Tellus (2009), 61B, 325–339
C
2008 The Authors
Journal compilation
C
2008 Blackwell Munksgaard
Printed in Singapore. All rights reserved
TELLUS
EARLINET observations of the 14–22-May long-range
dust transport event during SAMUM 2006: validation
of results from dust transport modelling
By D. M
¨
ULLER
1∗†
,B. HEINOLD
1
,M. TESCHE
1
,I. TEGEN
1
,D. ALTHAUSEN
1
,
L. ALADOS ARBOLEDAS
2
,V.AMIRIDIS
3
,A. AMODEO
4
,A. ANSMANN
1
,D.BALIS
5
,
A. COMERON
6
,G.D’AMICO
4
,E. GERASOPOULOS
3
,J. L. GUERRERO-RASCADO
6
,
V. FREUDENTHALER
7
,E. GIANNAKAKI
5
,B.HEESE
1
,M.IARLORI
8
,P. KNIPPERTZ
9
,
R. E. MAMOURI
10
,L.MONA
4
,A. PAPAYANNIS
10
,G.PAPPALARDO
4
,R.-M. PERRONE
11
,
G. PISANI
12
,V.RIZI
8
,M.SICARD
6
,N. SPINELLI
12
,A. TAFURO
11
and M . W IEG N E R
7
,
1
Leibniz
Institute for Tropospheric Research, Leipzig, Germany;
2
Andalusian Center for Environmental Studies, University of
Granada, Granada, Spain;
3
National Observatory of Athens, Athens, Greece;
4
Istituto di Metodologie per l’Analisi
Ambientale – Consiglio Nazionale delle Ricerche, Tito Scalo, Potenza, Italy;
5
Laboratory of Atmospheric Physics,
Aristotle University of Thessaloniki, Thessaloniki, Greece;
6
Universitat Polit
´
ecnica de Catalunya, Barcelona, Spain;
7
Meteorological Institute, Ludwig Maximilian University, Munich, Germany;
8
Dipartimento di Fisica, Universit
´
adegli
Studi - L’Aquila, L’Aquila, Italy;
9
Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany;
10
Physical Department, National Technical University of Athens, Athens, Greece;
11
Istituto Nazionale per la Fisica
della Materia, Universit
´
a degli Studi di Lecce, Italy;
12
Consorzio Nazionale Interuniversitario per le Scienze Fisiche
della Materia and Dipartimento di Scienze Fisiche - Universit
´
a degli Studi di Napoli “Federico II”, Naples, Italy
(Manuscript received 9 July 2008, in final form 20 October 2008)
ABSTRACT
We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral
Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun
photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and
with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the
far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ.
Ångstr
¨
om exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical
properties of the dust plume with a dust transport model. The model results and the experimental data show similar
times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees
in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust
could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases
we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one
reason for these deviations.
1. Introduction
This contribution is the companion paper to the publication
by Heinold et al. (2008) who present results from the regional
∗
Corresponding author.
e-mail: detlef@tropos.de
†
Now at: Atmospheric Remote Sensing Laboratory, Department of
Environmental Science and Engineering, Gwangju Institute of Science
and Technology, Gwangju, South Korea
DOI: 10.1111/j.1600-0889.2008.00400.x
dust model system LM-MUSCAT-DES (LM = Lokal Modell;
MUSCAT = MUltiScale Chemistry Aerosol Transport Model;
DES = dust emission scheme). The model is used to describe
the conditions of Saharan dust observed in Morocco during the
Saharan Mineral Dust Experiment (SAMUM) 2006. The model
simulates Saharan dust emission, the transport and deposition
of dust, and the effect of dust on the radiation balance (Heinold
et al., 2007). In our paper we extend the simulations on dust
transport into the far field of the North African source region.
The performance of the regional dust model system LM-
MUSCAT-DES was evaluated with data of particle optical
Tellus 61B (2009), 1 325
PUBLISHED BY THE INTERNATIONAL METEOROLOGICAL INSTITUTE IN STOCKHOLM
SERIES B
CHEMICAL
AND PHYSICAL
METEOROLOGY
326 D. M
¨
ULLER ET AL.
depths, extinction coefficients, and particle size distributions.
Data were collected with remote sensing and in-situ instrumenta-
tion in Morocco. Heinold et al. (2008) evaluated the performance
of the model for two time periods in May and June 2006 with
focus on dust properties observed over the Moroccan field sites
on 19 and 20 May 2006, and 3 and 4 June 2006. The authors find
rather good agreement between the modelled and the measured
dust optical thicknesses and dust particle size distributions. The
spatio-temporal evolution of the dust plumes in contrast was not
always reproduced. Another result of the study by Heinold et
al. (2008) is that the model finds the correct maximum value of
the dust extinction coefficient along the vertical scale of the dust
layer, if source and transport of the dust plume are correctly sim-
ulated. However the model does not reproduce well the strong
gradients of dust extinction coefficients that occurred at the top
of the dust layer during SAMUM 2006 (Heinold et al., 2008).
In summary, the model system is generally capable of describ-
ing the north Saharan dust cycle. In particular dust events related
to synoptic-scale meteorology and long-range transport of dust
agree well with the observations. However, the evaluation of the
model results with the large number of available observations in
proximity to dust source regions demonstrates the limits of the
regional dust model system (Heinold et al., 2008).
We extend the study by Heinold et al. (2008) into the far
field of the dust source on the basis of a long-range transport
event that began around 14 May 2006. At that time Saharan
dust was transported from Morocco to the Iberian peninsula and
from there across South Europe to Greece, where the dust plume
arrived around 20 May 2006. The dust plume was observed with
the lidars of the European Aerosol Research Lidar Network
(EARLINET) (B
¨
osenberg et al., 2003; Matthias et al., 2004)
and Sun photometers at the lidar stations.
Lidar observations in Spain, Italy, and Greece provide us with
vertically resolved information on the geometrical and optical
properties of the dust plume in the far field of the source region.
The observations at the SAMUM field site provide us with the
properties of the dust plume, just before it left the African source
region. Thus, this work also presents an extension of a previous
study of dust long-range transport observed within EARLINET
(Ansmann et al., 2003). In that previous case study dust was
carried across west and central Europe, but we did not have
information on the properties of the dust plume at its North
African source region. Our study also provides a useful link to
Saharan dust long-term observations which are carried out in the
framework of EARLINET since 2000 (Papayannis et al., 2005,
2008; Mona et al., 2006).
In our case study, the EARLINET sites provide us with a
valuable set of data for model validation. One has to keep in
mind that the comparison of modelled dust optical thickness
with satellite indices, provided by Ozone Monitoring Instrument
(OMI) (Levelt, 2002) and Meteosat Second Generation (MSG)
(Schmetz et al., 2002) can only be qualitative, as no quantitative
dust information can be obtained over land from remote sensing
with these instruments yet.
In Section 2, we summarize the methodology. In Section 3,
we describe the geometrical and optical properties of the dust
plume. We compare the properties of the dust plume over South
Europe to the same properties measured in Morocco. In Section
4, we compare our results for South Europe to the results from the
dust model simulations. In Section 5, we close our contribution
with a summary.
2. Methodology
2.1. EARLINET lidar stations
The European Aerosol Research Lidar Network (EARLINET)
is a network of 25 European lidar stations (status as of May
2008). Each lidar group performs observations on a routine
base several times per week since May 2000. EARLINET is
the follow-up network to the German lidar network that was op-
erational from September 1997 until April 2000 (B
¨
osenberg et
al., 2001). One goal of EARLINET is to establish a quantitative
database of both horizontal and vertical distribution of aerosols
on a continental scale. In addition to the regular observations,
lidar measurements are also carried out during so-called special
events as, for instance, large-scale transport of Saharan dust to
Europe.
Figure 1 shows the current distribution of EARLINET lidar
stations. The stations that carried out observations in the time
from 14 to 23 May 2006 are marked with yellow circles. A
Fig. 1. Location of EARLINET lidar stations (red bullets). Yellow
circles denote lidar stations that reported mineral dust in the
investigated timeframe. The lidar station that was operated in Morocco
during SAMUM is also shown (black bullet with yellow circle).
Tellus 61B (2009), 1
DUST TRANSPORT: MODEL VALIDATION WITH EARLINET LIDARS 327
description of the different instruments can be found in
B
¨
osenberg et al. (2003).
Briefly, most systems are multiwavelength lidars which allow
us to determine the particle backscatter coefficient β(λ)atseveral
measurement wavelengths λ. A detailed description on how that
analysis is performed can be found in Ansmann and M
¨
uller
(2005).
From these observations backscatter-related Ångstr
¨
om expo-
nents
˚
a
β
(λ
1
, λ
2
) are determined. This exponent describes the
spectral dependence of the backscatter coefficient. It is defined
as
˚
a
β
(λ
1
,λ
2
) = ln[β(λ
2
)/β(λ
1
)]/ ln(λ
1
/λ
2
). (1)
The expressions β(λ
1
)andβ(λ
2
) describe backscatter coef-
ficients at two different measurement wavelengths λ
1
and λ
2
,
respectively.
Several Raman lidars of the network measure the nitrogen
Raman signal at 387 nm (355-nm primary wavelength) and/or
607 nm (532 primary wavelength) in addition to the elastic
backscatter signals at the laser wavelengths. From these ob-
servations the volume extinction coefficient α(λ) and the vol-
ume backscatter coefficient of the particles can be determined
(Ansmann et al., 1992).
From β(λ)andα(λ) we determine the extinction-to-
backscatter ratio (lidar ratio) S (λ). This quantity is sensitive
to particle size and complex refractive index. Because this pa-
rameter contains the particle backscatter coefficient, the lidar
ratio also is sensitive to the geometrical shape of the particles.
In contrast, the particle extinction coefficient does not depend
on particle shape in a significant way (Mishchenko et al., 1997;
Kalashnikova and Sokolik, 2002; M
¨
uller et al., 2003).
Measurements of the particle extinction coefficient at two
wavelengths allow us to determine the extinction-related
Ångstr
¨
om exponent
˚
a
α
(λ
1
, λ
2
). This parameter is defined as
a
α
(λ
1
,λ
2
) = ln[α(λ
2
)/α(λ
1
)]/ ln(λ
1
/λ
2
). (2)
2.2. Sun photometers
Particle optical depth of the atmospheric column and column-
mean Ångstr
¨
om exponents were determined with Sun photome-
ters at all EARLINET lidar stations considered in our study,
except at L’Aquila. We add results of a Sun photometer station
in Rome (Rome Tor Vergata at 41.5
◦
N, 12.4
◦
E) which is about
113 km to the west of the lidar station in L’Aquila. Except for
a multi-filter rotational shadowband spectrometer at the EAR-
LINET station in Athens all other instruments are Sun photome-
ters operated by the Aerosol Robotic Network (AERONET).
The instrument type at the Athens stations is an MFR-7
Yankee (Env. System Inc., Turner Falls, MA). The spectrometer
provides 1-min averages of particle optical depth at five wave-
lengths (415, 501, 615, 675 and 867 nm). The methodology of
extracting particle optical depth, direct solar irradiance, and all
applied corrections is described in detail by Gerasopoulos et al.
(2003).
AERONET is a federated network of Sun photometer sta-
tions. The instrument characteristics are described in detail by
Holben et al. (1998). Briefly, spectral observations of Sun direct
irradiance are made at 340, 380, 440, 500, 670, 870, 940 and
1020 nm. Measurements of sky radiance are made at 440, 670,
870 and 1020 nm. Details of the calibration procedure of the
instruments are given by Holben et al. (1998, 2001).
From these signals one determines particle optical depth and
scattering phase functions. Microphysical properties such as par-
ticle size distributions and complex refractive indices are deter-
mined, too. A detailed description of the data analysis can be
found in Dubovik and King (2000). Details on error analysis are
given by Dubovik and King (2000) and Dubovik et al. (2000).
In this contribution we will only present particle optical depth
and Ångstr
¨
om exponents.
2.3. Instruments at the SAMUM field sites
Raman lidar and Sun photometer observations were carried out
in Morocco. The Raman lidars were operated at the SAMUM
field site at Ouarzazate (30.93
◦
N, 6.9
◦
W). The systems are de-
scribed in detail by Tesche et al. (2008) and Freudenthaler et al.
(2008). The lidar systems provide us with particle backscatter
and extinction coefficients, Ångstr
¨
om exponents, and lidar ra-
tios at the same measurement wavelengths that are used by the
EARLINET lidars. Linear particle depolarization ratios were
determined at 355, 532, 710 and 1064 nm. Data analysis and er-
ror analysis procedures for the lidar data are discussed in detail
by Tesche et al. (2008). We also operated one AERONET Sun
photometer at the field site.
2.4. Model
The simulations of Saharan dust transport presented here were
carried out with the regional dust modelling system LM-
MUSCAT-DES, for which a detailed description is given in
Heinold et al. (2007). The model consists of the mesoscale me-
teorological model Lokal Modell (LM) (Doms and Sch
¨
attler,
2002) which is provided by the German weather service
(Deutscher Wetterdienst, DWD), the online-coupled MUltiScale
Chemistry Aerosol Transport Model (MUSCAT) (Wolke et al.,
2004a,b), and a dust emission scheme which is based on the work
of Tegen et al. (2002). Dust emission, transport, and deposition
are simulated with MUSCAT with the use of the meteorological
and hydrological fields that are computed by the LM. Surface
properties (vegetation, surface roughness, soil texture, soil mois-
ture content) and the location of preferential dust sources are con-
sidered for dust flux calculations. Soil erosion by wind mostly
depends on the wind shear stress on the ground. Soil erosion oc-
curs when the surface friction velocity increases above a certain
soil-size dependent threshold friction velocity. We have lowered
Tellus 61B (2009), 1
328 D. M
¨
ULLER ET AL.
that threshold friction velocity by a factor of 0.66 in order to
compensate for lower model winds (Heinold et al., 2007). Lo-
cal wind systems, clouds, precipitation, and mesoscale convec-
tion are simulated depending on topography, subgridscale moist
convection is parameterized following Tiedtke (1989). The mod-
elled dust is transported as a passive tracer in five independent
size classes with diameter limits at 0.2, 0.6, 1.7, 5.3, 16 and
48 μm. The aerosol deposition parameterization in LM-
MUSCAT is adapted with respect to dust particle density and
washout efficiency. Dry deposition of dust is parameterized as
proposed by Zhang et al. (2001). For particles larger than 2
μm the removal from the atmosphere is mainly by gravitational
settling. Wet deposition, both in-cloud and subcloud removal,
is parameterized following Berge (1997) and Jakobson et al.
(1997). In the model, the radiative flux computation accounts
for the variability in the spatio-temporal distribution of mod-
elled dust aerosol, and the direct dust radiative effect can affect
meteorology and dust load (Helmert et al., 2007). Dust optical
properties are derived from Mie-scattering theory using the re-
fractive indices from laboratory measurements by Sokolik and
Toon (1999), assuming an internal mixture of 2% hematite and
98% kaolinite.
The dust model was run for the period from 9 May to 5
June 2006. In this contribution we focus on the long-range
dust transport case that occurred during SAMUM 2006 on the
days of 14–23 May, when Saharan dust was transported across
the Mediterranean basin. The model domain covers the area
13.86
◦
N, 25.35
◦
W – 47.78
◦
N, 38.16
◦
E. We use a horizontal res-
olution of 28 × 28 km and 40 vertical layers. The LM runs
are initialized using analysis fields from the global model GME
(Majewski et al., 2002). The LM runs are driven by 6-hourly
updated lateral boundary conditions from the GME. In order to
keep the meteorology of the regional model close to the anal-
ysis fields, the simulations were performed in 48-h cycles with
a spin-up time of 24 h for the LM. After this time MUSCAT is
coupled to compute dust mobilization and transport. At the first
cycle, the initial dust concentration is set to zero. The following
cycles are initialized using the modelled dust concentration from
the previous cycle.
For the model evaluation, the modelled dust concentration is
transferred to the dust optical thickness at 500 nm wavelength
with
τ =
j
k
3
4
Q
ext, 500
(j)
r
eff
(j)ρ
p
(j)
,c
dust
(j,k)z(k)
. (3)
The expression Q
ext, 500
(j) is the extinction efficiency at 500 nm
of the dust mode j, r
eff
(j) is the effective radius of dust particles
of mode j, c
dust
(j, k) is the dust concentration of the dust mode
j at the vertical level k and z(k) is the depth of the vertical
level k. For the evaluation of the simulated dust distribution, the
extinction efficiency Q
ext, 500
(j) is calculated from Mie-scattering
theory and the use of dust refractive indices from Sinyuk et al.
(2003).
Fig. 2. Selected 5-d backward trajectories started from gridpoints close
to the locations of the eight EARLINET stations Granada (G),
Barcelona (B), L’Aquila (A), Naples (N), Potenza (P), Lecce (L),
Athens (A) and Thessaloniki (T). The numbers indicate the trajectory
start and times (in UTC). The trajectories and the letters indicating the
stations are colour-coded with height above ground in m. The
trajectories are started from within the dust layer according to the lidar
profiles shown in Fig. 6.
3. Observations
3.1. Meteorological situation and dust distribution
The meteorological conditions of the main dust episodes during
the 2006 SAMUM field campaign are described in detail by
Knippertz et al. (2008) and are summarized here with the help of
5-d trajectories linking EARLINET stations in southern Europe
with dust sources in northern Africa (Fig. 2), and horizontal
distributions of the OMI aerosol absorption index (AI) (Fig. 3).
Blue areas in Fig. 3 indicate clouds that obscured the dust plume.
During 11–14 May 2006 an upper-level short wave trough
crossed northwestern Africa and triggered the formation of a lee
cyclone east of the Atlas Mountains (Knippertz et al., 2008). The
subsequent surge of cold air from the Mediterranean Sea accom-
panied by strong winds activated dust sources in western Tunisia
as well as eastern and central Algeria as indicated by the OMI
AI distribution on 14 May 2006 (Fig. 3a). Along the northern
flank of this cyclone, dust-laden air was advected into southern
Morocco as shown by the trajectories labeled ‘B’ and ‘G’ in Fig.
2. Between 15 and 17 May 2006 an upper-level ridge established
over northwestern Africa, with a surface high centered over the
eastern Atlas. Moderately dust-laden air from eastern and cen-
tral Algeria was transported with the anticyclonic flow to the
Moroccan coast and then towards the Iberian Peninsula (Figs.
2 and 3b). The dust plume passed over the two EARLINET
stations at Granada and Barcelona during this period as indi-
cated by both OMI AI and trajectories. By 18 May 2006 the
Tellus 61B (2009), 1
DUST TRANSPORT: MODEL VALIDATION WITH EARLINET LIDARS 329
Fig. 3. Horizontal distribution of the aerosol
index derived from OMI observations
(overpass at 13:45 local time). That index
indicates the presence of dust. The higher the
index the more likely dust was observed.
Model results (LM-Muscat-DES) of the
horizontal distribution of optical depth at
550 nm wavelength at 12 UTC are presented
in the right column. Shown are the results on
(a,b)14May,(c,d)16May,(e,f)18May,
(g, h) 20 May, and (i, j) 22 May 2006. The
abbrevations denote the following stations:
AT (Athens), BA (Barcelona), GR
(Granada), LA (L’Aquila), LE (Lecce), NA
(Naples), ORZ (Ouarzazate), PO (Potenza)
and TH (Thessaloniki).
dust plume covered the western Mediterranean and approached
the EARLINET station at L’Aquila (3c). Backward trajectories
that started on 19 May from the four Italian EARLINET stations
all show an anticyclonic track from western Algeria across the
Iberian Peninsula and suggest a connection to the dust event on
14 May (see Fig. 3a). On 20 May 2006 the dust plume stretched
across southern Italy into Greece (Fig. 3d), where parts of it
remained until 22 May 2006 (Fig. 3e). Backward trajectories
from Athens and Thessaloniki, started at 0:00 UTC 21 May,
show a path similar to the trajectories for the Spanish and Italian
stations, but the corresponding airmass was delayed by about
2–4 d. This path suggests a link with dust being mobilized over
Algeria in the aftermath of the event described by Knippertz et al.
(2008). Most of the dust transport occured in the lower half of
the troposphere as indicated by the gray shaded areas in Fig. 2.
3.2. Sun photometer observations of the dust plume over
South Europe
In this section we present results of Sun photometer observa-
tions. The data rather likely describe a mixture of mineral dust
with anthropogenic pollution. Anthropogenic pollution is ubiq-
uitous in the boundary layer over the South European lidar/Sun
photometer stations considered in this study. In Section 3.4, we
will present optical properties of the mineral dust plume in the
free troposphere on the basis of lidar observations. In that case
the data describe mineral dust that was rather unaffected with
anthropogenic pollution.
AERONET Sun photometer observations at the SAMUM
lidar field site at Ouarzazate showed daily-mean dust opti-
cal depths of 0.44 ± 0.04 at 500 nm on 13 May 2006, and
0.66 ± 0.19 on 14 May 2006. Ångstr
¨
om exponents from dust
Tellus 61B (2009), 1
