ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-5893-2017Three-dimensional evolution of Saharan dust transport towards Europe based on a 9-year
EARLINET-optimized CALIPSO datasetMarinouElenielmarinou@noa.grhttps://orcid.org/0000-0003-2631-6057AmiridisVassilishttps://orcid.org/0000-0002-1544-7812BinietoglouIoannishttps://orcid.org/0000-0002-0065-9791TsikerdekisAthanasiosSolomosStavrosProestakisEmannouilhttps://orcid.org/0000-0001-9547-3019KonstaDimitraPapagiannopoulosNikolaoshttps://orcid.org/0000-0001-7702-0710TsekeriAlexandraVlastouGeorgiaZanisProdromosBalisDimitrioshttps://orcid.org/0000-0003-1161-7746WandingerUllaAnsmannAlbertIAASARS, National Observatory of Athens, 15236 Athens, GreeceDepartment of Physics, Aristotle University of Thessaloniki,
54124 Thessaloniki, GreeceNational Institute of R&D for Optoelectronics, Magurele, RomaniaLaboratory of Atmospheric Physics, Department of Physics, University
of Patras, 26500 Patras, GreeceSchool of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, GreeceConsiglio Nazionale delle Ricerche, Istituto di Metodologie per
l'Analisi Ambientale (CNR–IMAA), Tito Scalo (PZ), ItalyLeibniz Institute for Tropospheric Research, 04318 Leipzig, GermanyDepartment of Physics, National and Kapodistrian University of Athens,
Athens, GreeceEleni Marinou (elmarinou@noa.gr)12May2017179589359197October201624November20166March201721March2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/5893/2017/acp-17-5893-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/5893/2017/acp-17-5893-2017.pdf
In this study we use a new dust product developed using
CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) observations and EARLINET (European Aerosol Research
Lidar Network) measurements and methods to provide a 3-D
multiyear analysis on the evolution of Saharan dust over North Africa and
Europe. The product uses a CALIPSO L2 backscatter product corrected with a
depolarization-based method to separate pure dust in external aerosol
mixtures and a Saharan dust lidar ratio (LR) based on long-term EARLINET
measurements to calculate the dust extinction profiles. The methodology is
applied on a 9-year CALIPSO dataset (2007–2015) and the results are
analyzed here to reveal for the first time the 3-D dust evolution and the
seasonal patterns of dust over its transportation paths from the Sahara
towards the Mediterranean and Continental Europe. During spring, the spatial
distribution of dust shows a uniform pattern over the Sahara desert. The
dust transport over the Mediterranean Sea results in mean dust optical depth
(DOD) values up to 0.1. During summer, the dust activity is mostly shifted
to the western part of the desert where mean DOD near the source is up to
0.6. Elevated dust plumes with mean extinction values between 10 and 75 Mm-1 are observed throughout the year at various heights between 2 and 6 km, extending up to latitudes of 40∘ N. Dust advection is
identified even at latitudes of about 60∘ N, but this is due to
rare events of episodic nature. Dust plumes of high DOD are also observed
above the Balkans during the winter period and above northwest Europe during
autumn at heights between 2 and 4 km, reaching mean extinction values up to
50 Mm-1. The dataset is considered unique with respect to its potential
applications, including the evaluation of dust transport models and the
estimation of cloud condensation nuclei (CCN) and ice nuclei (IN) concentration profiles. Finally, the product can be used to study dust dynamics during
transportation, since it is capable of revealing even fine dynamical
features such as the particle uplifting and deposition on European
mountainous ridges such as the Alps and Carpathian Mountains.
Introduction
Mineral dust is ubiquitous in the atmosphere and one of the main
contributors to the global aerosol load (Zender et al., 2004; Textor et al.,
2006), with almost half of the global dust emissions generated in Africa
(Huneuus et al., 2011). This has large consequences for air quality downwind
(Viana et al., 2002; Gobbi et al., 2007), for the radiative budget due to
scattering, absorption, and emission of solar and terrestrial radiation
(Balkanski et al., 2007), as well as for cloud formation and lifetime
(e.g., DeMott et al., 2003; Levin et al., 2005; Koren et al., 2010). These
effects depend strongly on the vertical distribution of dust. For example,
dust particles will have a stronger impact on shortwave radiation absorption
when they are located above bright clouds (Yorks et al., 2009; Winker et
al., 2013). Moreover, dust atmospheric lifetime is much longer in the free
troposphere than in the planetary boundary layer, and, upon entering the free
troposphere, dust particles can be transported across vast areas, altering
the geographic pattern of their impacts (Prospero and Lamb, 2003; Levin et
al., 2007; Ridley et al., 2012). Finally, the dust vertical distribution is
crucial for dust–cloud interactions (e.g., Mamouri and Ansmann, 2016;
Nickovic et al., 2016). Therefore, observing, monitoring and quantifying
the atmospheric dust burden and especially its vertical distribution is an
important step towards understanding the climatic role of dust (IPCC, 2013,
WG1, chap. 5, 7 and 9).
Lidar is the most prominent tool for aerosol profiling and has largely
contributed to our knowledge of the vertical distribution of the dust
optical properties (e.g., Liu et al., 2002; Ansmann et al., 2003; Balis et
al., 2004; Papayannis et al., 2008; Mona et al., 2012; Granados-Muñoz et al., 2016; Bovchaliuk et al., 2016). Polarization lidar observations greatly
expand the capabilities for dust detection, as non-spherical dust particles
have a distinct signature on the particle depolarization ratio (e.g., Z. Liu et
al., 2008; Tesche et al., 2009). In Europe, the European Aerosol Research
Lidar Network (EARLINET; Pappalardo et al., 2014) operates advanced lidar
systems employing depolarization techniques that have been invaluable for
dust research. Moreover, sophisticated methodologies developed in EARLINET
allow the complete characterization of different aerosol types including
dust (e.g., Papayannis et al., 2008) as well as the dust contribution to
the total aerosol load (Tesche et al., 2009).
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission
equipped with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)
instrument has been delivering aerosol and cloud profiles across the globe
for more than 10 years (Winker et al., 2009). This dataset offers the
possibility to characterize the three-dimensional spatial distribution of
aerosol as well as its temporal variation. CALIPSO is established as an
accurate and robust means for mineral dust identification from space (Liu et
al., 2008; Omar et al., 2009). The application of EARLINET methodologies on
CALIPSO observations can improve the observations for mineral dust research,
as already suggested and applied in Amiridis et al. (2013). Specifically,
this study retrieves the extinction of pure dust from CALIPSO with high
accuracy, applying the depolarization-based separation method introduced by
Tesche et al. (2009), coupled with a regionally uniform climatological LR
(lidar ratio) for calculating dust extinction. The latter is estimated from
long-term EARLINET measurements (Wandinger et al., 2010; Baars et al.,
2016). It has been shown that the EARLINET-optimized CALIPSO dust product
presented in Amiridis et al. (2013) is in better agreement with Aerosol
Robotic Network (AERONET) collocated measurements over the Sahara and Europe and
with Moderate Resolution Imaging Spectroradiometer (MODIS) measurements over
the Mediterranean for collocated cells with low cloudiness. This product is
considered as the first accurate dust retrieval from space, since dust
discrimination methods applied on passive sensors are based on the
separation of the fine from coarse particle mode (e.g., Kaufman et al.,
2005), delivering mostly biased DODs (dust optical depths) over the oceans due to the
contamination of the coarse mode by sea salt particles (Su et al., 2013).
Another advantage of the EARLINET-optimized CALIPSO dust product is its
capability to provide accurate dust retrievals over all surface types, since
CALIOP uses its own
light source, overcoming the surface reflectance limitations of passive
sensors (e.g., Hsu et al., 2004; Sayer et al., 2012).
Many studies have used satellite observations to derive dust properties over
the Mediterranean during the last 15 years. Most of them focus on the
horizontal distribution of dust using passive remote sensing techniques.
Antoine and Nobileau (2006) used SeaWIFS (Sea-viewing Wide Field-of-view
Sensor) observations to study the seasonal evolution and variability of dust
aerosols over the broader Mediterranean Sea during the period 1998–2004.
Alpert and Ganor (2001) and Israelevich et al. (2002) used the Total Ozone
Mapping Spectrometer (TOMS)
aerosol index (AI) product to study the
concentration of dust over the Middle East and the dust sources of northern
Africa, respectively. The MODIS instrument, onboard both Terra and Aqua
satellites, has been extensively used in studies of airborne mineral dust
over the Mediterranean Basin: Barnaba and Gobbi (2004) analyzed 1-year (2001) MODIS–Terra AOD (aerosol optical depth) at 550 nm observations and reported on the spatial
distribution and seasonal variability of aerosols, including dust, over southern Europe, with a focus over the Mediterranean region; Papayannis et
al. (2005) used MODIS–Terra data synergistically with lidar measurements and
dust model simulations and investigated the vertical distribution of
aerosols during dust outbreaks over Greece; Kosmopoulos et al. (2008) and
Papadimas et al. (2008) used MODIS–Terra and MODIS–Aqua to investigate the
seasonal and interannual variability of AOD at 550 nm over Athens (Greece)
and over the broader Mediterranean Sea, respectively; Marey et al. (2011)
analyzed 10 years of MODIS data synergistically with MISR (Multi-angle Imaging SpectroRadiometer)
and OMI (Ozone Monitoring Instrument), and they
produced a monthly climatology of aerosols over a domain covering the Nile
Delta and northeast Africa. With respect to CALIPSO, the 3-D distribution of
dust and its optical properties have been studied for specific cases (e.g.,
Amiridis et al., 2009; Mamouri et al., 2009; Marey et al., 2011; de Meij et
al., 2012; Nabat et al., 2012, 2013; Mamouri and Ansmann, 2015). Moreover,
Winker et al. (2013) provided a 3-D global aerosol climatology from 5-year
CALIPSO data, along with the global distribution of mineral dust, derived
using the ratio of columnar dust AOD to total AOD. Other studies offering a
global view of desert dust using CALIPSO are provided in D. Liu et al. (2008), Z. Liu et al. (2008), Adams et al. (2012), Yang et al. (2012), Tsamalis et al. (2013),
Huang et al. (2015a, b) and Gkikas et al. (2016). In particular, the
studies of Z. Liu et al. (2008), Yang et al. (2012) and Tsamalis et al. (2013)
examined the transatlantic Saharan dust transport, focusing on the optical
properties of dust, the influence of nearby clouds and the vertical
distribution of the Saharan Air Layer, respectively.
Huang et al. (2015a) assessed the inferred most probable heights of global dust and
introduced a separation method (Huang et al., 2015b) of anthropogenic
dust (produced by human activities on disturbed soils) and free-tropospheric
dust using CALIPSO and MODIS products. D. Liu et al. (2008), Adams et al. (2012) and Gkikas et al. (2016) used CALIPSO observations in order to
demonstrate the vertical structure of dust globally and/or above the
Mediterranean. All the aforementioned studies are based on standard CALIPSO
products with known limitations in accurately typing and quantifying the
optical properties of pure dust (Wandinger et al., 2010; Tesche et al.,
2013). The EARLINET-optimized dust CALIPSO product presented herein was
used in Georgoulias et al. (2016a) to apply aerosol typing on MODIS and
derive an aerosol climatology over the eastern Mediterranean.
To our knowledge, this is the first time that a 3-D pure-dust dataset is
statistically analyzed over the area of North Africa and Europe in order to
provide not only the horizontal but also the vertical patterns of Saharan
dust intrusion in the Mediterranean. The study domain is from 20
to 60∘ N and from 20∘ W to 30∘ E. More
specifically, we investigate the 3-D inter-seasonal variation and intensity
of dust transport patterns along with the interannual variations of DOD
above this region. The paper is organized as follows. In Sect. 2 the CALIPSO
lidar data are briefly introduced and the pure-dust retrieval scheme is
described in detail. In Sect. 3, the main findings are presented and
discussed; initially, the inter-seasonal variation and intensity of dust
transport patterns (e.g., DOD, dust layer heights) are presented (Sect. 3.1–3.3), and the representative extinction coefficient values inside the
dust plumes are derived (Sect. 3.4). In Sect. 3.5, the interannual
variation of dust is examined, while our summary and concluding remarks are
given in Sect. 4.
Data and methodologyCALIPSO product
CALIOP, flying onboard the joint NASA–CNES CALIPSO satellite, has delivered
global aerosol and cloud profiles since June 2006 (Winker et al., 2009).
CALIOP measures aerosol backscatter profiles at 532 and 1064 nm,
including parallel and perpendicular polarized components at 532 nm, at high
horizontal and vertical resolution. The data are processed to Level 2 (L2)
products, providing aerosol and cloud backscatter and extinction
coefficients at 532 and 1064 nm as well as the linear particle
depolarization ratio at 532 nm (Winker et al., 2009). First, the processing
algorithm separates the atmospheric scene in distinct atmospheric layers
(i.e., aerosol, cloud and surface returns; Vaughan et al., 2009). Then, for each
aerosol layer the algorithm determines an aerosol subtype (i.e., dust,
polluted dust, clean continental, polluted continental, marine and smoke)
based on a combination of information, such as the surface type, the layer
integrated attenuated backscatter, the depolarization ratio at 532 nm and
the aerosol layer height (Omar et al., 2009). The inferred subtype is used
to derive the appropriate lidar ratio, which is a crucial input for the subsequent
aerosol extinction retrieval (Young and Vaughan, 2009). Burton et al. (2013)
showed an 80 % successful detection of dust from CALIPSO, upon comparison
to underflights with the HSRL (High Spectral Resolution Lidar)
system of NASA. This score is considered very
high for aerosol typing purposes and is attributed to the depolarization
measurement capability of the CALIOP sensor. Finally, the L2 products are
aggregated to a gridded monthly mean Level 3 (L3) product, providing mean
profiles of extinction at 532 nm and mean AOD at a 2∘× 5∘ spatial grid resolution (Winker et al., 2013). The most recent
version of the L3 product (version 3), released in October 2015, includes
the correction of the AOD of cloudy scenes, the improved averaging of
individual types as proposed by Amiridis et al. (2013) and D. Liu et al. (2008), and corrections of signal artifacts responsible for high and low
biases as also observed in Papagiannopoulos et al. (2016).
EARLINET-optimized CALIPSO product
In this study, we make use of the EARLINET-optimized pure-dust extinction
product, monthly averaged at a horizontal resolution of 1∘× 1∘, based on the methodology described in Amiridis et
al. (2013). This product is a prominent outcome from the EARLINET–ESA
collaboration for the LIVAS database (LIdar climatology of Vertical Aerosol
Structure for space-based lidar simulation studies; Amiridis et al., 2015).
Unlike the original CALIPSO L3 product of 2∘× 5∘ resolution, the 1∘ resolution of LIVAS has been proven
very useful in supporting studies of the same spatial resolution,
specifically for the retrievals from passive satellite sensors and model
evaluation studies (e.g., Popp et al., 2016; Georgoulias et al., 2016b;
Tsikerdekis et al., 2017). In our methodology, the pure-dust backscatter
coefficient (βd) is decoupled from the total
aerosol backscatter (βp) based on depolarization
measurements (δp), assuming a particle
depolarization ratio value for pure dust (δd)
equal to 0.31 (Tesche et al., 2009). Typical dust δd values measured with lidars in field campaigns around the
globe are generally consistent with this value, showing little variation
independent of the source region (e.g., Sakai et al., 2000; Z. Liu et al.,
2008; Freudenthaler et al., 2009; Burton et al.,
2013; Groß et al., 2011, 2013, 2015;
Illingworth et al., 2015). During the SAMUM
(Saharan Mineral Dust Experiment) 1 and 2 campaigns, Saharan dust
δnd values varied between 0.27 and 0.35 at 532 nm (Ansmann et al., 2011), introducing a 4 % error in our calculations for
the dust separated backscatter values. Using this separation technique, we
avoid relying on the polluted dust and dust aerosol types used in CALIPSO
and, thus, eliminate possible misclassifications found in the CALIPSO L2 product
(Burton et al., 2013). A final correction is related to the particle linear
depolarization ratio, which is recalculated from L2 perpendicular and total
backscatter profiles, to improve the accuracy compared to the original
CALIPSO L2 version 3 product, affected by a known bug (Tesche et al.,
2013; Amiridis et al., 2013).
Quality control procedures and filtering applied in CALIPSO data.
1Screen out all features that are not aerosols2Set all clear air profile measurements to 0.0 km-13Samples below opaque cloud and aerosol layers are removed4Clear-sky mode: only measurements in which no clouds are in the column are considered5Large negative near-surface extinction filter: all level 2 aerosol extinction samples adjacent to the surface having a value less than -0.2 km-1 are ignored6Samples where aerosol extinction uncertainty is less than 99.9 km-1 are allowed7CAD score: only features having cloud–aerosol discrimination (CAD) scores between -100 and -20 are used8Only features having extinction QC flag values of 0, 1, 16 or 18 are allowed9Cirrus fringes: misclassified cirrus in the upper troposphere, coming from CAD artifacts, are removed10Remove measurements which are contaminated by surface values: extinction values near the surface less than -0.2 km-1 are ignored11Undetected surface-attached aerosol low bias filter (changed between CALIPSO L3 version 1 and version 3): samples classified as “clear air” lying beneath the lowest quality screened aerosol layer whose base is below 250 m from the local surface are ignored12Negative signal anomaly mitigation strategy: all level 2 aerosol extinction coefficients within 60 m of the planetary surface are excluded from level 3 calculations (new in L3 version 3)13All non-dust aerosol types detected in the cell are assigned with a value of 0.0 km-1Extra filters with more strict cloud screening14All profiles having cloud features anywhere in the column are removed15All profiles which fulfil the L3 CALIPSO “CAD score” or “Cirrus fringes” filters are removed
The quality control procedures and filtering criteria applied in the dataset
are summarized in Table 1. In brief, CALIPSO L3 version 3 screening
procedure is followed (Winker et al., 2013; CALIPSO L3-V3, 2015), and
additional filters are incorporated to ensure the use of only cloud-free
profiles. The additional methodology is as follows:
We remove all profiles with cloud features anywhere in the column.
We remove all profiles which fulfil the L3 CALIPSO “CAD score” or “Cirrus
fringes” filters (see also Table 1).
The pure-dust extinction coefficient is computed using a lidar ratio of 55 sr instead of the 40 sr used in the CALIPSO product (Omar et al., 2009; Lopes et
al., 2013). This value is representative of dust over Europe, mainly
originating from northwest Africa, as measured in coordinated
CALIPSO–EARLINET measurements (Pappalardo et al., 2010; Wandinger et al.,
2011), and is in excellent agreement with recent studies of dust
measurements both near the source (Tesche et al., 2009; Veselovskii et al.,
2016) and during long-range transport (Preißler et al., 2012; Kanitz et
al., 2013; Groß et al., 2015; Baars et al., 2016; Papagiannopoulos et al.,
2016). The individual backscatter coefficient profiles at 532 nm are
aggregated at a horizontal spatial resolution of 1∘× 1∘ and a vertical resolution of 60 m from -0.5 to 20.2 km
and 180 m from 20.2 to 30.1 km. This product height is referenced to above sea
level (a.s.l.) altitude.
Climatological vs. Conditional dust product
In this study, we calculate two separate dust products – the climatological
and the conditional.
The climatological dust product is based on Amiridis et al. (2013), with a
value of 0 km-1 assigned to the non-dust aerosol types when averaging
within a cell. This product, hereinafter, is referred to as the Climatological Dust
Extinction (Clim-DE), and the corresponding AOD as Dust AOD (DOD), and is
presented and discussed in Sect. 3.1–3.3. As already discussed in the
introduction, this product has been evaluated against AERONET data and is in
very good agreement with collocated measurements over the Sahara and Europe
(Amiridis et al., 2013). The averaging methodology has been adapted by the
L3-V3 CALIPSO product.
The conditional dust product is derived from averaging the CALIPSO dust
extinction coefficients where dust is present, ignoring non-dust
observations in the area. In particular, the clear air and non-dust aerosol
types detected in the cell are ignored (set as NaN values when averaging).
This product is referred to as the Conditional Dust Extinction coefficient
product (Con-DE) and is presented and discussed in Sect. 3.4.
The two products can be used for different applications. For example,
Clim-DE is representative of the dust contribution to the total aerosol load
and can be valuable in climatological studies. Moreover, the near-surface
DOD helps to estimate the natural aerosol contribution in the total aerosol
load close to the surface for air-quality applications. The Con-DE product,
on the other hand, provides a measure of the intensity of the dust plumes.
Dust product uncertainties
The sources of uncertainties for the pure-dust product are discussed in this
section. CALIOP is able to detect aerosol layers with AOD>0.005 and β>0.25 Mm-1 sr-1
(Winker et al., 2009). The uncertainty estimation of particulate backscatter,
extinction and AOD retrievals reported in the CALIPSO Level 2, version 3
data release, are based on the simplified assumption that all the
uncertainties are random, uncorrelated and produce no biases (Young, 2010).
More specifically, ignoring multiple scattering, the errors in the layer
optical depth calculations typically arise from three main sources: (a) signal-to-noise ratio within a layer, (b) calibration accuracy and (c) the
accuracy of the lidar ratio used for the extinction retrieval. The lidar
ratio uncertainty is the dominant contributor to the total uncertainties,
and the relative error in the layer optical depth is always at least as
large as the relative error in the lidar ratio of the layer and grows as
the solution propagates through the layer (CALIPSO L2-V3, 2010). In our
dataset the typical uncertainties in the CALIPSO Level 2 version 3 product
are between 30 and 100 % for the AOD, between 30 and 160 % for
the aerosol backscatter and extinction coefficient, and > 100 %
for the particle depolarization ratio.
Several studies report that CALIPSO underestimates the columnar AOD due to
undetected aerosol in the free atmosphere. For instance, Rogers et al. (2014) report a ∼0.02 AOD CALIPSO underestimation, when
compared to collocated airborne HSRL measurements over the North American
and Caribbean regions at night. In their data, the dust layers were
primarily non-opaque with extinction less than 1 km-1, so there were negligible multiple scattering effects.
These detection limits and uncertainties of CALIPSO products are propagated
to the dust product presented here.
As already described, the EARLINET-optimized CALIPSO dust product is derived
using the depolarization-based separation method, coupled with the selection
of a uniform climatological LR value. These steps introduce uncertainties in
the pure-dust product. In particular, the uncertainty in the selection of
the representative LR (55±11) is 20 % for the study area
(e.g., Wandinger et al., 2010; Baars et al., 2016, and references within). This
uncertainty in LR is less than half of the uncertainty of the generic LR in
the CALIPSO version 3 product (40±20 for dust layers and
55±22 for polluted dust layers). As already addressed in
several studies (e.g., Wandinger et al., 2010; Schuster et al., 2012; Amiridis
et al., 2013), the CALIPSO V3 dust extinction coefficient and AOD values are
about 30 % lower than those obtained from collocated ground-based Raman
lidar retrievals due to the low LR used in the CALIPSO aerosol retrievals.
Amiridis et al. (2013) applied the EARLINET LR for the pure-dust CALIPSO
cases above North Africa and Europe and compared with synchronous and
collocated AERONET measurements. The results showed an absolute bias on the
AOD of the order of -0.03, improving on the statistically significant
biases of the order of -0.10 reported in the literature for the original
CALIPSO product. The bias of -0.03 is similar to the low bias of
CALIPSO's column AOD due to undetected aerosol layers. In Kim et al. (2017),
they found a global mean undetected layer AOD of 0.0031±0.052 by
comparing 2 years of CALIPSO (L1-V4) and MODIS AODs.
Regarding the error induced from the application of the dust separation
method, this might be due to the selection of the particle depolarization
ratio of dust and the other aerosol types (marine, anthropogenic or smoke).
Tesche et al. (2009, 2011), and Ansmann et al. (2012) estimated that the
uncertainty in dust related backscatter coefficients is 15–20 % in
well-detected dessert dust layers and 20–30 % in less pronounces aerosol
layers. Moreover, we have calculated that the uncertainty of the dust
occurrences presented in Sect. 3.1 (“% dust/used overpasses”), might
be up to 8 % in latitude away from the sources, which is induced from the error in
the selection of the δnd value (0.03±0.04). Finally, an uncertainty induced in the dust product presented in this
work originates from the CALIPSO subtype selection algorithm. In this
version of our product, both dust and polluted dust observations are
considered polluted dust and the pure-dust component is separated using the
dust separation method. The other aerosol layers, which are characterized as
clean marine (CM), smoke (S), polluted continental (PC) or clean continental
(CC), are considered to be cases clear of dust and are not tested for a dust
component. This introduces negligible error in our analysis and is expected
to induce a negative bias in the parameter % dust/used overpasses
less than 8 %, mainly in areas above sea. In general, for Clim-DE and
Cond-DE products, the uncertainty of the dust extinction values close to the
surface and at high latitudes is < 54 %. At high altitudes and for
latitudes up to 45∘ N, the uncertainty of the values is < 20 %. Nevertheless, the standard deviation of the climatological products,
coming from the natural variability of the dust events, may exceed to a
large extent the uncertainty of the retrieval, reaching values as high as
100 and 200 %.
In the latest release of the CALIPSO Level 2 version 4 product (CALIPSO L2-V4,
2016), based on the CALIPSO team announcement, the accuracy of the original
CALIPSO product is increased and the uncertainty is reduced. This version is
based on a revised calibration approach which leads to an increase in the
total attenuated backscatter coefficients by ∼ 3 % overall
as compared to the version 3 values (CALIPSO L1-V4, 2016). Several bugs are
fixed and a major overhaul of the aerosol subtyping algorithms along with
revisions on the lidar ratio selections is applied.
Additional satellite and model dataset
s
The sixth version level-3 MODIS–Terra is a 1∘× 1∘
gridded aerosol dataset that is acquired from the NASA Giovanni system
(https://giovanni.sci.gsfc.nasa.gov/giovanni/). In the current
study the MODIS combined dataset of aerosol optical depth (that takes into
account the dark target, dark surface, and deep blue, bright surface,
measurements) was used for the period from 2007 to 2015 (Sayer et al., 2014).
Over the Mediterranean, MODIS–Terra v6 was evaluated against 23 AERONET
stations and was proven to score better than its predecessor MODIS–Terra v5
(Georgoulias et al., 2016b).
The MACC global dataset is a reanalysis product based on the Integrated
Forecast System (IFS) of the European Centre for Medium-Range Weather
Forecast (ECMWF) coupled with the chemistry transport model MOZART-3
(Kinnison et al., 2007). The horizontal resolution of the model is 80 km, and
it uses 60 vertical levels from the surface up to 0.1 hPa. MACC has been
used in numerous gas phase and particulate matter studies (Innes et al.,
2013; Katragkou et., 2015; Eskes et al., 2015; Flemming et al., 2015; Cuevas
et al., 2015; Georgoulias et al., 2017). The dust optical depth data used in
this study covers the period 2007–2012, and all MACC data are open to the
public (http://apps.ecmwf.int/datasets/data/macc-reanalysis/levtype=sfc/).
RegCM4 is an open source, area-limited, sigma-p vertical coordinated
regional climate model (Giorgi et al., 2012) based on the hydrostatic core
of the Penn State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model (MM5; Grell et al., 1994). The simulation
used in the current study is part of a previous research where the dust
optical depth of the model was evaluated against the dust climatological
product of this work, after it was fully spatially and temporally collocated
with the exact flyby of CALIPSO (Tsikerdekis et al., 2017). The simulation
covers the period from 2007 to 2014 with a horizontal resolution of 50 km and 18
vertical sigma-p levels.
Results and discussion
In Sect. 3.1–3.4, we examine the inter-seasonal variation and intensity of
dust transport patterns, from 2007 to 2015, for the domain 20∘ W
to 30∘ E and 20 to 60∘ N. In Sect. 3.1 we
provide the average climatological state of the seasonal dust distribution
at a spatial resolution of 1∘× 1∘. In Sect. 3.2 we give information on dust layer heights. In Sect. 3.3, we illustrate
the mean climatological vertical structure of dust reaching Europe. To
achieve that, the area of study is separated into five longitudinal zones
with a step of 10∘. In Sect. 3.4, we illustrate the vertical
intensity of the dust plumes, using again longitudinal zone maps. Finally,
in Sect. 3.5, we examine the interannual variation of dust.
Horizontal dust distribution
In this section, we provide the average climatological state of the seasonal
horizontal dust distribution derived from the CALIPSO dust product at a spatial
resolution of 1∘× 1∘ for the domain of North
Africa and Europe. The seasonal grouping used in this study is as follows:
from January to March (JFM), from April to June (AMJ), from July to
September (JAS) and from October to December (OND). In our study region,
March and October are considered transition months for Saharan dust
advection (e.g., Ganor, 1994; Guirado et al., 2014). This grouping is
based on the dominant patterns revealed from the maps of monthly mean DODs
(not shown): the events during February–March and October–November,
although rarer, are usually more intense than those of the other months.
This is further supported from a 10-year (2001–2011) analysis of African
dust outbreak PM10 (particulate matter with aerodynamic diameter lower than 10 µm) observations over the Mediterranean Basin (Pey et
al., 2013).
Geographical distribution of the seasonal dust occurrences
(a, c, e, d) and the mean DOD values (b, d, f, h) for the
3-month averages: January–March (a, b), April–June (c, d), July–September (e, f), and October–November (g, h),
and the domain between 20∘ W–30∘ E and 20–60∘ N for the period 2007–2015, measured with the CALIPSO
climatological dust product.
Regional statistics on mean dust optical depth, max values,
dust layer center of mass (CoM) and top height (TH) (a.s.l.), ratio of
dust observations to cloud-free observations, ratio of cloud-free
observations to total observations and domain boundaries.
Figure 1 shows the geographical distribution of dust occurrences (Fig. 1a,
c, e, g) and the corresponding mean DOD values for each season (Fig. 1b, d,
f, h). In order to provide a more quantitative representation of the
dataset, the domain is aggregated in six areas over the study region. The
main results and statistical parameters are provided in Table 2, and a map
with the domains is shown in Fig. 2. In particular, the information provided
is the mean and standard deviation of the DOD, the maximum values along
with the 95th percentile in parenthesis, the layer's center of mass and
top height along with their standard deviations (these parameters are
discussed in the next section), the percentage of the observations with DOD
greater than zero in the cloud-free observations, the percentage of the
cloud-free occurrences in the total observations provided by the CALIPSO
product (with 100 % as unity) and each domain's geographic extent.
Table 2 shows the impact of cloud contamination in our dataset. During AMJ,
JAS and OND, more than 80 % of the total observations are cloud free above
North Africa. The percentage is the same above the central–eastern Mediterranean
(C–E Med.), whereas above the central–western Mediterranean (C–W Med.) is
approximately 60–80 %. With increasing latitude, the cloud-free
sampling is reduced to percentages of ∼ 40–60 % in
latitudes greater than 45∘ N. During JFM, cloudy conditions
restrict our dataset in the greatest extent. During the same period, the
cloud-free cases used represent ∼ 80 % of the total
observations above North Africa, approximately 60–70 % above the
Mediterranean and ∼ 30 % in the domain between 45 and 60∘ N. In the areas (and seasons) where clouds do not dominate
(e.g., 70 % clear-sky conditions), our cloud-free product is considered
representative of the dust distribution. In areas where cloudy skies
dominate (e.g., 30 % clear-sky conditions), the clear-sky CALIPSO profiles
cannot be considered as representative of all meteorological conditions, so
the results should be used with caution.
Based on Fig. 1 and Table 2, the overall percentages of dust occurrences and
mean DOD values are greater during summer and spring months. During autumn
and winter the emission and transport of dust towards Europe is suppressed
due to the more effective removal processes and due to the atmospheric
dynamics favoring the transport of dust towards the Atlantic (e.g.,
Israelevich et al., 2002; Schepanski et al., 2009). More specifically,
during JFM (Fig. 1a, b) limited dust activity is observed almost uniformly
over the Sahara desert. The DOD remains below 0.13 roughly over the entire
study domain, with 75 % of the observations having DODs < 0.17,
95 % of the observation having DODs < 0.5 and extreme values with
DODs > 2. The dust occurrences decrease with latitude and the
presence of dust is approximately 70 % over Africa and the Mediterranean
region and decreases to lower than 50 % over northern Europe. The most
affected area during these months is eastern Mediterranean. The cyclone
formation over the central Mediterranean, which is affected by mid-latitude
depressions generated either in the Atlantic Ocean or in northwestern
Europe (e.g., Trigo et al., 1999; Maheras et al., 2001), results in the
transportation of dust from the Libyan Desert towards the Balkans, leading
to dust occurrences up to 70 % (Fig. 1a) along with mean seasonal DODs of
0.1–0.2 (Fig. 1b). In the domains between 10 and 30∘ E
and 30–40∘ N, 5 % of dust events are observed
with DODs > 0.41, 1 % with DODs > 0.95 and extreme
observations with DODs are up to 1.6. Similar mean values have been reported
in the literature for this period, along with extreme events characterized
by AOD values higher than 1 (Gerasopoulos et al., 2011). Moving northward,
mean DOD tends to decrease due to the increasing distance from the major
dust sources and also due to higher precipitation at the northern parts of
the study region that efficiently removes dust from the atmosphere (e.g.,
Moulin et al., 1998; Marrioti et al., 2002).
The six domains of the regional statistics provided on Tables 2 and
3.
Comparison of the seasonal spatial distribution of the optical depth
as received by (first column) pure-dust CALIPSO DOD product, (second column)
MODIS AOD product, (third column) MACC reanalysis DOD product, (fourth
column) RegCM4-simulated DOD product.
During AMJ (Fig. 1c, d) dust production occurs over the entire Saharan
desert, with mean DOD values of 0.26 ± 0.26 and occurrences of 86 %,
uniformly at latitudes between 20∘ N and 30∘ N. The
activated dust sources are located in the broad “dust belt” and are
usually associated with topographical lows in the arid regions and with the
intermountain basins (Prospero et al., 2002). The arrival of mid-latitude
extratropical cyclone systems from the Atlantic Ocean as well as
cyclogenesis at the Gulf of Genoa and/or at the northern African coast favors
dust transport over the central and eastern Mediterranean. Mean DOD over these
areas reaches values of 0.12 ± 0.20 (Fig. 1d) and extreme observations
observed with DODs up to 2.74. Dust is also present over central and
northern Europe with mean DOD up to 0.033 ± 0.062 and occurrence
percentages up to 61 % (Fig. 1c; Table 2), revealing that dust particles
can be transported far away from their sources under favorable
meteorological conditions.
During JAS (Fig. 1e, f), intense dust activity is prominently shifted to
the western part of the Sahara where dust occurrences are > 90 % and mean DOD near the sources is up to 0.6 (Fig. 1f). In the domain
between 10∘ W–00∘ and 20–35∘ N, the mean DOD is 0.43, with 25 % of the dust observations having DODs
> 0.69, 5 % > 1.2 and the extreme DODs up to 3 (Table 2). The migration of the ITCZ (Intertropical Convergence Zone)
towards higher latitudes and the dominance of trade wind patterns
(easterlies) benefit the transportation of dust towards the Atlantic Ocean
as seen also by the westward plumes in Fig. 1e and f. In the same period,
increased dust occurrences (83 %) are also found over the western
Mediterranean and South Italy. In the domain between 10∘ W–00∘ and 35–45∘ N, the mean DODs are 0.09 ± 0.14 with 5 % of the dust observations having DODs > 0.55
and extreme DODs up to 2.3.
During OND dust activity is significantly suppressed (Fig. 1g) except from
the southwest desert areas close to the Sahel where mean DOD lies in the
range 0.2–0.3 (Fig. 1h). In the domain between 10∘ W–00∘ and 20–30∘ N, the mean DODs are 0.43 ± 0.39 and extreme DODs up to 3 (Table 2).
In order to provide a more informative representation of the dust product,
we performed a comparison with MODIS AOD for the same period, the dust
optical depth of the MACC reanalysis for the period 2007–2012 and a RegCM4
simulation for the period 2007–2014 (Fig. 3). MODIS provides the AOD for all
natural and anthropogenic aerosol types. As a result the MODIS average value
for the whole period and domain (0.267) is 281 % bigger than our product
(0.095 ± 0.04). It is noted though that the values between the two
satellite products are very similar over the Sahara desert. On the contrary,
the corresponding average dust optical depth values of MACC (0.100) and
RegCM4 simulations (0.104) consider only dust and are in better agreement
with our product, with lower values by 5 and 8.6 %, respectively. The
95 % confidence interval of the mean for MACC is between 0.092 and 0.108,
and for RegCM4 it is between 0.099 and 0.108. Considering these ranges, the
discrepancies between the CALIPSO dust product and the two models are within the
combined uncertainty.
Dust optical depth is overestimated over Europe and the Mediterranean by MACC
and RegCM4 simulations compared to our product, in all seasons and
especially in the hot periods of AMJ and JJA. The reasons of these
discrepancies have to be further studied.
Vertical dust distribution
CALIPSO offers the ability to assess the vertical distribution of dust from
space. To facilitate the investigation of the vertical characteristics of
dust, two parameters are introduced the dust top height (TH) and the dust
center of mass height (CoM) (Mona et al., 2006, 2014;
Binietoglou et al., 2015). TH is defined as the height corresponding to the
altitude with 98 % of the dust extinction below it. CoM is the
extinction-weighted altitude given by the formula
CoM=∫ztzbzazdz∫ztzbazdz,
where zb and zt are the base and top altitude of the dust feature,
respectively, and α denotes the dust extinction coefficient at
altitude z. CoM provides a measure of the altitude where most of the dust
load is located. This parameter is considered ideal for comparisons with
aerosol layer height retrievals from passive remote sensing (e.g., IASI,
GOME-2A, Sentinel5P and the future Sentinel-4 and Sentinel-5 missions; Ingmann et al., 2012), since these retrievals are sensitive to the location
of the dust mass maximum within the layer (e.g., TROPOMI Aerosol Layer
Height product; Sanders et al., 2015).
Geographical distribution of the dust top height (a–d) and
the center of mass (e–h) in km a.s.e. measured with CALIPSO dust
product for the 3-month averages: January–March (a, e),
April–June (b, f), July–September (c, g), and
October–November (d, h), and the domain between
20∘ W–30∘ E and 20–60∘ N for the period
2007–2015.
Figure 4 shows the spatial distribution of TH and CoM for the four seasons.
In Table 2, the TH and CoM values above surface elevation (a.s.e.) are
accompanied with their standard deviations providing an indication of the
variability of the dust heights in the atmosphere of the study area. During
JFM dust resides in general below 3 km a.s.e. over land, with CoM at about
1.3 ± 1.6 km a.s.e. (Fig. 4a, b). Over the sea, several transport
paths are discernible especially over the eastern Mediterranean, with dust tops
traveling at 2.3 ± 1.9 km a.s.e. Over the eastern parts of the Sahara
during AMJ, TH and CoM are up to 4.2 ± 1.7 km and 2.4 ± 1.1 km a.s.e., respectively. Over the Mediterranean Sea and southern Europe, dust
tops extend at ∼ 2–3.5 km and CoM is ∼ 1–2 km a.s.e., with the central and eastern Mediterranean having the most elevated
plumes (Fig. 4c, d). The latitudinal slope of CoM denotes the latitudinal
transport of dust during AMJ from south to north. The highest TH values
(> 4.5 km) are found during the warm period (JAS) over
northwestern Africa and over the adjacent Atlantic Ocean region (Fig. 4e,
f). This is most likely attributed to the intrusion of the lower
tropospheric Atlantic monsoon, south of the ITCZ, and the development of
mesoscale convective systems that favor the elevation of dust at this
area (Bou Karam et al., 2008). The dust height decreases towards the eastern
part of the study region. In the interim, the dominance of the strong
Saharan high enables the mobilization of dust from the western part of
the Sahara towards the western Mediterranean and Europe. This pattern leads to
elevated dust at 3.0 ± 1.7 km a.s.e. and CoM at 1.6 ± 1.1 km a.s.e. over southern European countries and the Balkans. During OND the
horizontal pattern is similar to JJA, however, with much lower heights (Fig. 4g, h).
In general, our results are in agreement with lidar-based studies which have
been performed in several European sites. Papayannis et al. (2008) performed
an exhaustive analysis on Saharan dust particles over Europe using EARLINET
lidar profiles. They found that the dust layer center of mass extends from
3.0 to 3.8 km and the thickness ranges from 0.7 to 3.4 km. Moreover, Balis (2012) calculated the mean base and top of dust layers in the eastern
Mediterranean, Thessaloniki, to be around 2.5 ± 0.9 km and 4.2 ± 1.5 km, respectively. More recently, Mona et al. (2014) analyzed a
long dataset of Saharan dust intrusions over Potenza, Italy, and found a
mean layer center of mass of 3.5 ± 1.5 km.
Climatological dust cross sections
Regional statistics on the dust extinction coefficient for
altitudes between 0 and 2 km, 2 and 4 km and 4 and 6 km (a.s.l.).
To further illustrate the vertical dynamics of dust reaching Europe, the
area of study between 20∘ W and 30∘ E is separated into
five longitudinal zones of 10∘, covering latitudes from
20 to 60∘ N, and the results are presented as
latitude-height cross-section plots in Fig. 5, with the respective statistics
in Table 3. The vertical structure of the averaged Climatological Dust
Extinction coefficient (Clim-DE) for each of these five longitudinal zones
reveals several dust layers and strong seasonal variations. The two dashed
lines drawn in the cross-section plots show the number of dust observations
averaged for the extinction retrievals. The extinction values below the
higher dashed line correspond to > 18 dust observations (two dust
overpasses per season and year), whereas below the lower dashed line they correspond to
> 54 (two dust overpasses per month and year). The median surface
elevation is depicted with black color (and is labeled as NaN) in the
plots.
Geographical zonal distribution of the climatological dust
extinction coefficient values (Mm-1) measured by the CALIPSO dust product
for the regions 10 to 20∘ W (a–d), 0 to 10∘ W
(e–h), 0 to 10∘ E (i–l), 10 to 20∘ E
(m–p) and 20 to 30∘ E (q–t) for the latitudinal
regions from 10 to 60∘ N as illustrated by domain maps for the
3-month averages: January–March (a, e, i, m, q), April–June
(b, f, j, n, r), July–September (c, g, k, o, s) and
October–November (d, h, l, p, t). The median surface elevation is
depicted with black color.
Figure 5 shows that dust is always ubiquitous at heights close to the
surface throughout the year. The lower layers are representative of near-source dust activity and boundary layer processes. The spring and summer
peaks indicate the increased activity of Saharan dust sources (Moulin et
al., 1998; Schepanski et al., 2007). More specifically, for the area between
10 and 20∘ W over the Atlantic, extending from Africa
to the west of the Iberian Peninsula and Ireland (Fig. 5a–d), the presence of
elevated dust plumes is evident mainly during summer and for latitudes up to
30∘ N. During JFM the plume is located below 2 km height above
sea level (a.s.l.), while from spring to autumn the plume reaches a height
of 5 km a.s.l. and yields high values of extinction coefficient
(∼ 75 Mm-1) over Africa. Over the area from 0 to 10∘ W, extending from western Algeria, Morocco, the Iberian
Peninsula and the British Isles, we found Clim-DE values inside the Africa
mixing layer greater than 60 Mm-1for all seasons. Maximum values of
extinction are observed during summer months when dust is elevated up to 6 km with Clim-DE values around 120 ± 140 Mm-1 above N.
Africa and mean values exceeding 200 Mm-1 above the Algerian Desert
(Fig. 5g). These findings are in good agreement with more than 2 years of
AERONET observations in the Tamanrasset site, a strategic site for dust research
located in the heart of the Sahara (Guirado et al., 2014). A steep decrease in
extinction values is observed along the African coastline, with values of 20 Mm-1 above the southern part of the Iberian Peninsula (38–42∘ N) where dust is trapped by the Pyrenees. The distinct
decrease of extinction values across the African coastline is an indication
that dust is always present inside the rather deep Saharan boundary layer
while it is only occasionally transferred towards the Mediterranean when
atmospheric dynamics favor this kind of flow. At higher latitudes, the
CALIPSO dust extinction is drastically reduced but still observed at 1–2 km a.s.l., with mean Clim-DE values of 5 Mm-1. As discussed in detail
in Sect. 2.4, the uncertainty of the dust extinction values close to the
surface and at high latitudes is < 54 %, with the higher
uncertainty in this region mainly originating from the selection of the
δnd value during the dust separation step.
Moreover, the standard deviation, coming from the natural variability of the
dust events, is an order of magnitude higher than the mean values (Table 3).
Moving eastwards (0–10∘ E), elevated dust is trapped
topographically by the Alps (47–52∘ N), with values
> 10 Mm-1. As the dust-laden air-masses approach the
mountains, they decelerate and their dust concentrations increase
(Israelevich et al., 2012). Maximum values of extinction (> 50 Mm-1) are observed over northern Africa during summer (Fig. 5k). Close
to the Algerian sources, south of the Atlas Mountains (∼ 30∘ N), the extinction coefficient is greater than 200 Mm-1
close to the surface (Fig. 4k). The area south of the Atlas Mountains
(Fig. 5e, f, g, h) is characterized by haboob activity (Knippertz et al.,
2009; Solomos et al., 2012). These systems are generated from convective
outflows and contribute to the interannual burden of dust at this area. As
dust extends to higher latitudes (30–40∘ N) Clim-DE
decreases (< 75 Mm-1). Over the area between 10 and 20∘ E (Fig. 5m–p), similar patterns are observed. This region
includes the dust sources of Libya and the central Sahara, the central
Mediterranean, the eastern Alps and part of northern Europe. It is evident from
this figure that dust extinction over the central Mediterranean (35–45∘ N) is around 25 Mm-1throughout the year. As in the
previous western zonal section, the same pattern over the Alps is
encountered. Moving further eastwards, maximum values of Clim-DE are found
during spring. At the most eastern part of the study area (20–30∘ E; Fig. 5q–t), dust is trapped by the Carpathian Mountains
(45–49∘ N) especially during winter, highlighting once
more the role of topography. Significant dust presence is evident all over
the zonal section (until 60∘ N) and is mostly attributable to
elevated dust traveling along with the westerlies from western and central
parts of Europe towards the east. Above the Balkans and during JFM, values of 29 ± 65 Mm-1 are observed in the first 1.5 km and 10 ± 30 Mm-1 between 2.5 and 3.5 km. In AMJ and JAS, respectively, mean
values of ∼ 16 ± 40 Mm-1 and ∼ 9 ± 20 Mm-1 are observed at altitudes between 1.5 and 5 km. Over
Africa during winter and spring, the values of Clim-DE are higher
(> 45 Mm-1) compared to the ones observed during the other
two seasons (< 45 Mm-1), reaching high altitudes (5–6 km a.s.l.) during spring and summer. In summary, the obtained cross sections
for the five longitudinal zones indicate that higher extinction coefficient
values are observed near the source and at low altitudes, where dust
particles are efficiently deposited.
The above results are representative of the spatial distribution of dust
load as this is approximated by the aerosol extinction coefficient. In order
to provide the dust load in units that are more relevant for modeling
studies, we estimate here the dust mass concentration. The mass
concentration can be obtained from the optical properties of dust with an
uncertainty of 20–30 % (Ansmann et al., 2012; Mamouri and Ansmann, 2014).
For example, the Clim-DE values correspond to dust mass concentrations
> 75 µg m-3 above Africa throughout the year and
> 125 µg m-3 above West Africa during JAS. In southern
Europe and the Mediterranean, the corresponding values are > 17 µg m-3 in the first 2 km a.s.l. and ∼ 50 µg m-3 close to the surface. For latitudes greater than
45∘ N, values of ∼ 8 µg m-3 are the most
common.
The decreasing intensity with height and latitude of Clim-DE is
representative of the average dust distribution over the area. However, this
behavior is not representative of the distribution during dust episodes
over Europe. This is because the extinction coefficient values presented in
Fig. 5 for the Clim-DE product are produced by averaging both partially and
fully dominated dust cases. Conversely, the Con-DE product presented in the
next section describes the spatial patterns and the intensity of the dust
plumes during the dust episodes only.
Conditional dust cross sections
Geographical zonal distribution of the conditional dust extinction
coefficient values (Mm-1) measured by CALIPSO dust product for the
regions 10 to 20∘ W (a–d), 0 to
10∘ W (e–h), 0 to 10∘ E (i–l), 10 to
20∘ E (m–p) and 20 to 30∘ E (q–t) for
the latitudinal regions from 10∘ N to 60∘ N as illustrated by
domain maps for the 3-month averages: January–March (a, e, i, m, q), April–June (b, f, j, n, r), July–September (c, g, k, o, s) and October–November (d, h, l, p, t). The median terrain
elevation is depicted with black color.
The two dashed lines in Fig. 6 correspond to > 18 dust
observations (two dust overpasses per season and year) and > 54
dust observations (two dust overpasses per month and year) for the lower and
higher dashed line, respectively. Con-DE values derived from less than 4
dust observations (dO) in each cell are masked with grey color (and are
labeled as < 4dO) in the plots. The median surface elevation is
depicted with black color (same as in Fig. 5). Con-DE values are
significantly different from the Clim-DE, as seen in Fig. 5. Although Con-DE
has similar values to Clim-DE near the sources, where dust is always
present, above the Atlantic and the Mediterranean Con-DE is characterized by
significantly higher values. This is expected, since the two products differ
mostly over areas which are not dominated by dust.
In the vertical cross-section plots of Fig. 6 the patterns of Con-DE show
two distinct dust features: over the longitudinal zone from 20 to
30∘ E during summer (Fig. 6o) a distinctive feature is seen above
North Africa extending from the surface to ∼ 5 km a.s.l. and
a second feature is seen above the Mediterranean between 3 and 6 km a.s.l.
The two distinct layers are also identified in other regions and in other
seasons (e.g., Fig. 6a, l, p, s, t). These populations are linked to two
different processes: the near-surface dust at the southern parts of the
study region is connected to fresh emissions from the dust sources, while
the elevated plumes that extend north until 40∘ N are due to
the advection of dust, associated to the seasonality of the long-range
transport paths (Lelieveld et al., 2002; Israelevich et al., 2012; Huneeus
et al., 2016). This separation is enhanced as one moves from the west to the
east sectors. At the western part of the domain (10–20∘ W) the near-surface and elevated dust probably originates
from the same sources. Similar double layer patterns are found in all
seasons and over all areas with various characteristics. For example, during
JAS at the region extending from 0 to 10∘ W (Fig. 6g),
the generation of dust from the source region is much more intense than the
transportation of dust. For the same period, in the area 0 to
10∘ E, the dust transportation above the Mediterranean between 3
and 6 km height, originating from the intense source regions, becomes much
more important than the source emissions.
Moreover, the vertical cross-section plots in Fig. 6 show the rare but very
intense elevated dust plumes during JFM (Fig. 6a, e, i, m). During that period,
dust is advected between 1.5 and 4 km height a.s.l. with Con-DE values
> 45 Mm-1, equivalent to dust mass
concentrations > 75 µg m-3. The
intensity of the JFM dust episodes above the Balkans is also depicted (Fig. 6q): the Con-DE value is similar to other regions, but the dust plumes can
be thicker, extending from the ground until 4 km a.s.l. The trapping of
Saharan dust from the mountainous ridges of Europe (located between
40 and 50∘ N, e.g., the Alps 45–48∘ N) is also evident (e.g., Fig. 6i, m). The deceleration of
the transport air masses along the mountain ridges results in the
accumulation of dust at the windward slopes. Dry deposition of dust at these
areas also result in the formation of “brown snow” and albedo reduction,
with profound climatological implications (e.g., Fujita, 2007; Shahgedanova
et al., 2013). This phenomenon is more intense during the JFM period due to
the advection of dust at lower heights.
During AMJ (Fig. 5b, f, j, n, r) and JAS (Fig. 5c, g, k, o, s) the elevated dust above
the Mediterranean presents Con-DE values of 35–50 Mm-1
(58–83 µg m-3), in heights between 2 and 6 km and
up to latitudes of 40∘ N. The transport of dust during AMJ is
mostly due to the eastward propagation of N. Africa–Mediterranean low
pressure systems (Sharav cyclones). Dust is embedded in the cyclonic
circulation and the penetration to latitudes higher than 40∘ N is
limited. For latitudes 40 and 50∘ N during the warm
seasons (AMJ and JAS), the Con-DE values inside the transported dust plumes
are between 20 and 40 Mm-1 (33–67 µg m-3). Rare events, characterized by relatively higher Con-DE
(> 35 Mm-1 and > 58 µg m-3) between 2 and 5 km a.s.l., are observed over the British
Isles and Germany during OND (Fig. 6h, l). These events, caused by the
propagating low pressure systems over the east Atlantic, have been
documented in detail from the EARLINET community reporting extinction
coefficient values up to 200 Mm-1 inside dust plumes (Ansmann
et al., 2003; Müller et al., 2003). In the vertical cross-section plots
of Fig. 6, it is evident that dust reaches the upper levels of the
troposphere (> 8 km a.s.l.), with Con-DE values of
∼ 10 Mm-1 in all longitudinal zones and during
all seasons. Dust occurrence is very low, close to zero for heights greater
than 8 km a.s.l. during spring and summer and for heights greater than 6 km a.s.l. during autumn and winter. A quantitative representation of the
Clim-DE and Con-DE products is provided in Table 3. In this, regional
statistics of the two products, along with their standard deviation, are
provided for three altitudinal ranges (0–2, 2–4 and 4–6 km a.s.l.).
Interannual variability of dust
Geographical distribution of the deseasonalized trends (yr-1)
derived from monthly columnar DOD (a) and for five individual layers
(b–f), for the period 2007–2015, aggregated over
10∘× 10∘ grid cells. Hatched filled grid cells
depict the statistical significance trends with 99 % confidence.
Interannual variability of the DODs for the
10∘× 10∘ grid cells depicted in Fig. 7, for the
period 2007–2015.
In this section we present the CALIPSO-derived monthly mean DOD values, for
the total column and for five individual layers (0.18–0.5, 0.5–1, 1–2,
2–4 and 4–8 km), in order to study their interannual variability during
the 9-year period between 2007 and 2015. The selected layers are
representative for both near-surface and long-range transported dust plumes.
The data are aggregated on a 10∘× 10∘ cell over the
study region. Using a first-order autoregressive linear regression model on
the deseasonalized monthly DOD values (108 in total) as described in Zanis
et al. (2006), temporal trends of DOD were calculated. We note that 9 years are considered a small period for a robust trend calculation and it
would be interesting to extend this analysis with future measurements.
Figure 7 shows the geographical distribution of deseasonalized trends
(yr-1) for the columnar DOD (a) and for the five individual
layers (b–f). Hatched filled grid cells depict the statistical significance
trends with 99 % confidence. A decrease of ∼ 0.001 yr-1 (∼ 4 % yr-1) is evident
for the southern European cells (0–30∘ E, 40–50∘ N) (with these values being > 95 %
statistically significant). Examination of the five vertical layers shows a
similar decreasing pattern. The negative trends observed in the area (mainly
above North Africa and the Mediterranean) show a constant decrease throughout
the layers as well, although the trends are not statistically significant.
The small negative DOD trends (< 0.002 yr-1corresponding to < 5 % yr-1)
are in good agreement with the global decrease of dust estimated from a
161-year time series of dust from 1851 to 2011, created by projecting wind
field patterns onto surface winds from a historical reanalysis in Evan et
al. (2016). The agreement is also good with the global mean near-surface
dust concentration decrease of 1.2 % yr-1 reported in Shao
et al.'s (2013) paleoclimate research for the period 1984–2012, even though
Europe and North America are excluded from their trend analysis. In
comparison with studies relevant to the time period considered in this work,
the DOD decrease of 0.001 yr-1 over the northern coast of
Africa is in agreement with Floutsi et al. (2016), who based their analysis
on 12 years of MODIS-Aqua observations (2002–2014), reporting an average
decrease of 0.003 yr-1 for the coarse mode fraction of the
AOD over the broader Mediterranean Sea. Furthermore, over the same domain
the decreasing trend of DOD agrees well with the decrease of Saharan desert
dust episodes as reported by Gkikas et al. (2013). Regarding the AERONET
stations over the domain of northern Africa and Europe, Yoon et al. (2012)
reported trends of AOD at 440 nm along with the corresponding
ångström Exponents (440 and 870 nm). The documented negative trends
over the AERONET stations of Avignon (France), Dakar (Senegal) and Ispra
(Italy) are in agreement with the negative DOD reported here, although with
discrepancies in the magnitude, while trend disagreements are observed over
the AERONET station of Banizoumbou (Niger). The decreasing trends of DOD
observed over the domain northern of Africa and Europe coincide with the
documented downward AOD trends reported based on several satellite
observations of MODIS–Aqua, MODIS–Terra, MISR and SeaWiFS (Pozzer et al.,
2015; de Meij et al., 2012; Hsu et al., 2012; Georgoulias et al., 2016b).
Particularly in the most recent study of Georgoulias et al. (2016b),
using MODIS–Terra and MODIS–Aqua observations, negative statistically
significant trends are reported over Algeria, Egypt and the Mediterranean
and positive trends over the Middle East. Overall, for the Mediterranean they
reported an AOD trend of -0.0008 yr-1 for the MODIS–Terra
observations (2000–2015) and -0.0020 yr-1for the
MODIS–Aqua observations (2002–2015), with the trends being statistically
significant at the 95 % confidence level in both cases. A possible
increase is only found for the western Sahara areas (10–0∘ W, 20–30∘ N). However, the results for this cell are
not statistically significant. Figure 8 shows the DOD interannual
variability of the 20 individual areas, as it is calculated from the monthly
mean DODs. It is evident from this figure that the DOD values in 2008 are
relatively higher than the other years and in almost all the domains below
40∘ N. Similarly, relatively high values are observed in some of
these areas for the year 2010. Since these years are at the beginning of our
study period, they have a significant contribution on the negative trends
observed during the examined period.
Summary and conclusions
An optimized CALIPSO dust product was recently developed by Amiridis et al. (2013) using a regional correction for the Saharan dust lidar ratio derived
from EARLINET measurements. The same product is used here to provide the
three-dimensional dust distribution and its transport pathways across
northern Africa and Europe from 2007 to 2015. The study of the mean state
climatology shows strong seasonal shifts in dust source regions and
transportation pathways. The seasonal cycle of the dust transport is well
captured with the lowest values of DOD in winter and the highest values in
spring and summer. During summer and autumn, dust aerosols are mostly
confined to the source region, while during spring dust aerosols from the
Sahara are extended uniformly over the northern Sahara and are transported over the
Mediterranean and Europe. The dust extinction coefficient, CoM and TH
parameters are used to quantitatively describe the 3-D evolution of dust and
its seasonal variations. Over the source region of the Sahara Desert, dust CoM
and TH are higher during spring and summer and lower during winter. Dust
transport mechanisms are more efficient during summer when dust is often
lifted up to 6 km, coinciding with the deepest dust layer. Localized
regions of increased extinction coefficient values over mountains (the Alps,
the Pyrenees and the Carpathian Mountains) trace the aerosol transport
routes that decelerate in front of the mountain ranges. Rare and intense
events are observed above the Balkans during the winter period and above
northwest Europe during autumn. The interannual analysis revealed that DOD
trends during the study period are of the order of 0.001 yr-1 for
southern
Europe, showing constant decrease throughout the different layers.
The dust climatology presented here is of paramount importance in
understanding the three-dimensional production and transport of Saharan
dust, providing valuable information for better estimations of the dust
climatic impacts. The climatological and conditional products presented here
describe both the dust contribution to the total aerosol load over our
domain as well as the Saharan dust events recorded in the region,
respectively. Future work includes (i) the optimization of CALIPSO dust
retrievals based on measured dust LR from ground-based lidars and particle
depolarization ratio over extended regions of deserts in the Middle East and
China, to obtain a robust global climatology of dust; and (ii) the calculation
of cloud condensation nuclei (CCN) and ice nuclei (IN) concentrations from polarization
lidar measurements as suggested by Mamouri and Ansmann (2016), to provide a
quantification of the climatic effect of dust on cloud formation.
The CALIPSO data were obtained from the online archive of the ICARE Data and
Services center http://www.icare.univ-lille1.fr/archive
(CALIPSO Science Team, 2015; ICARE Data Center, 2016). MODIS data are
publicly available on the NASA Giovanni system (https://giovanni.sci.gsfc.nasa.gov/giovanni/). MACC data are publicly
available on http://apps.ecmwf.int/datasets/data/macc-reanalysis/levtype=sfc/. The regional climate model ReGCM4 code is available at https://gforge.ictp.it/gf/project/regcm/frs/. RegCM4 simulation data used
in this work are available upon request from Athanasios Tsikerdekis
(tsike@geo.auth.gr). The LIVAS database is publicly available
at http://lidar.space.noa.gr:8080/livas/. LIVAS
EARLINET-optimized pure-dust products are available upon request from Eleni
Marinou (elmarinou@noa.gr) and Vasilis Amiridis (vamoir@noa.gr).
The authors declare that they have no conflict of interest.
Acknowledgements
The authors acknowledge support through the following projects and research
programs:
BEYOND under grant agreement no. 316210 of the European Union Seventh
Framework Programme (FP7-REGPOT-2012-2013-1),
MarcoPolo under grant agreement no. 606953 from the European Union
Seventh Framework Programme (FP7/2007-2013),
ACTRIS under grant agreement no. 262254 of the European Union Seventh
Framework Programme (FP7/2007-2013),
ACTRIS-2 under grant agreement no. 654109 from the European Union's Horizon
2020 Research and Innovation Programme,
ITaRS under grant agreement no. 289923 of the European Union Seventh
Framework Programme (FP7/2007-2013),
ECARS under grant agreement No 602014 from the European Union's Horizon 2020
Research and Innovation programme,
EPAN II and PEP under the national action “Bilateral, multilateral and
regional R&T cooperations” (AEROVIS Sino-Greek project), and
A. G. Leventis Foundation scholarship.
The authors acknowledge EARLINET for providing aerosol lidar profiles
available under the World Data Center for Climate (WDCC; The EARLINET
publishing group 2000–2010, 2014a, b, c, d, e). We thank the AERONET PIs
and their staff for establishing and maintaining the AERONET sites used in
this investigation. CALIPSO data were obtained from the ICARE Data Center
(http://www.icare.univ-lille1.fr/). CALIPSO data were provided by NASA. We
thank the ICARE Data and Services Center for providing access to the data
used in this study and their computational center. We thank Jason Tackett
for his support during the algorithm development for the production of Level
3 CALIPSO products.Edited by: M. Tesche
Reviewed by: four anonymous referees
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