Realistic simulations of stellar surface convection with ANTARES: I. Boundary conditions and model relaxation

We have implemented open boundary conditions into the ANTARES code to increase the realism of our simulations of stellar surface convection. Even though we greatly benefit from the high accuracy of our fifth order numerical scheme (WENO5) the broader stencils needed for the numerical scheme complicate the implementation of boundary conditions. We show that the effective temperature of a numerical simulation cannot be changed by corrections at the lower boundary since the thermal stratification does only change on the Kelvin–Helmholtz time scale. Except for very shallow models this time scale cannot be covered by multidimensional simulations due to the enormous computational requirements. We demonstrate to what extent numerical simulations of stellar surface convection are sensitive to the initial conditions and the boundary conditions. An ill-conceived choice of parameters for the boundary conditions can have a severe impact. Numerical simulations of stellar surface convection will only be (physically) meaningful and realistic if the initial model, the extent and position of the simulation box and the parameters from the boundary conditions are chosen adequately.     

井水温度微动态及其形成机制

介绍了我国地震地下水温度观测概况及已获得的微动态现象, 即水温固体潮、 水温同震效应、 水温前兆异常等, 并对其形成机理做了理论分析。 提出水温微动态形成机理研究应从二个地温梯度、 二个对流、 一个传导、 一个扩散等热系统与热过程去考虑。     

Progress in studies on the structure and physical properties of the mantle in China

The studies on the structure and physical properties of the mantle by Chinese geophysicists from 2003 to 2007 are reviewed in this report. It mainly contains studies on the seismic velocity structure of the mantle,anisotropy of the mantle,mantle discontinuities,mantle convection and the physical properties of mantle. The review concerns mainly the contents,the methods used and the results of the studies. It can be found that new progress in the study on the structure and physical properties of mantle has been made in the last four years in China. In some preexis-tent areas much progress has been made,advanced methods have been adopted,extensive international co-operation has been conducted in many ways,and the scope of the co-operation has gradually expanded. More-over,some new fields appear as well.     

Rotating ‘star-in-a-box’ experiments

Using the radiation hydrodynamics code CO⁵BOLD in its ‘star-in-a-box’ setup, we have performed exploratory simulations of global convection in a rotating reference frame. The goal is to study the interaction of convection and rotation by direct numerical simulation. For these first experiments, we chose an idealized configuration (a scaled-down, fast rotating Sun) whose properties resemble those of red supergiants in some respect. We describe the setup and time evolution of these models, and discuss the particular problems we have encountered. Finally, we derive the resulting differential rotation pattern and meridional flow field by temporal and azimuthal averaging of the simulation data. We find anti-solar differential rotation for all cases studied so far. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The angular momentum transport in rotating turbulent convection

The angular momentum transport in rotating turbulent convection is simulated with the NIRVANA code for Taylor numbers up to 10⁶. The box consists of an unstable layer embedded in two stable overshoot layers. We find the expected anisotropies in the rotating anisotropic turbulence field: 〈u^′2_r〉 exceeds 〈u^′2_ϕ〉, and 〈u^′2_ϕ〉 exceeds 〈u^′2_θ〉. The resulting radial angular momentum transport is directed inwards and peaks in the middle of the convective layer. The resulting latitudinal angular momentum transport is equatorwards, peaks at the surface and is highly concentrated to the equatorial region. Apart from a factor of 2–3 the total amplitudes of the cross‐correlations are of the same order of magnitude. In the lower overshoot region (‘tachocline’) the cross‐correlations are negative. It is argued that the concentration of the latitudinal angular momentum transport towards the surface and towards the equator does not too strongly reduce its potential to produce rotation laws with accelerated equators. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

祁连山区大坂山隧道围岩的冻融状况分析

通过对严守地区隧道现场基本气象条件的分析,建立了隧道内空气与围岩对流换热及固体导热的综合模型;在隧道内空气以紊流为主的情况下,分析预报了祁连山区大坂山隧道开通运营后洞内温度及围岩冻结、融化状况,并与在隧道内空气以层流为主的情况下的已有结论进行比较。     

滇西北高原一次冰雹灾害天气过程成因诊断及预报

对2010年7月27日发生在滇西北宁蒗县境内一次局地降雹灾害过程进行诊断分析,结果表明:本次降雹过程的主要客观物理量条件是具备充沛的水汽条件、不稳定条件及较强的垂直上升运动条件,但过程前除具备降雹的不稳定条件外,水汽及动力条件均不满足;降雹过程前的数值预报产品分析表明,数值预报产品对本次过程的主要影响系统、水汽条件、垂直运动条件、不稳定条件均有一定的预测能力。因此,在分析主要影响系统及物理量诊断产品(包括数值预报产品)条件下,结合卫星云图及雷达回波可对局地冰雹灾害天气提前预报预警。     

一次强对流天气的诊断分析

通过对1998年4月19日发生在北海市银海区侨港、咸田、西塘等镇的一次飑线天气过境过程的大气环流特征风的垂直切变及物理量场、能量场的诊断分析,探讨这次强对流天气形成的原因。     

Reconstructing the IRAS Point Source Catalog Redshift Survey with a generalized PIZA

We study the influence of different magnetic boundary conditions on the generation of magnetic fields by turbulent convection. It is found that the structure and strength of the generated field in the vicinity of the boundary is strongly dependent on the choice of boundary conditions. In the convective interior, however, the solutions remain largely insensitive to the boundary conditions. In all cases the overall efficiency of the dynamo process remains high with a steady state magnetic energy density between 12 and 25 per cent of the turbulent kinetic energy, and peak field values exceeding the equipartition level. These results support the idea that the solar granulation may constitute a dynamo source for magnetic fields in the quiet photosphere.