Physicochemical properties of fine aerosols at Plan d'Aups during ESCOMPTE

The physical and chemical properties of aerosol particles were investigated at Plan d'Aups, one of the ESCOMPTE sites located in the St. Baume mountain area (700 m a.s.l.), 50 km east of Marseilles (France). The site is ideally located for assessing the vertical and horizontal extent of the pollution plume from the Marseilles–Berre area.Our study showed that polluted air masses from the Marseilles–Berre area are advected to Plan d'Aups in the early afternoon. Average daily concentration of particles reaches up to 40 μg m⁻³ while 1-h average particle number concentration is greater than 30,000 cm⁻³. Most of the particle mass is composed of SO₄²⁻ and organic carbon (OC). The chemical properties of the particles revealed that an additional source, possibly from the industrial area of Gardanne, contributes to the aerosol mass. This last source is characterised by significant emissions of elements, such as Zn, V, Al and Si.In addition to transport, we found that gas-to-particle conversion takes place at the interface between the free troposphere and the boundary layer. We estimated that on average, 30% of the particle number is accounted for by direct nucleation. This is potentially a major aerosol source to the free troposphere.     

大气边界层高度确定及应用研究进展

大气边界层高度是表征边界层特征的重要参量,影响边界层内水热、物质、能量的垂直分布,也是数值模拟、环境评估中的重要参数。从湍流运动、热力作用、动力作用以及物质分布等多视角总结了大气边界层高度的定义及确定方法,回顾了采用直接观测手段和遥感手段确定大气边界层高度的不同方法,对比了大气边界层高度不同获取手段的优缺点,梳理了大气边界层高度参数化方案,探讨了大气边界层高度确定中存在的问题,并提出未来相关研究和应用可能突破方向。     

Remote Sensing And Surface Observations Of The Response Of The Atmospheric Boundary Layer To A Solar Eclipse

On 11 August 1999, a near-total solar eclipse (80%) was observed in Campistrous, France. The influence of this particular event on the atmospheric boundary layer was observed with a UHF-RASS radar, a sodar and an instrumented mast. The changes in turbulence intensity, radar reflectivity, and temperature on the radiative budget are described in relation to collocated ground meteorological data. The impact of the eclipse induces a clear response of the atmosphere, with a time lag of 15 to 30 min, perceptible in several mean and turbulent meteorological variables up to the top of the atmospheric boundary layer.     

北京秋季一次降雪前污染天气的激光雷达观测研究

以2009年11月5~8日北京地区发生的一次特殊天气形势下的重污染天气过程为例,研究分析本次污染特点和大气边界层结构特征以及此天气过程的大气温度和相对湿度结构特点。激光雷达是探测大气边界层及气溶胶的一个高效工具,利用ALS300激光雷达系统测量信号,应用Fernald方法反演大气消光系数,根据反演的气溶胶消光系数的最大突变,即最大递减率的高度来确定大气边界层的高度。利用其观测的退偏比分析大气污染物特性。利用微波辐射计数据,确定大气温度和湿度时空特征。研究结果表明:在本次污染天气下,大气具有很强的逆温结构,逆温最大可达近1 K(100 m)-1,500 m以上的大气相对湿度很低,在这种天气特征下的大气边界层高度在400 m左右,非常稳定。污染结束降雪开始前,大气逆温结构消失,大气湿度大幅度增加,接近饱和。根据lidar(light detection and ranging)退偏比的分析,本次污染天气是一次典型的烟尘类颗粒物的污染,污染具有区域性特点。PM2.5(空气动力学当量直径小于等于2.5μm的颗粒物)与AOT(Aerosol Optical Thickness)之间有明显的线性关系,相关系数达到0.72。该lidar系统能够反演出秋季降雪前本次污染天气背景下北京城区上空的大气污染特性和大气边界层高度。     

Impact of terrain heterogeneity on near-surface turbulence structure

This study investigates the impact of terrain heterogeneity on local turbulence measurements using 18 months of turbulence data taken on a 30 m tower at the SIRTA mixed land-use observatory under varying stability conditions and fetch configurations. These measurements show that turbulence variables such as the turbulent kinetic energy or momentum fluxes are strongly dependent on the upstream complexity of the terrain (presence of trees or buildings, open field). However, using a detection technique based on wavelet transforms which permits the isolation of the large-scale coherent structures from small-scale background fluctuations, the study shows that, for all stability conditions, whatever the upstream complexity of the terrain, the coherent structures display universal properties which are independent of the terrain nature: the frequency of occurrence, time duration of the coherent structures, the time separation between coherent structures and the relative contribution of the coherent structures to the total fluxes (momentum and heat) appear to be independent of the upstream roughness. This is an important result since coherent structures are known to transport a large portion of the total energy. This study extends to all stability conditions a numerical study by Fesquet et al. [Fesquet, C., Dupont, S., Drobinski, P., Barthlott, C., Dubos, T., 2008. Impact of terrain heterogeneities on coherent structures properties: experimental and numerical approaches. In: 18th Symposium on Boundary Layers and Turbulence. No. 11B.1. Stockholm, Sweden., Fesquet, C., Dupont, S., Drobinski, P., Dubos, T., Barthlott, C., in press. Impact of terrain heterogeneity on coherent structure properties: numerical approach. Bound.-Layer Meteorol.] conducted in neutral conditions which shows that a reason for such behavior is that the production of local active turbulence in an internal boundary layer associated with coherent structure originating from the outer layer and impinging onto the ground is not sensitive to the nature of the terrain.     

On the Possible Impact of a Following-Swell on the Atmospheric Boundary Layer

A simple model of the atmospheric boundary layer over the ocean where the swell impact on the atmosphere is explicitly accounted for is suggested. The model is based on Ekman’s equations, where the stress in the wave boundary layer is split into two parts: the turbulent and wave-induced stress. The turbulent stress is parameterized traditionally via the eddy viscosity proportional to the generalized mixing length. The wave-induced stress directed upward (from swell to the atmosphere) is parameterized using the formalism of the wind-over-waves coupling theory. The model can be seen as an extension of the model by Kudryavtsev and Makin (J Phys Oceanogr 34:934–949, 2004) to the scale of the entire atmospheric boundary layer by including the Coriolis force into the momentum conservation equation and generalizing the definition of the mixing length. The regime of low winds for swell propagating along the wind direction is studied. It is shown that the impact of swell on the atmosphere is governed mainly by the swell parameter—the coupling parameter that is the product of the swell steepness and the growth rate coefficient. When the coupling parameter drops below − 1 the impact of swell becomes significant and affects the entire atmospheric boundary layer. The turbulent stress is enhanced near the surface as compared to the no-swell case, and becomes negative above the height of the inner region. The wind profile is characterized by a positive gradient near the surface and a negative gradient above the height of the inner region forming a characteristic bump at the height of the inner region. Results of the model agree at least qualitatively with observations performed in the atmosphere in presence of swell.     

2018年春季北京一次沙尘天气边界层特征及来源分析

2018年3月27—28日,内蒙古中东部、中国东北地区、华北等地出现一次大范围沙尘天气.28日凌晨,沙尘进入北京,受此影响北京出现了严重的污染天气.本文利用中国气象局地面常规观测资料、气溶胶激光雷达、风廓线雷达资料、生态环境部大气成分等资料分析了北京沙尘天气前后边界层特征、沙尘来源以及沙尘天气前后大气污染特征.结果表明...     

大气边界层物理研究进展(2012~2017年)

本文总结了2012~2017年中国科学院大气物理研究所大气边界层物理和大气化学国家重点实验室大气边界层物理研究的最新进展,主要包括不同下垫面（城市、青藏高原、草原、沙漠、湖泊、海洋等）大气边界层观测实验、大气湍流和阵风相干结构的理论研究以及大气数值模拟的参数化改进等,同时对未来几年内大气边界层物理的发展方向做了展望。     

一次深厚浓雾过程的边界层特征和生消物理机制

2007年12月13-14日,南京出现一次厚度达600 m、持续近14 h的浓雾过程,其中强浓雾阶段维持4 h.通过系留气球边界层探测系统、涡动协方差测量系统、雾滴尺度分布和自动气象站等外场试验资料分析了此次深厚浓雾过程的边界层结构特征和生消物理机制.结果表明,此次雾过程首先由地面辐射冷却形成贴地雾层,而后因低空平流冷却形成低云.在发展阶段,伴随低云不断下伸,贴地雾层不断抬升.在贴地雾层受到地面弱冷空气平流降温影响下,雾中微物理过程迅速发展,雾滴数密度、含水量、平均直径、最大直径等微物理参数在15 min内跃增,雾体爆发性升高,最终导致地面雾和低云上下贯通形成深厚雾层,地面能见度骤降至15 m以下.雾体爆发性增强时,地面垂直动量通量和向下长波辐射通量密度增大,净辐射趋于零.整个雾过程中,由于贴地层持续弱冷平流降温和上层雾阻碍了下层雾的辐射降温,二者的共同作用使贴地强逆温结构始终维持.     

Comparison of Kansas Data with High-resolution Large-eddy Simulation Fields

The surface layer of an atmospheric boundary layer (ABL) is most accessible to field measurements and hence its ensemble-mean structure has been well established. The Kansas field measurements were the first detailed study of this layer, providing numerous benchmark statistical profiles for a wide range of stability states. Large-eddy simulation (LES), in contrast, is most suitable for studying the mixed layer of the ABL where the energy-containing range of the vertical velocity field is well resolved. In the surface layer, typical large-eddy simulations barely resolve the energy-containing vertical-velocity fields and hence do not provide sufficient data for a detailed analysis.We carried out a nested-mesh simulation of a moderately convective ABL (-z_i/L = 8) in which the lower 6% of the boundary layer had an effective grid resolution of 512³. We analyze the LES fields above the 6th vertical grid level (z = 23 m) where the vertical velocity field has a well formed inertial subrange, for a detailed comparison with the Kansas results. Various terms in the budgets of turbulent kinetic energy, temperature variance, Reynolds stress, temperature flux, and some higher order moments are compared. The agreement is generally quite good; however, we do observe certain discrepancies, particularly in the terms involving pressure fluctuations.     

Atmospheric boundary-layer structure observed during a haze event due to forest-fire smoke

During a haze event in Baltimore, U.S.A. from July 6 to 8, 2002, smoke from forest fires in the Québec region (Canada), degraded air quality and impacted upon local climate, decreasing solar radiation and air temperature. The smoke particles in and above the atmospheric boundary layer (ABL) served as a tracer and provided a unique opportunity to investigate the ABL structure, especially entrainment. Elastic backscatter lidar measurements taken during the haze event distinctly reveal the downward sweeps (or wisps) of smoke-laden air from the free atmosphere into the ABL. Visualisations of mechanisms such as dry convection, the entrainment process, detrainment, coherent entrainment structures, and mixing inside the ABL, are presented. Thermals overshooting at the ABL top are shown to create disturbances in the form of gravity waves in the free atmosphere aloft, as evidenced by a corresponding ripple structure at the bottom of the smoke layer. Lidar data, aerosol ground-based measurements and supporting meteorological data are used to link free atmosphere, mixed-layer and ground-level aerosols. During the peak period of the haze event (July 7, 2002), the correlation between time series of elastic backscatter lidar data within the mixed layer and the scattering coefficient from a nephelometer at ground level was found to be high (R=0.96 for z =324 m, and R=0.89 for z=504 m). Ground-level aerosol concentration was at a maximum about 2 h after the smoke layer intersected with the growing ABL, confirming that the wisps do not initially reach the ground.     

北京冬季大风过程大气边界层特征分析

利用北京中国科学院大气物理研究所325 m气象观测塔的气象梯度资料和湍流资料,分析了2014年11月29日至12月5日北京两次大风过程中气象要素和湍流输送特征的变化。第一次大风过程的强度和持续时间均高于第二次大风过程。强烈的风速垂直切变主要集中在距地面100 m高度范围内,最强风速垂直切变达到0.31 s~(-1)。大风过程中,阵风系数呈现随高度减小的趋势,越接近地面,阵风系数愈大。阵风强度的变化与阵风系数相似,100 m以下高度时,阵风强度随高度增大而减小。大风过程自上而下改变边界层结构,平均动能、湍流动能和摩擦速度最先从上层(280 m)发生变化且迅速增加。近地层由于风速垂直梯度的显著差异,近地层垂直方向的湍流强度最大。大风时各功率谱在低频区(0.01 s~(-1))达到峰值,大风过后各高度的能量都有所下降。     

Ceilometer observations of aerosol layer structure above the Petit Lubéron during ESCOMPTE's IOP 2

A modified ceilometer has been used during the second Intensive Observation Period (IOP) of the “Expérience sur Site pour COntraindre les Modèles de Pollution atmosphériques et de Transport d'Émission” (ESCOMPTE) to perform continuous remote observations of aerosol accumulations in the first 3 km of the atmosphere. These observations encompassed an episode of intense particulate and photochemical pollution. The submicronic particles density, measured at an altitude of 600 m, went from a very low point of a few tens of particles per cubic centimeter (at the end of a Mistral episode in the free atmosphere) to a high point of more than 4500 particles per cubic centimeter (when pollutants were trapped by thermal inversions).The main result is that this instrument enables a fine documentation of the mixing layer height and of aerosol particles stratifications and circulation. Airborne aerosol measurements have been made above the mountainous region of Mérindol in order to validate in situ the remote sensing measurements. Ozone measurements near the summit of the mountains as well as in the valley were performed in order to correlate aerosol accumulation and ozone concentration. As a notable example, the two-layer aerosol stratification seen in the first 2 days of IOP 2b in that part of the ESCOMPTE domain confirms the results of another team which used backtrajectories. The low-altitude pollution for this timeframe had a local origin (the Fos industrial area), whereas above 500 m, the air masses had undergone regional-scale transport (from north-eastern Spain).The second major result is the highlighting of a pattern, in sea breeze conditions and in this part of the ESCOMPTE experiment zone, of nocturnal aerosol accumulation at an altitude of between 500 and 2000 m, followed by high ozone concentration the next day.     

大气边界层物理研究进展

本文总结了近4年来(2009～2012)中国科学院大气物理研究所大气边界层物理和大气化学国家重点实验室在大气边界层物理理论、观测实验、数值模拟和超声风速温度仪等仪器研制领域的主要研究进展,并对未来几年内大气边界层物理的发展方向提出了一些建议.     

Determination of the Atmospheric Boundary Layer Height from Radiosonde and Lidar Backscatter

The height of the atmospheric boundary layer is derived with the help of two different measuring systems and methods. From radiosoundings the boundary layer height is determined by the parcel method and by temperature and humidity gradients. From lidar backscatter measurements a combination of the averaging variance method and the high-resolution gradient method is used to determine boundary layer heights. In this paper lidar-derived boundary layer heights on a 10 min basis are presented. Datasets from four experiments – two over land and two over the sea – are used to compare boundary layer heights from both methods. Only the daytime boundary layer is investigated because the height of the nighttime stable boundary layer is below the range of the lidar. In many situations the boundary layer heights from both systems coincide within ±200 m. This corresponds to the standard deviation of lidar-derived 10-min values within a 1-h interval and is due to the time and space variability of the boundary layer height. Deviations appear for certain situations and depend on which radiosonde method is applied. The parcel method fails over land surfaces in the afternoon when the boundary layer stabilizes and over the ocean when the boundary layer is slightly stable. An automatic radiosonde gradient method sometimes fails when multiple layers are present, e.g. a residual layer above the growing convective boundary layer. The lidar method has the advantage of continuous tracing and thus avoids confusion with elevated layers. On the other hand, it mostly fails in situations with boundary layer clouds     

Ground-based remote sensing observation of the complex behaviour of the Marseille boundary layer during ESCOMPTE

Ground-based remote sensing systems have been used during the ESCOMPTE campaign, to continuously characterize the boundary-layer behaviour through many atmospheric parameters (wind, extinction and ozone concentration distribution, reflectivity, turbulence). This analysis is focused on the comparison of the atmospheric stratification retrieved from a UV angular ozone lidar, an Ultra High Frequency wind profiler and a sodar, above the area of Marseille, on June 26th 2001 (Intensive Observation Period 2b). The atmospheric stratification is shown to be very complex including two superimposed sea breezes, with an important contribution of advection. The temporal and spatial evolution of the stratification observed by the UV lidar and by the UHF radar are in good agreement although the origin of the echoes of these systems is quite different. The complexity of the dynamic situation has only partially been retrieved by a non-hydrostatic mesoscale model used with a 3 km resolution.     

宜昌冬季两次降水过程大气边界层的观测分析

利用宜昌2007年12月10-25日的加密观测资料,分析了两次低值系统经过宜昌时大气边界层的温湿风廓线结构及其日变化特征。结果表明：位温廓线具有明显的日变化特征,对流边界层在白天出现和发展,其高度可达600m,而稳定边界层在夜间出现和发展,其高度可达300m,降水会抑制对流边界层和稳定边界层的发展;湿度廓线结构及其日变化与对流边界层的发展有关,总体上湿度随高度减小,贴近地面的薄层湿度随高度减小较快,而混合层内湿度随高度变化较小,出现降水时,近地层的湿度有明显增加,大气边界层内湿度随高度快速平稳减小;风速廓线结构比较复杂,总体上风速随高度增大,在大气边界层低层有时会出现一个风速极大值,风速廓线没有明显的日变化特征,大气边界层内风向变化较大,但以偏东风为主。     

Large-Eddy Simulation of the Stable Atmospheric Boundary Layer using Dynamic Models with Different Averaging Schemes

Large-eddy simulation (LES) of a stable atmospheric boundary layer is performed using recently developed dynamic subgrid-scale (SGS) models. These models not only calculate the Smagorinsky coefficient and SGS Prandtl number dynamically based on the smallest resolved motions in the flow, they also allow for scale dependence of those coefficients. This dynamic calculation requires statistical averaging for numerical stability. Here, we evaluate three commonly used averaging schemes in stable atmospheric boundary-layer simulations: averaging over horizontal planes, over adjacent grid points, and following fluid particle trajectories. Particular attention is focused on assessing the effect of the different averaging methods on resolved flow statistics and SGS model coefficients. Our results indicate that averaging schemes that allow the coefficients to fluctuate locally give results that are in better agreement with boundary-layer similarity theory and previous LES studies. Even among models that are local, the averaging method is found to affect model coefficient probability density function distributions and turbulent spectra of the resolved velocity and temperature fields. Overall, averaging along fluid pathlines is found to produce the best combination of self consistent model coefficients, first- and second-order flow statistics and insensitivity to grid resolution.     

Statistics of Scalar Fields in the Atmospheric Boundary Layer Based on Large-eddy Simulations. Part 1: Free Convection

Convection in a quasi-steady, cloud-free, shear-free atmospheric boundary layer is investigated based on a large-eddy simulation model. The performed tests indicate that the characteristic (peak) values of statistical moments at the top of the mixed layer are proportional to the interfacial scales (from gradients of scalars in the interfacial layer). Based on this finding a parameterization is proposed for profiles of scalar variances. The parameterization employs two, semi-empirical similarity functions F_m(z/z_i) andF_i(z/z_i), multiplied by a combination of the mixed-layer scales and the interfacial scales.     

The Adaptation of the Atmospheric Boundary Layer to a Change in Surface Roughness

The adaptation of the atmospheric boundary layer to a change in the underlying surface roughness is an interesting problem and hence much research, theoretical, experimental, and numerical, has been undertaken. Within the atmospheric boundary layer an accurate numerical model for the turbulent properties of the atmospheric boundary layer needs to be implemented if physically realistic results are to be obtained. Here, the adaptation of the atmospheric boundary layer to a change in surface roughness is investigated using a first-order turbulence closure model, a one-and-a-half-order turbulence closure model and a second-order turbulence closure model. Perturbations to the geostrophic wind and the pressure gradients are included and it is shown that the second-order turbulence closure model, namely the standard k - model, is inferior to a lower-order closure model if a modification to limit the turbulent eddy size within the atmospheric boundary layer is not included within the model.