体回波的依次发生、发展。     

An approximate analytical solution for the deposition of heavy particles released from an elevated line source

This work re-examines and further develops an analytical solution for the deposition swath of heavy particles released in the atmosphere from an elevated source over uniform terrain, correcting the particle diffusivity for the crossing trajectory effect. The revised (approximate) analytical solution proves to be accurate within 20% over a wide range of micrometeorological conditions and particle size, despite its neglect of the turbulence component of the deposition flux. It compares very satisfactorily with experimental data and with the simulations of a Lagrangian stochastic model, provided the variable U(H)/w _g ≤7 (ratio of the mean horizontal wind speed at source height to the particle settling velocity). In this domain of validity, simple formulae relating the statistics of the deposition swath to U(H)/w _g are derived.     

The effect of mountainous topography on moisture exchange between the “surface” and the free atmosphere

Typical numerical weather and climate prediction models apply parameterizations to describe the subgrid-scale exchange of moisture, heat and momentum between the surface and the free atmosphere. To a large degree, the underlying assumptions are based on empirical knowledge obtained from measurements in the atmospheric boundary layer over flat and homogeneous topography. It is, however, still unclear what happens if the topography is complex and steep. Not only is the applicability of classical turbulence schemes questionable in principle over such terrain, but mountains additionally induce vertical fluxes on the meso-γ scale. Examples are thermally or mechanically driven valley winds, which are neither resolved nor parameterized by climate models but nevertheless contribute to vertical exchange. Attempts to quantify these processes and to evaluate their impact on climate simulations have so far been scarce. Here, results from a case study in the Riviera Valley in southern Switzerland are presented. In previous work, measurements from the MAP-Riviera field campaign have been used to evaluate and configure a high-resolution large-eddy simulation code (ARPS). This model is here applied with a horizontal grid spacing of 350 m to detect and quantify the relevant exchange processes between the valley atmosphere (i.e. the ground “surface” in a coarse model) and the free atmosphere aloft. As an example, vertical export of moisture is evaluated for three fair-weather summer days. The simulations show that moisture exchange with the free atmosphere is indeed no longer governed by turbulent motions alone. Other mechanisms become important, such as mass export due to topographic narrowing or the interaction of thermally driven cross-valley circulations. Under certain atmospheric conditions, these topographical-related mechanisms exceed the “classical” turbulent contributions a coarse model would see by several times. The study shows that conventional subgrid-scale parameterizations can indeed be far off from reality if applied over complex topography, and that large-eddy simulations could provide a helpful tool for their improvement.     

CFD simulation of airflow over a regular array of cubes. Part II: analysis of spatial average properties

In the first part of this study, results of a computational fluid dynamics simulation over an array of cubes have been validated against a set of wind-tunnel measurements. In Part II, such numerical results are used to investigate spatially-averaged properties of the flow and passive tracer dispersion that are of interest for high resolution urban mesoscale modelling (e.g. non resolved obstacle approaches). The results show that vertical profiles of mean horizontal wind are linear within the canopy and logarithmic above. The drag coefficient, derived from the numerical results using the classical formula for the drag force, is height dependent (it decreases with height). However, a modification of the formula is proposed (accounting for subgrid velocity scales) that makes the drag coefficient constant with height. Results also show that the dispersive fluxes are similar in magnitude to the turbulent fluxes, and that they play a very important role within the canopy. Vertical profiles of turbulent length scales (to be used in k–l closure schemes, where k is the turbulent kinetic energy and l a turbulent length scale) are also derived. Finally the distribution of the values around the mean over the reference volumes are analysed for wind and tracer concentrations.     

我国低钛月海型模拟月壤初始物质选择的地球化学依据

以Apollo14月壤样品和美国JSC-1模拟月壤的地球化学特征为基础,结合我国低钛玄武岩火山的分布,对我国不同地区新生代玄武岩的化学成分、年龄、储量等方面进行对比分析结果表明,滇西北金沙江地区和吉林辉南红旗林场-四海地区的玄武质火山喷发物的化学成分与Apollo14月壤样品和美国JSC-1模拟月壤相似,比较适合用于我国低钛月海玄武岩模拟月壤研制的初始物质。野外地质调查发现,吉林省辉南县金川镇红旗林场—四海一带产出的玄武质火山渣为距今1600 a的该玄武质火山岩的喷发物,其储量大、质地纯、粒度均匀,而且比滇西北金沙江地区最近一期的玄武质火山渣新鲜,最适合作为我国低钛月海玄武岩模拟月壤研制的初始物质。     

Modeling of gold mass transfer during the listwanitization and rodingitization using the example of the Ust’-Dep ophiolite complex in the upper Amur territory

Gold mass transfer with chloride and carbonate-chloride solutions was examined at the 300 and 400°C isotherms and P _tot = 1 kbar by means of experimental modeling and theoretical simulations. CO₂ was confirmed to suppress Au solubility in fluids. The low Au solubility (mAu < 10^?8) determined in the experiments explains the mechanism of its precipitation when serpentinites and listwanites interact with acidic mineralized solutions. Listwanitization, which was genetically related with the emplacement of orogenic granitoids, was determined to have overprinted serpentinites and rodingites and strongly affected Au transport in the oregeochemical system. The characteristics of the metasomatic processes in the Ust’-Dep ophiolites and the gold concentration in the rocks produced by these processes confirm this conclusion.     

湿斜压大气中暴雨中尺度系统发展的一种可能机制

根据暴雨中尺度系统发生时的大气运动特征,在绝热、无摩擦条件下,从湿斜压原始方程出发,考虑大气运动的涡度及散度演化不仅受动力场的制约,而且还受到热力场的约束,采用不同于传统研究涡度及散度方程的分析方法,导出了新形式的涡度及散度方程。在此基础上,分析了湿斜压大气中对流层中低层气流旋转与辐合持续增长的动力特征,初步揭示了湿斜压大气中暴雨中尺度系统发展的动力机制。     

1996年8月8日闽西地区特大暴雨过程分析

本文研究了9601号热带风暴减弱为低压环流后引起突发性大暴雨的成因。结果表明：9610号热带风暴减弱后的低压环流移支福建省南部地区,造成闽西地区湿斜压锋区和不稳定能量明显增强,在低压西侧辐合线所提供的福合上升运动激发下,中尺度对流云团迅速发展,导致远离低压中心的闽西突发大暴雨。闽西特殊的地形对特大暴雨的产生起了组织和增幅作用。另外,还通过逐时数字红外云团图的分析,揭示中尺度对流云团发展演变特征与突发性大暴雨的关系,供日常短时预报参考。     

Galactic Dynamo and Spiral Arms - 3D MHD Simulations

In the present project we investigate the evolution of a three-dimensional (3D), large-scale galactic magnetic field under the influence of gas flows in spiral arms and in the presence of dynamo action. Our principal goal is to check how the dynamical evolution of gaseous spiral arms affects the global magnetic field structure and to what extent our models could explain the observed spiral patterns of polarization B-vectors in nearby galaxies. A two-step scheme is used: the N-body simulations of a two-component, self-gravitating disk provide the time-dependent velocity fields which are then used as the input to solve the mean-field dynamo equations. We found that the magnetic field is directly influenced by large-scale non-axisymmetric density wave flows yielding the magnetic field locally well-aligned with gaseous spiral arms in a manner similar to that discussed already by Otmianowska-Mazur et al. 1997. However, an additional field amplification, introduced by a non-zero -term in the dynamo equations, is required to cause a systematic increase of magnetic energy density against the diffusive losses. Our simulated magnetic fields are also used to construct the models of a high-frequency (Faraday rotation-free) polarized radio emission accounting for effects of projection and limited resolution, thus suitable for direct comparisons with observations.