Baroclinic Planetary Boundary-Layer Model for Neutral and Stable Stratification Conditions

The temperature and wind profiles in abaroclinic atmospheric boundary layer (ABL) are investigated.Assuming stationary conditions, the turbulent state in the ABL forstable and neutral conditions is uniquely determined by the Rossbynumber, the external stratification parameter and two externalbaroclinic parameters. A simple two-layer baroclinic model isdeveloped. It consists of a surface layer (SL) and overlyingEkman-type layer. The system of dynamic and heat transfer equations isclosed using K-theory. In the SL the turbulent exchangecoefficient is consistent with the results of similarity theorywhile in the Ekman layer it is assumed constant. The universalfunctions in the resistance, heat and humidity transfer laws arededuced from the analytical solutions for the wind and temperatureprofiles. The solutions of the ABL resistance laws for theinternal ABL parameters, necessary for the calculations of the ABLprofiles, are approximated in terms of the external ABLparameters. Favourable agreement of model results with theavailable experimental data is demonstrated.     

Observations of atmospheric boundary layer evolution above the Gulf Stream frontal zone

Ship borne measurements of atmospheric boundary layer (ABL) parameters, sea-surface temperature and radar signals are analyzed to reveal the effects of the ABL transformation above the Gulf Stream temperature frontal zone. It was found that local changes in vertical gradients of wind speed and air temperature are well correlated with sub-mesoscale (~ 10 km) sea surface temperature variations. These effects are accompanied by appropriate variations in surface wind stresses that were identified from microwave backscatter.For steady atmospheric conditions the same effects were observed on spatial scales of 100 km, demonstrating positive radar signal contrast of the Gulf Stream warm waters with respect to surrounding Sargasso sea and shelf water areas. A simplified model of the ABL, accounting for an effect of spatial inhomogeneity by introducing an internal boundary layer, is used to analyze field observations. The model is able to reproduce both sub-mesoscale and mesoscale ABL evolution.     

农林复合带一维非静力大气边界层模式及其模拟分析

建立了一个农林复合带地区一维非静力大气边界层能量闭合模式，对1000m以下的大气边界层内的风、温、湿作了24h的预报，并对下垫面3种不同参数化方案(农作物、森林、无植被)的输出结果与实测值进行了分析和比较；同时通过敏感性试验，突出比较了农作物和森林下垫面对大气边界层垂直流场，湍流垂直交换和湍能的影响。结果表明，本模式能改善边界层风速、位温和湿度预报的模拟效果，下垫面植被对边界层气象要素大小和分布有显著的作用，对湍能垂直分布有一定影响。     

E − ε − 〈θ 2〉 turbulence closure model for an atmospheric boundary layer including the urban canopy

A modified three-parameter model of turbulence for a thermally stratified atmospheric boundary layer (ABL) is presented. The model is based on tensor-invariant parameterizations for the pressure–strain and pressure–temperature correlations that are more complete than the parameterizations used in the Mellor–Yamada model of level 3.0. The turbulent momentum and heat fluxes are calculated with explicit algebraic models obtained with the aid of symbol algebra from the transport equations for momentum and heat fluxes in the approximation of weakly equilibrium turbulence. The turbulent transport of heat and momentum fluxes is assumed to be negligibly small in this approximation. The three-parameter $E - \varepsilon - {\left\langle {\theta ^{2} } \right\rangle }$ model of thermally stratified turbulence is employed to obtain closed-form algebraic expressions for the fluxes. A computational test of a 24-h ABL evolution is implemented for an idealized two-dimensional region. Comparison of the computed results with the available observational data and other numerical models shows that the proposed model is able to reproduce both the most important structural features of the turbulence in an urban canopy layer near the urbanized ABL surface and the effect of urban roughness on a global structure of the fields of wind and temperature over a city. The results of the computational test for the new model indicate that the motion of air in the urban canopy layer is strongly influenced by mechanical factors (buildings) and thermal stratification.     

A moving grid finite-element model of the bulk properties of the atmospheric boundary layer

A moving-grid finite-element model has been developed to model numerically the vertically integrated properties of the atmospheric boundary layer (ABL) in one dimension. The model equations for mean wind velocity and potential temperature are combined with a surface energy budget and predictive equations for boundary-layer height to simulate both stable and unstable ABLs. The nodal position defining the top of the boundary layer is one of the model unknowns and is determined by boundary-layer dynamics. The finite-element method, being an integral method, has advantages of accurate representation of both bulk values and their vertical derivatives, the latter being essential properties of the nocturnal boundary layer. Compared with observations and results of other models, the present model predicts bulk properties very well while retaining a simple and economical form.Journal Paper No. J-12996 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project No. 2779.     

Observations of Coherent Turbulence Structures in the Near-Neutral Atmospheric Boundary Layer

Turbulence structures of high Reynolds number flow in the near-neutral atmospheric boundary layer (ABL) are investigated based on observations at Shionomisaki and Shigaraki, Japan. A Doppler sodar measured the vertical profiles of winds in the ABL. Using the integral wavelet transform for the time series of surface wind data, the pattern of a descending high-speed structure with large vertical extent (from the surface to more than 200-m level) is depicted from the Doppler sodar data. Essentially this structure is a specific type of coherent structure that has been previously shown in experiments on turbulent boundary-layer flows. Large-scale high-speed structures in the ABL are extracted using a long time scale (240 s) for the wavelet transform. The non-dimensional interval of time between structures is evaluated as 3.0–6.2 in most cases. These structures make a large contribution to downward momentum transfer in the surface layer. Quadrant analyses of the turbulent motion measured by the sonic anemometer (20-m height) suggest that the sweep motion (high-speed downward motion) plays a substantial role in the downward momentum transfer. In general, the contribution of sweep motions to the momentum flux is nearly equal to that of ejection motions (low-speed upward motions). This contribution of sweep motions is related to the large-scale high-speed structures.     

Atmospheric profiling with the UAS SUMO: a new perspective for the evaluation of fine-scale atmospheric models

For the first time, unmanned aerial system measurements collected by the small unmanned meteorological observer (SUMO) are used to evaluate atmospheric boundary layer (ABL) parameterization schemes embedded in the Advanced Weather Research and Forecasting model (AR-WRF). Observation sites were located in the vicinity of the almost idealized shaped mountain Hofsj?kull, Central Iceland. SUMO profiles provided temperature, relative humidity and wind data to maximum heights of 3?km above ground. Two cases are investigated, one with calm wind conditions and development of a convective ABL and one with moderate winds and gravity waves over Hofsj?kull. For the high-resolution simulation with AR-WRF, three two-way nested domains are chosen with a grid size of 9, 3 and 1?km. During its first meteorological test, SUMO has proved its great value for the investigation of the diurnal evolution of the ABL and the identification of mesoscale features residing above the ABL, such as subsidence.     

A Study of the Atmospheric Boundary Layer Structure During a Clear Day in the Arid Region of Northwest China

The local climate and atmospheric circulation pattern exert a clear influence on the atmospheric boundary layer （ABL） formation and development in Northwest China. In this paper, we use field observational data to analyze the distribution and characteristics of the ABL in the extremely arid desert in Dunhuang, Northwest China. These data show that the daytime convective boundary layer and night time stable boundary layer in this area extend to higher altitudes than in other areas. In the night time, the stable boundary layer exceeds 900 m in altitude and can sometimes peak at 1750 m, above which the residual layer may reach up to about 4000 m. The daytime convective boundary layer develops rapidly after entering the residual layer, and exceeds 4000 m in thickness. The results show that the deep convective boundary layer in the daytime is a pre-requisite for maintaining the deep residual mixed layer in the night time. Meanwhile, the deep residual mixed layer in the night time provides favorable thermal conditions for the development of the convective boundary layer in the daytime. The prolonged periods of clear weather that often occurs in this area allow the cumulative effect of the atmospheric residual layer to develop fully, which creates thermal conditions beneficial for the growth of the daytime convective boundary layer. At the same time, the land surface process and atmospheric motion within the surface layer in this area also provide helpful support for forming the particular structure of the thermal ABL. High surface temperature is clearly the powerful external thermal forcing for the deep convective boundary layer. Strong sensible heat flux in the surface layer provides the required energy. Highly convective atmosphere and strong turbulence provide the necessary dynamic conditions, and the accumulative effect of the residual layer provides a favorable thermal environment.     

Equilibrium Atmospheric Boundary-Layer Flow: Computational Fluid Dynamics Simulation with Balanced Forces

Forcing relationships in steady, neutrally stratified atmospheric boundary-layer (ABL) flow are thoroughly analyzed. The ABL flow can be viewed as balanced between a forcing and a drag term. The drag term results from turbulent stress divergence, and above the ABL, both the drag and the forcing terms vanish. In computational wind engineering applications, the ABL flow is simulated not by directly specifying a forcing term in the ABL but by specifying boundary conditions for the simulation domain. Usually, these include the inflow boundary and the top boundary conditions. This ‘boundary-driven’ ABL flow is dynamically different from its real counterpart, and this is the major reason that the simulated boundary-driven ABL flow does not maintain horizontal homogeneity. Here, first a dynamical approach is proposed to develop a neutrally stratified equilibrium ABL flow. Computational fluid dynamics (CFD) software (Fluent 6.3) with the standard \(k\) – \(\varepsilon \) turbulence model is employed, and by applying a driving force profile, steady equilibrium ABL flows are simulated by the model. Profiles of wind speed and turbulent kinetic energy (TKE) derived using this approach are reasonable in comparison with the conventional logarithmic law and with observational data respectively. Secondly, the equilibrium ABL profiles apply as inflow conditions to simulate the boundary-driven ABL flow. Simulated properties between the inlet and the outlet sections across a fetch of 10 km are compared. Although profiles of wind speed, TKE, and its dissipation rate are consistently satisfactory under higher wind conditions, a deviation of TKE and its dissipation rate between the inlet and outlet are apparent (7–8 %) under lower wind-speed conditions (2 m s \(^{-1}\) at 10 m). Furthermore, the simulated surface stress systematically decreases in the downwind direction. A redistribution of the pressure field is also found in the simulation domain, which provides a different driving pattern from the realistic case in the ABL.     

A Simple Parametrization for the Concentration Variance Dissipation in a Lagrangian Single-Particle Model

A new quasi-analytical mixed-layer model is formulated describing the evolution of the convective atmospheric boundary layer (ABL) during cold-air outbreaks (CAO) over polar oceans downstream of the marginal sea-ice zones. The new model is superior to previous ones since it predicts not only temperature and mixed-layer height but also the height-averaged horizontal wind components. Results of the mixed-layer model are compared with dropsonde and aircraft observations carried out during several CAOs over the Fram Strait and also with results of a 3D non-hydrostatic (NH3D) model. It is shown that the mixed-layer model reproduces well the observed ABL height, temperature, low-level baroclinicity and its influence on the ABL wind speed. The mixed-layer model underestimates the observed ABL temperature only by about 10 %, most likely due to the neglect of condensation and subsidence. The comparison of the mixed-layer and NH3D model results shows good agreement with respect to wind speed including the formation of wind-speed maxima close to the ice edge. It is concluded that baroclinicity within the ABL governs the structure of the wind field while the baroclinicity above the ABL is important in reproducing the wind speed. It is shown that the baroclinicity in the ABL is strongest close to the ice edge and slowly decays further downwind. Analytical solutions demonstrate that the \(\mathrm{e}\)-folding distance of this decay is the same as for the decay of the difference between the surface temperature of open water and of the mixed-layer temperature. This distance characterizing cold-air mass transformation ranges from 450 to 850 km for high-latitude CAOs.     

‘A Case Study on a Polar Cold Air Outbreak over Fram Strait using a Mesoscale Weather Prediction Model’

The mesoscale weather prediction model ’Lokal-Modell’ (LM) of the Deutscher Wetterdienst is applied to the situation of an Arctic cold air outbreak in the Fram Strait region in April 1998. Observations are available from a flight along 5⁰E carried out during the ARTIST campaign. Initial and time-dependent boundary data for the simulation are taken from a larger scale operational model system. Using the standard configuration of LM, the simulation reproduced the propagation of cold air and the characteristic structure of the atmospheric boundary layer (ABL) in fair agreement with the observations. However, a detailed comparison revealed three basic problems. Firstly, there is evidence that the available data on sea-ice conditions were insufficient approximations to the true state for several reasons. A modification of the sea-ice data towards observations revealed that parts of the discrepancies were due to the original sea-ice data. Secondly, a control run with the model in its standard configuration shows an insufficient warming of the ABL downstream of the ice edge due to underestimation of surface heat fluxes. A simple modification of the approach for the scalar roughness length resulted in the strongest benefit, while comparative studies showed only a slight sensitivity to different types of parametrisation of turbulent mixing or the inclusion of an additional moist convection parametrisation. Thirdly, in all the simulations the deepening of the convective ABL downstream of the ice edge is weaker than observed. This may be partly due to the thermal stratification above the ABL in the analysis data, which is more stable than observed; but it may also be a hint to the fact that processes near the inversion are insufficiently parametrised in mesoscale models with resolutions as used in LM. The simulated cloud layer in the convective ABL is similar to that observed with respect to condensate content, a sharply defined cloud top, a diffuse lower bound, and continuous light precipitation.     

Numerical simulation of the boundary layer above waves

A new approach to investigations of the structure of the boundary layer above waves is discussed. The approach is based on direct numerical simulation of wave motions in the boundary layer produced by a moving curved surface. Model equations are derived, which are the Reynolds equations in a curvilinear nonstationary system of co-ordinates, evolution equations for turbulent kinetic energy, and Kolmogorov's approximate similarity formulae relating the coefficient of turbulent viscosity to the dissipation of turbulent energy; the length scale is assumed to grow linearly with increasing distance from the surface. Principles of constructing the model numerical scheme are described. Results are given of modelling the structure of the boundary layer above a nonsteady surface, which, in a general case, is a superposition of progressive waves with assigned dispersion relations and amplitudes. Mechanisms of energy and momentum transfer to the surface, effects of density stratification and energy structure in the boundary layer are studied. Merits and demerits of the approach are discussed.     

复杂地形城市冬季边界层对气溶胶辐射效应的响应

作者着眼于城市气溶胶辐射效应与大气边界层的相互作用问题,针对地形复杂的兰州市及周边地区,开发应用了WRF(Weather Research and Forecasting,天气研究和预报)模式,使之与包含了大气气溶胶辐射效应和气溶胶粒子扩散的综合大气边界层数值模式嵌套起来.通过个例分析,揭示了冬季气溶胶辐射效应对边界层结构的定量影响.主要特征为夜间气溶胶的长波辐射效应使地面附近的气温增高,增温幅度为0.1～0.3 K/h,使低空(25～300 m)大气层冷却,降温幅度为0.08～0.15 K/h,风速在150 m以下减小;白天气溶胶的短波辐射效应使地面层内明显增温,1 h内升温约0.5 K,增温最大值在混合层顶500～600 m高度.受增温影响,垂直风场和水平风场随之调整,风速在450 m以下增大约0.1 m/s左右,而在450 m以上风速减小0.1 m/s左右.     

Large-Eddy Simulation of Coherent Flow Structures within a Cubical Canopy

Instantaneous flow structures “within” a cubical canopy are investigated via large-eddy simulation. The main topics of interest are, (1) large-scale coherent flow structures within a cubical canopy, (2) how the structures are coupled with the turbulent organized structures (TOS) above them, and (3) the classification and quantification of representative instantaneous flow patterns within a street canyon in relation to the coherent structures. We use a large numerical domain (2,560 m × 2,560 m × 1,710 m) with a fine spatial resolution (2.5 m), thereby simulating a complete daytime atmospheric boundary layer (ABL), as well as explicitly resolving a regular array of cubes (40 m in height) at the surface. A typical urban ABL is numerically modelled. In this situation, the constant heat supply from roof and floor surfaces sustains a convective mixed layer as a whole, but strong wind shear near the canopy top maintains the surface layer nearly neutral. The results reveal large coherent structures in both the velocity and temperature fields “within” the canopy layer. These structures are much larger than the cubes, and their shapes and locations are shown to be closely related to the TOS above them. We classify the instantaneous flow patterns in a cavity, specifically focusing on two characteristic flow patterns: flushing and cavity-eddy events. Flushing indicates a strong upward motion, while a cavity eddy is characterized by a dominant vortical motion within a single cavity. Flushing is clearly correlated with the TOS above, occurring frequently beneath low-momentum streaks. The instantaneous momentum and heat transport within and above a cavity due to flushing and cavity-eddy events are also quantified.     

Subsidence fronts

Approximate analytical solution of the coupled momentum and heat transfer equations are obtained, using an integral method, for natural convection boundary layers which are produced by a step change of surface temperature. The accuracy of the laminar solution is shown to be good, especially at high Prandtl Number (Pr). The corresponding solutions for the turbulent fluid yield a model of a boundary layer in which the horizontal flow strengthens downstream and is fed by downward motion from above. We call this system, which could possibly simulate the initiation of a sea breeze circulation, a subsidence front. Interesting properties of the solution include the control of the boundary-layer development by the product (PrS), whereS is the slope of the boundary layer, and also the generation of log-linear velocity and temperature profiles by heat flux divergence.     

Modelling the One-Dimensional Stable Boundary Layer with an E−ℓ Turbulence Closure Scheme

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.     

Impact of Different Aerosols on the Evolution of the Atmospheric Boundary Layer

The present work analyzes the effect of aerosols on the evolution of the atmospheric boundary layer (ABL) over Shangdianzi in Beijing.A one-dimensional ABL model and a radiative transfer scheme are incorporated to develop the structure of the ABL.The diurnal variation of the atmospheric radiative budget,atmospheric heating rate,sensible and latent heat fluxes,surface and the 2 m air temperatures as well as the ABL height,and its perturbations due to the aerosols with different single-scattering albedo (SSA) are studied by comparing the aerosol-laden atmosphere to the clean atmosphere.The results show that the absorbing aerosols cause less reduction in surface evaporation relative to that by scatting aerosols,and both surface temperature and 2 m temperature decrease from the clean atmosphere to the aerosol-laden atmosphere.The greater the aerosol absorption,the more stable the surface layer.After 12:00 am,the 2 m temperature increases for strong absorption aerosols.In the meantime,there is a slight decrease in the 2 m temperature for purely scattering aerosols due to radiative cooling.The purely scattering aerosols decrease the ABL temperature and enhance the capping inversion,further reducing the ABL height.     

大气边界层的初值形成

采用850毫巴和地面的大尺度天气状况和物理特性作为外参数,将大气边界层下层的地表层进行特殊处理,考虑了层结的作用,其上分成等距15层,考虑地转民随高度线性变化。上下层耦合,建立一个定常平衡初值模型,并将所得的初值作为第一级近似,再利用地面、300、600、900、1500米五个高度测风资料进行动力调整,试图建立一个更加接近实际状态的边界层初值。 文中利用所建立的模型,对大气边界层的一些物理参数的作用进行了分析,并对动力调整过程以及垂直分层的选取等作了讨论。 最后例举华北地区1966年8月23日08时边界层的各层次风场和垂直运动,清楚地显示出边界层内的细致结构。     

The unique,well posed reduced system for atmospheric flows: Robustness in the presence of small scale surface irregularities

It is well known that the primitive equations (the atmospheric equations of motion under the additional assumption of hydrostatic equilibrium for large scale motions) are ill posed when used in a limited area on the globe. Yet the atmospheric equations of motion for large scale motions are essentially a hyperbolic system that with appropriate boundary conditions should lead to a well posed system in a limited area. This apparent paradox was resolved by Kreiss through the introduction of the mathematical Bounded Derivative Theory (BDT) for any symmetric hyperbolic system with multiple time scales (as is the case for the atmospheric equations of motion). The BDT uses norm estimation techniques from the mathematical theory of symmetric hyperbolic systems to prove that if the norms of the spatial and temporal derivatives of the ensuing solution are independent of the fast time scales (thus the concept of bounded derivatives), then the subsequent solution will only evolve on the advective space and time scales (slowly evolving in time in BDT parlance) for a period of time. The requirement that the norm of the time derivatives of the ensuing solution be independent of the fast time scales leads to a number of elliptic equations that must be satisfied by the initial conditions and ensuing solution. In the atmospheric case this results in a 2D elliptic equation for the pressure and a 3D equation for the vertical component of the velocity.Utilizing those constraints with an equation for the slowly evolving in time vertical component of vorticity leads to a single time scale (reduced) system that accurately describes the slowly evolving in time solution of the atmospheric equations and is automatically well posed for a limited area domain. The 3D elliptic equation for the vertical component of velocity is not sensitive to small scale perturbations at the lower boundary so the equation can be used all of the way to the surface in the reduced system, eliminating the discontinuity between the equations for the boundary layer and troposphere and the problem of unrealistic growth in the horizontal velocity near the surface in the hydrostatic system.     

Effect of Surface Heterogeneity on the Boundary-Layer Height: A Case Study at a Semi-Arid Forest

We investigate the effects of an isolated meso-\(\gamma \)-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: \(28~\text{ km }^2\), forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (\(600~\text {W~m}^{-2}\) in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.