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1.
The nocturnal atmospheric boundary layer (ABL) poses several challenges to standard turbulence and dispersion models, since the stable stratification imposed by the radiative cooling of the ground modifies the flow turbulence in ways that are not yet completely understood. In the present work we perform direct numerical simulation of a turbulent open channel flow with a constant (cooling) heat flux imposed at the ground. This configuration provides a very simplified model for the surface layer at night. As a result of the ground cooling, the Reynolds stresses and the turbulent fluctuations near the ground re-adjust on times of the order of L/u τ , where L is the Obukhov length scale and u τ is the friction velocity. For relatively weak cooling turbulence survives, but when ReL=Lut/n <~100{Re_L=Lu_\tau/\nu \lesssim 100} turbulence collapses, a situation that is also observed in the ABL. This criterion, which can be locally measured in the field, is justified in terms of the scale separation between the largest and smallest structures of the dynamic sublayer.  相似文献   

2.
Some aspects of determining the stable boundary layer depth from sodar data   总被引：3，自引：2，他引：1  
The question of estimating the height of the stable boundary layer (SBL) based on digitalized vertical profiles of sodar signal intensity has been re-examined. A simple one-dimensional numerical boundary-layer model is used to compute vertical profiles of the temperature structure parameterC T 2 . It is shown that especially at the beginning of the night (when stratification is weak) one can not expect any significant profile structure in the upper part of the SBL if its depth is determined in terms of common turbulent height scales. From this it is concluded that the SBL-height will be underestimated early in the night when derived from the maximum gradient in the signal intensity profiles. Later in the night in contrast, the computations often show elevated maxima or even zones with reduced, and above them enhanced, vertical gradients ofC T 2 , from which a SBL-height can be deduced that compares well with other common height scales. The computed profiles ofC T 2 are shown to be in qualitative agreement with observed profiles of sodar signal intensity for several analysed cases from the HAPEX-MOBILHY experiment.Comparing different SBL-depth scales with sodar observations, it is demonstrated that most of them are often closely related to a sodar-derived SBL-height only during certain phases of the night. Thus the sodar-SBL-height can, after a transition period, be perhaps associated with the lower turbulent layer of the growing surface inversion during the first part and with the height of the low-level wind maximum during the second part of the night.  相似文献   

3.
We report on a novel approach for the Reynolds-averaged Navier-Stokes (RANS) modelling of the neutral atmospheric boundary layer (ABL), using the standard k-ek-{\varepsilon} turbulence model. A new inlet condition for turbulent kinetic energy is analytically derived from the solution of the k-ek-{\varepsilon} model transport equations, resulting in a consistent set of fully developed inlet conditions for the neutral ABL. A modification of the standard k-ek-{\varepsilon} model is also employed to ensure consistency between the inlet conditions and the turbulence model. In particular, the turbulence model constant C μ is generalized as a location-dependent parameter, and a source term is introduced in the transport equation for the turbulent dissipation rate. The application of the proposed methodology to cases involving obstacles in the flow is made possible through the implementation of an algorithm, which automatically switches the turbulence model formulation when going from the region where the ABL is undisturbed to the region directly affected by the building. Finally, the model is completed with a slightly modified version of the Richards and Hoxey rough-wall boundary condition. The methodology is implemented and tested in the commercial code Ansys Fluent 12.1. Results are presented for a neutral boundary layer over flat terrain and for the flow around a single building immersed in an ABL.  相似文献   

4.
Detailed measurements of profiles of mean and turbulent variablesthrough the nocturnal stable boundary layer over a valley arepresented. Two nights of data are analysed in detail, one with only aweakly stable boundary layer and one with a strongly stable boundarylayer. The weakly stratified night shows high levels of turbulence inwhich the flow remains attached to the valley and the boundary layeracts as a single coherent layer. On the strongly stratified night, twoflow regimes are identified: attached flow, as on the weaklystratified night, and decoupled flow in which the air in the valleybecomes so dynamically stable that there is no turbulent interactionwith the mean flow aloft. Because the valley is sloping, the decoupledlower stagnant air then forms a drainage current. We find that theFroude number evaluated at the hill height, FH = U(H)/N(H) H,diagnoses the flow regime: when FH = 2, the flow remainsattached and when FH 2 the flow in the valley becomesdecoupled from the flow aloft. The dynamics of the flow regimes areshown to be understandable in terms of the gradient Richardson number,which indicates the turbulent mixing. We show that the gradientRichardson number is a key parameter in diagnosing each flow regime.  相似文献   

5.
In an inversion-capped planetary boundary layer (PBL), the structure of the turbulent fluxes as well as the height of the inversion are determined by the interaction of turbulent mixing in the PBL, large-scale subsidence above the PBL and radiational cooling. Here the sensitivity of the inversion height and of the turbulent fluxes due to radiational processes is investigated with the aid of a three-layered model for a well mixed PBL. For an example of the Trade-Wind region, the inversion height (i.e., the difference between surface pressure and pressure at the inversion level) varies between 46 and 257 mb and the surface flux of moist static energy between 417 and 99 W m-2, if the (mean) radiative net flux divergence for both the inversion and the well-mixed layer is changed over a reasonable range of values. None of the parameterization schemes existing in the literature is able to describe these radiational effects in an appropriate way. This is due to the fact that these parameterizations are either not or not flexibly enough linked to the thermodynamical model parameter. Therefore the demand for an adequate parameterization of the radiational influence in a well-mixed PBL under a subsidence inversion is obvious.  相似文献   

6.
The formation mechanism of a cold sea-fog case observed over the Yellow Sea near the western coastal area of the Korean Peninsula is investigated using numerical simulation with a one-dimensional turbulence model coupled with a three-dimensional regional model. The simulation was carried out using both Eulerian and Lagrangian approaches; both approaches produced sea fog in a manner consistent with observation. For the selected cold sea-fog case, the model results suggested the following: as warm and moist air flows over a cold sea surface, the lower part of the air column is modified by the turbulent exchange of heat and moisture and the diurnal variation in radiation. The modified boundary-layer structure represents a typical stable thermally internal boundary layer. Within the stable thermally internal boundary layer, the air temperature is decreased by radiative cooling and turbulent heat exchange but the moisture loss due to the downward vapour flux in the lowest part of the air column is compensated by moisture advection and therefore the dewpoint temperature does not decrease as rapidly as does the air temperature. Eventually water vapour saturation is achieved and the cold sea fog forms in the thermal internal boundary layer.  相似文献   

7.
A case study of warm air advection over the Arctic marginalsea-ice zone is presented, based on aircraft observations with direct flux measurements carriedout in early spring, 1998. A shallow atmospheric boundary layer (ABL) was observed, which wasgradually cooling with distance downwind of the ice edge. This process was mainly connected with astrong stable stratification and downward turbulent heat fluxes of about 10–20 W m-2, but wasalso due to radiative cooling. Two mesoscale models, one hydrostatic and the other non-hydrostatic,having different turbulence closures, were applied. Despite these fundamental differences betweenthe models, the results of both agreed well with the observed data. Various closure assumptions had amore crucial influence on the results than the differences between the models.Such an assumption was, for example,the parameterization of the surface roughness for momentum (z0) and heat (zT). This stronglyaffected the wind and temperature fields not only close to the surface but also within and abovethe temperature inversion layer. The best results were achieved using a formulation for z0 that took intoaccount the form drag effect of sea-ice ridges together withzT = 0.1z0. The stability within theelevated inversion strongly depended on the minimum eddy diffusivity Kmin. A simple ad hocparameterization seems applicable, where Kmin is calculated as 0.005 timesthe neutral eddy diffusivity. Although the longwave radiative cooling was largest within the ABL, theapplication of a radiation scheme was less important there than above the ABL. This was related to theinteraction of the turbulent and radiative fluxes. To reproduce the strong inversion, it wasnecessary to use vertical and horizontal resolutions higher than those applied in most regional andlarge-scale atmospheric models.  相似文献   

8.
Scaling velocities relevant for turbulent flows in the planetary boundary layer are discussed. It is suggested that the scaling parameters should be determined by integrated bulk properties of the respective turbulent production terms. According to this concept, a new velocity scale, replacing the friction velocityu*, is proposed depending on bothu* and the geostrophic windu g . The convective velocity scalew* can be determined by the integral of the buoyancy production term and is therefore an appropriate velocity scale. Examination of Minnesota and Kansas data shows that these data do not give the possibility of verifying whether the new scaling velocity is more appropriate thanu*. This is because the range of variability of atmospheric stability during the field measurements is too small. However, theoretical considerations based on integrated properties of the turbulence, through the depth of the planetary boundary layer, are given in support of the new scaling velocity.  相似文献   

9.
The atmospheric boundary layer (ABL) model of Weng and Taylor with E−ℓ turbulence closure is applied to simulate the one-dimensional stably stratified ABL. The model has been run for nine hours from specified initial wind, potential temperature and turbulent kinetic energy profiles, and with a specified cooling rate applied at the surface. Different runs are conducted for different cooling rates, geostrophic winds and surface roughnesses. The results are discussed and compared with other models, large-eddy simulations and published field data.  相似文献   

10.
We present a scheme for parameterising scalar transfer in the urban boundary layer, which is divided into an inertial layer and a roughness layer. The latter is further divided into a shear layer and a canyon layer. In the inertial layer, scalar transfer is determined by turbulence related to canyon macroscopic features, while in the roughness layer, it is determined by shear-generated turbulence, canyon vortex and vortex-generated turbulence. We first describe a conceptual model for the canyon flow and the aerodynamic resistance network, and then estimate the resistances from the point of view of drag partition and vortex advection. The results are compared with the measurements from wind-tunnel experiments. It is found that for small canyon aspect ratio, σc, the transfer velocity increases with σc, reaching a maximum at around σc=0.5 and then decreases with σc. We also show that the scheme is not sensitive to adjustable parameters  相似文献   

11.
Direct numerical simulations of an Ekman layer are performed to study flow evolution during the response of an initially neutral boundary layer to stable stratification. The Obukhov length, L, is varied among cases by imposing a range of stable buoyancy fluxes at the surface to mimic ground cooling. The imposition of constant surface buoyancy flux , i.e. constant-flux stability, leads to a buoyancy difference between the ground and background that tends to increase with time, unlike the constant-temperature stability case where a constant surface temperature is imposed. The initial collapse of turbulence in the surface layer owing to surface cooling that occurs over a time scale proportional to \(L/u_*\), where \(u_*\) is the friction velocity, is followed by turbulence recovery. The flow accelerates, and a “low-level jet” (LLJ) with inertial oscillations forms during the turbulence collapse. Turbulence statistics and budgets are examined to understand the recovery of turbulence. Vertical turbulence exchange, primarily by pressure transport, is found to initiate fluctuations in the surface layer and there is rebirth of turbulence through enhanced turbulence production as the LLJ shear increases. The turbulence recovery is not monotonic and exhibits temporal intermittency with several collapse/rebirth episodes. The boundary layer adjusts to an increase in the surface buoyancy flux by increased super-geostrophic velocity and surface stress such that the Obukhov length becomes similar among the cases and sufficiently large to allow fluctuations with sustained momentum and heat fluxes. The eventual state of fluctuations, achieved after about two inertial periods (\(ft \approx 4\pi \)), corresponds to global intermittency with turbulent patches in an otherwise quiescent background. Our simplified configuration is sufficient to identify turbulence collapse and rebirth, global and temporal intermittency, as well as formation of low-level jets, as in observations of the stratified atmospheric boundary layer.  相似文献   

12.
To investigate the processes of development and maintenance of low-level clouds during major synoptic events, the cloudy boundary layer under stormy conditions during the summertime Arctic has been studied using observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and large-eddy simulations (LES). On 29 July 1998, a stable Arctic cloudy boundary-layer event was observed after the passage of a synoptic low pressure system. The local dynamic and thermodynamic structure of the boundary layer was determined from aircraft measurements including the analysis of turbulence, cloud microphysics and radiative properties. After the upper cloud layer advected over the existing cloud layer, the turbulent kinetic energy (TKE) budget indicated that the cloud layer below 200 m was maintained predominantly by shear production. Observations of longwave radiation showed that cloud-top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our LES results demonstrate the importance of the combination of shear mixing near the surface and radiative cooling at the cloud top in the storm-driven cloudy boundary layer. Once the low-level cloud reaches a certain height, depending on the amount of cloud-top cooling, the two sources of TKE production begin to separate in space under continuous stormy conditions, suggesting one possible mechanism for the cloud layering. The sensitivity tests suggest that the storm-driven cloudy boundary layer is possibly switched to the shear-driven system due to the advection of upper clouds or to the buoyantly driven system due to the lack of wind shear. A comparison is made of this storm-driven boundary layer with the buoyantly driven boundary layer previously described in the literature.  相似文献   

13.
When modelling the turbulent dispersion of a passive tracer using Reynolds-averaged Navier–Stokes (RANS) simulations, two different approaches can be used. The first consists of solving a transport equation for a scalar, where the governing parameters are the mean velocity field and the turbulent diffusion coefficient, given by the ratio of the turbulent viscosity and the turbulent Schmidt number Sc t . The second approach uses a Lagrangian particle tracking algorithm, where the governing parameters are the mean velocity and the fluctuating velocity field, which is determined from the turbulence kinetic energy and the Lagrangian time T L . A comparison between the two approaches and wind-tunnel data for the dispersion in the wake of a rectangular building immersed in a neutral atmospheric boundary layer (ABL) is presented. Particular attention was paid to the influence of turbulence model parameters on the flow and concentration field. In addition, an approach to estimate Sc t and T L based on the calculated flow field is proposed. The results show that applying modified turbulence model constants to enable correct modelling of the ABL improves the prediction for the velocity and concentration fields when the modification is restricted to the region for which it was derived. The difference between simulated and measured concentrations is smaller than 25% or the uncertainty of the data on 76% of the points when solving the transport equation for a scalar with the proposed formulation for Sc t , and on 69% of the points when using the Lagrangian particle tracking with the proposed formulation for T L .  相似文献   

14.
利用杭瑞高速公路洞庭湖大桥北岸测风塔的梯度风观测资料、三维超声风温仪资料以及岳阳气象站提供的逐小时气溶胶浓度和能见度观测资料,对湖南岳阳2017年1月28日的一次重度霾天气中的重污染过程的近地层物理量变化特征进行了分析,结果表明:（1）重污染来临前约130 min即28日01:50（北京时间,下同）,水平风速、垂直风速、高低层风切变都出现零值,大气处于静稳状态。重污染结束前180 min即28日09:00,上述物理量和高低层温度切变出现零值。（2）湍流强度在重污染来临前有强烈异常信号,其中水平纵向湍流强度异常信号最明显,于重污染发生前130 min出现异常峰值4.15,重污染结束前180 min出现异常峰值3.24。（3）湍流动能和动量通量都在重污染来临前130 min接近0.0 m2/s2,即湍流交换最弱,有利于污染物在近地面的持续堆积和重污染过程的发生。近地层的平均物理量和湍流特征量的异常信号的出现时间有较好的一致性,即出现在重污染来临前的130 min和结束前的180 min。揭示了重度霾污染天气的近地层物理量时间变化规律,着重分析了霾污染的生成、发展、消亡全过程的边界层湍流异常的前期信号,为深入认识霾污染天气进行有益的探索并为这类天气的预测预警提供科学依据。  相似文献   

15.
The contribution of radiative and turbulent processes to nocturnal atmospheric cooling has been studied using the experimental data of the ECLATS experiment which took place in the African Sahel; the radiative and turbulent fluxes were determined taking thermal advection into account. The turbulent cooling rate is predominant; it decreases strongly with altitude at the beginning of the night, which is the main cause of inversion formation.  相似文献   

16.
北京一次罕见夜间突发性强增温事件成因分析   总被引：4，自引：0，他引：4  
罗然  郑永光  陈敏 《气象》2020,46(4):478-489
夜间降温是正常的地表气温日变化,但统计表明北京及周边地区冬半年时常出现入夜后气温不降反升的现象,甚至出现了小时升温超过10℃的剧烈增温事件。这种增温具有明显的突发性,且很快转为降温,常对业务预报造成困扰。2010年11月26日夜间冷锋过境该区域造成了最强达12℃·h^-1的夜间急剧增温事件,与气候统计结果相比,该次增温幅度和影响区域范围非常罕见且极端。文章对其进行了详细诊断分析,使用的资料包括自动气象站、常规地面和探空、铁塔、卫星、风廓线雷达等观测资料和美国环境预报中心(NCEP)最终分析资料。分析结果表明:该次过程对流层低层有非常显著的冷平流;垂直速度诊断、卫星和风廓线雷达观测都表明,对流层中低层都存在显著的强下沉运动;铁塔观测和北京探空观测演变都表明增温过程中近地面大气有显著的湍流运动。分析此次罕见过程的机理包括三个方面:高原地区地面位温显著高于平原地区(二者最大可差10 K),是该次罕见增温事件形成的首要条件;强下沉运动使得低密度高位温空气强迫下沉到边界层,是增温必要条件;强湍流混合作用则是地面空气增温的必要机制。估算结果表明,边界层急流伴随的强湍流混合可引起约8℃的罕见地面空气增温。最后,给出了该次事件的机理概念模型。  相似文献   

17.
We present results of a technique for examining the scale-dependence of the gradient Richardson number, Ri, in the nighttime residual layer. The technique makes use of a series of high-resolution, in situ, vertical profiles of wind speed and potential temperature obtained during CASES-99 in south-eastern Kansas, U.S.A. in October 1999. These profiles extended from the surface, through the nighttime stable boundary layer, and well into the residual layer. Analyses of the vertical gradients of both wind speed, potential temperature and turbulence profiles over a wide range of vertical scale sizes are used to estimate profiles of the local Ri and turbulence structure as a function of scale size. The utility of the technique lies both with the extensive height range of the residual layer as well as with the fact that the sub-metre resolution of the raw profiles enables a metre-by-metre ‘sliding’ average of the scale-dependent Richardson number values over hundreds of metres vertically. The results presented here show that small-scale turbulence is a ubiquitous and omnipresent feature of the residual layer, and that the region is dynamic and highly variable, exhibiting persistent turbulent structure on vertical scales of a few tens of metres or less. Furthermore, these scales are comparable to the scales over which the Ri is less than or equal to the critical value of Ri c of 0.25, although turbulence is also shown to exist in regions with significantly larger Ri values, an observation at least consistent with the concept of hysteresis in turbulence generation and maintenance. Insofar as the important scale sizes are comparable to or smaller than the resolution of current models, it follows that, in order to resolve the observed details of small Ri values and the concomitant turbulence generation, future models need to be capable of significantly higher resolutions.  相似文献   

18.
利用南京地面站点2016—2017年黑碳气溶胶(Black Carbon, BC)和臭氧(O_3)逐小时观测资料,对比分析了不同季节BC与近地面O_3的关系。结果表明,高BC(高于平均值)影响下的O_3质量浓度值明显比低BC(低于平均值)影响下的O_3质量浓度值低,这种抑制作用在秋冬季明显高于春夏季,且BC与O_3的负相关性在秋冬季显著高于春夏季,而PM_(2.5)与O_3的负相关性不显著。利用WRF-Chem模式,对2017年12月个例开展BC反馈效应对O_3影响的数值模拟,结果再次证实BC对O_3存在负反馈影响。其影响机制是:BC可通过抑制边界层发展,使近地面NO_x积聚,从而减少臭氧的化学生成(VOCs控制区);BC可通过抑制边界层垂直湍流交换,减少边界层上部高O_3向下的湍流输送,从而减少近地面O_3;BC可通过减小近地面风速,减少O_3的平流输入,从而减少地面O_3。不同个例的主要控制因子不同。  相似文献   

19.
We present a new model of the structure of turbulence in the unstable atmospheric surface layer, and of the structural transition between this and the outer layer. The archetypal element of wall-bounded shear turbulence is the Theodorsen ejection amplifier (TEA) structure, in which an initial ejection of air from near the ground into an ideal laminar and logarithmic flow induces vortical motion about a hairpin-shaped core, which then creates a second ejection that is similar to, but larger than, the first. A series of TEA structures form a TEA cascade. In real turbulent flows TEA structures occur in distorted forms as TEA-like (TEAL) structures. Distortion terminates many TEAL cascades and only the best-formed TEAL structures initiate new cycles. In an extended log layer the resulting shear turbulence is a complex, self-organizing, dissipative system exhibiting self-similar behaviour under inner scaling. Spectral results show that this structure is insensitive to instability. This is contrary to the fundamental hypothesis of Monin--Obukhov similarity theory. All TEAL cascades terminate at the top of the surface layer where they encounter, and are severely distorted by, powerful eddies of similar size from the outer layer. These eddies are products of the breakdown of the large eddies produced by buoyancy in the outer layer. When the outer layer is much deeper than the surface layer the interacting eddies are from the inertial subrange of the outer Richardson cascade. The scale height of the surface layer, z s, is then found by matching the powers delivered to the creation of emerging TEAL structures to the power passing down the Richardson cascade in the outer layer. It is z s = u * 3 /ks, where u * is friction velocity, k is the von Kármán constant and s is the rate of dissipation of turbulence kinetic energy in the outer layer immediately above the surface layer. This height is comparable to the Obukhov length in the fully convective boundary layer. Aircraft and tower observations confirm a strong qualitative change in the structure of the turbulence at about that height. The tallest eddies within the surface layer have height z s, so z s is a new basis parameter for similarity models of the surface layer.  相似文献   

20.
A series of experiments were performed in a rotating cylindrical tank over a wide range of rotation rates in which convective turbulence was generated by a bottom-mounted heated plate in both homogeneous and stratified fluids. Measurements were made of the turbulent velocities in all three axes over the full depth of the chamber, and of the temperatures at the mid-depth near the centre of the tank. For even small rotation rates, the measurements showed that the turbulent velocities were weakly affected by rotation at all depths, but as the rotation rate increased, the deviation from the non-rotational scaling slowly and progressively increased until eventually the turbulent velocities were fully rotationally controlled. The results indicated that there was no sudden transition of the turbulent field from the non-rotational state (a function only of the surface buoyancy flux B and the depth z) to the rotational state (where the strength of the turbulent field is a function of only B and the Coriolis parameter f). Rather the transition was a smooth asymptotic one from one state to the other. Nevertheless, it was possible to parametrize this transition by a single value of the turbulent or small scale Rossby number, defined by Ro = (B/f3z2)1/3. Our measurements suggested a critical value of Roc ≈ 0.1, below which the turbulence was fully rotationally controlled and which was equivalent to a critical depth zc = (35 ± 15)(B/f3)1/2. Using typical oceanic values for B and f, the oceanic turbulence driven by surface cooling events becomes rotationally controlled only for depths greater than about 10 km, a depth which is greater than that of the bulk of the world's oceans. Thus, convective turbulence actively being generated by cooling of the ocean surface is best described by non-rotating turbulent velocity and length scales and is a function only of the surface buoyancy flux and the depth.  相似文献   

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