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1.
The heights of the daytime convective boundary layer (CBL), computed by a one-dimensional model for a bare soil surface at a semi-arid station,Anand, during the dry and hot summer month of May 1997, are presented. As input, the model requires surface heat flux, friction velocity and air temperature as functions of time. Temperature data at the one-metre level from a tower and sonic anemometer data at 9.5 m collected during the period 13–17 May 1997 in the Land Surface Processes Experiment (LASPEX-97) are used to compute hourly values of surface heat flux, friction velocity and Obukhov length following the operational method suggested by Holtslag and Van Ulden [J. Climate Appl. Meteorol. 22,517–529 (1983)]. The model has been tested with different values for the potential temperature gradient ( ) above the inversion. The model-estimated CBL heights comparefavourably with observed heights obtained from radiosonde ascents.  相似文献   

2.
The urbanization of Daegu metropolitan region has progressed rapidly over the last 100 years, but changing land-use resulting from that urbanization has also caused the regional circulations to evolve. The effect of 100 years of urbanization on the regional circulations was simulated for the analysis using an A2C (Atmosphere to Computational Fluid Dynamics) model. Calm synoptic conditions and a stable lapse rate were assumed for the initial model atmosphere, with a horizontal grid resolution of 1 km. The simulation demonstrated that land-use changes have affected the Daegu metropolitan area’s regional circulations by deepening the convective boundary layer (CBL) over the urbanized area, resulting in deviations from mean surface temperature and surface wind fields. The last 100 years have seen the following changes in the region: 1) the urbanized area has increased by around 15 times, while the number of forests and paddy fields has rapidly decreased; 2) surface wind speeds have decreased by about 25% and 35% during the day and at night, respectively, and severe changes in wind direction have been noted over the urbanized areas; 3) regional warming has been around 1.5°C and 4°C in the daytime and early morning hours, respectively (in particular, warming in the early morning hours suggests the presence of overnight heat islands), and 4) sensible heat flux and CBL height have increased by about 40 W m?2 and 500 m for the period, respectively.  相似文献   

3.
Six years of observations from a surface instrument site have been analysed to determine timings and factors influencing developmental changes in the near-surface wind and turbulent heat fluxes during the morning heating of the atmospheric boundary layer. A simple relationship has been found between near-surface wind speed and screen temperature, together with a predictive equation for the morning transition air temperature. Profile measurements from a probe mounted on a tethered balloon have beenused to supplement the surface data and study the processes underlying these surface relationships. The results have confirmed earlier work and have shown that both before and immediately after morning transition, almost all heating in the surface layer is due to turbulent diffusion from above. In order to explain the mechanisms involved in the relationships, a simple finite difference model has been run and validated against the profile data. The model predictions are compared with observations during both the morning and evening and the differences related to the different temperature profiles. Numerical forecasting rules for the surface wind speed and transition temperature are derived from the results.  相似文献   

4.
The effect of extensive terrestrial wind farms on the spatio-temporal structure of the diurnally-evolving atmospheric boundary layer is explored. High-resolution large-eddy simulations of a realistic diurnal cycle with an embedded wind farm are performed. Simulations are forced by a constant geostrophic velocity with time-varying surface boundary conditions derived from a selected period of the CASES-99 field campaign. Through analysis of the bulk statistics of the flow as a function of height and time, it is shown that extensive wind farms shift the inertial oscillations and the associated nocturnal low-level jet vertically upwards by approximately 200 m; cause a three times stronger stratification between the surface and the rotor-disk region, and as a consequence, delay the formation and growth of the convective boundary layer (CBL) by approximately 2 h. These perturbations are shown to have a direct impact on the potential power output of an extensive wind farm with the displacement of the low-level jet causing lower power output during the night as compared to the day. The low-power regime at night is shown to persist for almost 2 h beyond the morning transition due to the reduced growth of the CBL. It is shown that the wind farm induces a deeper entrainment region with greater entrainment fluxes. Finally, it is found that the diurnally-averaged effective roughness length for wind farms is much lower than the reference value computed theoretically for neutral conditions.  相似文献   

5.
We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.  相似文献   

6.
The convective boundary layer over pasture and forest in Amazonia   总被引：1，自引：0，他引：1  
Summary The coupling between different types of surface (tropical forest or grass) and the Convective Boundary Layer (CBL) has been investigated using observational (rawinsoundings) data collected over Rondônia in southwest Amazonia. The data reported here support the notion that deforestation may modify the dynamics of the boundary layer, in particular during the dry season. In this period the sensible heat fluxes are very high over pasture, creating a CBL around 550m deeper compared to that over the forest. The measurements showed the height of the fully developed CBL for pasture to be 1650m, compared to around 1100m for forest. During the wet season the height of the CBL is lower than during the dry season and has the same height (around 1000m) for forest and pasture sites. The CBL over pasture is hotter and drier than over forest during the dry season, but during the wet season the air temperatures and humidities are similar. Comparing the CBL growth during the dry and wet season, there is evidence that the CBL properties over the forest are not dependent on the surface characteristics, but over the pasture they are.  相似文献   

7.
Slope flow mechanisms are crucial for the transport of air pollutants in complex terrain. Previous observations in sloping terrain showed upslope flows filling the entire convective boundary layer (CBL) and reducing air pollution concentrations by venting air pollutants out of the CBL into the free atmosphere. During the Pacific 2001 Air Quality Field Study in the Lower Fraser Valley, British Columbia, Canada, we observed slope flows during weak synoptic winds, clear skies, and strong daytime solar heating. With a Doppler sodar we measured the three wind components at the foot of a slope having an average angle of 19° and a ridge height of 780 m. We operated a scanning lidar system and a tethersonde at a nearby site on the adjacent plain to measure backscatter of particulate matter, temperature, wind speed, wind direction, and specific humidity. Strong daytime upslope flows of up to 6 m s−1 through a depth of up to 500 m occurred in the lower CBL, but with often equally strong and deep return flows in the upper part of the CBL. The mass transport of upslope flow and return flow approximately balanced over a 4-h morning period, suggesting a closed slope-flow circulation within the CBL. These observations showed that air pollutants can remain trapped within a CBL rather than being vented from the CBL into the free atmosphere.  相似文献   

8.
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   

9.
Sodar has been used for about 20 years to determine mixing height. However, estimation of the height of a convective boundary layer (CBL) that exceeds the sodar-probing range is still an unsolved question. As one possible way, it is suggested that one adapt a simple mixed-layer model to sodar observations during the morning growth period of the CBL, when its top can be clearly detected. Results are compared with other methods for CBL-height estimation from sodar data that have been proposed in the literature. Finally, some prognostic aspects are discussed.  相似文献   

10.
A theoretical approach suggests that the surface heterogeneity on a scale of tens of kilometres can generate mesoscale motions that are not in a quasi-stationary state. The starting point of the theoretical approach is the equations of horizontal velocity and potential temperature that are low-pass filtered with a mesoscale cut-off wavelength. The transition of the generated mesoscale motions from a quasi-stationary state to a non-stationary state occurs when horizontal advection is strong enough to level out the potential temperature gradient on the surface heterogeneity scale. Large-eddy simulations (LES) suggest that the convective boundary layer (CBL) changes to a non-stationary state when forced by a surface heat-flux variation of amplitude of 100W m−2 or higher and a wavelength of the order of 10 km. Spectral analysis of the LES reveals that when the mesoscale motions are in a quasi-stationary state, the energy provided by the surface heat-flux variation remains in organized mesoscale motions on the scale of the surface variation itself. However, in a non-stationary state, the energy cascades to smaller scales, with the cascade extending down into the turbulence scale when the wavelength of the surface heat-flux variation is on a scale smaller than 100 times the CBL height. The energy transfer from the generated mesoscale motions to the CBL turbulence results in the absence of a spectral gap between the two scales. The absence of an obvious spectral gap between the generated mesoscale motions and the turbulence raises questions about the applicability of mesoscale models for studies on the effect of high-amplitude surface heterogeneity on a scale of tens of kilometres. The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

11.
The Role of Shear in the Morning Transition Boundary Layer   总被引：1，自引：1，他引：0  
We use large-eddy simulation (LES) to better define the early stages of the morning transition boundary layer. Previous LES studies relating to the morning transition boundary layer focus on the role of the entraining convective boundary layer (CBL). By using a combination of different domain sizes and grid lengths, the full evolution from the stable boundary layer (SBL) to the CBL is modelled here. In the early stages of the morning transition the boundary layer is shown to be a combination of a shallow mixed layer capped by a significant shear driven stable boundary layer (the so-called mixed CBL–SBL state). The mixed CBL–SBL state is the key to understanding the sensitivity to shear. Turbulent kinetic energy budgets also indicate that it is shear driven. The negative flux from the mixed CBL–SBL state extends much further above the minimum than is typically found for the CBL later in the day, and the depth of penetration scales as w m /N i , where w m is the combined friction and convective velocity scale and N i the static stability at the inversion top.  相似文献   

12.
南疆沙漠腹地大气边界层气象要素廓线分析   总被引：1，自引：0，他引：1  
利用塔中80m观测塔梯度系统采集的2006年8月、10月和2007年1月、4月的风、温度、湿度资料,结合气象站的同步气象资料,对南疆沙漠腹地近地层四季的晴天平均风速、温度、湿度廓线分布特征进行分析。结果表明,晴天平均风速白天随高度升高增加缓慢,夜间较快,低层风速白天比夜间大,高层则白天比夜间小,春夏季风速较大;四季平均温度廓线表现为夜间辐射型、早上过渡型、白天日射型和傍晚过渡型等四种类型,早、晚过渡时间四季各有不同,日最低、最高温度出现时间四季则相差不大;冬季夜间比湿随高度升高而增大,整个80m近地层表现为逆湿状态,其他季节逆湿一般出现在0．5—1m、1～2m、32—47m、63—80m等4个层次上,各逆湿层出现的时间各季节有所差异。  相似文献   

13.
The structures of mean flow and turbulence in the atmospheric surface boundary layer have been extensively studied on Earth, and to a far less extent on Mars, where only the Viking missions and the Pathfinder mission have delivered in-situ data. Largely the behaviour of surface-layer turbulence and mean flow on Mars is found to obey the same scaling laws as on Earth. The largest micrometeorological differences between the two atmospheres are associated with the low air density of the Martian atmosphere. Together with the virtual absence of water vapour, it reduces the importance of the atmospheric heat flux in the surface energy budget. This increases the temperature variation of the surface forcing the near-surface temperature gradient and thereby the diabatic heat flux to higher values than are typical on the Earth, resulting in turn in a deeper daytime boundary layer. As wind speed is much like that of the Earth, this larger diabatic heat flux is carried mostly by larger maximal values of T*, the surface scale temperature. The higher kinematic viscosity yields a Kolmogorov scale of the order of ten times larger than on Earth, influencing the transition between rough and smooth flow for the same surface features.The scaling laws have been validated analysing the Martian surface-layer data for the relations between the power spectra of wind and temperature turbulence and the corresponding mean values of wind speed and temperature. Usual spectral formulations were used based on the scaling laws ruling the Earth atmospheric surface layer, whereby the Earth's atmosphere is used as a standard for the Martian atmosphere.  相似文献   

14.
西北干旱区夏季大气边界层结构及其陆面过程特征   总被引：18，自引：3，他引：15  
张强  王胜 《气象学报》2008,66(4):599-608
在中国西北干旱区影响大气边界层形成和发展的气候环境和大气环流背景都具有一定特殊性.文中用外场观测试验资料,分析了位于西北干旱区的敦煌荒漠夏季大气边界层气象要素结构特征,发现该地区无论白天的对流边界层还是夜间的稳定边界层均比一般地区更深厚.在夏季晴天,夜间稳定边界层厚度超过900 m,最厚可以达到1750 m,其上的残余层一般能达到4000 m左右的高度;白天混合层最高达3700 m,混合层顶的逆温层顶盖的厚度大约450 m,甚至更厚,对流边界层厚度能够超过4000 m,对流边界层进入残余层后发展十分迅速.研究表明,白天深厚的对流边界层是夜间保持清晰而深厚的残余混合层的先决条件,夜间深厚的残余混合层又为白天对流边界层的发展提供了一个非常有利的热力环境条件.该地区经常性出现连续性晴天使得大气残余层的累积效应得以较长时间持续发展,创造了比较有利于大气对流边界层发展的大气热力环境条件.同时,该地区陆面过程和近地层大气运动特征也为这种独特的大气热力边界层结构提供了较好的支持.就该地区发展超厚大气对流边界层的物理机理而言,地表显著增温是强有力的外部热力强迫条件,近地层强感热通量提供了较充足的能量条件,较大的对流运动和湍流运动的速度是必要的运动学条件,大气残余层的累积效应提供了有利的热力环境条件.  相似文献   

15.
Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection. At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values of λ/z i (λ is the dominant horizontal scale of the turbulent eddies, z i is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL.  相似文献   

16.
During the Energy Balance EXperiment, the patch-by-patch, flood irrigation in a flat cotton field created an underlying surface with heterogeneous soil moisture, leading to a dry (warm)-to-wet (cool) transition within the cotton field under northerly winds. Moreover, the existence of an extremely dry, large bare soil area upstream beyond the cotton field created an even larger step transition from the bare soil region to the cotton field. We investigated the turbulence spectra and cospectra in the atmospheric surface layer (ASL) that was disturbed by large eddies generated over regions upstream and also influenced by horizontal advection. In the morning, the ASL was unstable while in the afternoon a stable internal boundary layer was observed at the site. Therefore, the turbulence data at 2.7 and 8.7 m are interpreted and compared in terms of interactions between large eddies and locally generated turbulence under two atmospheric conditions: the unstable ASL beneath the convective boundary layer (CBL) (hereafter the unstable condition) and the stable ASL beneath the CBL (hereafter the stable condition). We identified the influences of multiple sizes of large eddies on ASL turbulence under both stratifications; these large eddies with multiple sizes were produced over the dry patches and dry, large bare soil areas upstream. As a consequence of the disturbance of large eddies, the broadening, erratic variability, and deviation of spectra and cospectra, relative to those described by Monin–Obukhov similarity theory, are evident in the low- to mid-frequencies. Transfer of momentum, heat, and water vapour by large eddies is distinctly observed from the turbulence cospectra and leads to significant run-to-run variations of residuals of the surface energy balance closure. Our results indicate that these large eddies have greater influences on turbulence at higher levels compared to lower levels, and in the unstable ASL compared to the stable ASL.  相似文献   

17.
The morning development of the daytime convective boundary layer (CBL) during fine weather has been observed with an acoustic Doppler sodar of the C.R.P.E. In particular, the vertical profile of the vertical velocity third-order statistic W* 3 has been obtained. This quantity is a maximum near 0.3z I where z I, is the height of the CBL. The histogram of vertical velocity in the CBL shows a relationship between W 3 and the convective velocity W * and is useful for convective plume determination.  相似文献   

18.
城乡过渡地带低空温度平流和边界层特征的观测分析   总被引：4，自引：0，他引：4  
利用兰州河谷盆地城乡过渡区边界层观测资料,分析了该地区的温度平流和边界层特征。分析表明:(1)夜间热岛环流明显,白天则较弱;(2)夜间200m高度以下有较强的冷平流,在250～400m高度有较弱的暖平流,冷暖温度平流对测点上空边界层温度和层结变化有显著影响;(3)由声雷达确定的夜间边界层高度对应于Ri<1.0的高度,在这一高度范围内存在逆位温和强的风切变。本地区下垫面和复杂的地形导致夜间边界层高度随时间周期性地升高和降低,变化周期约3h。  相似文献   

19.
Model Simulations of the Boundary-Layer Evolution over an Arid Andes Valley   总被引：1，自引：1，他引：0  
The boundary layer of the Elqui valley in the arid north of Chile exhibits several interesting phenomena, such as a very shallow convective boundary layer (CBL) during the day. In the morning, warming is observed in and above the CBL, while the humidity decreases in the CBL. At midday, in and above the CBL of the valley, the temperature stagnates. In the afternoon in the CBL the temperature decreases and humidity increases, although the latent heat flux is very low. Because the characteristic features of the valley atmosphere are hard to interpret from observations alone, model simulations were applied. The simulations indicate that all components of the budget equations, i.e. the turbulent flux divergences, advection via the sea breeze, the upvalley and upslope wind systems, as well as subsidence, contribute to the evolution of the valley atmosphere.  相似文献   

20.
Summary In this study observations of the vertical structure of the Atmospheric Boundary Layer (ABL), recorded at a broad mountainous valley are presented. The vertical profiles of temperature, wind speed and direction up to a height of about 800 meters over the valley bottom have been measured and the temporal evolution of ABL structure of the area has been studied. Specifically, the mechanism of nocturnal inversion destruction during morning hours has been studied, which is of major importance in the study of the dispersion of air pollutants over the area. These observations suggest that the break up of nocturnal inversion during morning hours is mainly caused by a combined mechanism, the build up of the Convective Boundary Layer (CBL) and the presence of upslope winds, resulting to a continuous descent of the top of the nocturnal inversion.With 5 Figures  相似文献   

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