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281.
282.
Performance of various similarity functions for nondimensional wind and temperature profiles in the surface layer in stable conditions 总被引:3,自引:0,他引:3
The linear functions for non-dimensional wind and temperature profiles are commonly used to describe the surface layer fluxes in atmospheric models. However, their applicability is limited to smaller values of the stability parameter z/L (where z is the height above ground and L is the Obukhov length) i.e. z/L < 1.0. These linear functions have been modified (Webb 1970, Quart. J. Roy. Meteor. Soc. 96, 67–90; Clarke 1970, Quart. J. Roy. Meteor. Soc. 96, 91–114; Hicks 1976, Quart. J. Roy. Meteor. Soc. 102, 535–551; Beljaars and Holtslag 1991, J. Appl. Meteorol. 30, 327–341; Cheng and Brutsaert 2005, Boundary-Layer Meteorol. 114, 519–538) over the years for calculating fluxes when z/L > 1.0 under strongly stable conditions. In view of this, the objective of the present study is to analyze the performance of these similarity functions to compute surface fluxes in stable conditions.The meteorological observations from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) experiment are utilized for computing the surface fluxes in stable conditions. The computed fluxes are found to be reasonably close to those observed. The ratio of observed to computed fluxes reveals that the computed fluxes are close to the observations for all the similarity functions for z/L < 1.0 whereas the computed values show relatively a large scatter from observations for z/L > 1.0. The computed values of u and heat flux do not show significant differences from those observed at 99% confidence limit. The performance of all the similarity functions considered here is found to be comparable to each other in strongly stable conditions. 相似文献
283.
采用MODIS资料和美国发展的MODIS大气温、湿度廓线统计反演算法,估算大气温度、湿度廓线作为初始场,应用101层快速透过率模式(PFAAST)估算了大气透过率,并采用Newton非线性迭代算法反演中国西北荒漠戈壁地区大气温度廓线。结果表明:该方法对边界层高度及以上部分的大气温度反演得比较好,误差基本都在2 K范围内,边界层范围内的温度反演误差较大,反演误差与气溶胶光学厚度增量和地表温度估算误差呈显著正相关关系,与大气水汽混合比的关系较差。文中从敏感性试验和理论分析角度阐述了地表温度和气溶胶光学厚度估算误差对大气温度反演误差的影响,发现不同光谱波段的地表温度权重均随地表温度的增加有不同程度增加,地表温度反演误差增加将增加地表温度权重,提高地表温度估算误差有助于提高地表温度权重的精度;荒漠戈壁地区大气边界层中气溶胶浓度较高,光学厚度较大,使边界层大气透过率降低,进而降低卫星红外遥感波段的地表温度权重和空气温度权重。由于该模式没有很好地考虑边界层中沙尘气溶胶的影响,使卫星反演的大气透过率偏高,以至于高估地表温度权重和大气温度权重,使得反演的表面温度和空气温度偏低。该研究结合太阳光度计获得的光学厚度资料,采用统计方法对气溶胶效应引起的大气透过率误差和表面温度估算误差进行校正,并对物理算法进行本地化改进,实现了边界层温度廓线的反演。 相似文献
284.
Based upon comparisons between published experimental data and simulated results on the vertical sand flux distribution in
the saltation layer, Shao’s similarity saltation model has been greatly improved by correcting the average vertical particle
lift-off velocity and using a more suitable universal roughness length. By the improved model, the vertical sand flux profile
over the bare, dry and loose uniform sandy surface, which is quite representative of real desert surfaces, can be reproduced
very well. Meanwhile, the surface transport rate and the characteristic and average saltation heights have been simulated
and analyzed in detail, disclosing their relationships with friction velocity, particle size and roughness length, and the
possible underlying mechanisms. Besides, the average particle lift-off velocity and the average mean vertical aerodynamic
action upon the ascending particle, which determine the saltation process, are explicitly expressed by parameters involved
in the similarity model, and their relationships with friction velocity, particle size and roughness length are also described
concisely. The corrected average particle lift-off velocity makes it possible to investigate the characteristic particle trajectory,
whose initial velocity equals the average lift-off velocity, so as to estimate the average particle against surface impacting
velocity and the average aerodynamic action upon the saltation process. 相似文献
285.
J. F. Barlow G. G. Rooney S. von Hünerbein S. G. Bradley 《Boundary-Layer Meteorology》2008,127(2):173-191
Profiles of wind and turbulence over an urban area evolve with fetch in response to surface characteristics. Sodar measurements,
taken on 22 April 2002 during the Salford Experiment in the UK (Salfex), are here related to upstream terrain. A logarithmic
layer up to z = 65m was observed in all half-hour averaged profiles. Above this height the profile showed a different vertical gradient,
suggesting a change in surface cover upstream. The drag coefficient varied by a factor of two over only a 20° direction change.
Turbulence intensity (σ
x
) for each wind component (x) decreased with height, but the ratio suggested an underestimate of σ
u
compared to previous results. Mean urban and suburban cover fraction within the source area for each height decreased sharply
between z = 20 and 50m, increasing slightly above. The near-convergence of cover fractions thus occured for source areas of minimum
length ≈ 2,200 m. In comparison, the mean length scale of heterogeneity L
P was calculated from surface cover data to be 1,284 m, and the corresponding mean blending height h
b was 175 m. Finally, the mean streamline angle, α, was negative and the magnitude decreased with height. An exponential fit
to α for z ≤ 65m gave an e-folding height scale of 159 m. A simple relationship between this height scale and L
P was assumed, giving L
P ≈ 1,080 m, which is in reasonable agreement with the estimate from surface cover type. The results suggest that more emphasis
is required on modelling and measuring surface-layer flow over heterogeneous urban canopies. 相似文献
286.
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
287.
We derive a conceptual model of the flow over heterogeneous terrain consisting of patches with contrasting Bowen ratios. Upward
moving eddies are assumed to carry heterogeneous properties, whereas downward moving eddies carry homogeneous properties.
This results in a decorrelation of temperature and humidity as the contrast between the patches increases. We show that this
model is able to reproduce the relationship developed by Lamaud and Irvine (Boundary-Layer Meteorol. 120:87–109, 2006). Some
details differ from their expression but are in accordance with data obtained over African savannah. We extend the conceptual
model to a combination of any scalars, not necessarily linked through the surface energy balance (as is the case for temperature
and humidity). To this end we introduce a new parameter that describes the surface heterogeneity in surface fluxes. The results
of the current model can be used to predict the discrepancy between similarity relationships for different scalars over heterogeneous
terrain. 相似文献
288.
J. Sanjay 《Boundary-Layer Meteorology》2008,129(1):159-177
Data collected during the Land Surface Processes Experiment (LASPEX) in a semi-arid region of the state of Gujarat in north-west
India for a clear sky day (16 May 1997) are used to assess the performance of the atmospheric boundary-layer (ABL) and land-
surface parameterizations in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR)
Mesoscale Model (MM5). The ABL turbulence parameterizations examined are the Blackadar scheme coupled to a simple soil slab
model (SSM), and the Troen-Mahrt scheme coupled to SSM or to the more sophisticated Noah land-surface model (NSM). The comparison
of several two-way nested high resolution (9-km) MM5 short term 24-h simulations indicate that, although the model is able
to capture the trend in the observed data, the computed results deviate from observations. The NSM with a modest treatment
of vegetation outperforms the SSM in capturing the observed daily variations in surface heat fluxes and aspects of ABL structure
over the tropical land surface at local scales. Detailed analysis showed that, with the incorporation of observed local vegetation
and soil characteristics, the NSM reproduced a realistic surface energy balance and near-surface temperature. It is further
found that the coupling of the NSM with the Troen-Mahrt ABL scheme leads to excessive ABL mixing and a dry bias in the model
simulations. 相似文献
289.
选用河北省143个气象台站1975-2004年10 m高年平均风速资料,以及1990年和2000年人口普查资料,根据人口增长、台站迁移、仪器高度变化、台站微观环境变化等影响地面风速变化的台站历史信息,把所有气象台站分为4类,并分别对其进行比较分析.结果表明:河北省绝大多数台站风速变化呈减小趋势;城市化进程、台站观测环境等因素均在不同程度上对地面平均风速变化趋势产生了影响,其中台站所在城镇城市化程度是风速减小趋势不可忽略的原因,其影响程度约在1/4左右;台站观测环境因素中观测场附近微观环境变化对风速减小趋势具有重要影响,超过了区域背景风速减小趋势.台站观测环境因素对风速资料序列均一性的影响也不容忽视,至少有1/3的平均风速序列非均一性断点是由观测环境变化产生的. 相似文献
290.
A dataset of global ocean surface currents for 1999-2007 derived from Argo float trajectories: Comparison with surface drifter and TAO measurements
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A dataset of surface current vectors with error estimate from 1999 to 2007 is derived from the trajectories of the Array for Real-time Geostrophic Oceanography (Argo) drifting on surface over the global ocean. The error of the estimated surface currents is about 4.7 cm s-1 which is equivalent to the accuracy of the currents determined from the surface drifters. Geographically, the Argo-derived surface currents can fill many gaps left by the Global Drifter Program due to the greater number of floats, and can provide a complementary in situ observational system for monitoring global ocean surface currents. The surface currents from the Argo floats are compared with the surface drifter-derived currents and the Tropical Atmosphere Ocean program (TAO) measurements. The comparisons show good agreement for both the current amplitude and the direction of surface currents. Results indicate the feasibility of obtaining ocean surface currents from the Argo array and of combining the surface currents from Argo and the ocean surface drifters for in situ mapping of the global surface currents. The authors also make the dataset available to users of interest for many types of applications. 相似文献