29 research outputs found
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Investigations of boundary layer structure, cloud characteristics and vertical mixing of aerosols at Barbados with large eddy simulations
Large eddy simulations (LESs) are performed for the area of the Caribbean island Barbados to investigate island effects on boundary layer modification, cloud generation and vertical mixing of aerosols. Due to the presence of a topographically structured island surface in the domain center, the model setup has to be designed with open lateral boundaries. In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing, we use the cell perturbation method based on finite amplitude potential temperature perturbations. In this work, this method is for the first time tested and validated for moist boundary layer simulations with open lateral boundary conditions. Observational data obtained from the SALTRACE field campaign is used for both model initialization and a comparison with Doppler wind and Raman lidar data. Several numerical sensitivity tests are carried out to demonstrate the problems related to âgray zone modelingâ when using coarser spatial grid spacings beyond the inertial subrange of three-dimensional turbulence or when the turbulent marine boundary layer flow is replaced by laminar winds. Especially cloud properties in the downwind area west of Barbados are markedly affected in these kinds of simulations. Results of an additional simulation with a strong trade-wind inversion reveal its effect on cloud layer depth and location. Saharan dust layers that reach Barbados via long-range transport over the North Atlantic are included as passive tracers in the model. Effects of layer thinning, subsidence and turbulent downward transport near the layer bottom at zâââ1800 m become apparent. The exact position of these layers and strength of downward mixing is found to be mainly controlled atmospheric stability (especially inversion strength) and wind shear. Comparisons of LES model output with wind lidar data show similarities in the downwind vertical wind structure. Additionally, the model results accurately reproduce the development of the daytime convective boundary layer measured by the Raman lidar
1064âŻnm rotational Raman lidar for particle extinction and lidar-ratio profiling: cirrus case study
For the first time, vertical profiles of the 1064âŻnm particle extinction
coefficient obtained from Raman lidar observations at 1058âŻnm (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 1064âŻnm. 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 355âŻnm were
on average about 20â30âŻ% lower than the ones for 532 and 1064âŻnm. The
cirrus-mean extinction-to-backscatter ratio (lidar ratio) was 31âŻÂ±âŻ5âŻsr
(355âŻnm), 36âŻÂ±âŻ5âŻsr (532âŻnm), and 38âŻÂ±âŻ5âŻsr (1064âŻnm) in this single
study. We further discussed the requirements needed to obtain aerosol
extinction profiles in the lower troposphere at 1064âŻnm with good accuracy
(20âŻ% relative uncertainty) and appropriate temporal and vertical resolution
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Depolarization and lidar ratios at 355, 532, and 1064ânm 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 ÎŒm. The tropospheric smoke particles were much smaller (effective radius of 0.17 ÎŒm). Mass concentrations were of the order of 5.5 ÎŒgm-3 (tropospheric layer) and 40 ÎŒgm-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
Cloud top heights and aerosol layer properties from EarthCARE lidar observations: the A-CTH and A-ALD products
The high-spectral-resolution Atmospheric Lidar (ATLID) on the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE)
provides vertically resolved information on aerosols and clouds with unprecedented accuracy. Together with the Cloud Profiling Radar (CPR), the Multi-Spectral Imager (MSI), and the Broad-Band Radiometer (BBR) on the same platform, it allows for a new synergistic view on atmospheric processes related to the interaction of aerosols, clouds, precipitation, and radiation at the global scale.
This paper describes the algorithms for the determination of cloud top height and aerosol layer information from ATLID Level 1b (L1b) and Level 2a (L2a) input data. The ATLID L2a Cloud Top Height (A-CTH) and Aerosol Layer Descriptor (A-ALD) products are developed to ensure the provision of atmospheric layer products in continuation of the heritage from the CloudâAerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Moreover, the products serve as input for synergistic algorithms that make use of data from ATLID and MSI. Therefore, the products are provided on the EarthCARE joint standard grid (JSG).
A wavelet covariance transform (WCT) method with flexible thresholds is applied to determine layer boundaries from the ATLID Mie co-polar signal.
Strong features detected with a horizontal resolution of 1 JSG pixel (approximately 1âkm) or 11Â JSG pixels are classified as thick or thin clouds, respectively. The top height of the uppermost cloud layer together with information on cloud layering are stored in the A-CTH product for further use in the generation of the ATLID-MSI Cloud Top Height (AM-CTH) synergy product. Aerosol layers are detected as weaker features at a resolution of 11Â JSG pixels. Layer-mean optical properties are calculated from the ATLIDÂ L2a Extinction, Backscatter and Depolarization (A-EBD) product and stored in the A-ALD product, which also contains the aerosol optical thickness (AOT) of each layer, the stratospheric AOT, and the AOT of the entire atmospheric column. The latter parameter is used to produce the synergistic ATLID-MSI Aerosol Column Descriptor (AM-ACD) later in the processing chain. Several quality criteria are applied in the generation of A-CTH and A-ALD, and respective information is stored in the products. The functionality and performance of the algorithms are demonstrated by applying them to common EarthCARE test scenes. Conclusions are drawn for the application to real-world data and the validation of the products after the launch of EarthCARE.</p
Polarization lidar: an extended three-signal calibration approach
We present a new formalism to calibrate a three-signal polarization lidar and
to measure highly accurate height profiles of the volume linear
depolarization ratios under realistic experimental conditions. The
methodology considers elliptically polarized laser light, angular
misalignment of the receiver unit with respect to the main polarization plane
of the laser pulses, and cross talk among the receiver channels. A case
study of a liquid-water cloud observation demonstrates the potential of the
new technique. Long-term observations of the calibration parameters
corroborate the robustness of the method and the long-term stability of the
three-signal polarization lidar. A comparison with a second polarization
lidar shows excellent agreement regarding the derived volume linear
polarization ratios in different scenarios: a biomass burning smoke event
throughout the troposphere and the lower stratosphere up to 16 km in height, a
dust case, and also a cirrus cloud case.</p
Extreme levels of Canadian wildfire smoke in the stratosphere over central Europe on 21â22 August 2017
Light extinction coefficients of 500 Mmâ1, about 20 times higher than
after the Pinatubo volcanic eruptions in 1991, were observed by European
Aerosol Research Lidar Network (EARLINET) lidars in the stratosphere over
central Europe on 21â22 August 2017. Pronounced smoke layers with a 1â2 km
vertical extent were found 2â5 km above the local tropopause. Optically
dense layers of Canadian wildfire smoke reached central Europe 10Â days after
their injection into the upper troposphere and lower stratosphere which was
caused by rather strong pyrocumulonimbus activity over western Canada. The
smoke-related aerosol optical thickness (AOT) identified by lidar was close
to 1.0 at 532 nm over Leipzig during the noon hours on 22 August 2017.
Smoke particles were found throughout the free troposphere (AOT
of 0.3) and in the pronounced 2 km thick stratospheric smoke layer at an
altitude of 14â16 km (AOT of 0.6). The lidar
observations indicated peak mass concentrations of
70â100 ”g mâ3 in the stratosphere. In addition to the lidar
profiles, we analyzed Moderate Resolution Imaging Spectroradiometer (MODIS)
fire radiative power (FRP) over Canada, and the distribution of MODIS AOT and
Ozone Monitoring Instrument (OMI) aerosol index across the North Atlantic.
These instruments showed a similar pattern and a clear link between the
western Canadian fires and the aerosol load over Europe. In this paper, we
also present Aerosol Robotic Network (AERONET) sun photometer observations,
compare photometer and lidar-derived AOT, and discuss an obvious bias (the
smoke AOT is too low) in the photometer observations. Finally, we compare the
strength of this record-breaking smoke event (in terms of the particle
extinction coefficient and AOT) with major and moderate volcanic events
observed over the northern midlatitudes.</p
HETEAC â the Hybrid End-To-End Aerosol Classification model for EarthCARE
The Hybrid End-To-End Aerosol Classification (HETEAC) model for the Earth Clouds, Aerosols and Radiation Explorer (EarthCARE) mission is introduced. The model serves as the common baseline for the development, evaluation, and implementation of EarthCARE algorithms. It guarantees the consistency of different aerosol products from the multi-instrument platform and facilitates the conformity of broad-band optical properties needed for EarthCARE radiative-closure assessments. While the hybrid approach ensures that the theoretical description of aerosol microphysical properties is consistent with the optical properties of the measured aerosol types, the end-to-end model permits the uniform representation of aerosol types in terms of microphysical, optical, and radiative properties. Four basic aerosol components with prescribed microphysical properties are used to compose various natural and anthropogenic aerosols of the troposphere. The components contain weakly and strongly absorbing fine-mode and spherical and non-spherical coarse-mode particles and thus are representative for pollution, smoke, sea salt, and dust, respectively. Their microphysical properties are selected such that good coverage of the observational phase space of intensive, i.e., concentration-independent, optical aerosol properties derived from EarthCARE measurements is obtained. Mixing rules to calculate optical and radiative properties of any aerosol blend composed of the four basic components are provided. Applications of HETEAC in the generation of test scenes, the development of retrieval algorithms for stand-alone and synergistic aerosol products from EarthCARE's atmospheric lidar (ATLID) and multi-spectral imager (MSI), and for radiative-closure assessments are introduced. Finally, the implications of simplifying model assumptions and possible improvements are discussed, and conclusions for future validation and development work are drawn.</p
Tropospheric and stratospheric smoke over Europe as observed within EARLINET/ACTRIS in summer 2017
For several weeks in summer 2017, strong smoke layers were observed over Europe at numerous EARLINET
stations. EARLINET is the European research lidar network and part of ACTRIS and comprises more than 30
ground-based lidars.
The smoke layers were observed in the troposphere as well as in the stratosphere up to 25 km from Northern
Scandinavia over whole western and central Europe to the Mediterranean regions.
Backward trajectory analysis among other tools revealed that these smoke layers originated from strong wild fires
in western Canada in combination with pyrocumulus convection. An extraordinary fire event in the mid of August
caused intense smoke layers that were observed across Europe for several weeks starting on 18 August 2017.
Maximum aerosol optical depths up to 1.0 at 532 nm were observed at Leipzig, Germany, on 22 August 2017
during the peak of this event.
The stratospheric smoke layers reached extinction coefficient values of more than 600 Mmâ1 at 532 nm, a factor
of 10 higher than observed for volcanic ash after the Pinatubo eruption in the 1990s. First analyses of the intensive
optical properties revealed low particle depolarization values at 532 nm for the tropospheric smoke (spherical
particles) and rather high values (up to 20%) in the stratosphere. However, a strong wavelength dependence of
the depolarization ratio was measured for the stratospheric smoke. This indicates irregularly shaped stratospheric
smoke particles in the size range of the accumulation mode. This unique depolarization feature makes it possible
to distinguish clearly smoke aerosol from cirrus clouds or other aerosol types by polarization lidar measurements.
Particle extinction-to-backscatter ratios were rather low in the order of 40 to 50 sr at 355 nm, while values between
70-90 sr were measured at higher wavelengths.
In the western and central Mediterranean, stratospheric smoke layers were most prominent in the end of August
at heights between 16 and 20 km. In contrast, stratospheric smoke started to occur in the eastern Mediterranean
(Cyprus and Israel) in the beginning of September between 18 and 23 km. Stratospheric smoke was still visible in
the beginning of October at certain locations (e.g. Evora, Portugal), while tropospheric smoke was mainly observed
until the end of August within Europe.
An overview of the smoke layers measured at several EARLINET sites will be given. The temporal development
of these layers as well as their geometrical and optical properties will be presented