Vertical sodar measurements at different sites in summer

Measurements of the turbulent structure of the planetary boundary layer have been made with the vertical sodar ECHO-1 at different locations in summer. The backscatter returns are discussed in relation to some relevant parameters of the boundary layer (mixing depth, inversion regime, stability, heat flux). For the measurements during the experiment KOPEX-86, when simultaneous observations were made by acoustic sounder and an instrumented tower, the sodar data were compared with direct measurements.     

尘卷风对沙尘气溶胶的贡献及其与太阳辐射的关系

利用塔克拉玛干沙漠及周边地区观测资料,依据尘卷风热力学理论和方法,估算出在塔克拉玛干沙漠地区尘卷风对沙尘气溶胶贡献的年平均量为4.0× 106t,年最大量为5.0×106t,与该地区沙尘暴的沙尘气溶胶年均贡献量相当.依赖于太阳辐射强度季节变化,尘卷风的起沙量具有与地面温度同步的季节变化规律.     

Thermodynamic features of the atmospheric boundary layer during the summer monsoon

Characteristic variations in the thermodynamic parameters of the boundary layer at Pune (18°32′N, 73°51′E, 559m a.s.l.) have been studied using the aerological observations collected during the summer monsoon seasons of 1980 and 1981, and the temperature observations from aircraft during the summer monsoons of 1976, 1979–1981. This study showed suppression of the mixed layer, absence of inversion/stable layers and decreased convective instability in the lower layers during the period of active monsoon conditions. The reverse was observed during the periods of weak/break monsoon conditions. Temperature stratification of sub-cloud layer has been classified into four different categories depending on the extent of the mixed layer and the gradient of potential temperature in the overlying stable layer. It was observed that these categories cover all types of weather, conditions which prevailed during the monsoon season. The results are discussed with the possible association of the weather conditions prevailed during the active and break monsoon periods.     

Observed structure of the nocturnal urban boundary layer and its evolution into a convective mixed layer

The turbulence properties of the nocturnal urban boundary layer (UBL) and its transition to the convective mixed layer have been observed over the urban center of Sapporo, Japan. The nocturnal UBL was characterized by an elevated inversion layer and a near-neutral mixed layer below it. The inversion layer varied with time and showed a wavy pattern. Kelvin-Helmholtz instability (billow) was observed within the elevated inversion layer, the horizontal and vertical scales of this billow being evaluated to be of the order of the vertical scale of the elevated inversion layer. The transition from the nocturnal UBL to the convective mixed layer occured rapidly, within 1–1.5 h over the center of the urban area. At this time, the downward transport of sensible heat below the inversion base became important. The normalized turbulence intensities in the convective mixed layer were also examined.     

A large-eddy simulation of a line source in a convective atmospheric boundary layer—I. Dispersion characteristics

The dispersion of a passive line source in the convective atmospheric boundary layer is calculated with a large-eddy simulation. The results of the calculation are presented in the form of dispersion parameters. The total dispersion parameter is made up of two contributions: meandering and relative diffusion. Both contributions are calculated as a function of diffusion time and of source height. The results for vertical dispersion show that initially the contribution by meandering is large. Furthermore, the meandering component becomes larger as source height increases. These results can be interpreted in terms of the well-known looping plume in the convective boundary layer. The horizontal dispersion parameter is also influenced by meandering. However, in this case the lowest source has the largest meandering component. Finally, we consider the velocity statistics and demonstrate that they are consistent with the dispersion parameters.     

Observations and numerical simulation of the evolution of the tropical planetary boundary layer during total solar eclipses

Two convective boundary layer experiments were concluded in the tropics during total solar eclipses, one at Raichur, India on 16 February 1980 and the other at Tanjung Kodok, Java, Indonesia on 11 June 1983. Period of totality was about 3 min at Raichur and 5 min at Java. With the partial phase of the eclipse extending over a combined period of about 2-h before and after totality, there was sufficient time in these experiments for the atmospheric boundary layer to react to changes in solar radiation. Results from the Indian experiment indicated significant changes in the atmospheric stability during the eclipse with slightly stable conditions present after the second contact. Java observations showed similar results but with smaller effects. Changes in stability also caused changes in turbulence structure. In this paper, we present the observations made during these two experiments, as well as the results of numerical simulations using a one-dimensional, second-order closure PBL model. In terms of location, the Raichur site with approximately homogeneous conditions was better suited for the model simulations than Tanjung Kodok, which was located about 1 km downwind from the ocean.     

A large-eddy simulation of a line source in a convective atmospheric boundary layer—II. Dynamics of a buoyant line source

We consider a large-eddy simulation of a buoyant line source in a convective boundary layer. The motion of the line source as a function of time is influenced by two contributions: internal buoyancy (plume rise) and ambient turbulence. The advantage of large-eddy modelling with respect to laboratory and atmospheric experiments is that the simulations allow us to distinguish between these two contributions. For the part due to internal buoyancy we formulate an integral model for plume rise. A new feature in this model is the fact that plume looping, which is characteristic for plume dispersion in a convective boundary layer, promotes entrainment and therefore reduces plume rise. The contribution by ambient turbulence to plume motion is modelled in terms of the standard deviation of vertical velocity fluctuations. The results of these models are consistent with our large-eddy simulations.     

西南地区夏季云量与降水的关系特征分析

论文利用我国西南地区地面降水资料和ISCCP D2 云资料,通过SVD、合成分析等方法,分析研究了该区域夏季雨层云、高层云、深对流云与降水量、雨日的关系。结果表明：西南的贵州、重庆地区夏季的雨层云与雨日、雨量具有较好的关系,尤其是与雨日的相关系数高达0.73。除云南中、西部地区外,西南其余地区夏季高层云与雨量、雨日具有较好的关系,且相关系数与雨层云接近,反映此地区发生降水时,一般是雨层云和高层云相伴存在。夏季的深对流云也与雨量、雨日具有较好的关系,由于深对流云常产生阵性强降水,对降水量贡献较大,所以与雨量关系更为显著,其相关系数高达0.86,并且在三种降水性云系中与降水关系最好。     

Creation and fate of ozone layers aloft in Southern California

Data obtained during the 1987 Southern California Air Quality Study (SCAQS) were used to examine the creation and fate of ozone layers aloft. The ultimate objective was to estimate the potential impact of the layers on surface ozone concentrations. The smog episode of 13–15 July 1987 formed the basis for a case study. Several ozone layers aloft were created over the northern part of the Los Angeles Basin (e.g. over El Monte) during the day on 13 July. These layers were attributed to the action of slope flows (including convergence), convective elements transported into the inversion layer and undercutting by the seabreeze flow. A peak value of 34 pphm was observed aloft in one of the layers. The principal layers aloft were still present intact over the northern part of the Basin on the morning of 14 July. Their movement eastward (from areas in the vicinity of El Monte and Upland) was estimated by trajectory and isentropic analysis. The results indicated that a likely impact area of the ozone layer aloft near Upland during the day of the 14th was near San Bernardino. The ozone layer above El Monte on the morning of 14 July appears to have been incorporated into the mixed layer near or to the southeast of Riverside. A maximum ozone impact of 3–4 pphm was estimated although a portion of this impact was probably due to an overnight reservoir of surface-based precursors of ozone.     

Practical dispersion modelling for buoyant elevated sources in a tropical region

The worst-case meteorological scenarios for the air pollution impact of coal-fired power stations located in tropical Australia are usually those of convective or seabreeze states. Under such conditions maximum hourly ground-level concentrations occur within 5 km of typical elevated sources and are potentially important unless either low sulphur coal is burnt (the usual situation for Australian coals) or restrictions are placed upon neighbouring land use. In such conditions even slightly complex surrounding terrain can cause major changes in surface and lower-level wind and turbulence characteristics, although the resultant effects on ground-level concentrations are probably relatively small. The monitoring data bases of an inland and a coastal power station in tropical Queensland give similar qualitative results to recent EPRI studies. Concurrent detailed meteorological measurements show that convective scaling techniques order both data sets in a satisfactory manner. The convective dispersion model of Spillane (1985, CSIRO internal report) has also shown more predictive skill than conventional approaches and emphasizes the importance of gathering vertical velocity statistics for most sites. Some of the forthcoming developments in related Australian dispersion work are outlined.     

气象激光雷达的城市边界层探测

为了研究城市边界层结构变化特征,2005年7月利用气象激光雷达在南京城区做了城市边界层探测试验,就此次观测试验作了简要叙述并选取部分观测资料为示例,着重对气象激光雷达确定城市边界层高度垂直分布及逐时变化以及地面气象环境对边界层的日变化影响进行了初步分析.结果表明,主要由近地层对流混合形成的城市边界层高度具有典型的日变化特点,早晚比较低,日间有一个从低到高再到低的变化过程.提出了一种由边界层混合状态确定边界层高度的方法.分析结果表明,由气溶胶消光系数确定城市边界层高度比较准确.将MSL探测结果与同步的低空无线电探测仪的温度探测结果进行比较,结果表明,两者获得的实际廓线分布相当吻合,线性良好.就地表温度、辐射、湿度及云等气象要素对边界层垂直分布及时间变化的影响做了分析.结果表明,这些气象要素的分布对城市边界层高度的垂直分布及其逐时变化有明显影响.     

Simulation of dispersion in moderately complex terrain—Part B. The higher order closure dispersion model

A three-dimensional higher-order closure dispersion model is presented. The model is used to simulate dispersion from point sources in complex terrain. The model uses mean and turbulence quantities simulated with the fluid dynamic model presented in Part A to simulate dispersion in a polar coordinate system with its origin in the point source. Different turbulent length scales are used for the vertical and horizontal fluxes. Simulation results are compared with data from tracer experiments performed in southern Sweden, the Vänersborg-Trollhättan region. The tracer experiments were performed during convective atmospheric conditions as well as during very stable conditions. The geographical area has terrain features that exert forcing on the meso-γ-scale. Within the area there is a relatively flat agricultural area, forested hills, a river valley and an extended lake area. The terrain height relief is typically 80 m.The simulations with the dispersion model performed in the Vänersborg-Trollhättan region show good agreement with measured data in the region for convective atmospheric conditions as well as for very stable conditions.     

特强沙尘暴灾害性天气的研究及展望

总结特强沙尘暴天气的研究进展,梳理分析特强沙尘暴天气的基本特征以及动力学触发机制,提出进一步的研究问题,以期加深对特强沙尘暴天气过程机理的认识.分析结果表明特强沙尘暴天气事件的季节性分布特征显著,春季居多,且多发生在下午到傍晚时段,其典型特征为突发性、局地性,持续时间相对较短,对局地的大气环境质量短时产生重大影响.旺盛的地面混合层热对流胞运动和中尺度（反）气旋性涡旋过境是特强沙尘暴天气出现的必要条件.在沙尘源区,由于午后近地层温度升高,热对流加剧,近地层热层结极不稳定,易形成地面混合层热对流胞运动;沙尘源区的热对流胞与混合层上部的中尺度（反）气旋性涡旋耦合作用,涡量通过热对流胞下传促使近地层层结内的热对流胞发展成群发性旋转对流胞,特强沙尘暴天气出现.进一步理解特强沙尘暴的风沙流特征及能量快速耗散机制,提高预测预警能力,需要加强特沙尘暴的气象观测数据积累和多尺度数值模拟.     

Transport model calculations of NW-European methane emissions

Methane emissions of large parts of NW-Europe are derived from continuous concentration measurements at one sample site (Cabauw tower, The Netherlands) using the COMET transport model. This approach can be used as an independent check of emission estimates based on statistical information. COMET is a two-layer Lagrangian atmospheric transport model that describes the uptake of methane emissions in the mixing layer along a backward trajectory path and the exchange with a reservoir layer during mixing layer height changes. In inverse mode a Singular Value Decomposition (SVD) matrix inversion technique is used to calculate the emissions of a limited number of source areas using four years of 6-hourly concentration data (1993–1996) at Cabauw. The calculated emissions compare well with the current emission inventory data for the Netherlands and the surrounding countries. For the Netherlands a yearly emission of 757 Gg of methane is calculated, excluding the emissions from oil and gas mining at the North Sea continental shelf.     

The sensitivity of modeled sulfate wet deposition to the meteorological values used as input

Variations in the wet deposition of sulfate, resulting from uncertainties in key meteorological variables, have been calculated using a scavenging model with a steady-state meteorology and a time-dependent, non-linear chemical mechanism. Input data to initialize this model are representative of data measured during an actual storm event. The results are presented in terms of the deposition uncertainty that would result if the maximum prescribed uncertainty occurred simultaneously in all input meteorological variables. For stratiform clouds, sulfate deposition uncertainties of up to 33% of the computed value were calculated. For convective clouds, the sulfate deposition uncertainties ranged from near 30% at low grid-averaged precipitation rates to near 60% at higher precipitation rates. The largest contributor to deposition uncertainty was the area of convective cloud coverage. For a mixed convective-stratiform cloud system, uncertainties in deposition remained below 40% as long as the convective contribution to total precipitation was less than 75% of the total rainfall amount. In the mixed convective-stratiform system, the uncertainties in deposition were associated principally with uncertain areas of cloud coverage and uncertain precipitation contributions by each cloud type. Uncertainties in precipitation rate, cloud top height, and cloud base height were of secondary importance for the mixed cloud system and contributed roughly equally to the remaining deposition uncertainty. In all simulations, uncertainties calculated as the standard deviation from the known value are about half of the maximum value.     

北京地区冬夏季持续性雾-霾发生的环境气象条件对比分析

在北京地区,除冬季供暖期外盛夏也是雾-霾天气的高发季节,与我国南方不同.使用微波辐射仪、风廓线和常规气象探测资料、NCEP再分析资料以及大气成分观测结果,通过对比分析揭示了冬、夏季持续6 d的2个雾-霾过程形成和维持机制的异同.冬季雾-霾过程出现在高空西北气流、低层多短波活动的背景下,其形成和维持的主要机制是边界层内始终有逆温层、地面弱风场、底层湿度逐渐增大.逆温层昼高夜低、湿度昼小夜大是影响PM2.5质量浓度和能见度日变化的重要环境因子.在雾-霾天气持续期间地面弱风场能够维持主要源于冷空气势力弱、常不能影响到地面.此外,入夜后地面迅速辐射降温、边界层上层有暖平流以及空气过山后下沉增温在逆温层的形成中起了关键作用.然而,对于夏季持续性雾-霾天气,气溶胶区域输送、环境大气保持对流性稳定、空气的高饱和度是其发生的重要条件.在副热带高压长时间控制下对流层低层盛行偏南风,北京的PM2.5质量浓度随着偏南风风速增大升高.对流层底层系统性偏南风与北京附近的山谷风共同构成了从北京以南气溶胶累积地向北输送的机制.夏季雾-霾过程低层没有逆温,但是北京上空一直维持超过200 J·kG-1的对流抑制能量,它同样限制了污染物的垂直扩散.夏季自由对流高度也存在昼夜变化,其对PM2.5浓度和能见度的作用与逆温层高度升降相同.因此,冬、夏个例分别代表了2种不同类型的持续性雾-霾过程,导致差异的根本原因在于大气环流型.     

武清地区冬季一次重污染过程垂直分布特征

为研究京津冀地区重污染过程大气污染物的垂直分布特征,于2016年12月13日重污染前（11:49-12:18）和12月18日重污染期间（11:00-11:16）在北京市、天津市、河北省交界处的武清地区利用系留气球开展1 000 m以下的大气观测,探究污染物的垂直分布特征及对流边界层、覆盖逆温层和混合层等要素对重污染形成的影响.结果表明:①在重污染前,大气层结不稳定,ρ（PM_2.5）、ρ（NO_x）与ρ（O₃）随高度变化不明显,存在明显的垂直对流运动,有利于大气污染物的扩散;PM_2.5/PM₁₀[ρ（PM_2.5）/ρ（PM₁₀）]在800 m以下为0.60~0.80,在800~1 000 m以上大于0.90.②重污染期间,近地面大气层分为对流边界层（距地面0~150 m）、覆盖逆温层（150~370 m）、混合层（370~500 m）和自由大气（500 m以上）4个层次.③NO_x主要在对流边界层内聚积;高空O₃在向近地面扩散时受强混合层阻挡,在混合层出现一个小峰值;PM_2.5不仅在近地面聚积,而且在覆盖逆温层内聚积,ρ（PM_2.5）在覆盖逆温层内呈双峰（峰值分别出现在150和370 m）分布,其粒径集中在0.5~1.0 μm,属于积聚态气溶胶.研究显示,在不利扩散条件下,汽车排放、村镇居民供暖排放的污染物聚积及二次颗粒物的生成是重污染形成的重要因素.      

Estimates of maximum ground-level concentration in the convective boundary layer—the error in using The Gaussian distribution

This paper derives expressions for the maximum cross-wind integrated and ceterline concentrations at ground level for an elevated source in the convective boundary layer. These expressions account for the observation that the probability density function (pdf) of vertical velocities in the convective boundary is positively skewed with a negative mode. The estimates obtained with these formulae compare favorably with observations reported by Briggs (1993; Boundary-layer Met.62, 315–328).The paper also examines the error associated with using a symmetric Gaussian pdf instead of the skewed pdf to estimate the ground-level concentration. We show that the error is equivalent to an underestimation of a factor of almost 1.5 for the cross-wind integrated concentration and a factor of 1.3 for the ground-level centerline concentration.     

Model calculating annual mean atmospheric dispersion factor for coastal site of nuclear power plant

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Estimating the change of porosity in the saturated zone during air sparging

Introduction A ir sparging rem oves volatile organic conta- m inants from a saturated zone by com bining volatilization and aerobic biodegradation. A ir is injected below the w ater table through a slotted screen in a sparging w ell, and then rises to the…