A case study of turbulence in the stable nocturnal boundary layer

Velocity and signal intensity data during stable conditions in the nocturnal boundary layer (NBL) were obtained with a minisodar on two consecutive nights with similar mean conditions. There was little turbulence activity during the first night, but during the second night, continuous background Kelvin-Helmholtz waves and instabilities having a 2-min period grew and penetrated above the mean NBL height at approximately 60-min intervals. Enhanced ozone concentrations at the surface occurred during the active periods even though most mean meteorological parameters were unchanged. Vertical profiles of vertical velocity standard deviation, dissipation rate, and temperature variance destruction rate in the NBL were measured and analyzed separately according to levels of turbulence activity. Well-defined differences between inactive and active periods of a factor of two to four were found for each parameter. The temperature structure parameter flux was large and in opposite directions in the upper and lower part of the NBL during active periods of turbulence, but small during other periods.     

Radiation,evaporation and the maintenance of turbulence under stable conditions in the lower atmosphere

An expression is derived relating the critical flux Richardson number with the critical (gradient) Richardson number. In contrast to an earlier analysis by Townsend (1958), which is restricted to the atmosphere well outside the earth's boundary layer, the present treatment is intended specifically for turbulent flow in the lower atmosphere and it takes account of the effect of evaporation on the stability. The effect of radiation on the rate of destruction of the mean square of the temperature fluctuations is obtained by considering the radiative flux divergence in a stratified atmosphere and by using a simple functional relationship to represent empirical emissivity data.It was found that evaporation and radiation increase the critical Richardson number by a sensible amount depending on the atmospheric conditions, mainly temperature, humidity and the gradients. There is no definite critical Richardson number but rather a range between 0.25, below which turbulence is very likely, and somewhat higher than 0.5, above which turbulence is improbable. The value of the critical Richardson number can be expressed in terms of evaporation, radiation and the ratio ( _w /u _*) which also appears not to have a definite critical value. Evaporation and radiation cause the ratio ( _w /u ^*) to be larger than unity under neutral conditions. These results, based on the assumption of Reynolds' analogy,K ^H =K ^M , are consistent with the available experimental evidence.     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.     

Dissipation length in stable layers

An attempt is made, for an anticyclonic stable situation in the atmospheric boundary layer during November 1978, to estimate the dissipation length. The turbulent dissipation and the turbulent kinetic energy, as measured on a 100 m high mast, exhibit large variability, due to the sporadic turbulent structure of the stable layer, and associated variability of the Richardson number. A comparison with different parameterizations of the turbulent dissipation rate allows a validation of these parameterizations.     

Contrasting vertical structures of nocturnal boundary layers

This study analyzes eight levels of sonic anemometerdata collected on a 60-m towerduring CASES-99, toward the goal of understanding thevertical structure of thenocturnal boundary layer. Several different regimesare found. Thin boundarylayers are often observed where fluxes decrease with height and approximately vanish between 20 and 30 m aboveground. The flow above the thin boundary layeraccelerates and increasing shear oftengenerates significant turbulence in the middle ofthe night. Thisshear-generated turbulence is often stronger thanthat near the surface corresponding to an upside-downboundary layer. During these conditions,the turbulent transport of turbulence is downwardtoward the surface. The turbulence in this regimeshows features of z-less turbulence to the extentthat neither the height above groundnor the boundary-layer depth are primary scalingvariables. This layer isdifferent from a `residual layer' in thatturbulence is actively generated byshear associated with nocturnal accelerationsand often is stronger than that inthe surface-based boundary layer.In many cases, the turbulence does not varysignificantly across the towerlayer, implying that the boundary layer ismuch deeper than the 60-m towerlayer. Several case studies are presentedto illustrate the largevariation of vertical structure betweennights.     

A model study of the nocturnal boundary layer

In this paper, a second-order model is proposed for the study of the evolution of the nocturnal boundary layer (NBL). The model is tested against the Wangara data on atmospheric boundary layer. The computer results show ihat the model can simulate some important characters observed in the NBL, and that a kind of sudden change may occur in the developing process of NBL.     

Three-dimensional air pollutant modeling in the lower atmosphere

A steady-state, three-dimensional turbulent diffusion equation describing the concentration distribution of an air pollutant from an elevated point source in the lower atmosphere is solved analytically. The same formulation can be used to obtain solutions from line, area or other kinds of sources. The solution is developed for the cases in which the velocity, vertical and lateral diffusivities are given by the power law. The model preserves the beauty of analytical solution without sacrificing much on the accuracy of approximating the velocity and eddy diffusivities. Methods of evaluating the parameters, which are required for the model applications, are discussed. Results indicate that the ratio of the plume width to the plume length increases with decreasing stability and with increasing source height. These consequences are in response to the variations of the size of eddies in the vertical direction.     

低层大气光学厚度的一些特征

本文利用定时激光探测资料,分析了华北秋季晴天低层大气分层光学厚度的一些特征,并对大气光学厚度的计算方法和误差因素进行了讨论.初步分析结果表明:(1)低层大气光学厚度与气团属性有很密切的关系;(2)在白天,随对流发展、气溶胶由下向上输送的高度可超过3公里,最大输送在2公里以下;(3)消光系数的垂直结构具有明显的日变化;(4)光学厚度与地面水平能见度不呈简单的对应关系.     

A study on hydrogen peroxide in the atmosphere

Hydrogen peroxide (H2O2) concentrations were measured at two sites. One was on Lushan Mountain in May, 1987. The other was in Beijing in August, 1987 and in December, 1986. The automated fluorometric method was used for the determination of H2O2. The concentrations of H2O2 ranged from approximately 0.50 to 4.32 ppb on Lushan Mountain. The concentrations of H2O2 ranged from approximately 0.05 to 1.49 ppb in August in Beijing, and it was only 0.01 ppb in December in Beijing.     

Long-wave radiative flux divergence and nocturnal cooling of the urban atmosphere

Results from measurements of long-wave radiative flux divergence on calm, cloudless nights from within an urban canyon are presented. Results show the existence of three-dimensional (volume) radiative divergence in the canyon-air system. The results are compared with air temperature cooling rates (energy storage change) in the canyon. With calm, cloudless conditions, radiative divergence is the dominant mechanism controlling air temperature changes. In the early part of the night this results in air cooling, but radiative warming was commonly observed after midnight. Canyon-air volume radiative divergence is found to be considerably less than that observed previously above roof-level, and in rural areas. Measured cooling rates in the canyon are poorly predicted by a surface-oriented cooling approach. This again points to the importance of the atmospheric radiation balance.     

夜间低空急流的分析研究

本文利用北京325米气象塔取得的连续观测资料,分析了1979年9月5日夜间低空急流风温场的时空演变过程,这种低空急流的极值在夜间达到最大,其极值位置与逆温层顶的生长密切配合。低空急流是超地转的,超地转气流有一个惯性振荡周期。 低空急流的发展是起伏式的。急流的消散较快,当Ri ≤0.25达到切变不稳定的临界条件时加速崩溃。     

地面辐射雾和低层云对夜间边界层影响的数值研究

本文通过模式计算,讨论了夜间地面辐射雾和低层层积云的演变过程及其对边界层结构的影响.模式综合考虑了液态水和水汽对长波辐射通量的贡献.模式主要由两部分组成:(1)用湍流能量方程闭合的一维边界层模式;(2)长波辐射通量的计算.模式的计算结果与观测事实基本一致.     

Temperature stratification of the lower atmosphere over Moscow

Features of diurnal and annual cycles and of seasonal changes of temperature stratification in the lower 800-m air layer over Moscow are discussed on the basis of analysis of long-term data of acoustic (sodar) observations at Moscow State University (MSU). Of about 34 000 separate hours of height-time sweep of echo signal during 1988–2003, refined estimates are presented of occurrence frequencies of surface and elevated inversions, unstable stratification, etc., in Moscow. On the basis of long series of hourly sodar data and surface weather observations at MSU, special features of temperature stratification are considered under extremely low and high values of air temperature, wind speed, and relative humidity. A review is presented of existing data on temperature stratification in central Russia for the whole period of aerological observations; results of acoustic sounding are compared against the data obtained using other techniques.     

Advances in studies of the middle and upper atmosphere and their coupling with the lower atmosphere

Recent advances in studies of the middle and upper atmosphere and their coupling with the lower atmosphere in China are briefly reviewed. This review emphasizes four aspects: (1) Development of instrumentation for middle and upper atmosphere observation; (2) Analyses and observation of middle and upper atmosphere; (3) Theoretical and modeling studies of planetary wave and gravity wave activities in the middle atmosphere and their relation to lower atmospheric processes; (4) Study on the coupling between the stratosphere and the troposphere.     

A climatological study of the nocturnal planetary boundary layer

Seventy-five nights of fast-response wind and temperature data taken from a 300 m tower near Augusta, GA, were analyzed to determine the time-height structure of the nocturnal planetary boundary layer. The nights were selected from all four seasons over a wide range of synoptic conditions. Statistical summaries of Pasquill-Gifford stability, boundary-layer depth, nocturnal jet height, directional shear, gravity wave occurrence, and azimuthal meandering were obtained. The diversity of nocturnal conditions for the 75 cases resulted in histograms with broad peaks and slowly-varying distributions.To reduce the overall variance, we grouped the nights into two classes: steady nights and unsteady nights. Nights classified as steady maintained relatively uniform wind conditions. The data base was large enough to permit a further breakdown of the steady nights into three subclasses based on the height and strength of the wind maximum. Unsteady nights were more disturbed, showing time-dependent features in the wind field and were also divided into three subclasses, depending on the predominant features observed: microfrontal passage, trend, or variable conditions. Although the subclasses were based mainly on wind structure, they correlated well with other NPBL properties, such as mixed-layer depth and inversion strength. Thus, the classification procedure tended to group together nights with similar dispersion characteristics.     

稳定大气中烟流抬升高度的确定

本文通过Boussinesq近似的非线性动力方程组,得到了烟流抬升高度的解析表达式,讨论了地形、大气稳定度等物理因子对烟流抬升高度的影响。结果表明:地形的分布对烟流抬升高度的确定有一定影响,大气稳定度对高度的影响尤为明显。本文还讨论了理论公式的实用条件。