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

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.     

利用简化动力学近似分析黑河实验区的感热通量

本文利用简化动力学近似．运用统计和参数化两种方法分析了黑河实验区的感热通量．结果表明：不稳定条件下绿洲区的感热通量与近地面辐射温度(T3)和空气温度(Ta)之差符合幂定律方程；沙漠地区符合形式为H=f[u(T2-Ta)]的幂定律方程。利用从实验得到的热传输附加阻尼表示式kB^-1=0．2u(T2-Ta)用于对空气动力学阻尼(ra)的参数化，可适用于不同下垫面．并得到了比较满意的感热通量值．最后利用以上结果计算了实验区的潜热通量并做了比较分析。     

修正的质量通量积云对流方案及其模拟试验研究Ⅰ:方案介绍及对1991年洪涝过程的模拟

文中在综合比较各类积云对流参数化方案优缺点的基础上 ,主要参考陈伯民等修正的ECMWF质量通量积云参数化方案 ,对其进行简化和修改 ,发展了一个质量通量积云对流参数化方案 ,文中表示为 MFS(Mass Flux Scheme)。MFS是一种综合型的方案 ,既考虑了大尺度水汽辐合的重要性 ,又考虑了积云中的上升运动、下沉运动、环境中的补偿下沉运动 ,以及卷入、卷出和蒸发等 ,用总体云模式来描述积云与环境的相互作用 ,同时考虑了深对流和浅对流。将 MFS植入 NCAR区域气候模式 Reg CM2中 ,对 1 991年 5～ 7月江淮地区特大降水过程的夏季风气候特征和变化进行了模拟 ,并与 NCAR Reg CM2选用 Kuo方案 ,在同样初、边值条件和其它物理过程选择下的模拟结果进行了对比分析。分析结果表明 ,植入 MFS后的模式能够模拟这次极端的降水气候事件。在某些方面 ,如地表气温 ,降水的模拟上 ,植入 MFS后的模式的模拟结果要比原模式的结果更合理     

Numerical Simulations of Sheltering in Valleys: The Formation of Nighttime Cold-Air Pools

Numerical simulations of flow over two-dimensional valleys are conducted in order to study the occurrence of pools of cold air that form at the bottom of valleys during stable nighttime conditions. The results show that during strong surface radiative cooling and light-wind events, the near-surface potential temperatures that occur at the bottom of valleys can be several kelvin below the environmental mean. This is true for quite shallow valleys with depths and widths of 50 m and 1 km, respectively, and is a result of in situ sheltering at the valley bottom. For windier conditions or less rapid cooling, the cold-pool temperature contrasts are reduced. For shallow valleys the magnitude of the difference between the potential temperature at the bottom of the valley and the mean value increases with increasing valley depth. However there is a critical valley depth, beyond which the valley flow becomes decoupled from that aloft and there are no further increases in the potential temperature difference. This critical valley depth depends on the wind speed and radiative cooling rate and the results indicate it is a function of a non-dimensional valley depth (or inverse Froude number), which is itself a property of the undisturbed profiles of wind and stability.     

磁通量浮现驱动磁场重联的数值研究

应用二维时变可压缩磁流体动力学数值模拟,研究了双极－单极磁场中由于磁通量浮现驱动的磁场重联过程．结果表明,双极场与单极场间磁力线的重联形成上升的冷而密的等离子体团,磁场演变成鞭状结构．向上运动的等离子体团到达其最大高度后将回落和弥散.等离子体团最大上升速度达０．１４ＶＡ,等离子体最大上升速度达０．２７ＶＡ,ＶＡ为下边界处的Ａｌｆｖｅｎ速度．随着磁通量浮现幅度的增大,等离子体上升速度增加,重联过程发展得较快．背景等离子体β１值（β１为等离子体压力与磁压之比）越小,等离子体团中密度增量越大．磁Ｌｕｎｄｑｕｉｓｔ数Ｓ在１０３－１０６之间的改变对等离子体的速度和密度增量影响并不明显．与电阻撕裂模不稳定性引起的自发重联相比,磁通量浮现会更有效地驱动双极－单极场中的磁场重联过程,寻致日冕Ｈα冲浪和Ｘ射线喷流的形成．     

动态地幔柱尾管结构

动态地幔柱模式被广泛用于讨论地球科学中的一些重要课题,如巨大火成区的成因,冈瓦纳古陆解体的原因,板块内中小尺度动力过程的驱动因素等．但是这个基于实验研究而建立的模式中,忽略了地幔柱尾管特征及其作用．地幔柱尾管内温度和速度分布是研究地幔柱上升过程的必要条件．本文从控制尾管结构的基本方程出发,给出了一个定常轴对称地幔柱温度和速度分布的近似分析解．从而得到尾管结构的基本特征：影响尾管内温度分布的主导因素是地幔柱的热流通量,而尾管内上涌速度的大小则不仅取决于热流通量,主要是取决于地幔粘度随深度的变化方式．结果表明,对弱地幔柱,尾管的热损失可能是不可忽略的,而对强地幔柱,径向质量传递可能是不可忽略的.     

中山站地区大气边界层结构和湍流通量的输送特征

本文根据作者随中国第１１次南极考察队（１９９４－１９９６年）赴中山站夏考期间，使用ＴＭＴ（系留式气象塔）观测系统，在中山站、冰盖和海冰等三个点上观测到的气温、湿度、风速、风向和气压等大气要素的廓线资料，利用相似理论的通量－廓线关系，给出这三个观测点上的动量通量、曳力系数和感热通量，在近中性条件下，中山站粗糙度高度为２．９ｃｍ，冰盖与海冰点二者粗糙度高度差不多，分别为２．８×１０－２ｃｍ和３．６×１０－２ｃｍ，曳力系数则分别为５．０６×１０－３、１．３４×１０－３和１．４１×１０－３。结果表明，即使在中山站地区这样一个尺度范围内，由于下垫面的热力学特性、地形、地貌、地势和地理位置等的差异，无论是大气边界层结构还是湍流通量等的输送特性都有明显的差别，因此南极地区的大气边界层结构和冰（雪）－气交换特征都是比较复杂的，具有很强的局地性     

Surface heating in relation to air temperature,wind and turbulence in an urban street canyon

Wind and temperature measurements from within and above a deep urban canyon (height/width = 2.1) were used to examine the thermal structure of air within the canyon, exchange of heat with the overlying atmosphere, and the possible impacts of surface heating on within-canyon air flow. Measurements were made over a range of seasons and primarily analysed for sunny days. This allowed the study of temperature differences between opposing canyon walls and between wall and air of more than 15°C in summer. The wall temperature patterns follow those of incoming solar radiation loading with a secondary daytime effect from the longwave exchange between the walls. In winter, the canyon walls receive little direct solar radiation, and temperature differences are largely due to anthropogenic heating of the building interiors. Cool air from aloft and heated air from canyon walls is shown to circulate within the canyon under cross-canyon flow. Roofs and some portions of walls heat up rapidly on clear days and have a large influence on heat fluxes and the temperature field. The magnitude and direction of the measured turbulent heat flux also depend strongly on the direction of flow relative to surface heating. However, these spatial differences are smoothed by the shear layer at the canyon top. Buoyancy effects from the heated walls were not seen to have as large an impact on the measured flow field as has been shown in numerical experiments. At night canyon walls are shown to be the source of positive sensible heat fluxes. The measurements show that materials and their location, as well as geometry, play a role in regulating the heat exchange between the urban surface and atmosphere.     

Seasonal Variability in Circulation Pattern and Residence Time of Suo-Nada

The development and evolution of the persistent counterclockwise circulation in Suo-Nada have been studied in detail using a three-dimensional numerical model. The realistic circulation has been reproduced from the monthly climatological fields of salinity, temperature along the open boundary, wind, heat flux and buoyancy input from twenty-two major rivers surrounding the basin. The seasonal variation of fresh-water discharge from the rivers proved to be the most important forcing mechanism driving the circulation. The expansion and contraction of coastal low salinity water well corresponded with the development of counterclockwise circulation from spring to summer and dissipation from autumn to winter. The circulation was found to be vertically homogenous and quasi-barotropic. However, confined along the coastal zone is an estuarine flow (i.e., oppositely directed currents in the upper and lower layers) whose horizontal dimension conforms to the period of maximum buoyancy flux from the rivers. This indicates that the flow pattern in Suo-Nada can be separated into two distinct regions where the relative dominance of estuarine circulation plays a significant role. Furthermore, the monthly water exchange capacity for autonomous purification of the basin has been evaluated from the average residence time of conservative material based on the calculated current field. It is demonstrated that the kinetic energy of the basin is directly responsible in promoting water exchange in Suo-Nada. This revised version was published online in August 2006 with corrections to the Cover Date.     

北戴河海滩泥沙捕获实验及其初步结果分析

介绍了利用泥沙捕获器观测破波带泥沙垂直分布结构的现场实验方法和基本程序，以及利用实验结果计算泥沙通量的方法。研究表明，近岸带泥沙运移通量及其在垂向上的分布受破波带相对位置和海滩地形变化的影响。在破波点附近，波浪的搅动和流场作用强，泥沙运移通量增大，泥沙在波浪的作用下可以大量进入垂直水体以悬移和跃移的方式运移。在本实验中，破波点附近的泥沙在距海底100cm的垂直水体中运移，通量垂向向上逐渐减小。远离破波点，泥沙运移通量和进入垂直水体的高度明显下降。在地形变化复杂的有坝海滩，沙坝顶部的泥沙运移通量最大，泥沙进入垂直水体运移的机率增加，而在沙坝问的沟槽内，波浪和海流作用减弱，泥沙通量和垂向进入水体运移的比例下降。