Vertically resolved dust optical properties during SAMUM: Tinfou compared to Ouarzazate

Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground-based sites and by airborne high spectral resolution lidar. One of the sites, Tinfou, is located close to the border of the Sahara in Southern Morocco and was the main in situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for 1 d. The retrieved profiles of backscatter and extinction coefficients and particle depolarization ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross-section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions.     

EARLINET observations of the 14–22-May long-range dust transport event during SAMUM 2006: validation of results from dust transport modelling

We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ. Ångström exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical properties of the dust plume with a dust transport model. The model results and the experimental data show similar times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one reason for these deviations.     

Dust mobilization and transport in the northern Sahara during SAMUM 2006 – a meteorological overview

The SAMUM field campaign in southern Morocco in May/June 2006 provides valuable data to study the emission, and the horizontal and vertical transports of mineral dust in the Northern Sahara. Radiosonde and lidar observations show differential advection of air masses with different characteristics during stable nighttime conditions and up to 5-km deep vertical mixing in the strongly convective boundary layer during the day. Lagrangian and synoptic analyses of selected dust periods point to a topographic channel from western Tunisia to central Algeria as a dust source region. Significant emission events are related to cold surges from the Mediterranean in association with eastward passing upper-level waves and lee cyclogeneses south of the Atlas Mountains. Other relevant events are local emissions under a distinct cut-off low over northwestern Africa and gust fronts associated with dry thunderstorms over the Malian and Algerian Sahara. The latter are badly represented in analyses from the European Centre for Medium–Range Weather Forecasts and in a regional dust model, most likely due to problems with moist convective dynamics and a lack of observations in this region. This aspect needs further study. The meteorological source identification is consistent with estimates of optical and mineralogical properties of dust samples.     

Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006

During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10⁻³ to 5.2 × 10⁻³ at 450 nm and from 0.3 × 10⁻³ to 2.5 × 10⁻³ at 700 nm. Groupings of k values could be attributed to different source regions.     

Vertical profiling of Saharan dust with Raman lidars and airborne HSRL in southern Morocco during SAMUM

Three ground-based Raman lidars and an airborne high-spectral-resolution lidar (HSRL) were operated during SAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths. Aerosol Robotic Network (AERONET) Sun photometer observations and radiosoundings of meteorological parameters complemented the ground-based activities at the SAMUM station of Ouarzazate. Four case studies are presented. Two case studies deal with the comparison of observations of the three ground-based lidars during a heavy dust outbreak and of the ground-based lidars with the airborne lidar. Two further cases show profile observations during satellite overpasses on 19 May and 4 June 2006. The height resolved statistical analysis reveals that the dust layer top typically reaches 4–6 km height above sea level (a.s.l.), sometimes even 7 km a.s.l.. Usually, a vertically inhomogeneous dust plume with internal dust layers was observed in the morning before the evolution of the boundary layer started. The Saharan dust layer was well mixed in the early evening. The 500 nm dust optical depth ranged from 0.2–0.8 at the field site south of the High Atlas mountains, Ångström exponents derived from photometer and lidar data were between 0–0.4. The volume extinction coefficients (355, 532 nm) varied from 30–300 Mm⁻¹ with a mean value of 100 Mm⁻¹ in the lowest 4 km a.s.l.. On average, extinction-to-backscatter ratios of 53–55 sr (±7–13 sr) were obtained at 355, 532 and 1064 nm.     

State of mixing, shape factor, number size distribution, and hygroscopic growth of the Saharan anthropogenic and mineral dust aerosol at Tinfou, Morocco

The Saharan Mineral Dust Experiment (SAMUM) was conducted in May and June 2006 in Tinfou, Morocco. A H-TDMA system and a H-DMA-APS system were used to obtain hygroscopic properties of mineral dust particles at 85% RH. Dynamic shape factors of 1.11, 1.19 and 1.25 were determined for the volume equivalent diameters 720, 840 and 960 nm, respectively.
During a dust event, the hydrophobic number fraction of 250 and 350 nm particles increased significantly from 30 and 65% to 53 and 75%, respectively, indicating that mineral dust particles can be as small as 200 nm in diameter. Log-normal functions for mineral dust number size distributions were obtained from total particle number size distributions and fractions of hydrophobic particles. The geometric mean diameter for Saharan dust particles was 715 nm during the dust event and 570 nm for the Saharan background aerosol.
Measurements of hygroscopic growth showed that the Saharan aerosol consists of an anthropogenic fraction (predominantly non natural sulphate and carbonaceous particles) and of mineral dust particles. Hygroscopic growth and hysteresis curve measurements of the 'more' hygroscopic particle fraction indicated ammonium sulphate as a main component of the anthropogenic aerosol. Particles larger than 720 nm in diameter were completely hydrophobic meaning that mineral dust particles are not hygroscopic.     

Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006

Vertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9°N, –6.9°E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.     

Airborne measurements of dust layer properties, particle size distribution and mixing state of Saharan dust during SAMUM 2006

The Saharan Mineral Dust Experiment (SAMUM) was conducted in May/June 2006 in southern Morocco. As part of SAMUM, airborne in situ measurements of the particle size distribution in the diameter range 4 nm < D _p < 100 μm were conducted. The aerosol mixing state was determined below D _p < 2.5 μm. Furthermore, the vertical structure of the dust layers was investigated with a nadir-looking high spectral resolution lidar (HSRL). The desert dust aerosol exhibited two size regimes of different mixing states: below 0.5 μm, the particles had a non-volatile core and a volatile coating; larger particles above 0.5 μm consisted of non-volatile components and contained light absorbing material. In all cases, particles larger than 10 μm were present, and in 80% of the measurements no particles larger than 40 μm were present. The abundance of large particles showed almost no height dependence. The effective diameter D _eff in the dust plumes investigated showed two main ranges: the first range of D _eff peaked around 5 μm and the second range of D _eff around 8 μm. The two ranges of D _eff suggest that it may be inadequate to use one average effective diameter or one parametrization for a typical dust size distribution.     

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m⁻² per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m⁻² were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m⁻² for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.     

Vertical profiling of convective dust plumes in southern Morocco during SAMUM

Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9°N, 6.9°W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100–400 m. Most of the plumes (typically 50–95%) show top heights <1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3–4 km. Height-to-diameter ratio is mostly 2–10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5–26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (<0.3) favours plume evolution. Observed surface, 1 and 2–m air temperatures indicate that a difference of 17–20 K between surface and 2-m air temperature and of 0.9–1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2–7 m s⁻¹.     

沙尘辐射效应对天气和沙尘输送影响的数值模拟

现有的沙尘气溶胶数值预报模式大多是单向模式, 即只考虑气象场对沙尘气溶胶的输送作用, 而忽略沙尘气溶胶的辐射效应对气象场的反馈作用。本文利用一个考虑了沙尘气溶胶短波辐射效应的天气－沙尘双向模式, 对南疆盆地一次冷空气东侵而引起的沙尘天气进行数值模拟。通过比较考虑与不考虑辐射效应的两组试验, 分析了在沙尘暴－强浮尘过程中沙尘辐射效应对气象要素和沙尘输送的影响, 结果表明: 沙尘辐射效应的考虑明显改进后期强浮尘天气过程中海平面气压变化、地面气温日变化和气温垂直变化的预报; 前期沙尘暴过程沙尘辐射效应引起中低层主导风增大, 但同时使起沙量降低及沉降量减少; 沙尘辐射效应与地形的共同作用使沙尘的沉降量比起沙量减少的更显著, 从而使盆地内浮尘天气维持。     

Properties of dust aerosol particles transported to Portugal from the Sahara desert

Aerosol properties of mineral particles in the far field of an African desert dust outbreak were investigated that brought Saharan dust over the Mediterranean in different layers to Portugal. The measurements were performed inside the project Desert Aerosols over Portugal (DARPO) which was linked to the Saharan Mineral Dust Experiment (SAMUM). The maximum particle mass concentration was about 150 μg m⁻³ and the corresponding scattering coefficient was 130 M m⁻¹ which results in a mass scattering efficiency of 0.87 m² g⁻¹. The aerosol optical depth reached values up to 0.53 and the lidar ratio was between 45 and 50 in the whole dust loaded column. A comparison between particle size distributions and refractive indices derived from different instruments and models showed a general good agreement but some minor differences could also be observed. Measurements as well as calculations with a particle transport model suggest that there is a relatively higher concentration of very large particles in the upper region of the dust layer than on the surface which is likely connected with meteorological conditions at the observational site (Évora, Portugal).     

非结构网格空气质量模式对东亚强沙尘暴的初步模拟研究

基于中国科学院大气物理研究所自主研发的嵌套网格空气质量数值预报模式(NAQPMS)和英国帝国理工学院应用计算与建模小组(AMCG)研制的有限元流体模式(Fluidity),构建了非结构网格沙尘传输模式(Fluidity-Dust),并模拟再现了2010年3月19~22日东亚强沙尘暴整个暴发、演变的三维立体动态过程,从整体上对这次沙尘事件有了全新的直观认识和了解。通过利用FY-2D卫星沙尘反演资料及MODIS(Moderate Resolution Imaging Spectroradiometer)卫星反演的气溶胶光学厚度(AOD)资料,可对模拟结果进行整体上对比验证;同时,利用中国9个城市站点的PM10(空气动力学当量直径小于等于10μm的颗粒物,即可吸入颗粒物)地面观测资料以及日本多个站点的激光雷达资料,逐一对比分析了不同地区PM10的时空分布以及沙尘传输经过时的垂直分布情况;并与NAQPMS模式的沙尘模拟结果进行了模式间的对比分析。对比结果均表明:该模式具有较好的模拟能力,能很好地模拟再现整个沙尘暴过程,为今后进一步运用自适应变网格技术以实现对沙尘暴的高精度追踪模拟奠定了基础。不同模式比较是量化模拟不确定性的重要方法。以往沙尘输送模式比较研究表明:起沙量模拟的不确定性是沙尘暴数值模拟的最大不确定来源。本文通过两个具有相同起沙方案的模式对同一沙尘事件的模拟,发现不同的平流方案以及不同的沉降计算也会对沙尘过程模拟产生重要影响。     

A simulation of Asian dust events in March 2010 by using the ADAM2 model

Asian dust events occurred in Asia during March 2010 were simulated using the Asian Dust Aerosol Model 2 (ADAM2). The performance of the model for simulations of surface dust concentrations and dust event occurrences was tested at several monitoring sites located in the dust source region and the downstream region of Korea. The observed and modeled dust event occurrences at each monitoring site were defined with the hourly observed and modeled dust concentrations that were used to evaluate the performance of the model by constructing a contingency table for the dust event occurrence. It was found that the model simulated quite well the starting and ending times of dust events with their peak dust concentrations for most dust events occurred both in the dust source region and the downstream region of Korea. However, the model failed to simulate a few dust events observed in both regions mainly due to the inaccurate simulations of the meteorological fields. Inaccurate simulations of wind speeds have caused for the model to simulate dust events poorly in the dust source region whereas poor simulations of precipitation of the fifth-generation mesoscale model (MM5) model have led to miss dust events in the downstream region of Korea. The contingency table made with the hourly data for the dust event occurrence made it possible to evaluate the ADAM2 model for the simulation of the dust event occurrence. It was found that the model has the probabilistic simulation capability for dust events of about 78% with the hit rate of more than 83% and the false alarm rate of about 27% for the dust events occurred during March in 2010. The probabilistic capability of the model could be much improved by improving the meteorological model (MM5 model).     

2006年春季沙尘天气异常的气候背景分析及趋势预测检验

作者概述了2006年春季我国北方地区沙尘天气异常的观测特征.与2000～2005年同期相比,2006年春季,我国北方地区沙尘天气过程相对频繁,强度偏强.首先,利用NCEP再分析资料及台站观测资料,对可能影响2006年春季我国沙尘天气异常的气候背景进行了分析,结果表明:2006年春季北方地区冷空气势力较常年强,冷空气活动较频繁,这是造成2006年春季我国沙尘天气多于前几年的动力原因.此外,春季我国西北及内蒙古的沙源地区降水偏少,干旱少雨,加之春季气温偏高,在一定程度上也有利于沙尘天气的形成.利用IAP 年度数值气候预测系统对2006、2007年春季我国沙尘天气趋势的气候背景进行了预测,检验了系统对2006年沙尘趋势的预测能力,并对2007年春季沙尘天气趋势进行了展望.结果表明:预测系统较好地预测出2006年我国北方地区春季降水偏少、土壤偏干的状况,但对风场的预测与实况有一定的差距,由预测的气候距平结果推断2006年春季我国北方沙尘次数正常略偏少,与实况相反;对2007年春季的预测结果表明,2007年春季我国北方,特别是西北及内蒙古沙源地区土壤湿度偏湿,冷空气活动势力偏弱,不利于沙尘天气的形成,由此预测2007年春季我国北方沙尘趋势与常年相比略偏弱.     

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite's larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR's ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.     

Analysis of aircraft measurements of momentum flux in the subcloud layer over the tropical atlantic ocean during gate

The momentum flux data obtained by the gust probe aboard the NOAA DC-6 aircraft during GATE are analyzed. Vertical profiles are obtained for Phases I and III and correlated with vertical wind velocity profiles using the geostrophic departure method. Reasonable agreement is obtained using the horizontal equations of motion with negligible advective acceleration. The vertical profiles of momentum flux and wind speed variance compare well with the numerical model results of Deardorff (1972) and Wyngaard et al. (1974). Vertical distributions of power spectra for vertical eddy motion and cospectra corresponding to the momentum flux components are obtained along with the height variation of the dominant length scales of vertical eddy motion and the dissipation rate of turbulence kinetic energy. When normalized by mixed-layer similarity, these results agree well with previous determinations in the boundary layer over tropical oceans and over land.     

新疆不同降水观测资料的比较及其差异的可能原因

利用新疆地区89个气象站和38个水文站1961~2005年的降水资料,对比分析了这两类降水观测资料在表征新疆地区降水的时间演变特征上所存在的差异,进而探究了造成这种差异的原因,并在此基础上研究了增暖背景下新疆地区降水与地形(包括海拔高度、坡度和坡向)的关系。研究发现:1)从区域平均来看,水文站观测的年降水量(221.4 mm)明显高于气象站的观测值(152.1 mm),其差值为69.3 mm,但是二者具有相同的时空变化特征;2)气象站与水文站降水均值之间的差异,主要受海拔高度的影响,坡度和坡向的影响次之。新疆地区地形地貌复杂,降水时空变化较大,观测数据分布不均匀均可导致对降水的估算存在较大差异,因此多源降水数据相互融合是客观估算该地区降水量及其变化的一个途径。     

Global radiation within corn

Global and net solar radiation profiles were measured by traversing sensors at four heights in a square-sown plot and a row plot of field corn on four relatively cloudless days in August 1972. The fluxes and their vertical distributions are discussed. A numerical model of the short-wave radiation fluxes in a canopy is presented. Using leaf area index and fixed leaf radiative properties, calculated values of radiation are within 10% of measurements in most instances. The performance under predominantly beam or diffuse radiation is similar and model values of crop albedo compare well with values calculated from radiation measurements above the crops.     

Spectral aerosol optical depth characterization of desert dust during SAMUM 2006

The aerosol optical depth (AOD) in the range 340–1550 nm was monitored at Ouarzazate (Morocco) during the Saharan Mineral Dust Experiment (SAMUM) experiment in May–June 2006. Two different sun photometers were used for this purpose. The mean AOD at 500 nm was 0.28, with a maximum of 0.83, and the mean Ångström exponent (AE) was 0.35. The aerosol content over the site changed alternatively from very low turbidity, associated to Atlantic air masses, to moderate dust load, associated to air masses arriving in the site from Algeria, Tunisia and Libya. The dusty conditions were predominant in the measurement period (78% of data), with AOD (500 nm) above 0.15 and AE below 0.4. The spectral features of the AOD under dusty conditions are discussed. Air mass back trajectory analysis is carried out to investigate the origin and height patterns of the dust loaded air masses. The advection of dust occurred mainly at atmospheric heights below 3000 m, where east flow is the predominant. At the 5000 m level, the air masses originate mainly over the Atlantic Ocean. Finally the Optical Properties of Aerosols and Clouds (OPAC) model is used to perform a set of simulations with different aerosol mixtures to illustrate the measured AOD and AE values under varying dust concentrations, and a brief comparison with other measurement sites is presented.