An observational study of the structure of the nocturnal boundary layer

In an effort to describe the basic vertical structure of the nocturnal boundary layer, observations from four experiments are analyzed. During the night, the depth of significant cooling appears to increase with time while the depth of the turbulence and height of the low level wind maximum tend to remain constant or decrease with time. Since the inversion layer extends above the low level wind maximum and shear is small in the region of the low level jet, the Richardson number reaches a maximum at the jet level and then decreases again with height. As a result, turbulence is observed to be a minimum at the height of the low level wind maximum and then increases again above this height.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Part of this work was performed while a visiting scientist at Oregon State University.     

Entrainment Processes in the Convective Boundary Layer with Varying Wind Shear

Large-eddy simulations (LES) are performed to investigate the entrainment andthe structure of the inversion layer of the convective boundary layer (CBL) withvarying wind shears. Three CBLs are generated with the constant surface kinematicheat flux of 0.05 K m s^-1 and varying geostrophic wind speeds from 5 to 15m s^-1. Heat flux profiles show that the maximum entrainment heat flux as afraction of the surface heat flux increases from 0.13 to 0.30 in magnitude withincreasing wind shear. The thickness of the entrainment layer, relative to the depthof the well-mixed layer, increases substantially from 0.36 to 0.73 with increasingwind shear. The identification of vortices and extensive flow visualizations nearthe entrainment layer show that concentrated vortices perpendicular to the meanboundary-layer wind direction are identified in the capping inversion layer for thecase of strong wind shear. These vortices are found to develop along the mean winddirections over strong updrafts, which are generated by convective rolls and to appearas large-scale wavy motions similar to billows generated by the Kelvin–Helmholtzinstability. Quadrant analysis of the heat flux shows that in the case of strong windshear, large fluctuations of temperature and vertical velocity generated by largeamplitude wavy motions result in greater heat flux at each quadrant than that inthe weak wind shear case.     

On the Formation of an Elevated Nocturnal Inversion Layer in the Presence of a Low-Level Jet: A Case Study

We report on observed nocturnal profiles, in which an inversion layer is located at the core of a low-level jet, bounded between two well-mixed layers. High-resolution vertical profiles were collected during a field campaign in a small plain in the Israeli desert (Negev), distant 100 km from the eastern shore of the Mediterranean Sea. During the evening hours, the synoptic flow, superposed on the late sea breeze, forms a low-level jet characterized by a maximum wind speed of 12 m s ⁻¹ at an altitude of 150 m above the ground. The strong wind shear at the jet maximum generates downward heat fluxes that act against the nocturnal ground cooling. As a result, the typical ground-based nocturnal inversion is “elevated” towards the jet centre, hence a typical early morning thermal profile is observed a few hours after sunset. Since the jet is advected into the region, its formation does not depend on the presence of a surface nocturnal inversion layer to decouple the jet from surface friction. On the contrary, here the advected low-level jet acts to hinder the formation of such an inversion. These unusual temperature and wind profiles are expected to affect near-ground dispersion processes.     

Observation of a Self-Limiting,Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment.     

Ondes de gravite interfaciales marginalement instables dans la couche limite planetaire

Wavelike motions within a strong morning inversion of the planetary boundary layer were investigated experimentally using two atmospheric research aircraft: an Aerocommander 280FL and a Cessna 206. The Aerocommander aircraft, instrumented for the measurement of rapid fluctuations of temperature, water vapour density and air velocities, was flown horizontally at different levels within the inversion layer in order to document adequate data on the wave motion. An example of such motions observed on 8 June, 1974 is described and analyzed in the present paper. The aircraft records obtained within the inversion layer at about 600 m above the ground show large fluctuations of the meteorological variables with well-defined amplitudes and wavelengths.Spectra and cross-spectra of temperature, water vapour density and air velocities were computed and analyzed to determine characteristics of gravity waves according to the method described by Metcalf (1975). These spectra exhibit a sharp maximum associated with high coherences at a particular wavelength. In this narrow spectral band, phase angles ±90 ° are obtained between vertical velocity and temperature as well as between vertical and horizontal velocities. These features suggest that observed motions are horizontally propagating trapped or evanescent waves. They enable us to estimate true wavelengths (500 m), wave vector azimuths, intrinsic frequencies and phase velocities of these waves. These results appear to be mutually consistent. Furthermore, it is possible to confirm these latter with the detailed vertical profiles of the boundary layer provided by the Cessna aircraft making spiral soundings. In this regard, the vertical structure of the Brunt-Väisälä frequency confirms that the waves are everywhere evanescent except within a thin highly stable layer between the diurnal mixed layer and the overlapping inversion. Moreover, examination of the wind profiles reveals that the interfacial vertical wind shear might be a relevant parameter reducing phase velocities. Such a conclusion is also supported by the observed wave vector directions which appear to be closely parallel to the wind shear vectors at the 600-m level. Additional confirmation is found by comparing the observed wavelengths to those predicted by applying the hydrodynamical stability model of Hazel (1972) to the measured profiles. Although the wind shear clearly plays a role in wave development, local heat flux and temperature variance values show that in this case the instability is only a marginal and sporadic event embedded in nearly neutral waves. Accordingly, it is argued that the observed motions are interfacial waves at the inversion base level, the amplitude and wave vector azimuth of which are controlled by the vertical wind shear.

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Ondes de gravite interfaciales marginalement instables dans la couche limite planetaire

Résumé Des ondes de gravité interfaciales progressives sont étudiées à l'aide de deux avions dans la couche limite planétaire. Les spectres des vitesses et de la température fournissent les directions et vitesses de propagation. La comparaison des résultats aux profils verticaux du vent et de la température montre que ces ondes sont marginalement instables sous l'effet du cisaillement vertical du vent.

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IOPG, 12, avenue des Landais - 63001 Clermont-Ferrand Cedex.

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Complexe Scientifique des Cézeaux BP 45 - 63170 Aubiere.     

A numerical study of the marine stratocumulus cloud layer

A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.     

南京冬季平流雾的生消机制及边界层结构观测分析

利用系留飞艇边界层要素探测系统等设备,对2006年12月24q7日发生在南京地区的雾日边界层结构进行了综合探测,深入研究了这次平流雾的生消机制及边界层结构。结果表明：此次雾属于比较典型的平流雾,生成和维持主要决定于暖湿气流和系统性下沉运动,消散主要是干冷空气南下造成的;雾项下降阶段出现了双层结构,中层逆温是逆温主层,属于下沉逆温及平流逆温,主逆温层强中心始终位于雾顶附近或处于雾顶之下;风速随高度呈现多峰分布,中层急流与强度较弱的中上层和上层急流合并后,又与下层急流出现了一强一弱的波动;在风速较小时,风场趋于均匀化;雾消散时,低层风场趋于线性化;雾主要的水汽来源是暖湿气流;比湿场与风场有较好的时空分布对应性,主逆温层强中心也是逆湿强中心,风场与温度场共同主导了比湿场的时空分布。     

Low-level nocturnal wind maximum over the central Amazon basin

A low-level nocturnal wind maximum is shown to exist over extensive and nearly undisturbed rainforest near the central Amazon city of Manaus. Analysis of meteorological data collected during the 1985 and 1987 Amazon Boundary Layer Experiments (ABLE 2A and 2B) indicates the presence of this nocturnal wind maximum during both the wet and dry seasons of the Central Amazon Basin. Daytime wind speeds which are characteristically 3–7 m s^-1 between 300 and 1000 m increase to 10–15m s^-1 shortly after sunset. The wind speed maximum is reached in the early evening, with wind speeds remaining high until several hours after sunrise. The nocturnal wind maximum is closely linked to a strong low-level inversion formed by radiational cooling of the rainforest canopy. The night-time inversion extends up to 300 m with strong vertical shear of the horizontal wind below the inversion top and uniformly strong horizontal winds above the inversion top. Frictional decoupling of the air above the inversion from the rough forest below, however, is responsible for only part of the observed increase. Surface and low-level pressure gradients between the undisturbed forest and the large Amazon river system and the city of Manaus are shown to be responsible for much of the nocturnal wind increase. The pressure gradients are interpreted as a function of the thermal differences between undisturbed forest and the river/city. The importance of both the frictional decoupling and the horizontal pressure gradient suggest that the nocturnal wind maximum does not occur uniformly over all Amazonia. We suspect that stronger low-level winds are pervasive under clear skies and strong surface cooling and that, in many places (i.e., near rivers), local pressure gradients enhance the low-level nocturnal winds.     

The wind speed shear in the case of stable stratification and the scales of the similarity theory

The wind speed shear in the case of stable stratification in the linear part of the profile spreading high above the surface layer of constant flows is studied using the data of long-term sodar measurements in the atmospheric boundary layer. The wind speed shear in this part remains almost invariable during several hours at the significant change in parameters of the Monin-Obukhov theory. The length of this linear part can be associated with the layer of the critical Richardson number. In the case of the pronounced temperature inversion (with the positive gradient of more than 1°C per 100 m), the wind speed profile is close to the linear function in the most part of the nocturnal mixing layer. Proposed is a scale characterizing the height of the surface layer of constant flows.     

A numerical study of the effects of synoptic baroclinicity on stable boundary-layer evolution

The effects of synoptic baroclinicity on the evolution of the stable boundary layer are studied by using a numerical model in which the eddy exchange coefficients are determined from the turbulent kinetic energy and a local turbulent length scale. For model verification, several barotropic simulations are compared with those of higher-order closure models. The model predicts the existence of a value of geostrophic wind shear at which the nocturnal jet reaches its maximum intensity. The mechanism by which ageostrophic flow is generated and the role it plays in the development of the jet are explored. As baroclinicity increases, the directional shear in the wind near the level of the jet increases, thereby allowing the nocturnal inversion to grow to levels well above that of the jet.Journal Paper No. J-11109 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project 2521.     

Wind shear at tilted inversions

Vertical wind shear at a temperature inversion can be caused by baroclinicity associated with a tilt of the inversion. Four observational cases of tilted inversions are presented. The tilts on horizontal scales of 20–100 km range from 2–10 × 10^-3 and the vertical wind shear is between 1 and 25 m/s per 100 m. In general, there is remarkable agreement between observed and geostrophic wind shear.The observations show that the inversion tilt is particularly strong at the edges of mesoscale cloud fields. The Richardson number can reach subcritical values. Cloud fields may be surrounded by a cyclonically rotating wind field and cloud gaps by an anticyclonically rotating wind field.     

Laboratory and Numerical Studies of the Convective Boundary Layer Capped by a Strong Inversion

The convective boundary layer (CBL) with a wide range of stability is simulated experimentally using a thermally stratified wind tunnel, and numerically by direct numerical simulation (DNS). The turbulence structures and flow characteristics of various CBL flows, capped by a strong temperature inversion and affected by surface shear, are investigated. The various vertical profiles of turbulence statistics similar to those from the observed CBL in the field are successfully simulated in both the wind-tunnel experiment and in DNS. The comparison of the wind-tunnel data and DNS results with those of atmospheric observations and water-tank studies shows the crucial dependence of the turbulence statistics in the upper part of the layer on the strength of the inversion layer, as well as the modification of the CBL turbulence regime by the surface shear.     

边界层对流对示踪物抬升和传输影响的大涡模拟研究

利用"西北干旱区陆气相互作用野外观测实验"加密观测期间敦煌站的实测资料以及大涡模式, 通过一系列改变地表热通量和风切变的敏感性数值试验, 分析了地表热通量和风切变对边界层对流的强度、形式, 以及对对流边界层结构和发展的影响。模拟结果显示风切变一定, 增大地表热通量时, 由于近地层湍流运动增强, 向上输送的热量也较多, 使对流边界层变暖增厚, 而且边界层对流的强度明显增强, 对流泡发展的高度也较高。当地表热通量一定, 增大风切变时, 由于风切变使夹卷作用增强, 将逆温层中的暖空气向下卷入混合层中, 使对流边界层增暖增厚, 但是对流泡容易破碎, 对流的强度也较弱。另外通过在模式近地层释放绝对浓度为100的被动示踪物方法, 用最小二乘法定量地分析了地表热通量和风切变分别与示踪物抬升效率和传输高度的关系。分析结果表明, 风切变小于10.5×10-3 s-1时, 增大地表热通量加强了上层动量的下传, 使示踪物的抬升效率也线性增大;地表热通量小于462.5 W m-2时, 增大风切变减弱了边界层对流的强度, 从而使示踪物的抬升效率减弱。当风切变一定时, 示踪物的平均传输高度随地表热通量增加而增大, 而地表热通量一定, 只有风切变大于临界值时, 示踪物平均传输高度才随风切变的增加而增大, 而临界风速的大小由地表热通量决定。     

风廓线雷达资料在一次强对流天气过程中的应用

通过分析2010年6月28日广州市南沙区强对流天气过程的风廓线雷达资料发现：风廓线雷达测得的温度廓线资料可以很直观地显示边界层逆温及高温区域随时间的变化;风廓线雷达产品（垂直速度和信号噪声比）能清楚地反映降水的开始、结束以及降水的强度;风廓线雷达可以很好的监测到低空急流的发生发展及水平风的垂直切变．对强对流天气的预报有...     

雾霾天气个例气象条件对比分析

应用常规观测资料、NCEP 1°×1°再分析资料和L波段探空资料从环流形势、扩散条件和边界层特征3个方面对2013年两次雾、霾天气个例进行对比分析,结果表明:500hPa西北气流冷平流、地面弱风场、垂直速度呈弱上升-下沉的垂直分层特点和逆温是两次雾、霾天气出现和维持的共同特征。地面西北风、850hPa弱冷平流、近地层浅薄的接地逆温(100~200m)和湿层与霾天气对应,地面偏东风、850hPa暖平流、925hPa以下深厚的悬浮逆温(400m)和湿层与雾天气对应,霾过程较雾过程逆温强度强,上升运动高度高。消散时雾较霾下沉运动中心高度低,强度弱;霾消散时接地逆温特征变化不大,雾消散时悬浮逆温有底部抬升和大气稳定层结向中性层结转变的变化特征;但均有下沉气流接地、垂直风切变较强和高层低露点干空气下传到地面的特点。     

用探空资料检验中尺度数值模式对强对流天气的诊断分析能力

提高对流天气临近预报准确率的关键问题之一是了解大气的垂直稳定度和垂直风切变。中尺度数值模式产品提供了高时空分辨率的大气稳定度和垂直风切变信息，需要首先检验其精度才能进一步考虑其在对流天气预报中的应用。利用北京加密探空资料检验北京市气象局3km分辨率的MM5模式结果，对强对流天气的背景参数包括温湿风垂直廓线、对流有效位能CAPE和垂直风切变进行模式分析和预报与探空对比检验。结果表明：模式模拟的各种大气廓线中，风廓线和温度廓线都具有一定的参考价值，与实况有较好的一致性，但在廓线出现转折的地方，如：逆温层和风向转折时，模式预报较差。露点（湿度）廓线的预报误差较大，不能反映出真实水汽场的分布。因此，模式预报的深层（地面至500hPa）垂直风切变与探空具有较好的一致性，而模式给出的对流有效位能CAPE由于露点预报结果不理想，其值与实际偏差较大。因此模式输出的对流有效位能CAPE必须经过适当订正才能用于诊断强对流天气发生的可能性。     

Turbulence structure in temperature inversions and in convection fields as observed by doppler sodar

The structure of turbulence in an inversion layer and in an homogeneous convective field of the planetary boundary layer is described. In the first part of the paper, we validate the sodar estimates of turbulent dissipation, by using measurements with an hot-wire anemometric system in situ. Limitations of an ε measurement technique using structure function calculations are given, taking account of atmospheric properties and acoustic Doppler instrumental effects. By comparison between isopleths of backscattering intensity and of turbulent dissipation rates, we observe that in the early morning, turbulence is advected by mechanical turbulence generated by wind shear. The same mechanism seems to be operating in the case of an inversion layer capping thermal instability, when the convective activity is not too greatly developed. A turbulent kinetic energy budget is examined using aircraft, sodar, and tower measurements. This indicates a constant turbulent dissipation profile through a deep convective layer.     

景德镇市三类降雪天气概念模型研究

计算并分析了景德镇市出现雨夹雪、一般降雪、大雪（分别简称为Ⅰ类、Ⅱ类、Ⅲ类降雪）形势场、本站要素、层结资料,概括了3类降雪的天气学概念模型。分析结果表明：（1）3类降雪天气过程中,高空500 hPa有强盛的西南偏西气流,且随着降雪强度的增大,西南风速逐渐增大。地面则有较强冷空气堆积,位于贝加尔湖西部的冷高压中心气压在1 050 hPa以上,景德镇处于冷高压底部。925 hPa 30°N附近冷空气势力强,气温低。Ⅰ类降雪的主要影响系统是在江西省北部上空交汇的冷、暖平流,Ⅱ类、Ⅲ类则是850 hPa的切变线,出现Ⅲ类降雪时切变线位于景德镇附近,而出现Ⅱ类降雪时切变线则稍偏南,位于赣中。（2）地面气温变化是降水相态改变的关键,气温越低,越易出现降雪。从大雪至雪后雨,气温逐渐上升。（3）中低层的气温,Ⅱ类降雪较Ⅰ类低;近地面层气温,Ⅲ类降雪与Ⅱ类降雪接近;700 hPa和850 hPa层气温,Ⅲ类降雪比Ⅱ类降雪偏高。（4）Ⅰ类降雪常伴有逆温,但在向Ⅱ类降雪的转换过程中,逆温逐渐减弱,到Ⅲ类降雪时,逆温消失。（5）3类降雪均存在明显的风垂直切变,低层风弱,高层风强,且随着降雪强度的增大,表现愈加明显。     

延安市冬季边界层风、温场特征分布

本文利用1985年12月25日—1986年1月9日的实测资料,分析了延安市冬季边界层风、温场特征。结果表明:延安市冬季低层逆温持续时间长、强度大,逆温持续期近地面几十米表现为混合层结构。边界层风场十分复杂,时空变化大,地面常为小风状态,风向、风速沿垂直方向有明显切变,低层流场有显著渠道效应,山谷风明显。