An Improvement of the Louis Scheme for the Surface Layer in an Atmospheric Modelling System   总被引：1，自引：1，他引：0  

Yu Song 《Boundary-Layer Meteorology》1998,88(2):239-254

The Louis scheme for calculating the vertical eddy fluxes within the atmospheric surface layer is improved by broadening the original assumptions. In our approach, the momentum and heat transfer roughness lengths (z₀ and z_T respectively) can be different, and z₀ need not be negligibly small compared with the lowest height (z) in modelling. For these conditions, we choose more consistent wind and potential temperature profile forms, then derive new algorithms for calculating fluxes. Improvement is demonstrated for a wide range of z/L (L is the Obukhov length), z/z₀ and z₀ z_T, by comparing these fluxes with those derived from a theoretical surface-layer model. The improved algorithms can be used in atmospheric modelling systems for more varied surfaces and a wide range of atmospheric stability.  相似文献   

The Validity of Similarity Theory in the Roughness Sublayer Above Forests   总被引：1，自引：0，他引：1  

I. J. Simpson  G. W. Thurtell  H. H. Neumann  G. Den Hartog  G. C. Edwards 《Boundary-Layer Meteorology》1998,87(1):69-99

Flux-gradient relationships based upon similarity theory have been reported to severely underestimate scalar fluxes in the roughness sublayer above forests, as compared to independent flux estimates (for example, eddy covariance or energy balance measurements). This paper presents the results of a unique three-month investigation into the validity of similarity theory in the roughness sublayer above forests. Eddy covariance and flux-gradient measurements of carbon dioxide (CO²) exchange were compared above a mixed deciduous forest at Camp Borden, Ontario, both before and after leaf senescence. The eddy covariance measurements used a Li-Cor infrared gas analyzer, and the flux-gradient (similarity theory) measurements featured a tunable diode laser Trace Gas Analysis System (TGAS). The TGAS resolved the CO² concentration difference to 300 parts per trillion by volume (ppt) based upon a half-hour sampling period. The measured enhancement factor (the ratio of independent flux estimates, in this case eddy covariance, to similarity theory fluxes) was smaller and occurred closer to the canopy than in most previous investigations of similarity theory. Very good agreement between the eddy covariance and similarity theory fluxes was found between 1.9 and 2.2 canopy heights (h^c), and the mean enhancement factors measured before and after leaf senescence were 1.10 plusmn; 0.06 and 1.24 ± 0.07, respectively. Larger discrepancies were measured closer to the canopy (1.2 to 1.4 h^c), and mean enhancement factors of 1.60 ± 0.10 and 1.82 ± 0.11 were measured before and after leaf senescence, respectively. Overall, the Borden results suggest that similarity theory can be used within the roughness sublayer with a greater confidence than previously has been believed.  相似文献   

Landscape Roughness Parameters for Sherwood Forest – Experimental Results     

M. Bottema 《Boundary-Layer Meteorology》1998,89(2):285-316

The aim of this work is to present experimentally evaluated effective roughnesses (zoe) of a partly forested landscape. Although the ratio of boundary-layer height to obstacle size was only of the order of 50, there still seemed to exist a height range of 75–200 m where surface-layer similarity was approximately valid. Attempts were made to use conventional wind profile analysis to evaluate zoe, but the small height range and the large number of variables initially led to unacceptable uncertainties. Fixing the displacement height zd, rather than fitting it, reduced the data scatter to an acceptable level. The profile-derived roughness lengths zop obtained in this way were in good agreement with previous work, and with an alternative roughness length estimate zof for which flux-derived profile parameters u* and * were used. This implies that the profile-derived roughnesses were consistent with the measured surface-layer momentum flux. Comparison of both roughness estimates also yielded an improved estimate of the displacement height. Besides this, the authors tested a landscape roughness evaluation method which makes use of the gustiness parameter Tu = u/U in the surface layer. The results obtained by this method were in fair agreement with the profile-derived data. In previous work, the gustiness method was advocated because it could be used at relatively low levels, perhaps even within the roughness sub-layer. At the present measuring site, this was not the case as the gustiness method was only valid in an approximate way, and for a limited height range.  相似文献   

Optimized Computation of Transfer Coefficients in Surface Layer with Different Momentum and Heat Roughness Lengths     

Emmanuel Guilloteau 《Boundary-Layer Meteorology》1998,87(1):147-160

A new method for computing the surface transfer coefficients is proposed, based on state-of-the-art empirical flux-profile relationships. The influence of the roughness length ratio is first demonstrated with the classical iterative calculation method. Then a non-iterative algorithm is developed, taking into account the difference between momentum and heat roughness lengths.The new method is validated by comparison with the reference iterative computation. The large gain-in computer processing time (CPU) time gain for the calculation of surface fluxes in Eulerian grid models is finally assessed.  相似文献   

Impact of Atmospheric Surface-layer Parameterizations in the new Land-surface Scheme of the NCEP Mesoscale Eta Model   总被引：5，自引：1，他引：5  

Fei Chen  Zavisă Janjić  Kenneth Mitchell 《Boundary-Layer Meteorology》1997,85(3):391-421

We tested three atmospheric surface-layer parameterization schemes (Mellor-Yamadalevel 2, Paulson, and modified Louis), both ina 1-D mode in the new NCEP land-surface scheme against long-term FIFE and HAPEX observations, and in a coupled 3-D mode withthe NCEP mesoscale Eta model. The differences inthese three schemes and the resulting surface exchange coefficients do not, in general, lead to significant differences in model simulated surface fluxes, skin temperature, andprecipitation, provided the same treatment of roughness length for heat is employed.Rather, the model is more sensitive to the choice of the roughness length for heat. To assess the latter, we also tested two approaches to specifythe roughness length for heat: 1) assuming the roughness length for heat is a fixed ratio of the roughness length for momentum, and 2) relating this ratio to the roughness Reynolds number as proposed by Zilitinkevich.Our 1-D column model sensitivity tests suggested that the Zilitinkevich approach can improve the surface heat fluxand skin temperature simulations. A long-term test with the NCEP mesoscaleEta model indicated that this approach can also reduce forecast precipitation bias. Based on these simulations, in January 1996 we operationally implemented the Paulsonscheme with the new land-surface scheme of the NCEP Eta model, along with the Zilitinkevich formulation to specify the roughness length for heat.  相似文献   

Experimental Study of Mechanical Behaviour of Rock Joints Under Cyclic Loading   总被引：3，自引：5，他引：3  

M. K. Jafari  F. Pellet  M. Boulon  K. Amini Hosseini 《Rock Mechanics and Rock Engineering》2004,37(1):3-23

Summary Evaluation of the effects of small repetitive earthquakes on the strength parameters of rock joints in active seismic zones is of interest of the designers of underground constructions. In order to evaluate these effects, it is necessary to study the behaviour of rock joints under dynamic and cyclic loadings. This paper presents the results of a systematic study on the behaviour of artificial rock joints subjected to cyclic shearing. More than 30 identical replicas have been tested using triaxial compression devices under different conditions of monotonic and cyclic loading. At the first stage a few samples have been tested in monotonic loading modes under various confining pressures and rate of displacement. In the second series of tests, small cyclic loads were applied on the samples for increasing number of cycles, frequency levels and stress amplitudes. These were then followed by monotonic loading again. The variations of maximum and residual shear strengths for each test have been studied. The results show increase of shear strength as a result of the increase in confining pressure and they display decrease of shear strength due to the increase of rate of loading, number of cycles, frequency levels and stress amplitudes.  相似文献   

冬季北京城市近地层的气象特征   总被引：14，自引：1，他引：14  

陆龙骅  卞林根  程彦杰  高志球  汪瑛 《应用气象学报》2002,13(Z1):34-42

运用2001年1～3月北京大气边界层和大气化学综合试验期间,中国科学院大气物理研究所铁塔上所获得的8～320 m 15层风、温度和湿度梯度资料,对冬季北京城市边界层特征进行了诊断分析.结果表明,在冬季北京城市边界层中,平均而言地表粗糙度为1.34、零平均位移约为20 m;温度基本上随高度呈线性变化;风速随高度的变化并不总遵循对数关系,尤其是在午前和夜间,风速与高度之间对数关系的不显著率可达30%～40%.不能简单套用在Monin-Obuhov相似理论中由Businger-Dve风廓线层结订正获得的近地面层动量、热量和水汽湍流输送计算公式.  相似文献   

典型干旱区荒漠戈壁陆面参数的观测研究   总被引：29，自引：0，他引：29  

Zhang Qiang  Cao Xiaoyan  Wei Guoan  Huang Ronghui 《大气科学进展》2002,19(1):121-135

根据目前流行的陆面过程模式的需要，利用２０００年５－６月敦煌陆面过程野外观测实验加强期的观测资料，分析了西北典型干旱区荒漠戈壁的一些关键陆面过程和土壤参数的特征和规律。并且利用相对反射为权重加权平均，计算得到典型干旱区敦煌荒漠戈壁的平均反射率为０.２５５±０.０２１；剔除建筑物干扰后，利用对数平均法计算的粗糙度长度平均值为０，００１９±０。０００７１ ｍ；剔除绿洲影响后，用无降水影响的资料确定出土壤湿度影响因子为０．００４５；剔除降水影响后，用观测资料计算的敦煌典型干旱区荒漠戈壁的热容量平均值为１．１２×１０~６ｍ~（－３）Ｋ~（－１），比“黑河试验”在戈壁和在其它沙漠观测的有关值略小一些，但热扩散率和热传导率都比“黑河试验”在戈壁观测的值小一倍左右；观测的敦煌戈壁典型天气条件下的土壤含水量非常小，基本上不超过１％。  相似文献   

降水条件下的典型干旱区陆面特征模拟验证   总被引：6，自引：0，他引：6       下载免费PDF全文

王胜  张强  卫国安 《地球物理学报》2006,49(2):383-390

利用“我国西北干旱区陆－气相互作用观测试验"在敦煌双墩子戈壁站取得的观测资料及最近的一些研究成果对陆面模式中反照率、相似性函数及地表粗糙度(动量粗糙度,标量粗糙度)3个方面的参数化方案进行了改进,然后对一次典型降水过程的陆面特征及近地层的风、温、湿进行了模拟.结果表明：改进的模式能对降水条件下的干旱区陆面特征进行较好的模拟,其中对辐射、地表温度的模拟相当好,而对能量的模拟虽然还有要改进的地方,但总体结果令人满意;另外改进后的模式对近地层的温度和湿度的模拟也有明显的改善.  相似文献   

粗糙度研究的现状及展望   总被引：29，自引：6，他引：23  

李振山  陈广庭 《中国沙漠》1997,17(1):99-102

略述边界层剪切湍流各区域速度廓线规律和地面粗糙分类后,对各种地面粗糙度(包括沙质粗糙度、动力粗糙度、植被粗糙度、复杂地面粗糙度、有效粗糙度)的研究现状和计算方法作了简要综述,认为动力粗糙度算式中系数A和植被或有效粗糙度算式中位移高度d将会是进一步研究的重点。  相似文献