共查询到20条相似文献，搜索用时 31 毫秒
1.
On three-dimensionality of shelterbelt structure and its influences on shelter effects 总被引：5，自引：0，他引：5
Natural shelterbelts, unlike planar barriers, have a certain width, within which interactions among wind speed, drag force and pressure perturbations create a net sheltering effect. The variations of flow, drag force, permeability, and pressure perturbation for shelterbelts of different widths and different horizontal structures are numerically studied, and their influences on shelter efficiency are discussed. Comparisons are made of fourteen medium-dense shelterbelts, with the same overall leaf-area, that differ only in width or horizontal distribution of leaf-area density. The simulated results are consistent with both field observations and wind-tunnel measurements.The simulations demonstrate that the total drag force of the entire shelterbelt varies little with changes in width and structure. The results also show that shelter distance and the overall average wind speed reduction decrease only by 15–18% as width increases by a factor of 100, and changes even less for different internal structure. However, width greatly affects the location of minimum wind speed, pressure perturbation, and the permeability of shelterbelts. Horizontal changes of wind speed inside the uniform shelterbelts have four different patterns, which depend on shelterbelt width and height. The absolute pressure perturbation significantly decreases with increasing width. A possible cause of the insensitivity of shelter efficiency to width and internal inhomogeneous structure is the compensation between the effects of permeability and pressure perturbation on shelter efficiency. 相似文献
2.
AN ANALYTICAL ONE-DIMENSIONAL MODEL OF MOMENTUM TRANSFER BY VEGETATION OF ARBITRARY STRUCTURE 总被引：5，自引：2，他引：5
W. J. MASSMAN 《Boundary-Layer Meteorology》1997,83(3):407-421
An analyticalone-dimensional model of momentum transferby vegetation with variable foliage distribution,sheltering and drag coefficientis developed. The model relies on a simpleparameterization of the ratio of theabove-canopy friction velocity, u*, to thewind speed at the top of the canopy,u(h), to predict vegetation roughness length(z0) and displacement height(d) as functions of canopy height (h) and dragarea index. Model predictionsof d/h and z/h compare very favorably withobserved values.A model sensitivity analysis suggests that shelteringeffects for momentum transfertend to make canopies with non-uniform foliagedistribution resemble canopies withmore uniform foliage distribution and that anyinfluence wind speed has on d/hand z0/h is more likely to be related to theinfluence that wind speed may haveon u*/u(h) rather than the influence windspeed may have on the foliage dragcoefficient. Model results indicate that z0/hand d/h are sensitive to uncertaintiesin the numerical values of the model parameters,foliage density and distribution,sheltering effects and variations in drag coefficientwithin the canopy. In additionz0/h is also shown to be sensitive to thepresence or absence of the roughnesssublayer. Given the simplicity of the model it issuggested that it may be of usefor land surface parameterizations in large scalemodels. 相似文献
3.
A warm on-ice air flow from the open water over the Arctic sea ice in the Fram Straitwas, for the first time, systematically measured on 12 March 1998 by aircraft in thelowest 3 km over a 300-km long distance. The air mass modification and the processesinvolved are discussed.Over the water, air temperature was lower than water temperature so that a convectiveboundary layer (CBL) was present as initial condition. As soon as the CBL passed theice edge, a shallow stable internal boundary layer (IBL) was formed. In the residual CBL, turbulence and pre-existing convective clouds dissolved within about 20 km. Within about the same distance, due to the transition from unstable to stable stratification, the influence of surface friction increased in the IBL and decreased above the IBL with consequent generation of a low-level jet at IBL top. The IBL was strongly stratified with respect to both temperature and wind. The wind shear was around 0.1 s-1 so that the Richardson number in the IBL was subcritical and turbulence was generated. The IBL top grew to about 145 m over 230 km distance. The growth of the IBL was not monotonic and was influenced by (a) inhomogeneous ice surface temperatures causedby both different ice thickness and changes in the cloud conditions, and (b) leads in theice deck. At the front side of the on-ice flow, the air mass boundary between the warmair and the cold Arctic air was sharp (12 K over 10 km) at low levels and tilted withheight. Observations suggest that the stratified IBL was lifted as a slab on top of thecold air. 相似文献
4.
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. 相似文献
5.
浙江海岛台风和冬季大风阵风特征的对比分析 总被引：3，自引：1，他引：3
为了提高阵风预报准确率,利用2006—2016年浙江7个海岛气象站资料和ERA-interim资料,分析了台风和冬季大风的阵风因子与10 m稳定风速、风向、Brunt-Vaisala频率、总体理查逊数、边界层250~1 000 m风速及其与10 m稳定风速比值等的关系,对比两种大风系统阵风的主要成因差异,最后对冬季大风的阵风因子进行拟合。(1)从总体上,台风阵风因子比冬季大风要大0.1~0.2,波动幅度也一般比冬季大风偏大0.3~0.5。有些站点在稳定风速较大时,阵风因子随稳定风速变化不明显,而有的站点变化幅度较大。(2)站点不同方位的地表特征差异明显,导致台风和冬季大风的阵风因子在某个风向上有较统一的最大值和最小值,两者差值一般为0.2~0.3。(3)大气边界层台风样本主要表现为气流辐合上升及正涡度,而冬季大风样本主要表现为辐散下沉及负涡度,台风垂直速度、涡度和散度的强度均明显大于冬季大风样本;从Brunt-Vaisala频率来看,边界层750 m处冬季大风样本总体为静力不稳定,而台风样本总体为静力稳定;从总体理查逊数来看,台风样本和冬季大风样本两者边界层250 m处动力不稳定程度接近。(4)台风和冬季大风的阵风主要形成机制不同,冬季阵风与边界层上层气流向下动量传输引发的辐合辐散有关,而台风阵风可能更多与边界层气流的水平动量输送引发的辐合辐散有关。(5)基于风向、边界层1 000 m处风速和10 m稳定风速的冬季大风阵风因子的拟合模型,比仅考虑10 m稳定风速的拟合模型的绝对误差减少了20%~50%,误差方差也减少了10%~30%。 相似文献
6.
Regional climate models, such as RegCM3, generally show large biases in the simulation of western North Pacific (WNP) summer monsoon (WNPSM). In this study, the authors improved the simulation of WNPSM by applying the convection suppression criterion based on the averaged relative humidity from cloud base to cloud top. The simulated rainfall and monsoon circulation are significantly improved. The suppressed convective heating associated with the decrease in convective rainfall simulates a low-level anomalous anticyclone to its north. The anomalous anticyclone reduces the intensity of low-level southwesterly flow and the wind speed at 10 m. The reduction in wind speed at 10 m decreases the evaporation at sea surface. The less supply of water vapor from underlying ocean in turn favors less convective rainfall. The overestimation of simulated convective percentages and the cold bias of 2 m air temperature are also reduced. The different effects of convection suppression criterion in stand-alone RegCM3 and corresponding regional air–sea coupled model are also discussed. 相似文献
7.
Variation in wind velocity over water 总被引：1，自引：0，他引：1
Starting from the equations of motion and continuity, a theoretical model is deduced in this paper for the variation in wind velocity over water caused by abrupt changes in surface roughness and temperature when air flows from land to water, based on the consideration that the turbulent exchange coefficient varies with height and distance from the upwind edge. According to the computation of this model, the variation in wind velocity over water, as the drift of air is from land to water, occurs mainly in the first few kilometers from the upwind edge. The wind velocity over water increases to a maximum when the air over land is stable, it tends to moderate when neutral condition is reached, and least variation is shown in unstable condition. And when the air over land is unstable the wind velocity is less over water than over land in strong winds, but some-what greater in light winds. 相似文献
8.
A uniform,inviscid,incompressible fluid in a two-dimensional plane(x,z)is considered.Three principles:conservation of potential vorticity,conservation of absolute momentum,andconservation of mass are used for this study.If the initial mass field and the initial wind field donot satisfy geostrophic balance,then through geostrophic adjustment under suitable conditions,the frontogenesis will finally occur.Our work points out that the initial density distribution greatlyinfluences the frontal features.If the stratification in cold air is the same as that in warm air,twofrontogeneses will occur at top and bottom boundaries respectively.If the stratification in cold airis larger than that in warm air,the frontogenesis at the bottom boundary still exists,but the otherat the top boundary disappears.This result makes us further understand the mechanism of thefrontogenesis in the real atmosphere. 相似文献
9.
The present study investigates meteorological conditions for the day-to-day changes of particulate matter (PM) concentration in Beijing city during the period 2008–2015. The local relationship of PM concentration to surface air temperature, pressure, wind speed, and relative humidity displays seasonal changes and year-to-year variations. The average correlation coefficient with PM10 in spring, summer, fall, and winter is 0.45, 0.40, 0.38, and 0.30 for air temperature; –0.45, –0.05, –0.40, and –0.45 for pressure; 0.13, 0.04, 0.53, and 0.50 for relative humidity; and –0.18, –0.11, –0.45, and –0.33 for wind speed. A higher correlation with wind speed is obtained when wind speed leads by half a day. The heavily polluted and clean days, which are defined as the top and bottom 10% of the PM values, show obvious differences in the regional distribution of air temperature, pressure, and wind. Polluted days correspond to higher air temperature in all the four seasons, lower sea level pressure and anomalous southerly winds to the south and east of Beijing in spring, fall, and winter, and a northwest–southeast contrast in the pressure anomaly and anomalous southerly winds in summer. Higher relative humidity is observed on polluted days in fall and winter. The polluted days are preceded by an anomalous cyclone moving from the northwest, accompanied by lower pressure and higher air temperature, in all four seasons. This feature indicates the impacts of moving weather systems on local meteorological conditions for day-to-day air quality changes in Beijing. 相似文献
10.
山谷地形流场和扩散的数值研究 总被引：7，自引：4，他引：7
运用三维非表力E－ε闭合模式，模拟了山体和山谷地形下的流场、湍流场和不同位置低矮点源扩散的污染物浓度分布。发现在不稳定时湍能的热力产生率并非在任何位置都是主要的。当风速较大时，在山谷底部机械产生率也可能超过热力产生率。对污染扩散的分析表明：在近地面源条件下，稳定层结时如果出现小风，无论源在何处，都人造成一定范围内的严重污染，当污染源位于山前或山顶时，谷底的污染不剧烈。当污染源位于谷底时，无论何种层结、何种来流风速，都会造成山谷地区的严重污染。污染源位于山体沿来流方向的中心线上的谷底时，如果风速大，污染物沿回流输送；如果风速小，则沿来流方向输送。 相似文献
11.
为更好理解沿海区域近地面风速衰减规律及其内在机制并获取适用于业务观测风速数据的分析方法,通过引入内边界层厚度的发展机理,推导获得风速随离岸距离变化的数学解析拟合式。结合浙江省温州市境内一沿海区域6个自动气象站2014—2019年逐时风速观测数据应用该拟合式分析了风速随离岸距离的关系,结果表明不管是逐时风速还是逐日最大风速,其平均值均与离岸距离有着良好的负相关,并发现其衰减系数与风速有着密切关系。向岸流及离岸流的风速衰减特性均可以结合该拟合式用线性及幂数律拟合来体现,但后者可以更好地解释风速随离岸距离变化特征,并在较大风速(3~10 m/s)向岸流的背景条件下,获得合理可信的分析结果,说明该方法可以适用于近海岸区域风速观测数据的应用研究。 相似文献
12.
秦岭地区气溶胶对地形云降水的抑制作用 总被引：12，自引：3，他引：9
以华山站为影响站, 周围的西安、渭南和华阴作为对比站, 通过影响站与对比站降水之比——地形强化因子(Ro)的变化趋势以及Ro与能见度关系的分析, 定量研究了秦岭地区气溶胶对地形云降水的抑制作用。Ro的演变分析表明: 有观测以来Ro逐年递减, 减幅为14%～20%, 即影响站与对比站相比降水量减少了14%～20%; Ro的减少趋势与能见度递减、气溶胶递增相吻合, 说明气溶胶的增加抑制了地形云降水。Ro的递减主要是减少了中小雨 (日雨量小于30 mm) 的天数, 这种影响对浅薄的生命期较短的地形云降水作用更明显, 对于华山站, 30 mm以下的降水都会受到入云气溶胶的抑制作用, 而西安站为5 mm以下, 入云气溶胶浓度越高, 就有越厚的降水云受气溶胶影响而抑制降水; 在以动力强迫抬升为主的春秋季, 气溶胶抑制华山地形云降水20%左右, 最大可达25%; 在热对流条件下, 气溶胶对地形云和对平原地区云的抑制作用基本相当。不同风速风向下Ro的变化趋势表明, Ro递减随风速增大而加快, 迎风向 (240°～30°) 大风 (≥5 m/s) 时减少降水超过30%。由Ro与能见度关系的定量分析发现, 当能见度在14 km时Ro为1.8左右, 随着能见度的降低Ro逐渐减小, 当能见度小于8 km时,R0约为1.2, 减小了30%左右; 华山对于华阴的Ro与能见度呈线性关系, 相关系数达0.81。最后, 根据研究结果归纳出气溶胶抑制秦岭地区地形云降水的初步物理模型。 相似文献
13.
A uniform,inviscid,incompressible fluid in a two-dimensional plane(x,z)is considered.Three principles:conservation of potential vorticity,conservation of absolute momentum,and conservation of mass are used for this study.If the initial mass field and the initial wind field do not satisfy geostrophic balance,then through geostrophic adjustment under suitable conditions,the frontogenesis will finally occur.Our work points out that the initial density distribution greatly influences the frontal features.If the stratification in cold air is the same as that in warm air,two frontogeneses will occur at top and bottom boundaries respectively.If the stratification in cold air is larger than that in warm air,the frontogenesis at the bottom boundary still exists,but the other at the top boundary disappears.This result makes us further understand the mechanism of the frontogenesis in the real atmosphere. 相似文献
14.
根据FY-2G卫星遥感监测资料、常规气象观测资料和NCEP逐6h1°×1°再分析资料,分析2014年12月31日至2015年1月1日渤海大风过程中风速的3次波动特征以及影响系统,并对大风期间物理量进行了诊断分析,揭示了冷空气影响过程中渤海大风的突增以及波动性成因。结果表明:当冷空气影响渤海时,冷暖空气对比使低空锋区迅速加强,风力突增明显。大风期间高层深厚的冷平流自上而下形成了一条西北东南向后倾式的冷平流传输通道,平流分3次传送到底层对应着大风期间的3次波动峰值。整个过程动量下传起了重要的作用,下沉气流的径向度越大,高层下沉运动越强,对应地面的风速越大。 相似文献
15.
In this paper the mean wind speed profiles in the atmospheric surfacelayer over oasis, sand, and Gobi desert surfaces in the HEIhe river FieldExperiment (HEIFE) area are discussed for various stability conditions. Themathematical representation of mean wind speed profiles obtained fromdifferent sources are used to estimate the mean wind speeds, and the resultsare compared with observed wind speed data obtained at 20 m meteorologicaltowers over different ground surfaces. It is found that for unstable and neutral conditions, a compositeexponential profile and flux-profile relationship can be used to describe thewind speed profile in the atmospheric surface layer over homogeneoussurfaces, the composite exponential profile giving a relatively high accuracyin the estimation of wind speed profiles. For stable conditions, thecomposite exponential profile also can be used to estimate the wind speedprofile, but for very stable conditions, the accuracy is not good. For thoseconditions, flux-profile equations can give estimates of the wind speedprofile with relatively high accuracy. 相似文献
16.
计算并分析了景德镇市出现雨夹雪、一般降雪、大雪（分别简称为Ⅰ类、Ⅱ类、Ⅲ类降雪）形势场、本站要素、层结资料,概括了3类降雪的天气学概念模型。分析结果表明：（1）3类降雪天气过程中,高空500 hPa有强盛的西南偏西气流,且随着降雪强度的增大,西南风速逐渐增大。地面则有较强冷空气堆积,位于贝加尔湖西部的冷高压中心气压在1 050 hPa以上,景德镇处于冷高压底部。925 hPa 30°N附近冷空气势力强,气温低。Ⅰ类降雪的主要影响系统是在江西省北部上空交汇的冷、暖平流,Ⅱ类、Ⅲ类则是850 hPa的切变线,出现Ⅲ类降雪时切变线位于景德镇附近,而出现Ⅱ类降雪时切变线则稍偏南,位于赣中。（2）地面气温变化是降水相态改变的关键,气温越低,越易出现降雪。从大雪至雪后雨,气温逐渐上升。（3）中低层的气温,Ⅱ类降雪较Ⅰ类低;近地面层气温,Ⅲ类降雪与Ⅱ类降雪接近;700 hPa和850 hPa层气温,Ⅲ类降雪比Ⅱ类降雪偏高。（4）Ⅰ类降雪常伴有逆温,但在向Ⅱ类降雪的转换过程中,逆温逐渐减弱,到Ⅲ类降雪时,逆温消失。（5）3类降雪均存在明显的风垂直切变,低层风弱,高层风强,且随着降雪强度的增大,表现愈加明显。 相似文献
17.
The development and characteristics of coastal internal boundary layers were investigated in 28 tests. These were made at all seasons and in both gradient and sea-breeze flows but only during mid-day periods. Measurements of turbulence and temperature were taken from a light aircraft which flew traverses across Long Island at successive altitudes parallel to the wind direction. These were used to locate the boundary between modified and unmodified air as a function of height and distance from the coast. The same measurements plus tower measurements of wind, turbulence and temperature, pilot balloon soundings and measurements of land and water surface temperatures by a remote sensing IR thermometer were used to quantify the characteristics of the modified and unmodified air. The boundary layer slope was steep close to the land-water interface and became shallower with downwind distance. Growth of the boundary layer was initially slower with stable lapse rates upwind than with neutral or unstable conditions over the water. An equilibrium height was found in many tests except under conditions of free convection when the internal boundary layer merged into the mixed layer inland and with sea-breeze conditions. The equilibrium height depended on downwind conditions and was greater with low wind speeds and strong land surface heating than with stronger winds and small land-water temperature differences. Current theoretical models are not adequate to predict the height of the boundary layer at the altitudes and distances studied but reasonably good predictions were given by an empirical model developed earlier. Wind speed in the modified air averaged about 70% of that at the coast but turbulence levels were several times higher both near the surface and aloft. These findings have important implications for diffusion from coastal sites. 相似文献
18.
Hiroshi Ishida 《Boundary-Layer Meteorology》1990,52(4):335-348
Seasonal variations of the spectra of wind speed and air temperature in the mesoscale frequency range from 1.3 × 10-4 to 1.5 × 10-3 Hz (10 min to 2 h periods) have been studied through observations over land for one year. Spectrographs [time series of isopleths of spectral densities, f · S(f) vs f] of wind speed and air temperature contain occasional peaks that are attributed to short-lived mesoscale atmospheric activity with narrow frequency bands. Significant spectral peaks of wind speed were found in 19% of the total observations in winter, and in 15–16% in the other seasons; for air temperature, they occured in 12% of observations in autumn, and in 16–19% in the other seasons. The peaks most often occurred in the period range from 30 min to 1 h; most had durations less than 24 h. Mesoscale fluctuations of wind speed and air temperature were highly correlated, and in most cases, phase differences were 90–180 ° with air temperature leading wind speed. Significant spectral peaks of wind speed often occurred during northerly seasonal cold winds in winter, and accompanied tropical and/or mid-latitude cyclones in the other seasons. When the peaks occurred, wind speed was usually relatively high and the atmospheric surface layer was unstable. 相似文献
19.
The development and characteristics of coastal internal boundary layers were investigated in 28 tests. These were made at all seasons and in both gradient and sea-breeze flows but only during mid-day periods. Measurements of turbulence and temperature were taken from a light aircraft which flew traverses across Long Island at successive altitudes parallel to the wind direction. These were used to locate the boundary between modified and unmodified air as a function of height and distance from the coast. The same measurements plus tower measurements of wind, turbulence and temperature, pilot balloon soundings and measurements of land and water surface temperatures by a remote sensing IR thermometer were used to quantify the characteristics of the modified and unmodified air. The boundary layer slope was steep close to the land-water interface and became shallower with downwind distance. Growth of the boundary layer was initially slower with stable lapse rates upwind than with neutral or unstable conditions over the water. An equilibrium height was found in many tests except under conditions of free convection when the internal boundary layer merged into the mixed layer inland and with sea-breeze conditions. The equilibrium height depended on downwind conditions and was greater with low wind speeds and strong land surface heating than with stronger winds and small land-water temperature differences. Current theoretical models are not adequate to predict the height of the boundary layer at the altitudes and distances studied but reasonably good predictions were given by an empirical model developed earlier. Wind speed in the modified air averaged about 70% of that at the coast but turbulence levels were several times higher both near the surface and aloft. These findings have important implications for diffusion from coastal sites. 相似文献
20.
1971—2015年大连地区低风速气象特征分析 总被引：2，自引：0，他引：2
利用1971—2015年大连地区7个国家气象站的气象资料, 统计低风速条件下的累积频率、日变化、月变化和持续性等特征,分析低风速频率空间分布和年际变化特征。结果表明: (1)大连地区低风速频率较低,平均约20%,地区间差异显著,近海区域长海站最低,为8%,内陆的普兰店地区较高,达32%。(2)近45年,低风速频率呈增加趋势,大连、长海和普兰店站增加趋势显著,特别是近10年增幅更大。(3)大连站低风速频率具有显著的日变化,主要表现为白天偏低、中午时段最低,夜间高,半夜达到最高。(4)3—7月,大连地区低风速频率低;9月至次年2月较高,最大值出现在9月。(5)低风速持续时间长海站最长,持续10 h以上低风速频率达到27%,持续20 h以上接近9%,大连站低风速持续时长最短,持续4 h以下的占85%。 相似文献