198 research outputs found
Continuous monitoring of the boundary-layer top with lidar
International audienceContinuous lidar observations of the top height of the boundary layer (BL top) have been performed at Leipzig (51.3° N, 12.4° E), Germany, since August 2005. The results of measurements taken with a compact, automated Raman lidar over a one-year period (February 2006 to January 2007) are presented. Four different methods for the determination of the BL top are discussed. The most promising technique, the wavelet covariance algorithm, is improved by implementing some modifications so that an automated, robust retrieval of BL depths from lidar data is possible. Three case studies of simultaneous observations with the Raman lidar, a vertical-wind Doppler lidar, and accompanying radiosonde profiling of temperature and humidity are discussed to demonstrate the potential and the limits of the four lidar techniques at different aerosol and meteorological conditions. The lidar-derived BL top heights are compared with respective values derived from predictions of the regional weather forecast model COSMO of the German Meteorological Service. The comparison shows a general underestimation of the BL top by about 20% by the model. The statistical analysis of the one-year data set reveals that the seasonal mean of the daytime maximum BL top is 1400 m in spring, 1800 m in summer, 1200 m in autumn, and 800 m in winter at the continental, central European site. BL top typically increases by 100?300 m per hour in the morning of convective days
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Radiative effect of aerosols above the northern and southern Atlantic Ocean as determined from shipborne lidar observations
The direct solar radiative effect of aerosols over the Atlantic Ocean was investigated on the basis of aerosol Raman/polarization lidar observations aboard the research vessel Polarsternbetween Germany (50°N) and either South America (50°S) or South Africa (40°S) in 2009 and 2010. First, a case study of complex aerosol conditions with marine aerosol, dust, and smoke particles in the boundary layer and free troposphere is presented to demonstrate that detailed knowledge of aerosol layering (boundary layer, free troposphere) and aerosol mixing state is required for an accurate determination of the resulting radiative effects. A statistical analysis based on all lidar observations revealed the highest daily mean radiative effect (−43±59 W m−2at the surface, −14±18 W m−2at top of atmosphere) in the latitudinal belt from 0°N–15°N in the Saharan dust outflow region. Mean aerosol radiative effects of the polluted northern and clean southern midlatitudes were contrasted. In the northern midlatitudes, the averaged aerosol radiative effect of all simulations was −24±33 W m−2at the surface which is a factor of 1.6 higher than at similar southern hemispheric latitudes. The simulations based on the lidar observations are in good agreement with colocated pyranometer measurements
1064 nm rotational Raman lidar for particle extinction and lidar-ratio profiling: cirrus case study
For the first time, vertical profiles of the 1064 nm particle extinction
coefficient obtained from Raman lidar observations at 1058 nm (nitrogen and
oxygen rotational Raman backscatter) are presented. We applied the new
technique in the framework of test measurements and performed several cirrus
observations of particle backscatter and extinction coefficients, and
corresponding extinction-to-backscatter ratios at the wavelengths of 355,
532, and 1064 nm. The cirrus backscatter coefficients were found to be equal
for all three wavelengths keeping the retrieval uncertainties in mind. The
multiple-scattering-corrected cirrus extinction coefficients at 355 nm were
on average about 20–30 % lower than the ones for 532 and 1064 nm. The
cirrus-mean extinction-to-backscatter ratio (lidar ratio) was 31 ± 5 sr
(355 nm), 36 ± 5 sr (532 nm), and 38 ± 5 sr (1064 nm) in this single
study. We further discussed the requirements needed to obtain aerosol
extinction profiles in the lower troposphere at 1064 nm with good accuracy
(20 % relative uncertainty) and appropriate temporal and vertical resolution
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Depolarization and lidar ratios at 355, 532, and 1064 nm and microphysical properties of aged tropospheric and stratospheric Canadian wildfire smoke
We present spectrally resolved optical and microphysical properties of western Canadian wildfire smoke observed in a tropospheric layer from 5-6.5 km height and in a stratospheric layer from 15-16 km height during a recordbreaking smoke event on 22 August 2017. Three polarization/ Raman lidars were run at the European Aerosol Research Lidar Network (EARLINET) station of Leipzig, Germany, after sunset on 22 August. For the first time, the linear depolarization ratio and extinction-to-backscatter ratio (lidar ratio) of aged smoke particles were measured at all three important lidar wavelengths of 355, 532, and 1064 nm. Very different particle depolarization ratios were found in the troposphere and in the stratosphere. The obviously compact and spherical tropospheric smoke particles caused almost no depolarization of backscattered laser radiation at all three wavelengths ( 500nm). The stratospheric smoke particles formed a pronounced accumulation mode (in terms of particle volume or mass) centered at a particle radius of 350-400 nm. The effective particle radius was 0.32 μm. The tropospheric smoke particles were much smaller (effective radius of 0.17 μm). Mass concentrations were of the order of 5.5 μgm-3 (tropospheric layer) and 40 μgm-3 (stratospheric layer) in the night of 22 August 2017. The single scattering albedo of the stratospheric particles was estimated to be 0.74, 0.8, and 0.83 at 355, 532, and 1064 nm, respectively
Continuous vertical aerosol profiling with a multi-wavelength Raman polarization lidar over the Pearl River Delta, China
A dataset of particle optical properties of the highly polluted
atmosphere over the Pearl River Delta (PRD), Guangzhou, China, is presented
in this paper. The data were derived from the measurements of a
multi-wavelength Raman and depolarization lidar PollyXT and a
co-located AERONET sun photometer. The measurement campaign was conducted
from November 2011 to mid-June 2012. These are the first Raman lidar
measurements in the PRD that lasted for several months.
A mean value of aerosol optical depth (AOD) of 0.54 ± 0.33 was observed
by the sun photometer at 500 nm in the polluted atmosphere over this
megacity for the whole measurement period. The lidar profiles frequently show
lofted aerosol layers, which reach altitudes of up to 2 to 3 km
and, especially during the spring
season, up to 5 km. These layers contain between 12 and 56 % of the
total AOD, with the highest values in spring. The aerosol types in these
lofted layers are classified by their optical properties. The observed lidar
ratio values range from 30 to 80 sr with a mean value of
48.0 ± 10.7 sr at 532 nm. The linear particle depolarization ratio at
532 nm lies mostly below 5 %, with a mean value of
3.6 ± 3.7 %. The majority of the Ångström exponents lie
between 0.5 and 1.5, indicating a mixture of fine- and coarse-mode aerosols.
These results reveal that mostly urban pollution particles mixed with
particles produced from biomass and industrial burning are present in the
atmosphere above the Pearl River Delta. Trajectory analyses show that these
pollution mixtures arise mainly from local and regional sources
Surface matters: Limitations of CALIPSO V3 aerosol typing in coastal regions
In the CALIPSO data analysis, surface type (land/ocean) is used to augment the aerosol characterization. However, this surface-dependent aerosol typing prohibits a correct classification of marine aerosol over land that is advected from ocean to land. This might result in a systematic overestimation of the particle extinction coefficient and of the aerosol optical thickness (AOT) of up to a factor of 3.5 over land in coastal areas. We present a long-term comparison of CALIPSO and ground-based lidar observations of the aerosol conditions in the coastal environment of southern South America (Punta Arenas, Chile, 53° S), performed in December 2009–April 2010. Punta Arenas is almost entirely influenced by marine particles throughout the year, indicated by a rather low AOT of 0.02–0.04. However, we found an unexpectedly high fraction of continental aerosol in the aerosol types inferred by means of CALIOP observations and, correspondingly, too high values of particle extinction. Similar features of the CALIOP data analysis are presented for four other coastal areas around the world. Since CALIOP data serve as important input for global climate models, the influence of this systematic error was estimated by means of simplified radiative-transfer calculations
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Numerical simulations of optical properties of Saharan dust aerosols with emphasis on lidar applications
In the framework of the Saharan Mineral Dust Experiment (SAMUM) for the first time the spectral dependence of particle linear depolarization ratios was measured by combining four lidar systems. In this paper these measurements are compared with results from scattering theory based on the T-matrix method. For this purpose, in situ measurements—size distribution, shape distribution and refractive index—were used as input parameters; particle shape was approximated by spheroids. A sensitivity study showed that lidar-related parameters—lidar ratio Sp and linear depolarization ratio δp—are very sensitive to changes of all parameters. The simulated values of the δp are in the range of 20% and 31% and thus in the range of the measurements. The spectral dependence is weak, so that it could not be resolved by the measurements. Calculated lidar ratios based on the measured microphysics and considering equivalent radii up to 7.5μm show a range of possible values between 29 and 50 sr at λ = 532 nm. Larger Sp might be possible if the real part of the refractive index is small and the imaginary part is large. A strict validation was however not possible as too many microphysical parameters influence Sp and δp that could not be measured with the required accuracy
New-particle formation events in a continental boundary layer: first results from the SATURN experiment
International audienceDuring the SATURN experiment, which took place from 27 May to 14 June 2002, new particle formation in the continental boundary layer was investigated. Simultaneous ground-based and tethered-balloon-borne measurements were performed, including meteorological parameters, particle number concentrations and size distributions, gaseous precursor concentrations and SODAR and LIDAR observations. Newly formed particles were observed inside the residual layer, before the break-up process of the nocturnal inversion, and inside the mixing layer throughout the break-up of the nocturnal inversion and during the evolution of the planetary boundary layer.</p
Regional Saharan dust modelling during the SAMUM 2006 campaign
The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the
unique comprehensive data set of near-source dust properties during the 2006SAMUMfield campaign, the performance
of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size
distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations.
However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust
source placement by the model. While simulated winds and dust distributions are well matched for dust events caused
by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or
influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the
vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of
sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and
likely explains the absence of the observed sharply defined dust layers
An case of extreme particulate matter concentrations over Central Europe caused by dust emitted over the southern Ukraine
On 24 March 2007, an extraordinary dust plume was observed in the Central European troposphere. Satellite observations revealed its origins in a dust storm in Southern Ukraine, where large amounts of soil were resuspended from dried-out farmlands at wind gusts up to 30 m s?1. Along the pathway of the plume, maximum particulate matter (PM10) mass concentrations between 200 and 1400 ?g m?3 occurred in Slovakia, the Czech Republic, Poland, and Germany. Over Germany, the dust plume was characterised by a volume extinction coefficient up to 400 Mm?1 and a particle optical depth of 0.71 at wavelength 0.532 ?m. In-situ size distribution measurements as well as the wavelength dependence of light extinction from lidar and Sun photometer measurements confirmed the presence of a coarse particle mode with diameters around 2?3 ?m. Chemical particle analyses suggested a fraction of 75% crustal material in daily average PM10 and up to 85% in the coarser fraction PM10?2.5. Based on the particle characteristics as well as a lack of increased CO and CO2 levels, a significant impact of biomass burning was ruled out. The reasons for the high particle concentrations in the dust plume were twofold: First, dust was transported very rapidly into Central Europe in a boundary layer jet under dry conditions. Second, the dust plume was confined to a relatively stable boundary layer of 1.4?1.8 km height, and could therefore neither expand nor dilute efficiently. Our findings illustrate the capacity of combined in situ and remote sensing measurements to characterise large-scale dust plumes with a variety of aerosol parameters. Although such plumes from Southern Eurasia seem to occur rather infrequently in Central Europe, its unexpected features highlights the need to improve the description of dust emission, transport and transformation processes needs, particularly when facing the possible effects of further anthropogenic desertification and climate change
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