Variation Of Boundary Layer Height Research Articles

Atmospheric CO2 in the megacity Hangzhou, China: Urban-suburban differences, sources and impact factors

Limited observation sites and insufficient monitoring of atmospheric CO2 in urban areas restrict our comprehension of urban-suburban disparities. This research endeavored to shed light on the urban-suburban differences of atmospheric CO2 in levels, diurnal and seasonal variations as well as the potential sources and impact factors in the megacity of Hangzhou, China, where the economically most developed region in China is. The observations derived from the existing Hangzhou Atmospheric Composition Monitoring Center Station (HZ) and Lin'an Regional Atmospheric Background Station (LAN) and the newly established high-altitude Daming Mountain Atmospheric Observation Station (DMS), were utilized. From November 2020 to October 2021, the annual averages of HZ, LAN and DMS were 446.52 ± 17.01 ppm, 441.56 ± 15.42 ppm, and 422.02 ± 10.67 ppm. The difference in atmospheric CO2 mole fraction between HZ and LAN was lower compared to the urban-suburban differences observed in other major cities in China, such as Shanghai, Nanjing, and Beijing. Simultaneous CO2 enhancements were observed at HZ and LAN, when using DMS observations as background references. The seasonal variations of CO2 at LAN and DMS exhibited a high negative correlation with the normalized difference vegetation index (NDVI) values, indicating the strong regulatory of vegetation canopy. The variations in boundary layer height had a larger influence on the low-altitude HZ and LAN stations than DMS. Compared to HZ and LAN, the atmospheric CO2 at DMS was influenced by emissions and transmissions over a wider range. The potential source area of DMS in autumn covered most areas of the urban agglomeration in eastern China. DMS measurements could provide a reliable representation of the background level of CO2 emissions in the Yangtze River Delta and a broader region. Conventional understanding of regional CO2 level in the Yangtze River Delta through LAN measurements may overestimate background concentration by approximately 10.92 ppm.

Trans-boundary spatio-temporal analysis of Sentinel 5P tropospheric nitrogen dioxide and total carbon monoxide columns over Punjab and Haryana Regions with COVID-19 lockdown impact.

This study conducts a spatio-temporal analysis of tropospheric nitrogen dioxide (NO2) and total carbon monoxide (CO) concentrations in the Punjab and Haryana regions of India and Pakistan, using datasets from the Sentinel 5-Precursor (S5P) satellite. These regions, marked by diverse economic growth factors including population expansion, power generation, transportation, and agricultural practices, face similar challenges in atmospheric pollution, particularly evident in major urban centers like Delhi and Lahore, identified as pollution hotspots. The study also spotlights pollution associated with power plants. In urban areas, tropospheric NO2 levels are predominantly elevated due to vehicular emissions, whereas residential activities mainly contribute to CO pollution. However, precisely attributing urban CO sources is complex due to its longer atmospheric residence time and intricate circulation patterns. Notably, the burning of rice crop residue in November significantly exacerbates winter pollution episodes and smog, showing a more pronounced correlation with total CO than with tropospheric NO2 levels. The temporal analysis indicates that the months from October to December witness peak pollution, contrasted with the relatively cleaner period during the monsoon months of July to September. The severe pollution in the OND quarter is attributed to factors such as variations in boundary layer height and depletion of OH radicals. Furthermore, the study highlights the positive impact of the COVID-19 lockdown on air quality, with a significant decrease in NO2 concentrations during April, 2020 (Delhi: 59%, Lahore: 58%). However, the reduction in total CO columns was less significant. The study also correlates lockdown stringency with tropospheric NO2 columns (R2: 0.37 for Delhi, 0.25 for Lahore, 0.22 for Rawalpindi/Islamabad), acknowledging the influence of various meteorological and atmospheric variables. The research highlights the significant impact of crop residue burning on winter pollution levels, particularly on total CO concentrations. The study also shows the notable effect of the COVID-19 lockdown on air quality, significantly reducing NO2 levels. Additionally, it explores the correlation between lockdown stringency and tropospheric NO2 columns, considering various meteorological factors.

Temporal analysis of PM10 concentration with atmospheric boundary layer height variation in Ankara

Temporal analysis of PM10 concentration with atmospheric boundary layer height variation in Ankara

Lidar-based daytime boundary layer height variation and impact on the regional satellite-based PM2.5 estimate

Satellite-derived aerosol optical depth (AOD) is popularly used to infer ground-level PM2.5 concentration due to its wide coverage. The fact that aerosols are largely confined in the atmospheric boundary layer makes boundary layer height (BLH) an important scale factor for AOD-based PM2.5 estimates. Our recent ground-based lidar observations, nevertheless, indicate that aerosol particles are heterogeneously mixed within the boundary layer, and even frequently reside above BLH, forming the residual layers (RL)-like pattern. To better sort out the underlying mechanism behind the above-mentioned phenomenon and the impact on hourly ground-level PM2.5 estimates from satellite-based AOD, we firstly propose a novel notion of haze layer height (HLH), which is calculated from Micro-Pulse Lidar (MPL) profile. Combined analysis of 3.5-year ground-based lidar profiles, CE-318 AOD, and PM2.5 measurements show that the coefficient of determination (R2) between PM2.5 and AOD normalized with HLH increases from 0.49 to 0.61 for 90% of the dataset. Second, we applied HLH to an Auto-encoder-based Deep Residual Network (ADRN) and tested the effect on satellite AOD-based PM2.5 estimation within a 300 km range surrounding the MPL station. With the aid of the AOD imputation technique, a similar improvement of using HLH instead is found on the regional scale PM2.5 estimation, which can be demonstrated by the comparison with air quality measurements and other machine learning models. The results show that using ADRN with HLH achieves the highest performance (mean R2 = 0.87, RMSE = 10.12 μg/m3) among 4 machine learning models. This new approach, largely combining active and passive remote sensing data through artificial neural networks (CAPTA), shows improved accuracy and coverage of hourly PM2.5 estimation with aerosol vertical information and AOD calculation under clouds. In addition, further analysis showed that the average difference between morning and daily PM2.5 concentration could equate to an accuracy of 0.19–2.57 yrs. in terms of life expectancy, indicating that our new approach to the determination of PM2.5 from space sheds new insight into the assessment of the aerosol impact on public health.

Long-term variation of boundary layer height and possible contribution factors: A global analysis

Boundary layer height (BLH) plays an important role in regulating global weather/climate, as well as the dispersion and transportation of pollutants. Until now, however, the attribution and contributions of different controlling factors to BLH long-term variability and trends have not been quantified on a global scale. The long-term radiosonde dataset was used in this study to retrieve global BLH climatology; seasonal, diurnal, long-term variation and trends were analyzed over a 39-year period (1980–2018). Statistical results show that the global distribution of the BLH and its trend have apparent day-night differences. BLH during daytime is deeper during clear-sky conditions compared to cloudy sky conditions, indicating a significant effect of clouds; BLH during nighttime is deeper under cloudy conditions. BLH was also found to vary over different land types; dry and hot soil exhibits a deeper BLH than those of wet and cool soil. The long-term variation and trend of BLH are highly influenced by near-surface meteorological parameters. In particular, based on multiple linear stepwise regression models and the contribution calculation method, this investigation initiatively quantifies the influences of meteorological parameters on global BLH long-term variation and trend. Our results emphasized that a 10 m wind speed (WS) and low tropospheric stability (LTS) have significant contributions to long-term BLH variation; WS and LTS anomalies alternately dominated the contribution of the diurnal cycle of the BLH anomaly. Annual BLH recorded an average increasing trend (38.9–42.1 m/decade), and LTS is more dominant than WS from a contribution perspective, especially for increased BLH anomaly. Contributions from near-surface temperature (T) and relative humidity (RH) also play important roles. However, a decreasing WS trend dominated the decreased trends of BLH anomaly, accounting for nearly 40% of the total contribution.

A Comparison of Wintertime Atmospheric Boundary Layer Heights Determined by Tethered Balloon Soundings and Lidar at the Site of SACOL

High-precision and -resolution atmospheric boundary layer height (BLH) has received increasing attention in air pollution research in recent years. The low time resolution of sounding data is the main challenge to validate BLH retrieval from lidar observations. To resolve this issue, we conducted simultaneous tethered balloon sounding and lidar observations at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) during winter 2019–2020. The BLHs derived from the tethered balloon sounding data were 170, 210, 393, 676, 423, and 190 m at 02:00, 08:00, 11:00, 14:00, 17:00, and 20:00 (Beijing time), respectively. The diurnal evolution of BLH was reasonably captured by lidar observation-based wavelet covariance transform and ideal profile fitting methods, which exhibited correlation coefficients of 0.91 and 0.89, respectively, with the BLHs determined from tethered balloon sounding data. The lidar results slightly overestimated the BLHs, though all results were acceptable when considering both the absolute and relative errors with respect to BLHs from the tethered balloon data. Our results revealed high-precision and -resolution diurnal variations in BLH at SACOL in Northwest China and suggest the importance of validating lidar-based BLHs using simultaneous sounding data.

Atmospheric mercury in an eastern Chinese metropolis (Jinan)

Urban emissions are a major contributor to atmospheric Hg budgets. Continuous measurements of total gaseous mercury (TGM) and particulate-bound mercury (PHg) in PM2.5 were conducted from October 2015 to July 2016 in a metropolis, Jinan, in eastern China. Average TGM and PHg concentrations were 4.91±3.66ngm-3 and 451.9±433.4pgm-3, respectively, in the entire study period. During the winter heating period (HP), mean concentrations of TGM and PHg were 5.79ngm-3 and 598.7pgm-3, respectively, twice higher than those during the non-heating periods (NHPs). During the HP, TGM exhibited a distinct diurnal pattern with a peak in the morning and a minimum in the afternoon on less polluted days but a singular peak at midday on heavily polluted days. The diurnal variation of TGM during the NHPs was predominantly influenced by the variation in boundary layer height while during the HP by anthropogenic emissions. The ratio of PHg/PM2.5 in Jinan was one to two orders of magnitude larger than those elsewhere worldwide and those in soil and coal, which suggested the high enrichment of PHg in PM2.5 in Jinan. Correlation and principle component analysis results suggested that PHg and TGM had common combustion sources during the HP, whereas PHg resulted mainly from biomass burning and meteorological variations during the NHPs. High Hg concentrations in Jinan were mostly caused by emissions from coal-fired power plants, especially for those situated east of the sampling site. In addition, TGM and PHg concentrations significantly increased during haze and fog episodes, but decreased during a dust episode due possibly to strong ventilation conditions combined with partitioning of Hg between adsorption to PM2.5 and coarse dust particles.

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.

Boundary Layer Heights as Derived From Ground-Based Radar Wind Profiler in Beijing

The vertical structure of wind is a key factor in modulating air quality, from which the determination of boundary layer height (BLH) remains a major challenge. In this paper, we developed an improved threshold method to determine the BLH from radar wind profiler (RWP) measurements. The normalized signal-to-noise ratio (SNR) profiles were used instead of the original SNR profiles to avoid instrumental inconsistencies. Additionally, a peak filter was designed to indicate the BLH based on the maximum SNR by taking into account the multiple peaks in the SNR profile. This algorithm was then applied to the RWP measurements taken in the summer (June–July–August) of 2018 in Beijing to obtain the BLHs. Validation analyses suggested that the BLH retrievals from RWP exhibited high consistency with those from radiosondes, with an average correlation coefficient of 0.69 (0.66) and a root mean squared error of 0.39 (0.41) in the daytime (nighttime). Additionally, the major features of summertime BLHs in Beijing were examined. In particular, a distinct diurnal variation in BLH was observed with a peak (1630 ± 510 m) occurring at 0600 universal time coordinated (UTC) and a minimum (587 ± 343 m) at 2300 UTC. Therefore, the algorithm presented here has great potential to be applied to other regions to obtain reliable BLHs. The findings obtained here highlight the importance of vertical wind structure in air quality studies.

Elucidating the relationship between aerosol concentration and summertime boundary layer structure in central China

Wuhan, a megacity in central China, suffers from frequent aerosol pollution and is accompanied by meteorological factors at both synoptic and local scales. Partly due to the lack of appropriate observations of planetary boundary layer (PBL), the associations between synoptic conditions, PBL, and pollution there are not yet fully understood. Thus, systematic analyses were conducted using the fine-resolution soundings, surface meteorological measurements, and aerosol observations in Wuhan during summer for the period 2013–2016, in combination with T-mode principal component analysis and simulations of backward trajectory. The results showed that the variations of boundary layer height (BLH) not only modulated the diurnal variation of PM2.5 concentration in Wuhan, but also the daily pollution level. Five different synoptic patterns during summer in Wuhan were identified from reanalysis geopotential height fields. Among these synoptic patterns, two types characterized by northeasterly prevailing winds, were found to be associated with heavy pollution in Wuhan. Driven by the northeasterly winds, the polluted air mass from the heavily polluted regions could be easily transported to Wuhan, such as North China Plain and Yangtze River Delta. Such regional transports of pollutants must be partly responsible for the aerosol pollution in Wuhan. In addition, these two synoptic patterns were also featured by the relatively high cloud cover and low boundary layer height in Wuhan, which would favor the occurrence of pollution there. Overall, this study has important implications for understanding the important roles of meteorological factors in modulating aerosol pollution in central China.

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.

Diurnal and seasonal variations in surface methane at a tropical coastal station: Role of mesoscale meteorology

In view of the large uncertainties in the methane (CH4) emission estimates and the large spatial gaps in its measurements, studies on near-surface CH4 on regional basis become highly relevant. This paper presents the first time observational results of a study on the impacts of mesoscale meteorology on the temporal variations of near-surface CH4 at a tropical coastal station, in India. It is based on the in-situ measurements conducted during January 2014 to August 2016, using an on-line CH4 analyzer working on the principle of gas chromatography. The diurnal variation shows a daytime low (1898–1925ppbv) and nighttime high (1936–2022ppbv) extending till early morning hours. These changes are closely associated with the mesoscale circulations, namely Sea Breeze (SB) and Land Breeze (LB), as obtained through the meteorological observations, WRF simulations of the circulations and the diurnal variation of boundary layer height as observed by the Microwave Radiometer Profiler. The diurnal enhancement always coincides with the onset of LB. Several cases of different onset timings of LB were examined and results presented. The CH4 mixing ratio also exhibits significant seasonal patterns being maximum in winter and minimum in pre-monsoon/monsoon with significant inter-annual variations, which is also reflected in diurnal patterns, and are associated with changing synoptic meteorology. This paper also presents an analysis of in-situ measured near-surface CH4, column averaged and upper tropospheric CH4 retrieved by Atmospheric Infrared Sounder (AIRS) onboard Earth Observing System (EOS)/Aqua which gives insight into the vertical distribution of the CH4 over the location. An attempt is also made to estimate the instantaneous radiative forcing for the measured CH4 mixing ratio.

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.

Seasonal heterogeneity in ozone and its precursors (NOx) by in-situ and model observations on semi-arid station in Anantapur (A.P), South India

The continuous measurements of surface ozone (O3) and oxides of nitrogen (NOx) present in the ambient air were carried out on a semi-arid rural area in Anantapur [14.62° N, 77.65° E], South India during the period beginning from March 2012 to February 2013. The diurnal variation of surface ozone (O3) showed a minimum at sun rise and late evenings and maximum at noon time, due to slow titration of O3 with oxides of nitrogen (NOx). The monthly average minimum/maximum of O3 observed in August/March was 19.9 ± 0.3/55.8 ± 2.9 ppbv. The monthly average variation of NOx was found to be high (6.67 ± 0.8) in November and noticed low (2.07 ± 0.2) concentrations in May. The monthly mean maximum of O3 during day time (night time) was observed as 64.9 ± 5.3 (45.3 ± 4.1) ppbv in March, and mean low concentration was about 19.9 ± 1.02 (12.8 ± 0.9) ppbv in August. The monthly mean enhanced concentration of NOx during the day time/night time was about 5.0 ± 0.6/8.7 ± 0.9 in November and the fallen concentration was noticed 1.5 ± 0.2/2.7 ± 0.3 in May. The spatial distribution of O3 was associated with the air flow pattern, air mass trajectories and boundary layer heights. The water vapor content also played a significant role in the diurnal variation of O3 in addition to photo chemistry. The diurnal variation of UV radiation with O3 and NOx has been analyzed and the results are reported. And the diurnal and seasonal variations in boundary layer height have also been discussed. The monthly mean variation of aerosol optical depth (AOD) is similar to that of tropospheric ozone, with a correlation coefficient of r = 0.60. The diurnal and seasonal patterns of ozone were simulated using the chemical box model (NCAR Mater Mechanism) and when compared with the observed ones, the results are found to be in good agreement with the observed ones.

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.

Scattering properties of the atmospheric aerosol in Beijing, China

Measurements of aerosol scattering coefficient (σ sp) and PM 2.5 concentration obtained during June 2008 to May 2009 at urban, suburban, and rural sites in Beijing area. The mean value of σ sp during measurement period was 301 ± 307, 263 ± 263, 182 ± 201 Mm − 1 at Baolian (BL: urban site), Changping (CP: suburban site) and Shangdianzi (SDZ: rural site), respectively. The seasonal and diurnal patterns of σ sp were analyzed with the measurement data. The σ sp showed different seasonal and diurnal patterns at these three sites. The seasonal fluctuations inσ sp in urban area were mostly influenced by seasonal variability in both emissions and meteorological conditions, while the seasonal wind patterns seemed to dominate the σ sp in the suburban and rural areas. The diurnal activity of σ sp generally showed a bimodal, trimodal and a unimodal pattern at BL, CP and SDZ sites, respectively. The diurnal variation of boundary layer height companying with source activity was mainly dominated the diurnal variation of σ sp at urban and suburban sites. The mountain-valley breeze and boundary layer growth in SDZ region mostly dominated the diurnal variation of σ sp. The mass scattering efficiency of PM 2.5 was estimated at BL and SDZ, which showed a decreasing trend from urban to rural site with different seasonal variation at two sites. During fog and haze episodes, the lower northeasterly winds could result in significant spatial difference in σ sp, while the σ sp was spatially uniform under the influence of stronger southerly winds.

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.

Wintertime aerosol characteristics at a north Indian site Kharagpur in the Indo‐Gangetic plains located at the outflow region into Bay of Bengal

Keeping the importance of aerosol characterization in the out flow regions from the Indian subcontinent in view, a campaign mode observation on aerosol physical properties was made at Indian Institute of Technology campus, Kharagpur located under the vent region in the Indo‐Gangetic plains during the winter month of December 2004. The aerosol spectral optical depths and near‐surface mass concentrations were high with a mean aerosol optical depth of 0.7 at 500 nm and a percent share of fine mode particle concentration as high as 90. However, the share of the BC aerosol to fine mode aerosol was consistently 10%, which is typical of an urban location. The vertical profiles of aerosol backscatter intensity derived using a micropulse lidar show that the boundary layer height variation accounts for the day‐to‐day variability in the surface mass concentrations. The negative correlation between aerosol backscatter intensity at two representative altitudes above and below the boundary layer implicates only vertical redistribution of aerosols. The lidar data also suggest that no aerosol transport has taken place over the location to account for the day‐to‐day variability. The forward trajectories at three representative altitudes with source point at the observing site indicate a possible aerosol transport from the outflow regions into Bay of Bengal, southern peninsular India and Arabian Sea. The results were discussed in light of the earlier mobile campaign observations on the spatial variability of aerosol physical properties over the peninsular India.

The Relation Between Ageostrophy and Boundary-Layer Structure in the Western-Pacific Trades (Wake Island)

Eight years (1980–1987) of Wake Island rawinsonde data are used to derive atmospheric boundary layer (ABL) depth, integrated boundary-layer moisture, and a measure of boundary-layer ageostrophy. The variability in these processes controls the accumulation of moisture and heat in the tradewind regions and their transport to regions of intense convection. Preliminary analyses using different methods reveal quasi-periodic signals in these data in the 30–60 days range. Cross correlation calculations in this intraseasonal range show that these ABL variables are coherent with each other and with the low-level flow. The integrated ABL variables and the ABL height exhibit local in-phase relationships. At higher frequencies, the analyses show intense diurnal variation of boundary-layer height but only a weak diurnal signal in integrated ABL properties. At the lower frequency range, the analyses show a significant reduction in the amplitude of the seasonal and intraseasonal variation in ageostrophy during the strong El-Nino event of 1982/1983. The results clearly establish a relationship between integrated water vapour and divergent ABL processes (Ekman pumping/suction) in which shallower (deeper) ABLs are associated with mass and moisture divergence (convergence) and higher (lower) sea-level pressure. A possible interpretation in terms of a remote dynamic response of the trade inversion and ABL processes to equatorial deep convection is suggested.