Variation Of Layer Height Research Articles

Impact of ice-free oases on particulate matter over the East Antarctic: Inferences from the carbonaceous, water-soluble species and trace metals

The present study reports on carbonaceous, water-soluble species and trace metals in PM10 aerosols collected at Bharati station during the austral summer (December 17, 2016 to February 2, 2017) over East Antarctica. Organic matter was the predominant among all measured species followed by nss-Ca2+ and nss-SO42-. Two distinct air masses that arrive from high-altitude-troposphere (HTAs) and low-altitude-troposphere (LTAs) regions which clearly showed significant differences in concentrations of measured chemical and metal species as inferred from the air mass backward trajectory analysis and mixed layer height variations. Based on significant positive correlations among chemical species, aerosol liquid water content (ALWC), and PM10 mass, we confirmed that the aqueous-phase formation of secondary aerosols followed by atmospheric processing are possible sources in HTAs while primary emissions associated with soil/dust from ice-free regions and in-situ emissions are major sources in LTAs. This result further supported by significant (p < 0.05) higher concentrations of specific trace metals (for example, Fe, Ti, and Pb) in LTAs. Moreover, higher cation-to-anion (Σ+/Σ−) ratios suggesting the alkaline nature of aerosols at Bharati. The present study demonstrates that ice-free regions over East Antarctica may act as major sources of particulate matter, thus significant implications toward climate change over the global environment.

Diagnostic analysis of wintertime PM2.5 pollution in the North China Plain: The impacts of regional transport and atmospheric boundary layer variation

Air pollution has recently become a regional problem in the North China Plain, which is heavily populated with industrialized city clusters. The local air quality is frequently affected by air pollutant transport in this region, as well as other meteorological conditions. This study aims to reveal the roles of air pollutant transport and atmospheric boundary layer variation in the development of air pollution episodes. Two cities (Dezhou and Cangzhou in Shandong and Hebei Provinces, respectively) near the centre of the North China Plain are chosen, and PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm) concentrations over two winters (January 2016 and December 2017 to January 2018) are evaluated. Both meteorological and air quality observational data are used to construct hourly wind and PM2.5 concentration fields. High-quality atmospheric boundary layer soundings provide detailed information on boundary layer height and its diurnal evolution. A simple box model is employed to simulate variation in PM2.5 levels in these two cities by using the spatial gradients and temporal variation rates in the diagnostic fields of wind and PM2.5 concentrations. Two prominent factors affecting PM2.5 concentration variation are identified by comparing the simulated and observed diurnal variation in PM2.5 concentration. The first one is the entrainment of upper-layer air pollutants into the boundary layer in the morning. And the second one is the temporary accumulation of PM2.5 during the day-to-evening period. After incorporating these two effects, the simulated PM2.5 is comparable with the observed data. The correlation coefficients between the simulation and observation are 0.84 and 0.77 (P < 0.01) for Cangzhou and Dezhou, respectively, and the normalized mean biases are −0.04 and −0.09, respectively. The relative contributions of different processes affecting PM2.5 concentrations during air pollution episodes are assessed with the model simulations. The percentage contributions for local emissions, deposition, horizontal advection, temporary accumulation, and boundary layer height variation are from 27% to 45%, −28% to −20%, 46% to 76%, 13% to 40%, and −6% to 22%, respectively. Local emission and regional air pollutant transport are the main causes of these episodes. Temporary accumulation of PM2.5 during the day-to-evening period is also an important contributor to the episodes. In addition, the entrainment shows varying contributions to PM2.5 increase with a maximum rate of 30% during the PM2.5 pollution episodes in the North China Plain.

Wear Behavior of 3D Printed PLA Surfaces for Superhydrophobic Interaction

Polylactic Acid (PLA) is a biodegradable thermoplastic polymer known for its widescale application in bio-tribology. A known problem with PLA is its high level of water adsorption due to its hydrophilic nature which can lead to growth of mold and other fungal derivatives due to water stagnation. In our investigation we developed a re-entrant pattern using a standard fused-deposition modelling (FDM) 3D printer with the aim to achieve surfaces which are superhydrophobic in nature. The layer heights of the re-entrant profiles were systematically modified while keeping all other printing parameters constant to achieve a multi-edge ladder effect. The samples were then investigated for their superhydrophobic characteristics by measurement of Contact and Tilt Angles and thereafter characterized for wear resistance using a ball-on-disc tribometer. A fixed low rpm setting was used to eliminate the effect of temperature. Wear parameters including Wear Depth and Coefficient of Friction were recorded after each cycle using until the sample’s re-entrant structure was damaged and no longer supported water-repellent behavior shown by a considerable decrease in water contact angle. The effect of layer height variation in the re-entrant profiles for effective wear resistance was established as a preliminary study for our subsequent research in the area. It can be concluded that the layer height plays a vital role in achieving superhydrophobicity and has a direct influence on the wear resistant of the surface.

Resolution, energy and time dependency on layer scaling in finite element modelling of laser beam powder bed fusion additive manufacturing

The Laser Beam Powder Bed Fusion (PBF-LB) category of Additive Manufacturing (AM) is currently receiving much attention for computational process modelling. Major challenges exist in how to reconcile resolution, energy and time in a real build, with the practical limitations of resolution (layer height and mesh resolution), energy (heat format and magnitude) and time (heating and cooling step times) in the computational space. A novel thermomechanical PBF-LB process model including an efficient powder-interface heat loss mechanism was developed. The effect of variations in layer height (layer scaling), energy and time on the temperature and stress evolution was investigated. The influence of heating step time and cooling step time was characterised and the recommended ratio of element size to layer scaling was presented, based on a macroscale 2D model. The layer scaling method was effective when scaling up to 4 times the layer thickness and appropriately also scaling the cooling step time. This research provides guidelines and a framework for layer scaling for finite element modelling of the PBF-LB process.

Picosecond Pulsed Laser Ablation of Liquid Covered Stainless Steel: Ef-fect of Liquid Layer Thickness on Ablation Efficiency

Under liquid laser ablation is a material removal technique in which a focused laser beam passes through a liquid layer on top of the surface of a sample to be processed. When compared to laser ablation without a liquid layer, material (re)deposition around ablated regions, as well as the size of the Heat Affected Zone (HAZ) is decreased. In addition, the ablation efficiency of the process, in terms of the amount of material removed per pulse, can be optimized by careful variation of the height of the liquid layer: a liquid layer height variation as small as a few tens of millimeters already has a measurable effect on the amount of ablated material. In studies reported in existing literature, the required liquid layer height is typically realized by pouring a pre-defined amount of liquid on top of the sample surface. Surface tension, however, causes the air-liquid interface at the boundaries of the domain to deviate from the planar interface away from the boundaries, which affects the accuracy with which the liquid layer height can be determined. To the best of our knowledge, these accuracy issues have not been studied in previous research. Therefore, an experimental set-up is proposed which circumvents the issues of a curved free surface. Next, a 7 picosecond pulsed laser source (M2≤1.3) at a wavelength of 515nm was employed to study the efficiency of laser ablation of stainless steel for a range of liquid layer heights. Our findings provide a more detailed quantification of crater depth as a function of liquid layer height than is available through existing literature.

Surface Meteorological Conditions and Boundary Layer Height Variations During an Air Pollution Episode in Nanjing, China

AbstractThe evolution of planetary boundary layer (PBL) was investigated using observations from a laser ceilometer, an eddy covariance system, and an automatic meteorological station in the north of Nanjing city during an air pollution episode in 2016–2017 winter. Based on 7‐day observation under clean to polluted day, we recorded the temporal variations of backscatter signals observed by the ceilometer, and then intercompared the planetary boundary layer height (PBLH) retrieved from individual methods. The results show that backscatter signal gradient, standard deviation, and wavelet transform analysis methods generated similar PBLH values and PBL diurnal variation patterns. Moreover, the PBL structure varied diurnally, with distinct patterns corresponding to clean and polluted days. Based on these measurements, the relationships between PBLH, weather conditions, and contaminants were analyzed. Results show that on clean days, strong surface turbulence exchange makes the PBL fully developed and makes the PBLH increase sharply after sunrise, with a maximum of 1,483 m; on polluted days, stable synoptic conditions and the weaker wind speeds facilitated the accumulation of air pollutants, leading to smaller net surface radiation and weaker turbulence. Consequently, these conditions during polluted days led to lower PBLH values, which were typically less than 900 m.

The role of Boundary Layer Height (BLH) variations on pollution dispersion over a coastal station in the Southwest Peninsular India

We report the variability of Boundary Layer Height (BLH) and Ventilation Coefficient (VC) over a coastal station in southwest Peninsular India from vertical soundings. Altitude profile of Virtual Potential Temperature (θv) (derived from balloon-borne GPS radiosonde observations) was used to study the seasonal changes of BLH over Thumba (8.5°N, 77°E), a coastal station in southwest Peninsular India. Variability in the BLH is maximum during the pre-monsoon season and minimum during the winter, whereas the VC was found to be highest in the summer and lowest during winter. It is revealing the excellent efficiency in the dispersion of pollutants during the summer monsoon season. Conversely, low VC during winter and post-monsoon seasons showed great potential for pollution at this site. Role of VC on the dispersion of pollution was also examined using surface mass concentration of Black and Organic Carbon (BC and OC), sulphate and columnar Aerosol Optical Depth (AOD). High value of VC lowers the surface mass concentrations of BC, OC and sulphate during winter and maximises during summer monsoon and decreases the pollution potential.

Vertical Distributional Characteristics of Inorganic Ions of PM2.5 at Mt. Huashan, Inland China

PM2.5 was simultaneously collected using a high-volume sampler at 4 h intervals from the mountainside and the foot of the mountain in the Mt. Huashan region, inland China, during the summer of 2016, and the samples were analyzed for inorganic ions, to investigate the chemical characteristics and vertical distribution of the fine particles in the region. The results showed that the concentrations of PM2.5 were (46.9±38.2) μg·m-3 and (76.0±44.3) μg·m-3 on the mountainside and at the foot of Mt. Huashan, respectively. The concentrations of inorganic ions in PM2.5 was higher at the foot of the mountain than on the mountainside, with the order of the mass concentrations of the major ionic species being SO42- > NO3- > NH4+ > Ca2+. Among all the inorganic ions, SO42-, NO3-, and NH4+ are the dominant species, accounting for 89% and 85% of the total, on the mountainside and at the foot, respectively. The fine particulate NH4+ existed mostly in the forms of (NH4)2SO4 and NH4NO3 on the mountainside, and in the forms of NH4HSO4 and NH4NO3 at the foot of the mountain. Mass concentrations of PM2.5 and its major components on the mountainside showed clear diurnal variations, with maximums between 12:00-16:00, mainly due to the transport of the pollutants from the ground surface by the planetary boundary layer height variation and the valley breeze. In contrast, the diurnal variations of PM2.5 and its major components at the foot are characterized with two maxima, peaking between 08:00-12:00 in the daytime and 00:00-04:00 in the nighttime, respectively, mainly due to the increased emissions from both the morning rush-hour traffic and the nighttime on-road heavy-duty vehicles. Acidity of the fine particles was estimated by using the equivalent ratio of anions to cations and the thermodynamic equilibrium model ISORROPIA Ⅱ. Both methods showed that the acidity of PM2.5 at the ground surface site is stronger than that on the mountainside in the Mt. Huashan region.

Impact of vegetative emissions on urban ozone and biogenic secondary organic aerosol: Box model study for Berlin, Germany

Tropospheric ozone and particulate matter affect human health and cause vegetation stress, dysfunction and damages. In this study we investigate the effect of increasing urban vegetation i.e. tree species on atmospheric chemistry, a potential urban management strategy to counteract high levels of local pollutants such as ozone, OH and PM10 caused by e.g. traffic. We use an extended version of an atmospheric chemistry box model including detailed gas-phase chemistry, mixing layer height variation and secondary organic aerosol calculations based on observations for Berlin, Germany. It is shown to accurately simulate the observed ozone volume mixing ratios during the intensive measurement period in July 2014 (BAERLIN2014) if basic parameters such as nitrogen oxides, meteorological conditions, PM10 concentrations as well as volatile organic compounds (VOCs) are considered as 1 h resolved datasets. Based on this setup the effects of changing present day vegetation mixture by 24 different relevant tree species and of urban greening is tested to elucidate benefits and drawbacks in order to support future urban planning. While the present day vegetation causes boundary layer ozone to decline slightly at 35 °C, individual tree types alter the ozone production rate and concentration as well as the secondary organic aerosol mass in different ways. Our results suggest that trees intensively emitting isoprene such as black locust, European oak and poplar result in higher ozone and total PM10 concentrations than at present, while tree species emitting primarily monoterpenes such as beech, magnolia and wayfaring trees yield less of both. This is in line with the similar behaviour of OH concentration and new particle formation rates. Thus, for future urban planning including urban greening, consideration of the beneficial and harmful aspects of tree species need to ensure that citizens benefit from and are not being negatively affected by climate adaptation strategies.

Boundary Layer Characteristics over Aindanda Low-Mountain Pass of Kathmandu Valley, Nepal

The early monsoon time boundary layer characteristics prevailing over Aindanda low-mountain pass of Kathmandu valley has been continuously monitored for the period of 11 to 24 June 2013. The study reveals that the Aindanda pass channels regional air masses from the western neighboring valley up into the Kathmandu valley as westerly/ northwesterly winds during the daytime whereas it drains air mass out of the valley during night-time. The speed of the westerly/northwesterly wind over the pass often exceeds 6.5 ms-1 during the late afternoon. Nighttime mixing layer height (MLH) was highly fluctuating with an average around 300m whereas daytime MLH was suppressed limiting it in between 290-450m above the ground in early part of the day but reduced to 210-270m during the late afternoon. Comparison of diurnal variation of mixing layer height at Aindanda with that of the central area of the valley floor strongly suggests that air mass intruding into the Kathmandu valley through this pass is a cool density flow over the weakly stratified mixed layer of valley. The structure of the wind channeled through this pass indicates the possibility of making hydraulic jump in the western part of the Kathmandu valley, particularly, during the late afternoon time.Journal of Institute of Science and Technology, 2015, 20(2): 22-30

Boundary Layer Characteristics over the Central Area of the Kathmandu Valley as Revealed by Sodar Observation

The characteristic behavior of prevailing boundary layer over the central area of the Kathmandu valley was continuously monitored by deploying a monostatic flat array sodar during the period of 03 to 16 March 2013. Diurnal variation of wind and mixing layer height were chosen to describe the boundary layer activities over the area by considering the day of 12 March 2013 as the representative day for the period of observation. The study shows that central area of the valley remains calm or windless under stable stratification throughout the night and early morning frequently capped by northeasterly or easterly wind aloft. Strong surface level thermal inversion prevails during the period up to the height of 80m above the surface. This inversion tends to lift up as the morning progresses and reaches to the height of 875 m or so close to the noontime. Intrusion of regional winds as westerly/northwesterly and the southerly/southwesterly from the western and southwestern low-mountain passes and the river gorge in the afternoon tends to reduce the noontime mixing layer height to about 700 m. The diurnal variation of wind and mixing layer height suggest that Kathmandu valley possesses a poor air pollution dispersion power and hence the valley is predisposed to high air pollution potential.Journal of Institute of Science and Technology, 2015, 20(1): 28-35

Typhoon kinematic and thermodynamic boundary layer structure from dropsonde composites

AbstractThe data from 438 Global Positioning System dropsondes in six typhoons are analyzed to investigate the mean atmospheric boundary layer structure in a composite framework. Following a recent study on boundary layer height in Atlantic hurricanes, we aim to quantify characteristics of boundary layer height scales in Western Pacific typhoons including the inflow layer depth (hinflow), height of the maximum tangential wind speed (hvtmax), and thermodynamic mixed layer depth. In addition, the kinematic and thermodynamic boundary layer structures are compared between the dropsonde composites using data in typhoons and hurricanes. Our results show that similar to the hurricane composite, there is a separation between the kinematic and thermodynamic boundary layer heights in typhoons, with the thermodynamic boundary layer depth being much smaller than hinflow and hvtmax in the typhoon boundary layer. All three boundary layer height scales tend to decrease toward the storm center. Our results confirm that the conceptual model of Zhang et al. (2011a) for boundary layer height variation is applicable to typhoon conditions. The kinematic boundary layer structure is generally similar between the typhoon and hurricane composites, but the typhoon composite shows a deeper inflow layer outside the eyewall than the hurricane composite. The thermodynamic structure of the typhoon boundary layer composite is warmer and moister outside the radius of maximum wind speed than the hurricane composite. This difference is attributed to different environmental conditions associated with typhoons compared to the hurricanes studied here.

Forest Filter Effect: Role of leaves in capturing/releasing air particulate matter and its associated PAHs

Plants play a key role in removing particulate matter and their associated Semi-volatile Organic Compounds (SVOCs) from the atmosphere. Understanding the processes involved in particle capture by vegetation is essential to understand the interactions between SVOCs, particles and plants. In the present study Two Photon Excitation Microscopy (TPEM) was used to visualise particle matter uptake and encapsulation, together with its distribution on leaf/needle surface of different broadleaf (cornel and maple) and conifer species (stone pine). Phenanthrene accumulation, the number of particles associated with this compound and its migration from particles into the leaf cuticle was also identified and quantified. Species-specific deposition velocities were estimated to model temporal PM10 leaf/needle accumulation and to investigate the role of Planet Boundary Layer (PBL) height variation in influencing PM10 flux to plants. Particles at the leaf/needle surface were visualised to range in size from 0.2 to 70.4 μm, but cuticular encapsulation was negligible for particles larger than 10.6 μm, which were removed by a washing procedure. Phenanthrene concentration varied between ≈5 and ≈10 ng g−1 dw according to plant species and between ≈10 and ≈200 ng g−1 dw depending on needle age; this compound was visualized to migrate from particles into the adjacent leaf cuticle. Species-specific deposition velocity range between 0.57 and 1.28 m h−1 and preliminary simulations showed that the diel variability of PBL structure influenced the temporal PM10 flux and leaf/needle concentration, e.g. during daytime hours characterized by high PBL height, PM10 accumulated on cornel leaves was about 65% lower than the amount accumulated during night time. The capability of vegetation to capture particles from the atmosphere, retain, encapsulate them into the cuticle and release them to soil and/or lower biomass, highlighted the value of vegetation in removing pollutants from the atmosphere and influencing their environmental fate.

A method to estimate concentrations of surface-level particulate matter using satellite-based aerosol optical thickness

We propose a new method to estimate surface-level particulate matter (PM) concentrations by using satellite-retrieved Aerosol Optical Thickness (AOT). This method considers the distribution and variation of Planetary Boundary Layer (PBL) height and relative humidity (RH) at the regional scale. The method estimates surface-level particulate matter concentrations using the data simulated by an atmospheric boundary layer model RAMS and satellite-retrieved AOT. By incorporation MODIS AOT, PBL height and RH simulated by RAMS, this method is applied to estimate the surface-level PM2.5 concentrations in North China region. The result is evaluated by using 16 ground-based observations deployed in the research region, and the result shows a good agreement between estimated PM2.5 concentrations and observations, and the coefficient of determination R 2 is 0.61 between the estimated PM2.5 concentrations and the observations. In addition, surface-level PM2.5 concentrations are also estimated by using MODIS AOT, ground-based LIDAR observations and RH measurements. A comparison between the two estimated PM2.5 concentrations shows that the new method proposed in this paper is better than the traditional method. The coefficient of determination R 2 is improved from 0.32 to 0.62.

Single particle analysis of ambient aerosols in Shanghai during the World Exposition, 2010: two case studies

ATSI Model 3800 aerosol time-of-flight mass spectrometer (ATOFMS) was deployed for single-particle analysis in Shanghai during the World Exposition (EXPO), 2010. Measurements on two extreme cases: polluted day (1st May) and clean day (25th September) were compared to show how meteorological conditions affected the concentration and composition of ambient aerosols. Mass spectra of 90496 and 50407 particles were analyzed respectively during the two sampling periods. The ART-2a neural network algorithm was applied to sort the collected particles. Seven major classes of particles were obtained: dust, sea salt, industrial, biomass burning, organic carbon (OC), elementary carbon (EC), and NH4-rich particles. Number concentration of ambient aerosols showed a strong anti-correlation with the boundary layer height variation. The external mixing states of aerosols were quite different during two sampling periods because of different air parcel trajectories. Number fraction of biomass burning particles (43.3%) during polluted episode was much higher than that (21.6%) of clean time. Air parcels from the East China Sea on clean day diluted local pollutant concentration and increased the portion of sea salt particle dramatically (13.3%). The large contribution of biomass burning particles in both cases might be an indication of a constant regional background of biomass burning emission. Mass spectrum analysis showed that chemical compositions and internal mixing states of almost all the particle types were more complicate during polluted episode compared with those observed in clean time. Strong nitrate signals in the mass spectra suggested that most of the particles collected on polluted day had gone through some aging processes before reaching the sampling site.

Measurements of molecular hydrogen and carbon monoxide on the Trainou tall tower

We present 2 yr (October 2008 to September 2010) of in situ measurements of molecular hydrogen (H2) and carbon monoxide (CO) sampled at the tall tower of Trainou, France (47.96°N, 02.11°E, 131 masl, sampling height: 50, 100 and 180 m). Radon-222 (222Rn) measurements were added in May 2009. Background seasonal cycles, based on afternoon values, exhibit an amplitude of 45 and 60 ppb for H2 and CO, respectively, for the three different heights (50, 100 and 180 m above ground). The vertical gradient also shows seasonal variations with a maximum (during the night) of 20 and 45 ppb for H2 and CO, respectively.We also observe diurnal cycles for H2 and CO for the three different heights. In the afternoon, the mixing ratios at the three different heights are similar and are comparable with maritime background stations, such as Mace Head (Ireland). The diurnal cycle of 222Rn follows the boundary layer height variations, with maximum values in the morning. Throughout the year but especially in summer and autumn, the H2 mixing ratio shows nighttime depletion, with the lowest values at 06:00 UTC, due to soil uptake and the low boundary layer height. Using a simple box approach and the Radon-Tracer-Method, the H2 deposition velocity is calculated for the catchment area of Trainou.We find a mean value for the H2 deposition velocity of 2.6 ± 0.9 10-2 cm-2 s-1. During wintertime, H2 and CO are sometimes strongly correlated leading to a H2/CO ratio around 0.25. This ratio is lower than the ratio from traffic emissions, thus highlighting the mixing of sources in this area.

Seasonal variability in mixed layer height and its impact on trace gas distribution over a tropical urban site: Ahmedabad

Abstract Altitude profiles of virtual potential temperature ( θ v ) and specific humidity ( q ) derived from meteorological data obtained from balloon-borne radiosonde ascents are used to investigate the seasonal variations in mixed layer height over Ahmedabad (23.03°N, 72.54°E), an urban site located on the western part of India. A total of 82 balloon ascents were conducted fortnightly in the morning hours during a period of about four years spanning from April 2003 to July 2007. Analysis of the vertical profiles of θ v and q reveals a systematic seasonal variability in the mixed layer height (MLH), showing the decreasing trend from summer–monsoon to winter season. The MLH is observed to be maximum (∼ 1170 m) in summer–monsoon while a minimum (∼ 160 m) is observed during the winter months. In general, the mixed layer height was found to be highly variant during pre-monsoon and summer–monsoon seasons. This variability is observed to be comparatively lower in the post-monsoon and winter months. Effects of MLH have been investigated on the variations in surface ozone and MOPITT derived surface and vertical distributions of CO. The reverse trend is observed in surface ozone and CO with mixed layer seasonal variability. The impact of MLH over CO vertical distributions is observed up to an altitude of 3–4 km.

Reconstruction of spatial structure of thin layer in sporadic E region by using VHF coherent scatter radar

On the basis of interferometer technique implemented at a VHF coherent scatter radar, a method of measuring tilted angle and true horizontal extent of a thin layer structure containing 3‐m field‐aligned irregularities (FAIs) in ionospheric sporadic E (Es) region is proposed in this article. The essential part of this method is to measure the length and the orientation of the striation‐like echo pattern resolved by interferometry technique, which is the result of the interception of the layered structure by a fan‐like effective antenna beam pattern defined by the product of the physical antenna beam and the aspect sensitivity of the backscatter from Es FAIs. From the projections of the striated echo pattern on mutually orthogonal planes, the zenith and azimuth angles of the unit vector normal to the layer can be estimated in accordance with a set of line equations describing the length and the orientation of the echo pattern. It is found that the Es thin layer for the present case is primarily tilted in the northeast‐southwest direction with a maximum zenith angle of 11°. In addition, quasi‐periodic variations in layer height and tilted angle with a period of approximately 3–5 min were observed, in which the peak‐to‐peak amplitudes of the layer height variation were in a range of 0.3–1.2 km. These features provide solid evidences that the thin Es layer was modulated by a quasi‐periodic oscillation not only in the layer height, but also in the tilt of the layer surface. Interferometer measurement reveals that the layer is in an anisotropic shape with major axis aligned in the northwest‐southeast direction making an angle of approximately 45° with respect to the west direction. The width (or minor axis length) of the thin layer varied with time in a range 8.8–13 km, and the axial ratio of the anisotropy is no less than 1.73. Although the azimuth alignment of the anisotropic thin layer is in harmony with that of the Es layer instability proposed by Cosgrove and Tsunoda (2002), the measured Doppler velocity, drift direction of the FAIs, and interferometer‐resolved horizontal extent of the layer do not favor the theoretical predictions. In light of the fact that the oscillating period is larger than Brunt‐Vaisala period in Es region and the amplitude of the layer modulation is consistent with that of a propagating gravity wave, it suggests that the oscillation was very likely associated with a short‐period gravity wave with period of 3–5 min propagating in the northeast‐southwest direction.

Microphysical and optical properties of Arctic mixed-phase clouds. The 9 April 2007 case study.

Abstract. Airborne measurements in Arctic boundary-layer stratocumulus were carried out near Spitsbergen on 9 April 2007 during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign. A unique set of co-located observations is used to describe the cloud properties, including detailed in situ cloud microphysical and radiation measurements along with airborne and co-located spaceborne remote sensing data (CALIPSO lidar and CloudSat radar). CALIPSO profiles indicate cloud top levels at temperature between −24°C and −21°C. In situ measurements confirm that the cloud-top lidar attenuated backscatter signal along the aircraft trajectory is linked with the presence of liquid water, a common feature observed in Arctic mixed-phase stratocumulus clouds. A low concentration of large ice crystals is also observed up to the cloud top resulting in significant CloudSat radar echoes. Since the ratio of the extinction of liquid water droplets to ice crystals is high, broadband radiative effects near the cloud top are mostly dominated by water droplets. CloudSat observations and in situ measurements reveal high reflectivity factors (up to 15 dBZ) and precipitation rates (1 mm h−1). This feature results from efficient ice growth processes. About 25% of the theoretically available liquid water is converted into ice water with large precipitating ice crystals. Using an estimate of mean cloud cover, a considerable value of 106 m3 h−1 of fresh water could be settled over the Greenland sea pool. European Centre for Medium-Range Weather Forecast (ECMWF) operational analyses reproduces the boundary layer height variation along the flight track. However, small-scale features in the observed cloud field cannot be resolved by ECMWF analysis. Furthermore, ECMWF's diagnostic partitioning of the condensed water into ice and liquid reveals serious shortcomings for Arctic mixed-phased clouds. Too much ice is modelled.

Variation of dust layer height in the northern Taklimakan Desert in April 2002

LIDAR observations were conducted in the northern Taklimakan Desert to investigate the vertical distribution of dust in April 2002. During the observation period, a dust outbreak occurred on 13 April, remaining over the desert for several days. Despite only slight variations being observed in dust layer height before the dust event, marked diurnal variation in dust layer height was observed after the dust event. In this study, we conducted two numerical simulations to investigate differences in the variation exhibited by the dust layer heights before and after the dust event, and assessed the influence of meteorological conditions on this variation. The simulated results show that the clearly diurnal variation in dust layer height is strongly influenced by local circulation which is affected by the characteristic topography and synoptic conditions of the Tarim Basin.