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1.
Non-stationary turbulence can invalidate eddy flux calculations. Two-hour longrecords of wind velocity, temperature and humidity are classified as stationaryor non-stationary based on the behaviour of the flux as a function of Reynoldsaveraging period; a number of indicators of stationarity are investigated. Thetwo-hour Maritime Continent Thunderstorm Experiment wind datasets are notcompletely stationary, as indicated by the lack of a spectral gap, but can beclassified as approximately stationary, when the mean wind speed is greaterthan 3.8 m s-1, or the standard deviation of true wind direction is 10°, or the ratio of horizontal wind variance to wind speed is <0.25. In the stationary case the calculated friction velocity exhibits a 7% decrease on average when the Reynolds averaging period is doubled, while data classified as non-stationaryexhibit an increase of 32%. There is little non-stationary behaviour in the kinematicheat fluxes, and is independent of the non-stationarity of the friction velocity. Thekinematic heat fluxes show small decreases (around 3%) when the Reynolds averagingperiod is doubled. 相似文献
2.
余锦华 《热带气象学报(英文版)》2005,11(1):31-41
1 INTRODUCTION Being one of the important factors that govern the track and intensity change of the tropical cyclone (TC) [1, 2], topographic features are closely related with topographic parameters and the speed of latitudinal flows. As shown in a statistical study by Brand et al., with its passage through the islands of the Philippines, the TC begins to decrease the mean maximum surface wind speed from 47 m/s to 45 m/s 4 hours before reaching the Philippines. They also find that when … 相似文献
3.
Turbulence Intensity Parameters over a Very Complex Terrain 总被引：1，自引：1，他引：0
Cintya A. Martins Osvaldo L. L. Moraes Otávio C. Acevedo Gervásio A. Degrazia 《Boundary-Layer Meteorology》2009,133(1):35-45
Detailed knowledge of turbulence structure is important for the understanding of atmospheric phenomena in the boundary layer,
especially over complex terrain. In the present study, turbulence intensity parameters are analyzed for different conditions
regarding stability, wind speed and wind direction over a mountainous region. The purpose of the analysis is to verify whether
the observed parameters follow Monin–Obukhov similarity theory (MOST), despite the terrain heterogeneity. The dataset was
collected during an experimental campaign at the Nova Roma do Sul site, in southern Brazil, with a micrometeorological tower
located near a sharp slope, approximately 400 m high. The results show that the normalized standard deviations of the vertical
velocity component as well as the normalized standard deviation of temperature follow Monin–Obukhov similarity for all stability
regimes, regardless of the wind direction. However the normalized standard deviation of the horizontal components of the turbulent
velocity obeys the similarity relationship only for a limited range of the stability parameters. 相似文献
4.
The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u
* /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ
w
/U) is the smallest over the downslope surface whereas that of the u component (σ
u
/U) has a minimum over the upslope surface. Consequently, σ
w
/u
* has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ
u
/u
* has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface. 相似文献
5.
Renmin Yuan Minseok Kang Sung-Bin Park Jinkyu Hong Dongho Lee Joon Kim 《Meteorology and Atmospheric Physics》2011,110(3-4):123-133
An expanded planar-fit (PF) method over complex terrain is presented and applied to coordinate rotation of the eddy-covariance (EC) flux and vertical velocity estimation. Theoretical analysis indicates that PF coefficients depend on wind direction, and an expression of vertical velocity is deduced. We applied the theory using 1?year of observations from the KoFlux site in the Gwangneung Forest in Korea and investigated the influence of wind direction on the PF method. Then, we performed an expansion of the PF method to consider dependence of PF coefficients on wind direction and applied the PF method to every sector. The results show that the PF coefficients and tilt angles over complex terrain vary with wind direction. Two hundred 30-min data sets are sufficient to derive stable PF coefficients over hilly terrain for each sector. The relative difference in eddy-covariance flux between the general planar fit (GPF) and sector planar fit (SPF) is less than 10% for the scalar flux and about 18% for friction velocity. Vertical velocity and vertical advection (VA) terms were also calculated and compared using SPF and GPF methods, and a normal distribution and diurnal trend of real vertical velocity on clear days are presented. 相似文献
6.
水平风作用下雨滴水平速度的数值仿真 总被引：1，自引：0，他引：1
为了研究雨滴在水平风作用下的水平移动情况,在分析水平风作用下雨滴受力情况的基础上,通过对曳力系数与雷诺数对应关系的研究,采用数值仿真的方法分别对海平面大气条件和不同大气条件下雨滴水平运动速度进行仿真.结果表明:当有水平风作用时,雨滴的水平运动速度不等于风的速度,而是随雨滴直径和水平风速的变化而变化;在水平风作用下,雨滴的水平移动速度可以在较短的时间内(一般小于15 s)达到一个稳定的值;在水平风速相同的情况下,气压越高、温度越低,雨滴达到平衡时的水平移动末速度相对越大,反之则越小.这些结论对基于图像采集原理的光学降水自动观测仪器进行雨滴图像拼合有重要的应用价值. 相似文献
7.
利用１９８９年１２月１６日－１９９０年１月１４日在重庆市用三轴风速仪所测得的风速资料，计算了小风条件下重庆近地面层的湍流参数，欧拉时间自相关系数，湍流时间积分尺度，湍流强度，摩擦速度，湍流应力等湍流特征量，以了解重庆地区小风条件下的近地面层的湍流场特征。 相似文献
8.
Larry Mahrt 《Boundary-Layer Meteorology》2010,135(1):1-18
The relationship of turbulence quantities to mean flow quantities, such as the Richardson number, degenerates substantially for strong stability, at least in those studies that do not place restrictions on minimum turbulence or non-stationarity. This study examines the large variability of the turbulence for very stable conditions by analyzing four months of turbulence data from a site with short grass. Brief comparisons are made with three additional sites, one over short grass on flat terrain and two with tall vegetation in complex terrain. For very stable conditions, any dependence of the turbulence quantities on the mean wind speed or bulk Richardson number becomes masked by large scatter, as found in some previous studies. The large variability of the turbulence quantities is due to random variations and other physical influences not represented by the bulk Richardson number. There is no critical Richardson number above which the turbulence vanishes. For very stable conditions, the record-averaged vertical velocity variance and the drag coefficient increase with the strength of the submeso motions (wave motions, solitary waves, horizontal modes and numerous more complex signatures). The submeso motions are on time scales of minutes and not normally considered part of the mean flow. The generation of turbulence by such unpredictable motions appears to preclude universal similarity theory for predicting the surface stress for very stable conditions. Large variation of the stress direction with respect to the wind direction for the very stable regime is also examined. Needed additional work is noted. 相似文献
9.
The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the mixing-length concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited mixing-length already proposed for non-logarithmic wind profiles, while large errors characterize traditional mixing-length formulations. 相似文献
10.
When the wind speed decreases below a certain value (1–2 m s-1) meandering (low frequency horizontal wind oscillations) starts to prevail. In these conditions it becomes difficult to define a precise mean wind direction and to estimate the airborne dispersion. To study the wind and turbulence characteristics during meandering, two sonic anemometer datasets, containing hourly wind observations, were analysed: the first one, lasting 1 year, was recorded in complex terrain (Graz, Austria) and the second one, lasting about 1month, was recorded in a rather flat area (Tisby, Sweden). It was found that meandering seems to exist under all meteorological conditions regardless of the stability or wind speed and it was confirmed that meandering sets a lower limit for the horizontal wind component variances. Further, it was found that the autocorrelation functions of the horizontal wind components, computed for the low wind cases, show an oscillating behaviour with the presence of large negative lobes. Two different relationships from the literature, and relevant to these oscillatory aspects, were fitted to the data. They contain two parameters: one associated and relevant to the classical integral time scale and the second with meandering occurrence. Based on these relationships, expressions for the mean square displacement of particles y2(t) were also derived. 相似文献
11.
M. W. Rotach 《Boundary-Layer Meteorology》1993,65(1-2):1-28
12.
Meteorological data of velocity components and temperature have been measured on a mast of height 4.9 m at one site in the Heihe River Basin Field Experiment (HEIFE) conducted in west China. Mean and individual turbulence parameters, power spectra/cospectra, phase angles and their changes withfetch downwind of a change in surface roughness were analyzed. The turbulence characteristics depend strongly on the prevailing wind direction, which in turn is associated with changes in the upwind surface roughness pattern. The results show that values of horizontal velocity standard deviations sigma;u,v scaled with local friction velocity u under different stratifications are larger than those over flat terrain, while the values of w/u have the same values as over flat terrain. The differences between variance values of the horizontal velocity components, u and v, over inhomogeneous terrain were found to be significantly smaller than those over flat terrain. Since energy densities of the w spectra, uw and wT cospectra at low frequencies are relatively lower than those of longitudinal velocity spectra, total energies of w spectra, uw and wT cospectra tend to be in equilibrium with the local terrain. The values of phase angles at the low frequency end of the frequency showed obvious differences associated with changes of roughness. 相似文献
13.
Xuezhong Wang Banghui Hu Hong Huang Ju Wang Gang Zeng Yanke Tan Li Zou 《Acta Meteorologica Sinica》2017,31(6):1133-1148
The traditional method for computing the mean displacement in latitude–longitude coordinates is a spherical meridional–zonal resultant displacement method (MRDM), which regards the displacement as the resultant vector of the meridional and zonal displacement components. However, there are inhomogeneity and singularity in the computation error of the MRDM, especially at high latitudes. Using the NCEP/NCAR long-term monthly mean wind and idealized wind fields, the inhomogeneity in the MRDM was accessed by using a great circle displacement computing method (GCDM) for non-iterative cases. The MRDM and GCDM were also compared for iteration cases by taking the trajectories from a three-time level reference method as the real trajectories. In the horizontal direction, the GCDM assumes that an air particle moves along its locating great circle and that the magnitude of the displacement equals the arc length of the great circle. The inhomogeneity of the MRDM is evaluated in terms of the horizontal distance error from the products of wind speed, lapse time, and angle difference from the GCDM displacement orient. The non-iterative results show that the mean horizontal displacement computed through the MRDM has both computational and analytical errors. The displacement error of the MRDM depends on the wind speed, wind direction, and the departure latitude of the air particle. It increases with the wind speed and the departure latitude. The displacement magnitude error has a four-wave pattern and the displacement direction error has a two-wave feature in the definition range of the wind direction. The iterative result shows that the displacement magnitude error and angle error of the MRDM and GCDM with respect to the reference method increase with the lapse time and have similar distribution patterns. The mean magnitude error and the angle error of the MRDM are nearly twice as large as those of the GCDM. 相似文献
14.
Cao Shuyang Akira Nishi Kimitaka Hirano Shigehira Ozono Hiromori Miyagi Hiromori Kikugawa Yuji Matsuda Yasuo Wakasugi 《Boundary-Layer Meteorology》2001,101(1):61-76
An actively controlled wind tunnel equipped with multiple fansand airfoils has been developed, mainly for the purpose of reproducing the atmospheric boundary layer (ABL) for wind engineering applications. Various fluctuating flows can be achieved in this wind tunnel by altering the input data of the fans and airfoils through computer control. In this study, the ABL is physically simulated in this wind tunnel, and particular attention ispaid to the simulation of the profile of Reynolds stress. The method of generating the fluctuating flow and the experimental results of reproducing the ABL are presented. As the results show, the spatial distribution of Reynolds stress is satisfactorily simulated, and the profiles of other statisticalturbulent parameters, such as mean velocity, turbulent intensity, integral scale and power spectrum are successfully reproduced simultaneously. 相似文献
15.
R.P. Angle 《大气与海洋》2013,51(3):110-125
Real terrain is complex and its effects on the atmosphere are not well understood. The two‐dimensional change‐of‐roughness problem is investigated in an experimental study of airflow from mustard to fallow. The surface friction velocity is found to overshoot and return slowly to the equilibrium value. The wind profile in the modified region is more nearly logarithmic than the Peterson model predicts. The simple Elliot theory is in fair agreement with the wind profile data. Values for the ratio of the standard deviation of the vertical wind to the friction velocity reveal no variation with stability and are in close agreement with the value for laboratory flows. Values for the ratio of the standard deviation of the longitudinal wind component to the friction velocity are comparable to other recently determined atmospheric values and show a variation attributable to large‐scale terrain properties. 相似文献
16.
“黄蜂”边界层风速分析——
台风平均风速的时距选取与强度估计 总被引：1，自引：0，他引：1
通过对热带风暴(TC)"黄蜂"登陆的一次边界层观测所得到风速资料的分析计算,取得不同时距下平均风速序列.比较不同时距选取时登陆TC的平均风速、风向和台风强度,发现1 min平均比10 min平均水平最大风速大25.30%,风向变化不大,TC强度指数1 min与10 min相差2%～4%.时距选取较小时,TC平均风速和强度变大,所测得风速区域也变大,而风向与时距的选取没有明显关系.只是在阵风变化比较大的情况下选取小的时距可以更好的表示这种特征.所以时距选取需要对所研究问题的需要进行分析,对不同的问题选取不同的时距以得到更好的结果. 相似文献
17.
It is shown that the ratio of standard deviation of lateral velocity to the friction velocity, /u
*, and therefore wind direction fluctuations, are sensitive to mesoscale terrain properties. Under neutral conditions, /u
* is almost 40% larger in rolling terrain than over a horizontal surface. In the lee of a low mountain, the fluctuations may be 2.5 times as strong as over horizontal terrain. In contrast, vertical velocity fluctuations are little influenced by mesoscale terrain features.Now with Air Weather Service, Offutt AFB, Omaha, Nebraska. 相似文献
18.
Turbulent flow over a very rough,random surface 总被引：1，自引：1，他引：1
A knowledge of the nature of turbulent flow over very rough surfaces is important for an understanding of the environment of crops, forests, and cities. For this reason, a wind-tunnel investigation was carried out on the variations in mean velocity, Reynolds shear-stress, and other turbulence quantities in a deep turbulent flow over a rough surface having a fair degree of randomness in the shapes, sizes, and positions of its elements.There was a layer close to the surface with considerable variations in both mean velocity and shear-stress, and the horizontal scale over which the mean velocity varied was much larger than the average distance between roughness elements. Above this layer, whose depth was of the order of the spacing between roughness elements, shear stress was constant with height, and the velocity profile had a logarithmic form. The usefulness of both mean profile and eddy-correlation methods for estimating fluxes above very rough terrain is discussed in the light of these findings. 相似文献
19.
We have proposed in a previous study a non-stationary wind model to represent the typhoon record as a summation of a time-varying mean wind speed (TVM) and a stationary turbulence. This note further suggests a quantitative scheme, rather than the previous qualitative method, to find the best TVM for any given wind record. Trial TVMs are first extracted from the wind record by a data-processing technique named empirical mode decomposition. For each TVM, its corresponding turbulent component is computed by removing the TVM from the original wind record, and the degree of stationarity of the turbulence component is checked. The best TVM is taken as the one that leads to the maximum degree of stationarity. The degree of stationarity of turbulence is quantified by two indicators: ?? the ratio of horizontal wind variability and wind speed; and ?? the ratio of friction velocity at different Reynolds averaging periods. The applicability of the suggested scheme is validated with 550 typhoon and 3300 monsoon records of 10 minute duration and at different measurement heights. Threshold values for the two stationary indicators ?? and ?? are determined using field measurements and their sensitivities to the Reynolds averaging periods are discussed. Observations in this study demonstrate that the suggested scheme is proper for finding the TVM of a wind record. For a stationarity quantification of 10 minute duration record, the ?? indicator with 30 second Reynolds averaging period is recommended. 相似文献
20.
利用2007—2017年余姚地区44个气象自动站观测数据和欧洲中心ERA5再分析资料,对余姚地区日极大风分布进行了统计分析,并提出一种本地化的经验阵风预报模型(雷暴日除外)。研究发现:余姚地区日极大风呈正态分布,风力峰值为4～5级,平均每年大风日占比达7.03%,累计大风时次占比3.32%,年平均大风日26 d。风力越小,阵风与平均风之间的线性拟合效果越好。影响阵风预报的因子主要有10 m平均风速、925 hPa风速、地面粗糙度、摩擦速度和3 h变压。6级及以下阵风预报中,业务中的阵风经验系数1.4容易造成余姚本地阵风预报偏低,适用于余姚本地阵风预报经验系数为1.775;6级以上阵风预报经验方程考虑了平均风速、垂直动量下传、水平动量传输、地面摩擦和海拔高度订正,其与925 hPa风速的平方呈正相关,与摩擦速度呈自然指数相关,经验方程相对于经验阵风系数的预报拟合优度提升了59.61%;经验阵风预报方程通过了2018—2019年的数据检验,该方程对1～2级阵风预报偏高,3～5级效果最好,6级阵风的预报偏低;6级以上的阵风等级预报准确率达55.2%。地形摩擦作用在冷空气大风与台风大风过程中尤为重要,这两类过程阵风系数分布类似,但台风带来的动量下传比冷空气更为明显。 相似文献