Modelling considerations for examining the mean and unsteady flow in a simple urban-type street canyon

With very few exceptions, just about all limited area models (LAMs) used in operational NWP and regional climate modeling use the Davies (Q J R Meteorol Soc 102:405–418, 1976) relaxation lateral boundary conditions (LBCs), even though they make no effort to respect the basic mathematics of the problem. While in the early stages of the primitive equation LAM development in the seventies numerous schemes have been proposed and tested, LAM communities have eventually for the most part settled on the relaxation LBCs with few questions asked. An exception is the Eta model used extensively at NCEP and several other centers, in which the Mesinger (Contrib Atmos Phys 50:200–210, 1977) LBCs are used, designed and based on knowledge available before the introduction of the relaxation scheme. They prescribe variables along the outermost row of grid points only; all of them at the inflow points and one less at the outflow points where the tangential velocity components are extrapolated from inside of the model domain. Additional schemes are in place to suppress separation of gravity-wave solutions on C-subgrids of the model’s E-grid. A recent paper of Veljovic et al. (Meteor Zeitschrift 19:237–246, 2010) included three 32-day forecasts done with both the Eta and the relaxation LBCs and the comparison of some of their verification results. Here we extend this experiment by three additional forecasts to arrive at an ensemble of six members run with both schemes, and present a more complete discussion of results. We in addition show results of one of these forecasts in which the linear change of relaxation coefficients was replaced by the change following the recommendation of Lehmann (Meteorol Atmos Phys 52:1–14, 1993). We feel that the results of our two verification schemes strongly suggest the advantage of the Eta over the conventional relaxation scheme, thereby raising doubts as to the justification for its use.     

The Volumetric Particle Approach for Concentration Fluctuations and Chemical Reactions in Lagrangian Particle and Particle-grid Models

A new approach is proposed to predict concentration fluctuations in the framework of one-particle Lagrangian stochastic models. The approach is innovative since it allows the computation of concentration fluctuations in dispersing plumes using a Lagrangian one-particle model with micromixing but with no need for the simulating of background particles. The extension of the model for the treatment of chemically reactive plumes is also accomplished and allows the computation of plume-related chemical reactions in a Lagrangian one-particle framework separately from the background chemical reactions, accounting for the effect of concentration fluctuations on chemical reactions in a general, albeit approximate, manner. These characteristics should make the proposed approach an ideal tool for plume-in-grid calculations in chemistry transport models. The results are compared to the wind-tunnel experiments of Fackrell and Robins (J Fluid Mech, 117:1–26, 1982) for plume dispersion in a neutral boundary layer and to the measurements of Legg et al. (Boundary-Layer Meteorol, 35:277–302, 1986) for line source dispersion in and above a model canopy. Preliminary reacting plume simulations are also shown comparing the model with the experimental results of Brown and Bilger (J Fluid Mech, 312:373–407, 1996; Atmos Environ, 32:611–628, 1998) to demonstrate the feasibility of computing chemical reactions in the proposed framework.     

Evaluation of Two Energy Balance Closure Parametrizations

A general lack of energy balance closure indicates that tower-based eddy-covariance (EC) measurements underestimate turbulent heat fluxes, which calls for robust correction schemes. Two parametrization approaches that can be found in the literature were tested using data from the Canadian Twin Otter research aircraft and from tower-based measurements of the German Terrestrial Environmental Observatories (TERENO) programme. Our analysis shows that the approach of Huang et al. (Boundary-Layer Meteorol 127:273–292, 2008), based on large-eddy simulation, is not applicable to typical near-surface flux measurements because it was developed for heights above the surface layer and over homogeneous terrain. The biggest shortcoming of this parametrization is that the grid resolution of the model was too coarse so that the surface layer, where EC measurements are usually made, is not properly resolved. The empirical approach of Panin and Bernhofer (Izvestiya Atmos Oceanic Phys 44:701–716, 2008) considers landscape-level roughness heterogeneities that induce secondary circulations and at least gives a qualitative estimate of the energy balance closure. However, it does not consider any feature of landscape-scale heterogeneity other than surface roughness, such as surface temperature, surface moisture or topography. The failures of both approaches might indicate that the influence of mesoscale structures is not a sufficient explanation for the energy balance closure problem. However, our analysis of different wind-direction sectors shows that the upwind landscape-scale heterogeneity indeed influences the energy balance closure determined from tower flux data. We also analyzed the aircraft measurements with respect to the partitioning of the “missing energy” between sensible and latent heat fluxes and we could confirm the assumption of scalar similarity only for Bowen ratios $\approx $ 1.     

Application of a Bivariate Gamma Distribution for a Chemically Reacting Plume in the Atmosphere

The joint concentration probability density function of two reactive chemical species is modelled using a bivariate Gamma distribution coupled with a three-dimensional fluctuating plume model able to simulate the diffusion and mixing of turbulent plumes. A wind-tunnel experiment (Brown and Bilger, J Fluid Mech 312:373–407, 1996), carried out in homogeneous unbounded turbulence, in which nitrogen oxide is released from a point source in an ozone doped background and the chemical reactions take place in non-equilibrium conditions, is considered as a test case. The model is based on a stochastic Langevin equation reproducing the barycentre position distribution through a proper low-pass filter for the turbulence length scales. While the meandering large-scale motion of the plume is directly simulated, the internal mixing relative to the centroid is reproduced using a bivariate Gamma density function. The effect of turbulence on the chemical reaction (segregation), which in this case has not yet attained equilibrium, is directly evaluated through the covariance of the tracer concentration fields. The computed mean concentrations and the O₃–NO concentration covariance are also compared with those obtained by the Alessandrini and Ferrero Lagrangian single particle model (Alessandrini and Ferrero, Physica A 388:1375–1387, 2009) that entails an ad hoc parametrization for the segregation coefficient.     

Dynamics and predictability of a low-order wind-driven ocean–atmosphere coupled model

The dynamics of a low-order coupled wind-driven ocean–atmosphere system is investigated with emphasis on its predictability properties. The low-order coupled deterministic system is composed of a baroclinic atmosphere for which 12 dominant dynamical modes are only retained (Charney and Straus in J Atmos Sci 37:1157–1176, 1980) and a wind-driven, quasi-geostrophic and reduced-gravity shallow ocean whose field is truncated to four dominant modes able to reproduce the large scale oceanic gyres (Pierini in J Phys Oceanogr 41:1585–1604, 2011). The two models are coupled through mechanical forcings only. The analysis of its dynamics reveals first that under aperiodic atmospheric forcings only dominant single gyres (clockwise or counterclockwise) appear, while for periodic atmospheric solutions the double gyres emerge. In the present model domain setting context, this feature is related to the level of truncation of the atmospheric fields, as indicated by a preliminary analysis of the impact of higher wavenumber (“synoptic” scale) modes on the development of oceanic gyres. In the latter case, double gyres appear in the presence of a chaotic atmosphere. Second the dynamical quantities characterizing the short-term predictability (Lyapunov exponents, Lyapunov dimension, Kolmogorov–Sinaï (KS) entropy) displays a complex dependence as a function of the key parameters of the system, namely the coupling strength and the external thermal forcing. In particular, the KS-entropy is increasing as a function of the coupling in most of the experiments, implying an increase of the rate of loss of information about the localization of the system on its attractor. Finally the dynamics of the error is explored and indicates, in particular, a rich variety of short term behaviors of the error in the atmosphere depending on the (relative) amplitude of the initial error affecting the ocean, from polynomial (at ² + bt ³ + ct ⁴) up to exponential-like evolutions. These features are explained and analyzed in the light of the recent findings on error growth (Nicolis et al. in J Atmos Sci 66:766–778, 2009).     

A new nonlinear analytical model for canopy flow over a forested hill

A new nonlinear analytical model for canopy flow over gentle hills is presented. This model is established based on the assumption that three major forces (pressure gradient, Reynolds stress gradient, and nonlinear canopy drag) within canopy are in balance for gentle hills under neutral conditions. The momentum governing equation is closed by the velocity-squared law. This new model has many advantages over the model developed by Finnigan and Belcher (Quart J Roy Meteorol Soc 130: 1–29 2004, hereafter referred to as FB04) in predicting canopy wind velocity profiles in forested hills in that: (1) predictions from the new model are more realistic because surface drag effects can be taken into account by boundary conditions, while surface drag effects cannot be accounted for in the algebraic equation used in the lower canopy layer in the FB04 model; (2) the mixing length theory is not necessarily used because it leads to a theoretical inconsistency that a constant mixing length assumption leads to a nonconstant mixing length prediction as in the FB04 model; and (3) the effects of height-dependent leaf area density (a(z)) and drag coefficient (C _d ) on wind velocity can be predicted, while both a(z) and C _d must be treated as constants in FB04 model. The nonlinear algebraic equation for momentum transfer in the lower part of canopy used in FB04 model is height independent, actually serving as a bottom boundary condition for the linear differential momentum equation in the upper canopy layer. The predicting ability of the FB04 model is largely restricted by using the height-independent algebraic equation in the bottom canopy layer. This study has demonstrated the success of using the velocity-squared law as a closure scheme for momentum transfer in forested hills in comparison with the mixing length theory used in FB04 model thus enhancing the predicting ability of canopy flows, keeping the theory consistent and simple, and shining a new light into land-surface parameterization schemes in numerical weather and climate models.     

Effects of Temporal Discretization on Turbulence Statistics and Spectra in Numerically Simulated Convective Boundary Layers

Six state-of-the-art large-eddy simulation codes were compared in Fedorovich et al. (Preprints, 16th American Meteorological Society Symposium on Boundary Layers and Turbulence, 2004b) for three airflow configurations in order to better understand the effect of wind shear on entrainment dynamics in the convective boundary layer CBL). One such code was the University of Oklahoma large-eddy simulation (LES) code, which at the time employed a second-order leapfrog time-advancement scheme with the Asselin filter. In subsequent years, the code has been updated to use a third-order Runge–Kutta (RK3) time-advancement scheme. This study investigates what effect the upgrade from the leapfrog scheme to RK3 scheme has on turbulence statistics in the CBL differently affected by mean wind shear, also in relation to predictions by other LES codes that participated in the considered comparison exercise. In addition, the effect of changing the Courant number within the RK3 scheme is investigated by invoking the turbulence spectral analysis. Results indicate that low-order flow statistics obtained with the RK3 scheme generally match their counterparts from simulations with the leapfrog scheme rather closely. CBL growth rates due to entrainment in the shear-free case were also similar using both timestepping schemes. It was found, however, that care should be given to the choice of the Courant number value when running LES with the RK3 scheme in the sheared CBL setting. The advantages of the largest possible (based on the stability criterion) Courant number were negated by degrading the energy distribution across the turbulence spectrum. While mean profiles and low-order turbulence statistics were largely unaffected, the entrainment rate was over-predicted compared to that reported in the original code-comparison study.     

Formulation of a tropical town energy budget (t-TEB) scheme

This work describes the tropical town energy budget (t-TEB) scheme addressed to simulate the diurnal occurrence of the urban heat island (UHI) as observed in the Metropolitan Area of Rio de Janeiro (MARJ; ?22° S; ?44° W) in Brazil. Reasoning about the tropical urban climate have guided the scheme implementation, starting from the original equations from Masson (Bound-Lay Meteorol 94:357–397, 2000). The modifications include (a) local scaling approaches for obtaining flux–gradient relationships in the roughness sub-layer, (b) the Monin-Obukhov similarity framework in the inertial sub-layer, (c) increasing aerodynamic conductance toward more unstable conditions, and (d) a modified urban subsurface drainage system to transfer the intercepted rainwater by roofs to the roads. Simulations along 2007 for the MARJ are obtained and compared with the climatology. The t-TEB simulation is consistent with the observations, suggesting that the timing and dynamics of the UHI in tropical cities could vary significantly from the familiar patterns observed in mid-latitude cities—with the peak heat island intensity occurring in the morning than at night. The simulations are suggesting that the thermal phase shift of this tropical diurnal UHI is a response of the surface energy budget to the large amount of solar radiation, intense evapotranspiration, and thermal response of the vegetated surfaces over a very humid soil layer.     

Analytically Modelling Mean Wind and Stress Profiles in Canopies

An analytical model for mean wind profiles in sparse canopies (W. Wang, Boundary-Layer Meteorol 142:383–399, 2012) has been further developed, with (1) an explicit solution being derived, and (2) a linear term being added to the $K$ -closure scheme to improve the shear-stress parametrization when the contribution of non-local transport is significant. Results from large-eddy simulations and from laboratory experiments are used to evaluate the model and adjust model parameters, showing that the model can well simulate canopy wind and stress profiles not only for sparse-canopy scenarios, but also for dense-canopy scenarios. The analytical solution converges exactly to the standard surface-layer logarithmic wind profile in the case of zero canopy density, and tends to an exponential wind profile for a dense canopy.     

Comment on “Kinetics of the reactions of Cl atoms with 2-buten-1-ol, 2-methyl-2-propen-1-ol, and 3-methyl-2-buten-1-ol as a function of temperature” by Rodriguez et al. (J. Atmos. Chem. (2008) 59:187–197)

The Arrhenius expressions and the data plotted in Figure 2 of Rodriguez et al. 2008 give rate coefficients of approximately 2?×?10^-8 cm³ molecule^-1 s^-1 at 255 K. Such values are approximately two orders of magnitude larger than expected from simple collision theory (Finlayson-Pitts and Pitts 1986). The rate coefficients reported at sub-ambient temperatures are substantially greater than the gas kinetic limit and are not physically plausible. The rate coefficients reported by Rodriguez et al. imply a long range attraction between the reactants which is not reasonable for reaction of neutral species such as chlorine atoms and unsaturated alcohols. We also note that the pre-exponential A factors (10^-23-10^-20) and activation energies (?15 kcal mol^-1) are not physically plausible. We conclude that there are large systematic errors in the study by Rodriguez et al. (Atmos Chem 59:187–197, 2008).     

The influence of isoprene peroxy radical isomerization mechanisms on ozone simulation with the presence of NOx

Isoprene peroxy radical isomerizations (1,5- and 1,6-H shifts) have recently been proposed as important pathways regenerating and recycling HO_x (OH?+?HO₂) in the atmosphere under low-NO_x conditions (Peeters et al. Phys. Chem. Chem. Phys. 28: 5935?C5939 2009; da Silva et al. Environ. Sci. Technol. 44:250?C256 2010). Evaluation and comparison of the isoprene peroxy radical isomerization mechanisms from recent studies have been performed against isoprene-NO_x experiments conducted in the UNC dual outdoor smog chambers. Five different kinetic mechanisms were tested in this study, including the original Master Chemical Mechanism (MCM) v3.1; two modified MCM mechanisms both implementing isoprene peroxy radical isomerization reactions but with different rate coefficients; the Carbon Bond 6 (CB6) mechanism; and the ISO-UNC mechanism. Sensitivity analyses of the unsaturated hydroxyperoxy aldehydes (HPALDs) reaction mechanisms under fast isomerization have also been performed. The results indicate that the fast isomerization mechanism and the mechanisms with high OH yields from HPALDs photolysis both significantly enhance HO_x estimates with increasing isoprene/NO_x ratios. However, O₃ predictions, as well the isoprene decay rates are substantially overestimated. Our results suggest that given the current state of our knowledge, it is difficult to improve both HO_x levels and maintain reasonable O₃ simulations using the Peeters et al. (Peeters et al. Phys. Chem. Chem. Phys. 28: 5935?C5939 2009) mechanism.     

WRF模式中QNSE方案的湍流长度尺度系数的调整试验研究

边界层参数化方案中湍流混合对数值模拟起着重要的作用,湍流混合作用的恰当描述对于温度、湿度、风场以及降水的准确模拟至关重要。我国长江中下游流域人口密集,暴雨灾害频发,很有必要寻找一种适合该地区降水模拟的边界层参数化方案。本文运用WRF（Weather Research and Forecasting）中尺度数值模式,以QNSE（Quasi-Normal Scale Elimination）边界层参数化方案为基础,将其中湍流混合长度尺度系数调整为可变参数。本文将Noh et al.（2003）提出的Prandtl公式与Janji?提出的修正湍流长度尺度系数的方法相结合,通过考虑非局地项的强迫、地表稳定度与边界层高度对湍流长度尺度系数的影响,强调大气的动力结构特征与热力结构特征对湍流混合的共同影响,从而提高QNSE边界层参数化方案在不同地理环境下的模拟能力。文章通过进行长江中下游地区的典型暴雨试验,将调整参数后的QNSE方案与原方案进行比较,重点分析调整参数后的方案与原方案对关键基本气象要素场、边界层结构特征以及降水的模拟能力,并将模拟结果与观测结果进行对比,结果表明:调整参数后的方案一定程度上改进了地表温度、边界层结构以及降水的模拟效果。进一步研究表明,调整参数后的方案主要通过改变边界层混合缓解水汽混合比、位温模拟方面的误差。     

Including the Drag Effects of Canopies: Real Case Large-Eddy Simulation Studies

We use the mesoscale meteorological model Meso-NH, taking the drag force of trees into account under stable, unstable and neutral conditions in a real case study. Large-eddy simulations (LES) are carried out for real orography, using a regional forcing model and including the energy and water fluxes between the surface (mostly grass with some hedges of trees) and the atmosphere calculated using a state-of-the-art soil-vegetation-atmosphere-transfer model. The formulation of the drag approach consists of adding drag terms to the momentum equation and subgrid turbulent kinetic energy dissipation, as a function of the foliage density. Its implementation in Meso-NH is validated using Advanced Regional Prediction System simulation results and measurements from Shaw and Schumann (Boundary-Layer Meteorol, 61(1):47?C64, 1992). The simulation shows that the Meso-NH model successfully reproduces the flow within and above homogeneous covers. Then, real case studies are used in order to investigate the three different boundary layers in a LES configuration (resolution down to 2 m) over the ??Lannemezan 2005?? experimental campaign. Thus, we show that the model is able to reproduce realistic flows in these particular cases and confirm that the drag force approach is more efficient than the classical roughness approach in describing the flow in the presence of vegetation at these resolutions.     

Reply to the Comment by Mauder on “How Well Can We Measure the Vertical Wind Speed? Implications for Fluxes of Energy and Mass”

In Kochendorfer et al. (Boundary-Layer Meteorol 145:383–398, 2012, hereafter K2012) the vertical wind speed $(w)$ measured by a non-orthogonal three-dimensional sonic anemometer was shown to be underestimated by $\approx $ 12 %. Turbulent statistics and eddy-covariance fluxes estimated using $w$ were also affected by this underestimate in $w$ . Methodologies used in K2012 are clarified here in response to Mauder’s comment. In addition, further analysis of the K2012 study is presented to help address questions raised in the comment. Specific responses are accompanied with examples of time series, calculated correlation coefficients, and additional explanation of the K2012 methods and assumptions. The discussion and analysis included in the comment and in this response do not affect the validity of the methods or conclusions presented in K2012.     

The contribution of timescales to the temperature response of climate models

Both the magnitude and timescale of climate change in response to anthropogenic forcing are important consideration in climate change decision making. Using a familiar, yet simple global energy balance model combined with a novel method for estimating the amount of gain in the global surface temperature response to radiative forcing associated with timescales in the range 10⁰?C10³?years we show that the introduction of large-scale circulation such as meridional overturning leads to the emergence of discrete gain?Ctimescale relationships in the dynamics of this model. This same feature is found in the response of both an intermediate complexity and two atmosphere?Cocean general circulation models run to equilibrium. As a result of this emergent property of climate models, it is possible to offer credible partitioning of the full equilibrium gain of these models, and hence their equilibrium climate sensitivity, between two discrete timescales; one decadal associated with near surface ocean heat equilibration; and one centennial associated with deep ocean heat equilibration. Timescales of approximately 20 and 700?years with a 60:40 partitioning of the equilibrium gain are found for the models analysed here. A re-analysis of the emulation results of 19 AOGCMs presented by Meinshausen et al. (Atmos Chem Phys Discuss 8:6153?C6272, 2008) indicates timescales of 20 and 580?years with an approximate 50:50 partition of the equilibrium gain between the two. This suggests near equal importance of both short and long timescales in determining equilibrium climate sensitivity.     

Non-isothermal scavenging of highly soluble gaseous pollutants by rain in the atmosphere with non-uniform vertical concentration and temperature distributions

We suggest a non-isothermal one-dimensional model of precipitation scavenging of highly soluble gaseous pollutants in inhomogeneous atmosphere. When gradients of soluble trace gases’ concentrations and temperature in the atmosphere are small, scavenging of gaseous pollutants is governed by two linear wave equations that describe propagation of a scavenging and temperature waves in one direction. If wash-down front velocity is much larger than the velocity of the temperature front, scavenging is determined by propagating scavenging front in the atmosphere with inhomogeneous temperature distribution. We solved the derived equation by the method of characteristics and determined scavenging coefficient and the rates of precipitation scavenging for wet removal of sulfur dioxide using measured initial distributions of trace gases and temperature in the atmosphere. It is shown that in the case of exponential initial distribution of soluble trace gases and linear temperature distribution in the atmosphere, scavenging coefficient in the region between the ground and the position of a scavenging front is proportional to rainfall rate, solubility parameter in the under-cloud region, adjacent to a bottom of a cloud and to the growth constant in the formula for the initial profile of a soluble trace gas in the atmosphere. The derived formula yields the same value of scavenging coefficient for sulfur dioxide scavenging by rain as field estimates presented by McMahon and Denison (Atmos Environ 13:571–585, 1979). It is demonstrated that in the case when the altitude variation of temperature in the atmosphere is determined by the environmental lapse rate, scavenging coefficient increases with height in the region between the scavenging front and the ground. In the case when altitude temperature variation in the atmosphere is determined by temperature inversion, scavenging coefficient decreases with height in a region between the scavenging front and the ground. Theoretical predictions of the value of the scavenging coefficient for sulfur dioxide washout by rain and of the dependence of the magnitude of the scavenging coefficient on rain intensity are in good agreements with the atmospheric measurements of Martin (Atmos Environ 18:1955–1961, 1984).     

Monsoon intraseasonal oscillation and land�Catmosphere interaction in an idealized model

A simple coupled model is used in a zonally-symmetric configuration to investigate the effect of land?Catmosphere coupling on the Asian monsoon intraseasonal oscillation. The atmospheric model is a version of the Quasi-equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature. The land model is the simple one-layer model SLand. The complete nonlinear version and a linear version of the model are used to understand how land?Catmosphere interaction influences the northward-propagating intraseasonal oscillation that has been documented in the atmospheric model (Bellon and Sobel in J Geophys Res 113, 2008a, J Atmos Sci 65:470?C489, 2008b). Our results show that this interaction damps the intraseasonal variability in most cases. The small heat capacity of land surfaces is the main factor that intervenes directly in the dynamics of the intraseasonal oscillation and explains the damping of intraseasonal variability. But in a few peculiar cases, the small heat capacity of land can also cause a strong interaction between the intraseasonal oscillation and the mean state via the nonlinearity of precipitation, that enhances the monsoon intraseasonal variability. High land albedo indirectly influences the intraseasonal variability by setting the seasonal mean circulation to conditions unfavorable for the monsoon intraseasonal oscillation.     

Amendment to “Analytical Solution for the Convectively-Mixed Atmospheric Boundary Layer”: Inclusion of Subsidence

In Ouwersloot and Vilà-Guerau de Arellano (Boundary-Layer Meteorol. doi:10.1007/s10546-013-9816-z, 2013, this issue), the analytical solutions for the boundary-layer height and scalar evolutions are derived for the convective boundary layer, based on the prognostic equations of mixed-layer slab models without taking subsidence into account. Here, we include and quantify the added effect of subsidence if the subsidence velocity scales linearly with height throughout the atmosphere. This enables analytical analyses for a wider range of observational cases. As a demonstration, the sensitivity of the boundary-layer height and the potential temperature jump to subsidence and the free tropospheric stability is graphically presented. The new relations show the importance of the temporal distribution of the surface buoyancy flux in determining the evolution if there is subsidence.     

上海徐家汇地区台风“灿鸿”与良态风风速特征

上海徐家汇地区建筑分布密集且高低不一,是典型的具有非均一下垫面特征的城市地貌。本文利用该地区的地理信息研究了粗糙度长度的分布规律,并基于80m高度的风速实测数据,对台风"灿鸿"和良态风作用下的平均风速、湍流强度、阵风因子等参数与粗糙度长度之间的关系进行了分析。结果表明:不同风向角对应的计算扇区内建筑物高度、分布密度的差别导致了粗糙度长度值随风向角发生明显变化,但是变化幅度随着计算扇区的增大而减小;台风"灿鸿"作用下的平均风速最大值大于良态风,两者对应的粗糙度长度变化范围差别甚微,但是台风作用下的粗糙度长度中位数较小且分布相对集中;台风"灿鸿"作用下各向湍流强度均随着平均风速的增加呈明显的减小趋势,但不随粗糙度长度变化;良态风作用下,各向湍流强度不随平均风速变化,而随着粗糙度长度的增加而增加;台风"灿鸿"和良态风作用下,各向阵风因子均随湍流强度的增加而增大,但前者作用下的阵风因子略大于后者。     

Structure Inclination Angles in the Convective Atmospheric Surface Layer

Two-point correlations of the fluctuating streamwise velocity are examined in the atmospheric surface layer over the salt flats of Utah’s western desert, and corresponding structure inclination angles are obtained for neutral, stable and unstable conditions. The neutral surface-layer results supplement evidence for the invariance of the inclination angle given in Marusic and Heuer (Phys Rev Lett 99:114504, 2007). In an extension of those results it is found that the inclination angle changes drastically under different stability conditions in the surface layer, varying systematically with the Monin–Obukhov stability parameter in the unstable regime. The variation is parametrized and subsequently can be used to improve existing near-wall models in the large-eddy simulation of the atmospheric surface layer.