A study of temperature fluctuations in the atmospheric boundary layer

The spectral density of temperature fluctuations in the boundary layer has been studied in the range 0.2 to 2 Hz. It is shown that the temperature spectrum is strongly variable from one minute to the next and that the spectral slope tends to increase with the standard deviation of temperature fluctuations and with the stability of the medium. It is shown that C _T ² values computed from short time series (30 s) tend to be smaller than the C _T ² values computed from several minutes of record.Formerly at Laboratoire de Météorologie Dynamique du C.N.R.S. (Paris).     

Temperature and humidity spectra in the atmospheric surface layer

Temperature and humidity spectra have been measured at 3 and 12 m above the ground, together with profiles of wind, temperature and humidity, and flux measurements. Both temperature and humidity spectra appear to follow Monin-Obukov similarity as well as Kolmogorov's prediction for the inertial subrange. The standard deviations of temperature and humidity fluctuations support Monin-Obukov similarity and the predictions of local free convection. The spectral constants for the inertial subrange have been estimated as 0.8 for temperature and 0.6 for humidity.     

Scaling the atmospheric boundary layer

We review scaling regimes of the idealized Atmospheric Boundary Layer. The main emphasis is given on recent findings for stable conditions. We present diagrams in which the scaling regimes are illustrated as a function of the major boundary-layer parameters. A discussion is given on the different properties of the scaling regimes in unstable and stable conditions.     

成都地区大气边界层逆温特征分析

针对污染气象条件对大气环境影响的问题,利用2010—2012年成都地区探空资料的温度数据,系统研究了成都地区逆温的结构及分布特征。结果表明:2010—2012年成都地区整体以贴地逆温出现频率最高、厚度最大及强度最强,其次为低悬逆温和高悬逆温。不同类型逆温出现的频率、厚度及强度也存在一定的季节差异,贴地逆温春季出现频率最高,厚度最大,其他各季差异不显著;冬季逆温最强,夏季最弱。3类逆温的日变化明显,08时逆温出现的频率和厚度普遍大于20时,且08时逆温强度大于20时。分析成都地区大气边界层逆温层特征,对了解成都地区污染物扩散规律具有重要的意义。     

Scale effects in wind tunnel modeling of an urban atmospheric boundary layer

Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations.     

Small-scale atmospheric structure deduced from measurements of temperature,humidity and refractive index

Measurements have been made with fast-response multi-channel temperature, humidity and refractive index sensors flown to 2000 m on a tethered balloon to investigate small-scale fluctuations important in radio-wave scattering, their relation to atmospheric parameters, and their spatial variation in both one and three dimensions. Data from the three types of sensors at one point were consistent for frequencies up to about 8 Hz. Power spectra of data at various heights were computed over 0.1 to 10 Hz and generally showed slopes (on a log-log plot) close to - 5/3 above 1 Hz but ranged from –1.5 to – 3.5 at lower frequencies; in this range (f < 1 Hz) slopes were close to – 5/3 for negative Richardson number (Ri), provided temperature gradients were steeper than –1.1 °C 100 m^–1 and wind shears > 1.4 x 10^–2 s^–1 approx. Steeper slopes were generally associated with stable atmospheric conditions but no precise relation to the above parameters was found. Spectral density was a maximum for Ri –0.75.Cross-correlations of 0.5 were frequently observed between sensors 1 m apart in orthogonal directions; in the vertical, examples of negative correlation of vapour pressure were occasionally found over this spacing. Using four sensors spaced in line over 9 m, cross-spectrum phase calculations of drift speeds were found to be consistent with measured wind speeds. The ratio of identification distance (coherence=0.6) to scale size of irregularities ranged from 0.25 to 0.5 with no apparent relation to height or meteorological parameters.     

On similarity in the atmospheric boundary layer

A similarity theory for the atmospheric boundary layer is presented. The Monin-Obukhov similarity theory for the surface layer is a particular case of this new theory, for the case of z 0. Universal functions which are in agreement with empirical data are obtained for the stable and convective regimes.On leave from Institute of Environmental Engineering, Warsaw Technical University, 00653 Warsaw, Poland. Present address, Department of Geological and Geophysical Sciences, University of Wisconsin, Milwaukee, WI 53201 U.S.A.     

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.     

大气边界层研究进展

基于2013—2015年6—8月“第三次青藏高原大气科学试验（TIPEX-Ⅲ）”和常规气象业务探空观测资料、欧洲中期天气预报中心（ECMWF）第5代再分析（ERA-5）数据以及“国际卫星云气候计划（ISCCP）”云量资料,采用统计分析和物理量诊断分析方法,研究了夏季青藏高原（简称高原）大气对流边界层高度东西差异对高原地区天气尺度环流的影响。结果表明：高原对流边界层高度东西差异表现出明显的日变化,且这种差异呈现西高东低的分布特征,主要由西部对流边界层高度明显增大所致。当中午对流边界层高度东西向差异增大时,午后地面虚位温6 h变差呈西部高、东部低的特征,且西部变化更明显,高原西部对流边界层内温度升高,东部略降低,并伴随着高原西部对流边界层内气压降低、高层升高且低压系统较浅薄,而东部低层气压升高;高原低层东高西低的气压场特征产生了异常东西向气压梯度,引起高原中部低层出现偏南风异常,伴随着西部的低层异常辐合和高层异常辐散;同时,浅薄的低压有助于当地低云发展。     

Current problems in the stratocumulus-topped atmospheric boundary layer

Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.     

Entrainment effects in the well-mixed atmospheric boundary layer

We discuss the structure and evolution of a cloud-free atmospheric boundary layer (ABL) during daytime over land, starting from a shallow ABL at sunrise and developing into a deep ABL with strong convection in the afternoon. The structure of the turbulence in the lower half of a convective ABL capped by an inversion is reasonably well understood. Less is known about the details of the turbulence in higher regions affected by entrainment, because of the difficulty in taking turbulence measurements there. For the evolution in time of the height of the ABL and its mean potential temperature mixed-layer models have been developed that give satisfactory agreement with observations. It has been shown that for many practical applications accurate knowledge of forcing functions and boundary conditions is more important than a refinement of the entrainment hypothesis. Observations show that the assumption of well-mixedness of first-order moments of conservative variables is not valid for all quantities. A simple similarity relation for the inclusion of the effect of entrainment on the shape of the vertical profiles is given.     

Parametric relations for the atmospheric boundary layer

Some parametric relations for the atmospheric planetary boundary layer (PBL) are suggested for possible use in the various atmospheric circulation and air quality models, as well as in other applications. These are for parameterizing the mean wind and temperature profiles, the vertical fluxes of momentum, heat and moisture, the variances of velocity fluctuations and length and time scales in the PBL. The parametric relations for the PBL height, the vertical velocity at the top of the PBL and the total energy dissipation in the PBL are also discussed. Experimental and/or theoretical bases for the various parametric relation are given. Some of the suggested parameterizations should be considered as tentative, until they are properly validated.     

The urban boundary layer in Montreal

Horizontal and vertical sampling of the atmosphere has provided new information on the form of Montreal's urban heat island. The horizontal pattern under clear skies with light winds shows a major heat island, with marked gradients at the periphery, and a multicellular inner core. Retarded urban cooling rates in the evening yield a maximum heat-island intensity around midnight. Combined horizontal and vertical temperature surveys show that under conditions of strong rural stability, the lowest layers of the urban atmosphere become progressively modified as air moves toward the centre of the city. The change in the form of the potential temperature profile is in good agreement with Summers' internal boundary-layer hypothesis. In Montreal differing heights of heat and SO₂ emission appear to produce more than one internal layer. SO₂ observations, and heat input calculations reveal two major emission sources in Montreal; one associated with an industrial complex, and the other with the downtown core.     

Turbulent diffusivity and diurnal variations in the atmospheric boundary layer

Diurnal variations of the vertical profiles of wind and temperature have been surveyed, and the diffusivity and the dimensionless gradient function in the atmospheric boundary layer have been estimated. Even in the middle of the atmospheric boundary layer (e.g., below a height of 442 m), the vertical wind profile normalized by the surface friction velocity has approximately a universal profile function different from that in the surface boundary layer. Under strong stability conditions, the dimensionless gradient function has a value of about 9.     

Determination of vertical temperature profiles for the atmospheric boundary layer by ground-based microwave radiometry

A useful method for remote sensing of vertical temperature profiles in the atmospheric boundary layer is described. From angular measurements of brightness temperature at 58 GHz, profiles have been inferred up to an altitude of 700 m. Calculations were done with an iterative inversion procedure (Smith et al., 1972) using Twomey-type smoothing (Twomey, 1963). It is shown how an initial-guess profile can be directly derived from the radiation measurements using, a nomogram.     

Coherent structures in the very stable atmospheric boundary layer

This study examines the structure of horizontal modes (meandering, vortical modes or fossil turbulence) in a layer of intermittent turbulence occurring at the top of a strongly stratified nocturnal inversion layer as observed by fast response aircraft data. The spatial variation of the coefficients of the principal components identify regular coherent structures with mainly horizontal motions. Conditional sampling is formulated in terms of this spatial variation. The quasi-horizontal motions are characterized by relatively sharp edges (transition zones) where horizontal convergence or divergence, small-scale turbulence and vertical fluxes seem to be concentrated. Zones of horizontal divergence appear to be associated with ejection of cold air from the underlying surface inversion while the convergent zones might be due to random collisions between horizontal modes.     

Thermal plumes and turbulence spectra in the atmospheric boundary layer

Experimental observations on the temperature and wind fields above flat grassy terrain have been obtained with an instrumented 92-m tower during intervals of strong insolation about midday. The turbulence characteristics of the air confirm that free convection prevailed at heights between 16 and 48 m, with some tendency for departure at higher levels. The spectra of temperature and vertical velocity contain gaps at wave numbers in the range 0.01–0.025 m^–1. These are attributed to natural thermal plumes that act as sources of extra energy input to the Kolmogorov-Obukhov-Corrsin scheme of turbulence in or at the low-wave number limit of the inertial subrange. Modified forms of the K-O-C spectral laws for thermally unstable air are derived which agree with the observed spectra over the whole range of wave numbers examined, and which contain the spectral gap at wave numbers corresponding to the thermal plume diameters.     

A TKE-dissipation model for the atmospheric boundary layer

The dissipation, , of turbulent kinetic energy (TKE) is a key parameter in atmospheric boundary-layer (ABL) models. Besides being a sink for momentum, it is often used together with the TKE to define an internal turbulence time scale for closure relations. A prognostic formulation for the dissipation of TKE is formulated, based on isotropic tensor modeling methods. The formulation is coupled to a level 2.5 second-order closure model and evaluated against measurements taken in horizontally homogeneous conditions, as well as against a tailored length-scale formulation. A formulation suitable for convective as well as neutral and stable ABLs is suggested.On leave from Department of Meteorology, Uppsala University, P.O. Box 516, S-751 20 Uppsala, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A parameterization of the stable atmospheric boundary layer

Two formulations of the stable atmospheric boundary layer are proposed for use in weather forecasting or climate models. They feature the log-linear profile near the surface, but are free from the associated critical Richardson number. The diffusion coefficients in the Ekman layer are a natural extension of the surface layer. They are locally determined using wind shear in one case and turbulent kinetic energy in the other. The parameterizations are tested in a one-dimensional model simulating the evolution of the nocturnal boundary layer with and without radiative cooling. Both formulations give very similar results, except near the top of the boundary layer where the transition to the free atmosphere is smoother with the wind shear formulation. A distinctive feature of these schemes is that they retain their simulating skill when resolution is reduced. This is verified for a wide range of situations. In practice, this means that there is no need for a large-scale model to have a level below 50 m or so.