Numerical Simulation of the Novaya Zemlya Bora

The development of the bora in case of strong southeastern wind in the area of Novaya Zemlya in the winter-spring of 2016 is simulated using the WRF-ARW numerical atmosphere circulation model with high spatial resolution. The features of wind speed and air temperature fields are considered which define the formation of the intensive near-surface flow, the bora, over the lee western slope of the mountain range. It is demonstrated that the bora development leads to the air temperature rise over the eastern part of the Barents Sea, to the increased surface heat fluxes, and to the formation of the cloudless zone over the sea westward of Novaya Zemlya. It was found that the main reason for the bora development is the high stability of the atmospheric boundary layer over the Kara Sea. It is shown that in case of western wind the Novaya Zemlya archipelago does not exert considerable influence on the air exchange in the Kara Sea area.     

Simulation of bora in Novorossiysk

The bora event observed in Novorossiysk on February 6–8, 2012 was analyzed using the WRF numerical model of regional atmospheric circulation. The main meteorological parameters are reproduced. It is demonstrated that the maximum wind speed is reached in the area of the lee slope directly over the bay. It is obtained that the bora development is accompanied by intensive waves associated with the flowing around coastal mountains whose regime changes in time. Computed and measured changes in the surface wind speed are in a good agreement.     

Similarity theory and resistance laws for the planetary boundary layer

Methods are developed for the determination of parameters of the atmospheric planetary boundary layer, within the framework of similarity theory based on the external parameters — wind velocity at the upper boundary of the layer, its thickness, air temperature difference between the upper and the lower boundaries, roughness of the underlying surface, and buoyancy forces. The form of the resistance laws is discussed. Determination of the thickness of the stationary and horizontally homogeneous (Ekman) boundary layer is analyzed and generalizations of the latter are suggested for non-stationary and inhomogeneous boundary layers.     

Gustiness of the Novorossiysk bora

The development of the Novorossiysk bora on February 5–6, 2010 is simulated with high spatial and temporal resolution using the regional model of atmospheric circulation. Considered are typical features of the wind speed and air temperature fields. Singled out are two regimes defining the type of the air flowing around the mountain ridge and the temporal variability (gustiness) of the surface wind speed.     

Numerical Simulation and Observational Analysis of the Bora of Pag’s Ribs

The severe bora case that lasted from 13 to 15 November 2004 has been selected for the analysis of the bora of Pag’s ribs, which occurs in the northern part of the eastern Adriatic coast over the Pag island area (Croatia). According to the measurements from automatic stations, the MM5 numerical model is successful in the 10-min mean wind speed prediction at 10-m height. The vertical analysis of the wind speed and potential temperature also gave satisfactory results. At the commencement of the bora the modelled wind had a magnitude of 20ms⁻¹ at 10-m height in the Pag island area, which sharply attenuated in the cross direction and to the open sea. In this way the model has proved successful in predicting the characteristics of the bora of Pag’s ribs. Potential vorticity (PV) at 600m has lower values within PV banners than during the developed bora. The consequence is a strong jet emanating from the nearest gap. The vertical cross-sections through the centre of the gap point out a permanent hydraulic-like flow. At the time of the bora of Pag’s ribs the highest modelled turbulent kinetic energy is found in the jump-like region above the inversion and within the boundary layer along the lower boundary, ranging from 6–8m² s⁻². It is concluded that the dissipation in the hydraulic jumps and the wave breaking regions are the reasons for PV generation.     

Testing the method for surface air temperature refinement based on a complex of models of the atmospheric boundary layer and local heat and water budgets

A method is considered for space-time refinement of surface air temperature obtained from the atmospheric general circulation model (GCM) of the Hydrometeorological Center of Russia over a limited area by means of use of a complex of the atmospheric boundary layer models and surface heat and water budget model. The latter describes all major processes of heat and water exchange at the underlying surface and within the soil and vegetation using 14 external parameters for different types of landscapes and soils. The side and upper boundary conditions for the local model are provided from the GCM model. Different statistical estimates of the modeling results show possibility and reliability of the refinement for time changes of the quantity under calculation as well as for development of averaged fields reflecting small-scale inhomogeneities of the landscape.     

Large-eddy simulation of turbulent flow above and within a forest

A large-eddy simulation has been performed of an atmospheric surface layer in which the lower third of the domain is occupied by a drag layer and heat sources to represent a forest. Subgridscale processes are treated using second-order closure techniques. Lateral boundaries are periodic, while the upper boundary is a frictionless fixed lid. Mean vertical profiles of wind velocity derived from the output are realistic in their shape and response to forest density. Similarly, vertical profiles of Reynolds stress, turbulent kinetic energy and velocity skewness match observations, at least in a qualitative sense. The limited vertical extent of the domain and the artificial upper boundary, however, cause some departures from measured turbulence profiles in real forests. Instantaneous turbulent velocity and scalar fields are presented which show some of the features obtained by tower instrumentation in the field and in wind tunnels, such as the vertical coherence of vertical velocity and the slope of structures revealed by temperature patterns.     

Wind flow over ridges in simulated atmospheric boundary layers

The flows over four two-dimensional triangular hills and three two-dimensional bell-shaped hills have been investigated in a simulated rural atmospheric boundary layer modelled to a scale of 1:300: Further measurements were made over two of the triangular hills in a simulated rural boundary layer of 1: 3000 scale and in a simulated urban boundary layer modelled to a scale of 1:400. The effect of the model hill surface roughness was also investigated. Flow measurements were restricted to the mean velocity U, RMS velocity fluctuations u and the energy spectra for the streamwise velocity component Measurements were made at a number of longitudinal positions in the approach flow, over the model hills and downstream of the model hills. For each model hill, the crest was the region of largest mean velocity and smallest velocity fluctuations. The largest mean velocities over the model hills occurred for hills of intermediate slope rather than for the steepest hills. A decrease in the scale of the simulated atmospheric boundary layer led to a reduction in the amplification factors at the hill crests, whereas an increase in the surface roughness of the approach flow resulted in increased amplification factors at the hill crests.     

Large-Eddy Simulation of Katabatic Flows

A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.     

Numerical model of a non-steady atmospheric planetary boundary layer,based on similarity theory

A numerical model of a non-stationary atmospheric planetary boundary layer (PEL) over a horizontally homogeneous flat surface is derived on the basis of similarity theory. The two most typical turbulence regimes are reproduced: one corresponding to a convectively growing PBL and another corresponding to a stably stratified quasi-equilibrium PBL. The PBL is treated as a unit. Hence, its evolution is characterized by temporal changes of intrinsic parameters, such as the PBL depth, the vertically averaged values of potential temperature, specific humidity and components of wind velocity, the near-surface values of heat, water vapor and momentum fluxes. The internal structure of the PBL is considered self-similar. This allows one to represent the interaction between the air flow and the underlying surface by means of universal heat/mass transfer and resistance laws. Numerical experiments on the diurnal variations of meteorological fields in the lower 2 km layer confirm the ability of the model to reproduce the main features of the phenomena, known from observations.On leave from: Insitute of Limnology, Academy of Sciences of the Russia, SU-196199, St. Petersburg, Russia.     

A nonstationary nocturnal drainage flow model

The evolution and structure of the steady state of an idealized nocturnal drainage flow over a large uniformly-sloping surface are studied using a nonstationary model with a height-dependent eddy diffusivity profile and a specified surface cooling rate. The predicted mean velocity and temperature profiles are compared with Prandtl's stationary analytical solutions based on the assumption of a constant eddy diffusivity in the drainage layer. The effects of important physical parameters, such as the slope angle, surface cooling, atmospheric stability, and surface roughness, on the steady drainage flow are investigated.Affiliated with Oak Ridge Associated Universities (ORAU).     

沙漠-绿洲大气边界层结构的数值模拟

利用美国NCAR新版中尺度MM5V3．6非静力平衡模式,采用三重嵌套的模拟方法,模拟研究了沙漠绿洲的环流及边界层特征。结果表明：在没有西风气流背景的影响下,绿洲沙漠环流和大气边界层结构是对称的。沙漠绿洲改变了原有沙漠地区环流结构及温、湿场的分布,绿洲上空大气下沉,沙漠上空大气上升,从而产生了绿洲上空大气冷干,沙漠上空大气暖湿的边界层特征。绿洲边缘形成了由干到湿的强湿度梯度带围绕着绿洲,起到了保护绿洲的作用。在有西风背景气流的影响下,绿洲沙漠环流和大气边界层结构是非对称的。但是,西风背景气流的存在可以破坏绿洲系统对称的环流结构,不利于绿洲系统的稳定发展。     

不稳定边界层下地形重力内波

水槽实验及线性理论研究表明,当低层大气处于近中性或不稳定时,如果地形引起的动力扰动足够强,地形扰动可在上部稳定层结中激发出重力内波,波动反过来影响低层流场,引起动量输送。低层大气处于近中性或不稳定时,地形波同样对大气运动可产生波阻,这应引起模式工作者的重视。最后讨论了大气粘性对中性或不稳定层结下地形波的影响。     

Similarity criteria for the application of fluid models to the study of air pollution meteorology

Similarity criteria for modeling atmospheric flows in air and water are reviewed. It is shown that five nondimensional parameters plus a set of nondimensional boundary conditions must be matched in model and prototype. The neglect of the Rossby number can lead to serious errors in modeling of diffusion in a prototype with a length scale greater than about five kilometers. The Reynolds number, the Peclet number and the Reynolds-Schmidt product criteria may be neglected if the model flow is of sufficiently high Reynolds number. The Froude number criterion appears to be the most important. The complete specification of boundary conditions is found to be nebulous, but is discussed in some detail. Over-roughening of the model surface may be necessary to satisfy a roughness Reynolds number criterion. Both air and water appear to be suitable fluids to use as modeling media.On assignment from the National Oceanic and Atmospheric Administration, United States Department of Commerce.     

Similarity of large Reynolds number boundary layers

Similarity will exist for both the mean velocity and the turbulent shear stress of a turbulent boundary layer if the following conditions are met: (1) The surface shear is constant. (2) The boundary layer thickness parameters vary linearly with downstream distance. (3) The flow Reynolds number approaches infinity.These conditions may be nearly obtained in atmospheric type boundary layers. Measurements are presented from the Colorado State University Atmospheric Simulation Wind Tunnel which demonstrate the similarity. The wind-tunnel Reynolds number is from one to two orders of magnitude smaller than those of an atmospheric boundary layer.     

Turbulent dissipation of cold air lake in a basin

Summary Problems of turbulent dissipation of a cold air lake (CAL) and the inversion layer bordering CAL on the upper boundary are presented and studied with the compound model. In wintertime such cold air lakes can persist for days even if rather strong winds are blowing above them. The required conditions for CAL dissipation are removed processes of its formation or maintenance, as well as a sufficiently strong invasion of turbulence in the inversion layer from above down-wards. By this, the inversion layer at first becomes stronger and dissipation is stopped, until the increase of turbulent kinetic energy of the upper flow enables further dissipation. Such turbulent dissipation process is shown by the model for typical conditions and for different initial values of the relevent variables.With 6 Figures     

Numerical Simulation Experiment of Land Surface Physical Processes and Local Climate Effect in Forest Underlying Surface

Based on the basic principles of atmospheric boundary layer and plant canopy micrometeorology, a forest underlying surface land surface physical process model and a two-dimensional atmospheric boundary layer numerical model are developed and numerical simulation experiments of biosphere and physiological processes of vegetation and soil volumetric water content have been done on land surface processes with local climate effect. The numerical simulation results are in good agreement with realistic observations, which can be used to obtain reasonable simulations for diurnal variations of canopy temperature, air temperature in canopy, ground surface temperature, and temporal and spatial distributions of potential temperature and vertical wind velocity as well as relative humidity and turbulence exchange coefficient over non-homogeneous underlying surfaces. It indicates that the model developed can be used to study the interaction between land surface process and atmospheric boundary layer over various underlying surfaces and can be extended to local climate studies. This work will settle a solid foundation for coupling climate models with the biosphere.     

A simple model of drainage flow on a slope

The concept of a cold air ‘Parcel’ is introduced for describing the bulk properties of drainage flow. By means of a model based on the momentum and sensible heat transports under calm conditions, the thickness h and velocity u of the Parcel are derived in simple forms. It is shown that h and u correspond to the inversion height and maximum velocity of actual drainage flow. The governing parameters for h and u are the length and vertical drop of the slope, potential temperature difference between the ambient atmosphere and the Parcel, aerodynamic condition of the slope surface expressed by the mean bulk coefficients, and ambient stability. The mean bulk coefficients depend on the roughness lengths for the velocity and potential temperature profiles and are decreasing functions of the slope length. The Parcel Model agrees qualitatively with Manins and Sawford's (1979) model under neutral ambient stratification. But agreement is not so good under stable conditions. The thickness and velocity of drainage flow predicted by the Parcel Model agree with observations on slopes several tens of meters to several hundred kilometers long.     

Wind-Tunnel Simulation of Weakly and Moderately Stable Atmospheric Boundary Layers

The simulation of horizontally homogeneous boundary layers that have characteristics of weakly and moderately stable atmospheric flow is investigated, where the well-established wind engineering practice of using ‘flow generators’ to provide a deep boundary layer is employed. Primary attention is given to the flow above the surface layer, in the absence of an overlying inversion, as assessed from first- and second-order moments of velocity and temperature. A uniform inlet temperature profile ahead of a deep layer, allowing initially neutral flow, results in the upper part of the boundary layer remaining neutral. A non-uniform inlet temperature profile is required but needs careful specification if odd characteristics are to be avoided, attributed to long-lasting effects inherent of stability, and to a reduced level of turbulent mixing. The first part of the wind-tunnel floor must not be cooled if turbulence quantities are to vary smoothly with height. Closely horizontally homogeneous flow is demonstrated, where profiles are comparable or closely comparable with atmospheric data in terms of local similarity and functions of normalized height. The ratio of boundary-layer height to surface Obukhov length, and the surface heat flux, are functions of the bulk Richardson number, independent of horizontal homogeneity. Surface heat flux rises to a maximum and then decreases.     

Random-walk model studies of the transport and diffusion of pollutants in katabatic flows

The flow and turbulence quantities governing dispersion in katabatic flows vary with both height and downslope distance. This variation cannot be accounted for in conventional plume dispersion models. In this study, three random-walk models of varying complexity are formulated to simulate dispersion in katabatic flows, and their strengths and weaknesses are discussed. The flow and turbulence parameters required by these models are determined from a high-resolution two-dimensional katabatic flow model based on a turbulent kinetic energy closure. Random-walk model calculations have been performed for several values of source height and slope angle to examine the influence of these parameters on dispersion. Finally, we simulated the perfluorocarbon and heavy methane tracer releases for Night 4 of the 1980 ASCOT field study over a nearly two-dimensional slope in Anderson Creek Valley, California. The observed peak concentrations are generally well-predicted. The effects of the pooling of the drainage air could not be taken into account in our katabatic flow model and, consequently, the predicted concentrations decay much more rapidly with time than the observed values.