A planetary boundary-layer numerical model containing third-order derivative terms

In this paper, the third-order derivative of velocity with respect to height is included in the traditional motion equations of the neutral PBL. The nonlinear equations are solved numerically to obtain the vertical distribution of wind in the PBL and some PBL characteristic parameters. Reasonable simulations of the Leipzig wind profile using these parameters show the success of this kind of nonlocal closure in a real PBL simulation.     

A variable K planetary boundary-layer model

The steady-state, homogeneous and barotropic equations of motion within the planetary boundary layer are solved with the assumption that the coefficient of eddy viscosity varies as K(Z) = K _O(1–Z/h) ^p , where h is the height of the bounday layer and p is a parameter which depends on atmospheric stability. The solutions compare favourably with observed velocity profiles based on the Wangara data.     

Incorporation of planetary boundary layer dynamics in a numerical model of long-range air-pollutant transport

In this paper, measurements of the first 150 m of the atmospheric boundary layer obtained by a high-frequency acoustic mini-sounder are compared with measurements obtained by a full complement of instruments including sonic anemometers mounted on the Boulder Atmospheric Observatory tower. The acoustic mini-sounder, starting as low as 6 m from the ground, measures in the monostatic mode the profiles of the vertical wind speed, w, and of the temperature structure parameter, C _T ² with enhanced height resolution of the order of 1 m and time resolution of the order of 30 s. The results of the comparison show that the high-frequency mini-sounder is an effective atmospheric boundary-layer profiler that is also portable and relatively inexpensive. Measurements of the spectrum of C _T ² are presented that provide information on the local isotropy of the temperature field. Statistics of the variability of C _T ² in both stable and unstable conditions are also given. The sounder's capabilities are further demonstrated by some detailed observations of the structure and time evolution of a thermal plume root at noon and of a nocturnal, stably stratified layer in which a dynamic instability develops. The plume starts at a height of less than 5 m, possesses substantial internal structure, and includes vertical velocities in excess of 2 m s^-1.     

Eddy exchange coefficients in numerical models of the planetary boundary layer

Specification of the eddy exchange coefficients is perhaps one of the most difficult problems in the numerical modeling of the planetary boundary layer. These coefficients have been computed from finite-difference analogs to analytical expressions associated with surface boundary-layer similarity theory, which is based on observations in an equilibrium surface layer. This procedure leads to erroneous results in the region above the surface layer and in a non-equilibrium surface layer. In addition, differencing problems arise in regions of small vertical wind shear. A new turbulence transport model has been obtained through the closure procedures for the transport equations of the Reynolds stress and the turbulent length scale. The new approach could be used to calculate Reynolds stresses and eddy exchange coefficients throughout a non-neutral planetary boundary layer under non-equilibrium conditions.     

Some numerical urban boundary-layer studies

Two types of models, describing respectively the thermal and the dynamic structure of the urban boundary layer are presented. The influence of density and height of urban buildings, urban traffic, man-made heat flux, changes of albedo and existence of an aerosol layer are tested. The models give a possibility of explaining the influence of selected factors on the atmospheric state over an urban area.On sabbatical at Meteorological Department, University Uppsala, Sweden.     

A method for solving the planetary boundary-layer equations

A method for solving the nonlinear three-dimensional steady-state equations which govern planetary boundary layer flow is described. The method is applicable to air motions over terrain with horizontally varying surface roughness, temperature and moisture. It can also be applied to a physical system consisting of the air and the sea (or earth) boundary layers taken together. Examples of calculations for selected cases of terrain variations are presented. Convergence of the method has been assessed by determining the degree to which the solutions satisfy the set of equations.The results of the calculations show that the method produces qualitatively realistic distributions of the different meteorological variables.Contribution No. 1251 from the University of Miami, Rosenstiel School of Marine and Atmospheric Sciences.     

Diurnal variations in planetary boundary-layer winds over land

Windsonde data gathered over a nine-year period at three stations in the Southeast U.S. are stratified by season and by time of observation to provide average profiles depicting the diurnal variations in low-level winds. Significant variations are found (especially during the summer months) in wind speed, angle between wind direction and isobars, and the various terms of the kinetic energy budget equation. A qualitative model of the diurnal variations in planetary boundary-layer winds (over land) is developed. From a thesis submitted to the Graduate Faculty of Colorado State University in partial fulfillment of the requirements for the degree, Doctor of Philosophy.     

智能网格SCMOC及多模式降水预报对比

以三源融合网格实况降水分析资料CMPAS为参照,基于二分法经典检验、预报评分综合图和面向对象MODE检验等方法,对比分析2021年智能网格预报SCMOC以及ECMWF全球、CMA-Meso中尺度模式在秦岭及周边地区的降水预报表现,主要结论如下：1)ECMWF能够很好地刻画日平均降水量、日降水量标准差以及地形影响下降水量、降水频次的空间分布特征,但对于0.1 mm以上量级的降水预报频次远高于观测,暴雨预报频次低于观测,SCMOC、CMA-Meso日降水量大于等于0.1 mm的降水频次和暴雨频次预报更好;SCMOC不足在于降水的空间精细分布特征描述能力相对较弱。2)ECMWF预报的大于等于0.1 mm降水频次日峰值出现时间整体较观测偏早3 h左右,CMA-Meso、SCMOC与观测总体吻合较好。3)三种产品24 h降水量大于等于0.1 mm的TS(Threat Score)评分数值上基本一致,但降水预报表现的特征显著不同,SCMOC成功率高、命中率低,漏报多、空报少,ECMWF、CMA-Meso则相反;24 h、3 h大雨以上量级降水SCMOC的TS评分、成功率、命中率一致优于其他两种产品...     

The influence of the earth's rotation on planetary boundary-layer turbulence

The planetary boundary layer (PBL) differs from other simple boundary layers in that it forms on the earth's rotating surface. While the effect of the earth's rotation on the mean wind vector of the PBL is well known, the rotational influence on PBL turbulence is not yet established. In the present work, the latter effect is investigated using numerical models that account for the influence of the earth's rotation on the turbulence. It is found that the earth's rotational influence on PBL turbulence is negligible, and therefore does not need to be included in turbulence models used to simulate PBL flows.     

A subgrid-scale model for large-eddy simulation of planetary boundary-layer flows

A long-standing problem in large-eddy simulations (LES) of the planetary boundary layer (PBL) is that the mean wind and temperature profiles differ from the Monin-Obukhov similarity forms in the surface layer. This shortcoming of LES has been attributed to poor grid resolution and inadequate sub-grid-scale (SGS) modeling. We study this deficiency in PBL LES solutions calculated over a range of shear and buoyancy forcing conditions. The discrepancy from similarity forms becomes larger with increasing shear and smaller buoyancy forcing, and persists even with substantial horizontal grid refinement. With strong buoyancy forcing, however, the error is negligible.In order to achieve better agreement between LES and similarity forms in the surface layer, a two-part SGS eddy-viscosity model is proposed. The model preserves the usual SGS turbulent kinetic energy formulation for the SGS eddy viscosity, but it explicitly includes a contribution from the mean flow and a reduction of the contributions from the turbulent fluctuations near the surface. Solutions with the new model yield increased fluctuation amplitudes near the surface and better correspondence with similarity forms out to a distance of 0.1–0.2 times the PBL depth, i.e., a typical surface-layer depth. These results are also found to be independent of grid anisotropy. The new model is simple to implement and computationally inexpensive.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Near-surface wind estimates using statistics from a planetary boundary-layer model

This paper shows the possibilities of a procedure for estimating near-surface wind statistics, by means of the numerical integration of a simple boundary-layer model with a second-order turbulent closure. Standard and easily available synoptic data are used as initial and boundary conditions. The development of this methodology is impelled by increasing requirements of a quick and precise knowledge of the wind characteristics in many regions of South America, which confronts the serious limitation of a reduced number of extended observational series, scattered over a vast continent. In order to evaluate the methodology, near-surface wind statistics from observed data at two locations are compared with model output statistics. Relative errors are about 0.2 for daily mean velocities and about 0.1 for weekly mean velocities, observed and computed time series being highly correlated in both cases. Calculated frequency distribution of wind directions is in good agreement with the observed one, and the absolute mean error in the daily mean wind direction is about 20 deg. Even though a wide variety of large-scale synoptic situations has been indirectly considered through boundary conditions, basic model output statistics resemble fairly well those observed at different levels between the surface and 100 m.     

数值预报AI气象大模型国际发展动态研究

数值预报是研究地球系统的重要工具,有助于加深科学家对大气、海洋、气候和环境等复杂系统之间相互作用和变化过程的理解,在防灾减灾、气候变化和环境治理等方面发挥着不可或缺的作用。随着模式复杂度和分辨率的提高,传统数值模式在气候变化研究和气候预测方面取得了迅速的进展,但也面临一些挑战,需要得到数据同化、集合耦合、高性能计算和不确定性分析等多方面的支持。而近年来,“AI+气象”的交叉研究在气象领域引起了广泛关注。基于多种深度学习架构的人工智能大模型,依托强大的计算资源和海量的数据进行训练,能够以新的科学范式进行高效数值预报。气象大模型不断涌现,一些科技公司如华为、英伟达、DeepMind、谷歌、微软等,以及国内外高校如清华大学、复旦大学、密歇根大学、莱斯大学等发布了多个涵盖临近预报、短时预报、中期预报和延伸期预报等不同领域的气象大模型。这标志着人工智能与气象领域的交叉融合已经达到新的高度。尽管气象大模型在现阶段取得了较大突破,但其发展仍然面临弱可解释性、泛化能力不足、极端事件预报强度偏低、智能预报结果过平滑、深度学习框架能力需要拓展等诸多挑战。     

A planetary boundary-layer study in the Mackenzie Valley,Canada

Detailed wind velocity profiles were obtained by means of a rocket-sonde technique to a height of about 700 m at a site in the Canadian Northwest Territories. Less detailed temperature observations were also made using a balloon sonde. The site was some 100 km east of the easternmost range of the Rocky Mountains. The observations took place in mid-February when the overall atmospheric static stability was considerable. The results showed the presence of an arctic, atmospheric ‘thermocline’ some 500 m above ground, which sloped up or down considerably, with the generators of isothermal surfaces usually parallel to the nearby mountains, in the manner of upwelled or downwelled thermoclines in the ocean near shore. There was often strong baroclinic flow parallel to the mountain range. Noticeable frictional effects were confined to a near-ground layer always less than 100 m and mostly no more than 10 m in height. An Ekman-type boundary layer could only be identified in about one-third of the velocity profiles. The non-dimensionalized depth coefficient of such layers was close to 0.1, the geostrophic drag coefficient about 2.5×10^?4.     

WRF ARW and CHIMERE models for numerical forecasting of surface ozone concentration

Results of joint calculations with meteorological WRF ARW model and chemistry transport CHIMERE model are considered as a basis of the modern system of the air quality assessment and forecasting. The system was designed in the Russian Hydrometeorological Center and Institute of Applied Physics of the Russian Academy of Sciences. Detailed prognostic information about the atmosphere state provided by the WRF ARW was used in the CHIMERE model for describing the air mass transport processes, chemical transformation, and pollution deposition. Results of retrieval and forecast of surface ozone concentration as one of main air quality indicators are under consideration. Calculations of ozone concentrations for different configurations of a prognostic system differ in resolution of model grid and in the way the boundary conditions are prescribed.     

The use of mesoscale numerical models to assess wind distribution and boundary-layer structure in complex terrain

Mesoscale models which can be used to assess wind and turbulent structure in complex terrain are overviewed. The different types of models — diagnostic and prognostic are discussed and the significant physical processes which each can handle realistically are reviewed. Examples of specific applications of these models are presented.     

Numerical studies of neutrally stratified planetary boundary-layer flow above gentle topography

A numerical model of planetary boundary-layer flow above two-dimensional gentle topography is developed as an extension of the surface-layer model described by Taylor (1977). Comparisons are made with surface-layer predictions for flow over Gaussian hills; and the flow at various angles above hills, valleys and escarpments is modelled. Some simple case studies of the influence of gentle two-dimensional topography on pollutant dispersion are made which indicate relatively minor effects on surface pollutant concentrations in comparison with results for dispersion above a plane surface.     

A grid nesting method for large-eddy simulation of planetary boundary-layer flows

A method for performing nested grid calculations with a large-eddy simulation code is described. A common numerical method is used for all meshes, and the grid architecture consists of a single outer or coarse grid, and nested or fine grids, which overlap in some common region. Inter-grid communication matches the velocity, pressure and potential temperature fields in the overlap region. Resolved and sub-grid scale (SGS) turbulent fluxes and kinetic energy on the fine grid are averaged to the coarse grid using a conservation rule equivalent to Germano's identity used to develop dynamic SGS models.Simulations of a slightly convective, strong shear planetary boundary layer were carried out with varying surface-layer resolutions. Grid refinements in the (x, y, z) directions of up to (5, 5, 2) times were employed. Two-way interaction solutions on the coarse and fine meshes are successfully matched in the overlap region on an instantaneous basis, and the turbulent motions on the fine grid blend smoothly into the coarse grid across the grid interface. With surface-layer grid nesting, significant increases in resolved eddy fluxes and variances are found. The energy-scale content of the vertical velocity, and hence vertical turbulent fluxes, appear to be most influenced by increased grid resolution. Vertical velocity spectra show that the dominant scale shifts towards higher wavenumbers (smaller scales) and the magnitude of the peak energy is increased by more than a factor of 3 with finer resolution. Outside of the nested region the average heat and momentum fluxes and spectra are slightly influenced by the fine resolution in the surface layer. From these results we conclude that fine resolution is required to resolve the details of the turbulent motions in the surface layer. At the same time, however, increased resolution in the surface layer does not appreciably alter the ensemble statistics of the resolved and SGS motions outside of the nested region.     

A one-dimensional photochemical model of the troposphere with planetary boundary-layer parameterization

The results from a one-dimensional photochemical model of the troposphere representative of summertime conditions at Northern Hemisphere mid-latitudes are presented. A parameterization of mixing processes within the planetary boundary layer (PBL) has been incorporated into the model for both the daytime convective PBL and the formation of the nocturnal PBL. One result of the parameterized PBL is that the concentrations of some trace species in the free troposphere are 20–30% higher than when mixing processes are described by a vertical eddy diffusion coefficient which is held constant with respect to height and time.The calculations indicate that the lifetime of the oxides of nitrogen (NO _x =NO+NO₂) against photochemical conversion to nitric acid (HNO₃) during summertime conditions is on the order of 6 h. This lifetime is short enough to deplete most of the NO _x in the PBL, resulting in the finding that other reactive nitrogen species (HNO₃ and peroxyacetyl nitrate) are more abundant than NO _x throughout the free troposphere, even though NO _x is the most abundant reactive nitrogen species at the surface. The effects of the inclusion of anthropogenic nonmethane hydrocarbon (NMHC) chemistry are also discussed. The inclusion of NMHC chemistry has a pronounced effect on the photochemistry of tropospheric oxone and increases thein situ column production by more than 30%.     

A numerical simulation of boundary-layer flows near shelterbelts

We have developed a shelterbelt boundary-layer numerical model to study the patterns and dynamic processes relating to flow interaction with shelterbelts. The model simulates characteristics of all three zones of airflow passing over and through shelterbelts: the windward windspeed-reduction zone, the overspeeding zone above the shelterbelt, and the leeward windspeed-reduction zone. Locations of the maximum windspeed reduction and recirculation zone, as well as the leeward windspeed-recovery rate are well simulated by the model. Where comparisons with field measurements and wind-tunnel experiments were possible, the model demonstrated good performance for flows over and through shelters ranging from almost completely open to almost solid. The dynamic pressure resulting from the convergence and divergence of the flow field alters the perturbation pressure field. The disturbed pressure controls not only the formation of the separated flow but also the location of maximum windspeed reduction, streamline curvature, speed-up over the shelterbelt, and leeward windspeed recovery rate. The interaction of pressure with the flow produces complex flow patterns, the characteristics of which are determined, to a great extent, by shelterbelt structure.