Simulation of the Askervein Flow. Part 1: Reynolds Averaged Navier–Stokes Equations (k ∈ Turbulence Model)

The neutrally stratified flow over the Askervein Hill was simulatedusing a terrain-following coordinatesystem and a two-equation(k - ) turbulence model. Calculations were performed on awide range of numerical grids to assess, among other things, theimportance of spatial discretization and the limitations of theturbulence model. Our results showed that a relatively coarse gridwas enough to resolve the flow in the upstream region of the hill;at the hilltop, 10 m above the ground, the speed-up was 10% lessthan the experimental value. The flow's most prominent feature wasa recirculating region in the lee of the hill, which determinedthe main characteristics of the whole downstream flow. This regionhad an intermittent nature and could be fully captured only in the caseof a time-dependent formulation and a third-order discretization ofthe advective terms. The reduction of the characteristic roughnessnear the top of the hill was also taken into account, showing theimportance of this parameter, particularly in the flow close to theground at the summit and in the downstream side of the hill.Calculations involving an enlarged area around the Askervein Hillshowed that the presence of the nearby topography affected the flowneither at the top nor downstream of the Askervein Hill.     

Surface-Layer Characteristics in the Stable Boundary Layer with Strong and Weak Winds

An extensive meteorological dataset obtained from the plumevalidation experiment conducted by the Electric Power Research Institute (EPRI) atKincaid during 1980–1981 is analysed for studying the characteristic differences in thesurface-layer parameters in strong and weak wind stable conditions. The surface-layerparameters are computed using the similarity functions _m and _h proposed byBeljaars and Holtslag. The weak winds are characterized using the geostrophic wind speedas well as the wind speed at the 10-m level. The surface fluxes are found to be finitein weak wind conditions.Empirical formulations for the eddy diffusivities of momentum(K_M) and heat (K_H), and drag (C_D) and heat exchange (C_H) coefficients, as powerlaw functions of the bulk Richardson number (Ri_B), are proposed under both strong andweak wind conditions. Results are close to those based on observations taken from the IndianInstitute of Technology low wind diffusion experiment, the Land surface processes experiment,the Hanford diffusion experiment, the Cabauw field experiment and the Cooperative Atmospheric SurfaceExchange Study 1999 (CASES-99) experiment. In addition, the fluxes obtained fromthe proposed empirical relations are in good agreement with those based on similarity theory as wellas the turbulence measurements taken from the CASES-99 experiment.     

Seismic Diagnostics on Stellar Convection Treatment from Oscillation Amplitudes of p-modes

The excitation rate P of solar p-modes is computed with a model of stochastic excitation which involves constraints on the averaged properties of the solar turbulence. These constraints are obtained from a 3D simulation. Resulting values for P are found 4.5 times larger than when the calculation assumes properties of turbulent convection which are derived from an 1D solar model based on Gough (1977)'s formulation of the mixing-length theory (GMLT). This difference is mainly due to the assumed values for the mean anisotropy of the velocity field in each case.Calculations based on 3D constraints bring the P maximum closer to the observational one.We also compute P for several models of intermediate mass stars (1 M 2 M).Differences in the values of P _max between models computed with the classical mixing-length theory and GMLT models are found large enough for main sequence stars to suggest that measurements of P in this mass range will be able to discriminate between different models of turbulent convection.     

Laboratory Simulation Of Vertical Plume Dispersion Within A Convective Boundary Layer

Mesoscale measurements of the vertical dispersion coefficient ₂ by using a composite turbulence water tank were validated through a comparison with CONDORS (Convective Diffusion Observed with Remote Sensors) field data, and were analysed with respect to the intensity of the thermal flux, mechanical turbulence, and plume release height.It seems possible to correct the plume _z values for different release heights below 0.5z_i (z_i is the mixing height) by applying an equation expressing the height dependency of turbulence intensity. The downwind distance where the plume's mass centre height approaches its final level was also analysed with respect to the above three parameters, and an empirical equation to estimate the downwinddistance derived.     

土壤—植物—大气系统水分运行的界面过程研究

本文从水文循环的微观角度出发，针对大田土壤－大气系统中的水分运行与转化，研究了ＳＰＡＣ各界面上水分与能量的交换过程，旨在通过各界面上水分运行与生态环境因子相互作用关系，探索各界面水分、能量通量的计算与人工调控的可能途径，为农业节水提供理论依据。     

Observation and interpretation of cometary low frequency waves

The first observations of cometary wave activity were carried out in 1985/1986 by several space missions (ICE, VEGAs 1 and 2, Suisei, Sakigake, Giotto) in the environments of comets Giacobini-Zinner and Halley. The interpretation of thesein situ field (and particle) measurements fostered investigations on (among other topics) wave generation that, leaving aside the inherently nonlinear (but related) problem of the eventual formation of a cometary bow shock wave, explored the free energy available in two specific features of the velocity distributions of the newborn particle populations: their parallel (with respect to the IMF direction) drift in the solar wind frame and perpendicular ring-like organization. Analytical and simulation works looked into the influence of the solar wind and cometary newborn parameters on the instabilities and the ensuing, or associated (as evidenced by wave observations), nonlinear phenomenology. Comprehensive reviews have described the experimental and theoretical results obtained in this cometary wave research until 1992 and identified outstanding problems warranting further attention. Here, only a cursory revisit to the Giacobini-Zinner/Halley era of low frequency wave observation and interpretation shall be made: rather, attention shall be predominantly focussed on the new implications to cometary wave research of the recent Giotto encounter with comet Grigg-Skjellerup on July 10 of 1992. The three visited comets, starting with their gas production rates, had different characteristics that showed up in thein situ observations. Yet, with the important exception of the Grigg-Skjellerup encounter, the interpretation of the wave activity measurements could be made in terms of common basic generation mechanisms adapted to the relevant properties of the appropriate plasma environment. New aspects emerged in the last Giotto cometary mission: the smaller gas production rates yield a scale length for the neutral gas density that is not (much) larger than the gyration distance of a heavy newborn ion (estimated by the product of the solar wind speed and the ion cyclotron period). As a consequence of this inhomogeneity, the velocity distribution of the heavy newborn ions exhibits gyrophase organization, i.e. nongyrotropy. This new source of free energy, albeit briefly mentioned in a few studies preceding the Grigg-Skjellerup mission, was not investigated in the context of the Giacobini-Zinner and Halley encounters. Since the last Giotto observations strongly suggest that nongyrotropy plays a prominent role in wave generation as the comet Gigg-Skjellerup nucleus is approached and its stability characteristics have only seldomly been analyzed, the review shall emphasize the wave generation capabilities of particle populations with gyrophase organization.     

晴空湍流(CAT)对大尺度和中尺度动力强迫过程的响应

该文利用中尺度客观分析资料，对θ坐标系下Ri数倾向方程中的非湍流变形强迫场进行了诊断分析，以揭示晴空湍流（CAT）对大尺度和中尺度动力强迫过程的响应关系。计算结果表明:晴空湍流往往出现在对流层高层或高空急流附近。在层结稳定的高层大气薄层中，Ri数较小的空气微团遇有较强的非湍流变形强迫场时，由于动力强迫和Ri的耦合作用，往往导致晴空湍流的发生。     

论中尺度运动对大尺度模式网格平均的次网格垂直通量的影响

较系统地阐述了中尺度运动对大尺度模式网格平均的次网格通量的影响方式和作用机制 ,提出了在大尺度模式中参数化次网格中尺度通量和地表次网格通量中尺度加强的思想方法 ,初步分析了这些参数化方法的缺陷和局限性 ,对几个在参数化中尺度通量和地表次网格通量中尺度加强时应重点考虑的关键因子提出了建议。最后 ,讨论了参数化非均匀大气的网格平均的次网格通量的一些新的想法及相关问题     

Atmospheric surface layer characteristics with MONTBLEX data

Data obtained from the 30m high MONTBLEX tower installed at the Indian Institute of Technology, Kharagpur are described. Data on wind speed and direction, temperature and humidity were recorded during the first week of July 1989 at six levels on the tower. They indicate some of the atmospheric surface layer characteristics. Using two levels of tower data involving wind speed and temperature a profile method was used for computing the surface fluxes of heat and momentum. A method for calculating the surface roughness length using the data was also used to obtain the vertical windspeed profile. The computed wind profile was compared with observations, while the surface roughness length was compared with values obtained by a least square fit. The computed surface fluxes were compared with theoretical values obtained by an energy budget method which uses only wind data at one level. The agreement was reasonably good.