河西走廊下垫面粗糙度实测值与模拟值的差异性分析   总被引：5，自引：4，他引：1       下载免费PDF全文

胡文超  张文煜  张宇  郭振海  刘欣 《高原气象》2010,29(1):51-55

利用甘肃省河西地区17个风塔观测资料和风向风速标准差法,计算了风塔处的下垫面粗糙度,并与中尺度数值模式ARPS中使用的粗糙度参数进行了比较分析。结果表明,数值模式中使用的粗糙度值和实况具有一定差异,其使用的粗糙度定义方案并不能准确地反映出下垫面粗糙度和非均一性特征。差异最大出现在草原下垫面上,可达375%,且差异程度随下垫面植被复杂程度增加。  相似文献   

青藏高原东部土壤冻融过程中地表粗糙度的确定   总被引：4，自引：1，他引：3       下载免费PDF全文

尚伦宇  吕世华  张宇  奥银焕  罗斯琼  李锁锁  王少影 《高原气象》2010,29(1):17-22

利用黄河源区气候与环境综合观测研究站2006年10月—2007年4月的湍流观测资料和一种新的方法,计算了青藏高原东部玛曲地区土壤冻融过程中的地表粗糙度。结果表明:所用的计算粗糙度的方法是可行的,玛曲土壤未冻结阶段、冻结阶段和融化后阶段的地表粗糙度分别为3.23×10-3m,2.27×10-3m和1.92×10-3m,地表粗糙度呈逐渐减小的趋势。三阶段地表粗糙度有明显区别,以前将冬季前后的粗糙度取为定值的计算会导致一定的误差。  相似文献   

The Effect of Stratification on the Aerodynamic Roughness Length and Displacement Height   总被引：1，自引：0，他引：1  

S. S. Zilitinkevich  I. Mammarella  A. A. Baklanov  S. M. Joffre 《Boundary-Layer Meteorology》2008,129(2):179-190

The roughness length, z _0u , and displacement height, d _0u , characterise the resistance exerted by the roughness elements on turbulent flows and provide a conventional boundary condition for a wide range of turbulent-flow problems. Classical laboratory experiments and theories treat z _0u and d _0u as geometric parameters independent of the characteristics of the flow. In this paper, we demonstrate essential stability dependences—stronger for the roughness length (especially in stable stratification) and weaker but still pronounced for the displacement height. We develop a scaling-analysis model for these dependences and verify it against experimental data.  相似文献   

Temporal and Spatial Variations of the Aerodynamic Roughness Length in the Ablation Zone of the Greenland Ice Sheet   总被引：2，自引：1，他引：1  

C. J. P. P. Smeets  M. R. van den Broeke 《Boundary-Layer Meteorology》2008,128(3):315-338

To understand the response of the Greenland ice sheet to climate change the so-called ablation zone is of particular importance, since it accommodates the yearly net surface ice loss. In numerical models and for data analysis, the bulk aerodynamic method is often used to calculate the turbulent surface fluxes, for which the aerodynamic roughness length (z ₀) is a key parameter. We present, for the first time, spatial and temporal variations of z ₀ in the ablation area of the Greenland ice sheet using year-round data from three automatic weather stations and one eddy-correlation mast. The temporal variation of z ₀ is found to be very high in the lower ablation area (factor 500) with, at the end of the summer melt, a maximum in spatial variation for the whole ablation area of a factor 1000. The variation in time matches the onset of the accumulation and ablation season as recovered by sonic height rangers. During winter, snow accumulation and redistribution by snow drift lead to a uniform value of z ₀≈ 10⁻⁴ m throughout the ablation area. At the beginning of summer, snow melt uncovers ice hummocks and z ₀ quickly increases well above 10⁻² m in the lower ablation area. At the end of summer melt, hummocky ice dominates the surface with z ₀ > 5  ×  10⁻³ m up to 60 km from the ice edge. At the same time, the area close to the equilibrium line (about 90 km from the ice edge) remains very smooth with z ₀ = 10⁻⁵ m. At the beginning of winter, we observed that single snow events have the potential to lower z ₀ for a very rough ice surface by a factor of 20 to 50. The total surface drag of the abundant small-scale ice hummocks apparently dominates over the less frequent large domes and deep gullies. The latter results are verified by studying the individual drag contributions of hummocks and domes with a drag partition model.  相似文献   

The Parameterisation of Scalar Transfer over Rough Ice     

C. J. P. P. Smeets  M. R. van den Broeke 《Boundary-Layer Meteorology》2008,128(3):339-355

We test a surface renewal model that is widely used over snow and ice surfaces to calculate the scalar roughness length (z _s ), one of the key parameters in the bulk aerodynamic method. For the first time, the model is tested against observations that cover a wide range of aerodynamic roughness lengths (z ₀). During the experiments, performed in the ablation areas of the Greenland ice sheet and the Vatnajökull ice cap in Iceland, the surface varied from smooth snow to very rough hummocky ice. Over relatively smooth snow and ice with z ₀ below a threshold value of approximately 10^?3 m, the model performs well and in accord with earlier studies. However, with growing hummock size, z ₀ increases well above the threshold and the bulk aerodynamic flux becomes significantly smaller than the eddy-correlation flux (e.g. for z ₀ = 0.01 m, the bulk aerodynamic flux is about 50% smaller). Apparently, the model severely underpredicts z _s over hummocky ice. We argue that the surface renewal model does not account for the deep inhomogeneous roughness sublayer (RSL) that is generated by the hummocks. As a consequence, the homogeneous substrate ice grain cover becomes more efficiently ‘ventilated’. Calculations with an alternative model that includes the RSL and was adapted for use over hummocky ice, qualitatively confirms our observations. We suggest that, whenever exceedance of the threshold occurs (z ₀  >  10^?3 m, i.e., an ice surface covered with at least 0.3-m high hummocks), the following relation should be used to calculate scalar roughness lengths, ln (z _s /z ₀)  =  1.5  ? 0.2 ln (Re _*)  ? 0.11(ln (Re _*))².  相似文献   

A straightforward approach for using single time domain simulations to assess characteristic extreme responses     

L.V.S. Sagrilo  Z. Gao  E.C.P. Lima 《Ocean Engineering》2011,38(13):1464-1471

Short-term wave design approach of marine structures, using nonlinear time domain simulations, is a design procedure that is recognized by various modern standard codes. One of the most challenging points of this approach is the evaluation of the characteristic extreme values for response parameters used in the design check equations. The most straightforward and recommended way to evaluate a response characteristic value is by fitting an extreme value probability distribution to the N-sample of extreme values extracted from N independent time domain simulations with duration equal to the short-term period indicated by the code, which is usually taken as 3 h. However, this procedure would not be practical for some types of marine structures, such as risers and mooring lines, under numerous design load cases and demanding huge finite element models. A more feasible approach would be to assess the response extreme value distribution using only a single short-term time domain simulation with duration shorter than 3 h. But reduced time simulations always introduce some additional statistical uncertainty into the extreme values estimates. This paper discusses a workable way of properly taking into account the statistical uncertainty associated with the simulation length in the assessment of a characteristic short-term extreme response value based on a single time series.  相似文献   

独立坐标系模型对长度变形影响的研究     

李东  毛之琳  廖文兵  陈敏 《测绘与空间地理信息》2014,(9):79-81

针对海拔较高的城市建立或改造独立坐标系,存在着如何选择模型及各模型对长度变形的影响等问题,通过算例对常见椭球变换和比例缩放模型进行较为全面系统的分析,归纳出常见模型在不同投影带区域、不同投影面高度对长度变形的影响差异,并得出有益的结论。  相似文献   

城市下垫面空气动力学参数的确定   总被引：10，自引：1，他引：9  

李倩  刘辉志  胡非  洪钟祥  李爱国 《气候与环境研究》2003,8(4):443-450

分析近10年北京325m气象塔常年观测资料,研究城市化发展对城市大气边界层动力学结构和特征的影响.结果表明,空气动力学粗糙度和零平面位移总体是逐年递增的.气象塔西南方向的高层建筑群是20世纪90年代初期逐渐建成,在SW方向,空气动力学粗糙度和零平面位移在90年代增加明显,增加的幅度1997年和1999年之间较大,这一点与气象塔西南方向城市化的加快相符合.而在NE、SE和NW方向,从1987年到1994年空气动力学粗糙度和零平面位移增加明显,1994年以后变化较小.  相似文献   

基于低成本单频u-blox接收机的GPS/BDS定向性能分析     

李江卫  马立烨  吴仁攀  孙伟  吴多 《大地测量与地球动力学》2019,39(3):279-284

针对高精度GNSS定向应用场景，通过实验对比对低成本单频u-blox接收机的数据质量和解算精度。结果表明，u-blox接收机GPS、BDS观测值的信噪比略低于测量型接收机；伪距精度分别为0.91 m、0.56 m，相位精度分别为1.35 cm、1.20 cm。在静态观测环境下，u-blox的定向精度可以达到航向0.2°/m和俯仰0.4°/m；动态环境下解算结果稍差，但也可以达到航向0.3°/m和俯仰0.6°/m，略低于高成本测量型接收机单频数据的实时动态定向精度。  相似文献   

Sharks caught in the KwaZulu-Natal bather protection programme,South Africa. 14. The smooth hammerhead shark Sphyrna zygaena (Linnaeus)     

ML Dicken  H Winker  MJ Smale  G Cliff 《African Journal of Marine Science》2018,40(2):157-174

The current study provides long-term catch-rate, biological and feeding data for smooth hammerhead sharks, Sphyrna zygaena, caught in South Africa’s KwaZulu-Natal bather protection programme. In total, 2 512 S. zygaena were caught in net installations between 1978 and 2014, and 72 S. zygaena were caught on drumlines between 2007 and 2014. There was no significant log-linear year trend in the net catch rate over time (slope = 0.0054, t = 1.808, p = 0.07). However, there was a significant temporal increase in mean size of the captured sharks (slope = 0.0012, t = 3.502, p < 0.001). A quasi-Poisson generalised additive mixed model showed that increasing latitude, winter months, colder sea temperatures and the deployment of drumlines all had a significant positive effect on the catch rate of sharks in nets. The size frequency of the catch was unimodal, with significantly more females caught in the nets and more males on the drumlines. The majority (93.1%) of all sharks caught were immature and measured between 80 and 120 cm precaudal length. Teleosts and cephalopods dominated the sharks’ diet in terms of all dietary indices. The prey species consumed indicate that immature S. zygaena are feeding primarily within the pelagic zone of shallow coastal habitats.  相似文献