Introduction
The Sahara is the world's largest desert. In summertime, a large thermal low
develops in the lower troposphere over the western Sahara in response to the
radiative warming of the surface . This Saharan heat low
plays a pivotal role in the atmospheric regional circulation
e.g.. The Sahara is also
known as the largest source of dust, making dust a key element of the Saharan
climate . In a recent study, noticed
that the increase in aerosol optical depth (AOD) often coincided with the
position of the heat low over the western Sahara. It is well known that
dust is an important component of the Earth's climate system through its
interaction with radiation (e.g. ). Based on experiments on
sensitivity to dust load, they demonstrated the increase in the thickness of
the heat low with dust. In addition to this direct radiative impact, they
found an indirect effect – the intensification of the circulation in the
Sahel, i.e. the monsoon flow and African Easterly Waves (AEWs). Such a remote
impact of dust on the regional circulation has already been observed over
western Africa e.g., as well as over
other dust-prone regions such as north-western China (e.g. ).
The activation of dust is mainly controlled by soil conditions, surface
characteristics and surface wind speed. Because of the persistence of surface
characteristics (including dry soil conditions) over the Sahara, the main
factor that influences the dust activation here is the low-level dynamics. In
the recent years, several mechanisms have been identified, among which are
certain synoptic features like the harmattan wind, Sharav cyclones
, AEWs and the nocturnal monsoon flow
. Some other mechanisms of dust activation are mesoscale
processes, in particular the breakdown of the nocturnal low-level jet in the
morning and the density currents formed by deep
convection in the afternoon . The recent
capability to run high-resolution models over a large domain has led to
process-oriented studies on the role of moist convection in mobilizing dust
e.g.. Strong wind gusts at the
leading edge of cold pools are known to generate dust. However, their role is
a matter of debate because of the difficulties in detecting them by satellite
in the presence of large cloud anvils and because of the inability of low-resolution
models to generate such convective systems and their associated mesoscale
circulations see the review by.
An investigation of the characteristics of the dust properties in the Saharan
heat low region was one of the motivations of the Fennec programme. Fennec is
an international programme aiming at a better understanding of the Saharan
climate system . In June 2011, a field campaign was
organized to measure key properties (i.e. the dynamics, thermodynamics and
composition) of the Saharan atmosphere. Two aircraft were operated over
northern Mauritania and Mali and ground-based observations
were made over Zouerate, Mauritania , and Bordj Mokhtar, Algeria
. A description of the airborne observations
together with an overview of the main Fennec results obtained so far is given
by .
An aspect of the Fennec programme not described so far is the numerical
effort to forecast dusty conditions for the guidance of aircraft. During the
2011 field campaign, four sets of dust forecasts were specifically designed
for Fennec. They were produced every day using three different models. Two
dust forecasts were made with models running with grid spacing of 24 and
20 km. With such a grid spacing, convection is a subgrid-scale
process for which the grid-scale effects are expected to be represented by a
convection parameterization. The two other dust forecasts were made with
models running with grid spacing of 5 km. In these forecasts, no
parameterization was used for deep convection, which was permitted to be
explicitly represented. This allowed these high-resolution models to forecast
the density currents associated with thunderstorms, while the low-resolution
models were more apt to miss these features crucial for dust emission.
The paper presents an intercomparison of the dust forecasts made for the 2011
Fennec field campaign. The objectives of this intercomparison were to look
for any potential systematic error in the forecasts, to identify the wind
regimes leading to dust emission, to estimate their relative contribution to
the total dust emission over the western Sahara, and to evaluate the ability
of the models to reproduce the key processes for mobilization and transport.
The dust evaluation was made using two key variables: the AOD retrieved from
satellite and ground-based observations and the dust extinction coefficient
derived from airborne lidar measurements. The structure of the Saharan
atmospheric boundary layer (SABL) was also assessed using dropsondes launched
over the western Sahara, i.e. in northern Mauritania and northern Mali.
The paper is organized as follows. Section 2 describes the models and the
observations. Section 3 provides an overview of the model performance.
Section 4 presents a discussion on the dominant wind regimes leading to dust
emission. Section 5 gives an assessment of the model skill over the western
Sahara, where the heat low was located in the second half of June. Section 6
concludes the paper.
Models and observations
Models
The four sets of dust forecasts compared here were performed with three
limited-area models, ALADIN (Aire Limitée Adaptation Dynamique
INitialisation; ), AROME (Applications of Research to
Operations at MEsoscale; ) and Meso-NH .
ALADIN and AROME are two spectral models used operationally for weather
prediction by Météo-France and other national weather services. They
share the same semi-implicit, semi-Lagrangian advection scheme as the
European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated
Forecast System (IFS) and the Météo-France global model ARPEGE
(Action de Recherche Petite Echelle Grande Echelle; ).
Meso-NH is the non-hydrostatic mesoscale atmospheric model of the French research community. It is a grid-point model using a fourth-order centred
advection scheme for the momentum components and the piecewise parabolic
method advection scheme for other variables.
ALADIN is a hydrostatic model, while AROME and Meso-NH are non-hydrostatic.
All three models use the same parameterizations for surface processes
and radiation, that is, the Interactions between Soil, Biosphere and
Atmosphere (ISBA) surface scheme , the Rapid Radiative
Transfer Model (RRTM) parameterization for longwave, and the
two-stream formulation originally employed by for
shortwave. In addition, AROME and Meso-NH share their physical
parameterizations for microphysics, turbulence and shallow convection. More
details on ALADIN, AROME and Meso-NH can be found in ,
and , respectively.
The limits of the model domains are shown in Fig. . The
horizontal grid spacing was 24 km for ALADIN and 5 km for
AROME. ALADIN and AROME used a terrain-following vertical grid in pressure
coordinates, with 60 and 41 vertical levels, respectively. The vertical
levels were separated by 35 m above the ground and by 1 km in
the upper troposphere. For these two models, initial and boundary conditions
were taken from operational large-scale ARPEGE forecasts at 18:00 UTC.
ALADIN and AROME were integrated forward for 72 and 48 h,
respectively. For Meso-NH, two horizontal grid spacings were employed, 20 and
5 km, leading to two sets of forecasts, named MNH20 and MNH05,
respectively, hereafter. They shared the same terrain-following vertical grid
in altitude coordinates, with 70 levels ranging from 40 m above
ground to 27 km. These two grid configurations covered the same domain as
shown in Fig. . Meso-NH was initialized by the ECMWF analysis at
00:00 UTC and run for 24 h using the ECMWF forecasts for the lateral
boundary conditions. All the forecasts started from aerosol-free conditions.
Then, dust prognostic variables at the end of a given 24 h forecast
were passed on as initial conditions at the start of the next 24 h
forecast. Model outputs were saved every 3 h, with some diagnostics being
missed for a few models. The comparison examines the first day of forecasts,
i.e. between lead times of 9 and 30 h for ALADIN and AROME and 3 and
24 h for Meso-NH. Because of the 5 km grid spacing, explicit
deep convection was permitted in AROME and MNH05 and no parameterization was
employed for subgrid deep convection.
Topography of northern Africa. The locations of the domains of ALADIN,
AROME and Meso-NH are indicated with cyan, blue and red lines, respectively.
The black rectangle shows the area of interest where most of the SAFIRE
Falcon 20 data were acquired. The location of the three AERONET stations used
in this study are shown with black dots.
The three models share the same dust prognostic scheme described in
. Dust fluxes are calculated from wind friction velocities
using the Dust Entrainment and Deposition (DEAD) model . The
physical basis of the model is taken from , in which dust
fluxes are calculated as a function of saltation and sandblasting processes.
The horizontal saltation flux is first calculated based on the wind friction
speed deduced from the wind speed at the first level. Then, the dust
vertical flux equals the total horizontal saltation mass flux weighted by the
sandblasting efficiency. The dust emissions are forced directly by the
surface flux parameters of the ISBA surface scheme, and then distributed into
the atmosphere. In this parameterization, the three log-normal modes are
generated and transported by the log-normal aerosol scheme of the ORganic and
Inorganic Log-normal Aerosols Model ORILAM;. The initial
dust size distribution contains three modes with median diameters of 0.078,
0.64 and 5.0 µm and standard deviations of 1.7, 1.6 and 1.5,
respectively, as defined by . Dust loss occurs through
sedimentation and rainout in convective clouds. Radiative properties of dust
are calculated within ORILAM, which is coupled online to the radiation
scheme.
The three models used different versions of DEAD. ALADIN used the version
improved by to better take the size distribution of
erodible material into account. In this version, the surface soil size
distribution depends on the soil texture following and
the sandblasting efficiency is parameterized following . AROME
and Meso-NH used the original version of DEAD. The surface soil size
distribution was assumed to be uniform without any restriction on the number
of particles available for saltation. The sandblasting efficiency was
calculated following , in which the sandblasting
efficiency depends on the clay fraction available in the soil (up to a limit
of 20 %). AROME and Meso-NH also differed by a mass tuning factor
used to estimate the horizontal saltation flux. This factor depends on the
dynamics and grid spacing of the meteorological model. Therefore, it needs to
be tuned. Its value was 1.6×10-3 cm-1 in MNH20, 1.0×10-3 cm-1 in MNH05 and 15×10-3 cm-1 in
AROME.
Observations
The forecast intercomparison took advantage of the unique observation data
set obtained from the SAFIRE (Service des Avions Français
Instrumentés pour la Recherche en Environnement) Falcon 20. The aircraft
was equipped with the LEANDRE Nouvelle Génération (LNG) backscatter
lidar . The profiles of atmospheric extinction coefficient
at 532 nm were retrieved using a standard lidar inversion method that
employs a backscatter-to-extinction ratio of 0.0205 sr-1 (see
, for a more detailed description of the inversion
method). The retrievals had an estimated uncertainty of 15 %, and a
resolution of 2 km in the horizontal and 15 m in the
vertical. The Falcon 20 was also equipped with a dropsonde capability. It was
based at Fuerteventura (Canary Islands, Spain) from 1 to 23 June 2011. The
aircraft flew over the western African coast during the maritime phase (2 to
12 June; ) and over the Sahara during the heat low phase (13
to 30 June). When the aircraft was cruising, the flight altitude was
maintained constant at approximately 11 km above mean sea level. The
reader is referred to for a comprehensive description of the
instrumentation on board the Falcon 20 as well as the complete list of the
flights.
The forecast intercomparison also took advantage of the Aerosol Robotic
Network (AERONET) stations located in the Sahara (Zouerate, Bordj Mokhtar and
Tamanrasset, Fig. ). It is worth noting that the AERONET stations
at Zouerate and Bordj Mokhtar were installed specifically for the 2011 Fennec
field campaign. We used AOD at 500 nm and the 440–870 nm
Ångström exponent from the three stations. For the purpose of comparison
with 3-hourly forecast outputs, the AERONET products were averaged within
a time window of 3 h. The Ångström exponent characterizes the AOD
dependence on wavelength and provides information on the aerosol type and
size. The purely dust cases correspond to the lowest values of the
Ångström exponent. In general, dust is characterized by low values of
the Ångström exponent, less than 0.4.
To obtain an assessment of the models performance at the regional scale, we
used satellite-based AOD retrievals from the Moderate Resolution Imaging
Spectroradiometer (MODIS) on board Terra and Aqua and the Multi-angle
Imaging SpectroRadiometer (MISR) on board Terra. Terra and Aqua cross the
Equator at around 10:30 and 13:30 local time, respectively. The MODIS AOD at
550 nm was obtained from the Deep Blue collection 6 over bright surfaces
such as deserts . The MISR AOD used here is the monthly AOD
average (level 3 product) taken in the green band (558 nm). MODIS Deep Blue
algorithm is based on observations in the blue wavelengths of the visible
spectrum (412 and 470 nm), while MISR uses four narrow spectral bands
centred at 446, 558, 672, and 866 nm and nine distinct zenith angles. AOD
products are then referenced to 550 and 558 nm for MODIS and MISR,
respectively. Comparisons of AOD between the Deep Blue algorithm and AERONET
sun photometers showed a general agreement within 20–30 % for AOD
. A global comparison of MISR and AERONET AODs showed that
63 % of the MISR AODs fell within the envelope of ±0.05 or
±20 % × AOD of AERONET values . Products
were on a 1∘ latitude–longitude grid for MODIS and 0.5∘
latitude–longitude grid for MISR. The global AOD coverage with MISR was
achieved in 9 days due to its narrow swath of 360 km while MODIS with
a 2400 km wide swath achieved a near-global AOD coverage on a daily
basis. As a result, the mean number of retrievals for June 2011 was 28 for
MODIS and 5 for MISR. have shown that, over northern Africa and
during the Fennec campaign, AODs retrievals from these sensors were sensitive
to meteorological conditions as well as to the emissivity of underlying
surfaces.
Aerosol optical depth around 12:00 UTC in June 2011 from
(a) MODIS, (b) MISR, (c) ALADIN,
(d) AROME, (e) MNH20 and (f) MNH05. The solid line
shows the 600 m altitude. The black rectangle shows the domain of analysis.
The locations of the three AERONET stations used in this study are shown with
stars.
Assessment of AOD
Overall evaluation against satellite observations
We first provide a comparison of monthly mean forecast AODs at
500 nm for June 2011 against equivalent quantities derived from
satellite observations (Fig. ). The forecast AODs were averaged
at 12:00 UTC, the closest output time to the observations. Note that the
dust load follows a diurnal cycle over western Africa . The
single daytime observation from either the Aqua or the Terra satellite led to
an undersampling of the dust cycle. This undersampling, combined with the
effect of the cloud masking, strongly affected the reliability of the AOD.
For MODIS retrievals, estimated an AOD underestimation of
0.28 over the convective regions and an AOD overestimation of 0.1 over
morning source areas.
MODIS showed an AOD maximum of 1 in the Bodélé depression over
northern Chad, known as one of the most intense sources of dust in the world.
This area of large AOD extends westward over the Erg of Bilma (north-eastern
Niger). Large AOD values were also found by MISR in approximately the same
locations, but with different magnitudes (e.g. 1 for MISR, compared to 0.6 for
MODIS over the Erg of Bilma; 18∘ N, 14∘ E). From the
southern flanks of Adrar des Iforas to the Atlantic coast, the two retrievals
agreed in the meridional location of the AOD maximum, over the Sahara. But
they differed strongly in its magnitude, with values around 0.6 for MODIS and
over 1 for MISR. Another area of disagreement was the north-western edge of
the Hoggar Mountains and the Tademaït Plateau
(28∘ N, 2∘E), where MODIS AOD retrievals were larger than
0.5, while MISR showed values less than 0.4. However, MODIS and MISR agreed on the
AOD range (between 0.3 and 0.5) over the Grand Erg Occidental between the
Hoggar and Atlas mountain ranges. In a comparison of satellite observation of
Saharan dust source areas, found large discrepancies
between MODIS and the Spinning Enhanced Visible and Infrared Imager
(SEVIRI). As in Fig. a, found the large
frequency of MODIS AOD larger than 0.5 in the Sahel region. Their result
contrasted with the dust source activation derived from SEVIRI, for which a
large strip of activation was found along the southern Algerian border with
Mali and Mauritania. By contrast, MISR shows higher AODs further north,
particularly over the central Sahara, in better agreement with the
SEVIRI-derived source regions. This is likely related to the overpass time of
MISR, which is closest to the time of source activation as the result of the
breakdown of the nocturnal low-level jet in the morning.
All the forecasts agreed with each other in showing a strip of large AOD
around 18∘ N, consistent with both MODIS and MISR. The forecasts
differed significantly, however, in the range of AOD. ALADIN showed the
lowest value around 0.4, a little lower than MNH20, with values around 0.6.
The two convection-permitting forecasts exhibited much higher values, up to
1, which matched the largest MISR values. The difference in MNH20 and MNH05
suggests that these high AOD values were caused by processes better
represented at high resolution. This concerned wind acceleration by both
topographical channelling and dust uplift at the leading edge of density
currents related to thunderstorms. The convection-permitting models also
delivered the best forecasts by capturing the meridional gradient of AOD
observed by MISR over the western Sahara (the rectangle in Fig.
delimits the area within which most of the aircraft operation took place in
June 2011; see ). There, the average AOD was the lowest for
ALADIN (0.3) and the highest for MNH05 (0.7).
Over the Grand Erg Occidental, MNH20 and MNH05 showed much larger AODs than
ALADIN and AROME, with AOD values of 0.7 for Meso-NH, compared to 0.4 for the
other two models. The contrast between these two sets of forecasts is attributed to wind speed (as discussed in the next section), in relation to the difference in the dynamical core and/or the initial conditions
provided by different global models. Note that the models with the smallest
domain (AROME and Meso-NH) show a lack of AOD at the eastern boundary. The
reduced coverage by these models can be a strong limitation as dust can
travel from Sudan to western Africa . Lastly, the maximum over
the Bodélé depression appears to be missed by all the models. This is
also true for ALADIN despite the corrections implemented in the revised DEAD
version. This failure could be attributed to an underestimation of
near-surface wind speed as previously noted in a model intercomparison study
dedicated to Bodélé .
Comparison of AOD at AERONET stations
We now consider comparisons of forecast AOD against AERONET AOD at
500 nm using time series from the three stations located in the
Sahara (Fig. ). The corresponding MODIS values are also
shown, together with the 440–870 nm Ångström exponent from
AERONET. As described by , June 2011 can be separated into two
distinct periods with different meteorological conditions. In the first
period, the so-called maritime phase from 2 to 12 June, the western Sahara
was under the influence of synoptic disturbances originating from the
Atlantic, while the heat low was located around 15∘ E. During the
second period, the so-called heat low phase, from 13 to 30 June, the heat low
migrated in a westward location at 5–10∘ W and a series of AEWs
propagated across the western Sahara.
Time evolution of AOD (left vertical axis) from AERONET, MODIS,
ALADIN, AROME, MNH20 and MNH05 (see legend box) and of the Ångström exponent
(right vertical axis) from AERONET (crosses) at (a) Zouerate,
(b) Bordj Mokhtar and (c) Tamanrasset.
At Zouerate (Fig. a), the change in AOD was particularly
representative of the westward migration of the heat low in the western
Sahara. During the maritime phase (2–12 June), AOD values were lower than
0.2, while the Ångström exponent shows values larger than 0.4. Because of
the prevalence of north-westerlies, these low values of AOD were probably due
to sea salts coming from the Atlantic. Near-zero values of AOD were correctly
forecast by all the models while the MODIS retrieval overestimated AOD
strongly with values around 0.3 for a few days. From 13 June onwards,
Zouerate was primarily affected by dust advecting with the north-easterlies
. AERONET AOD increased in the range between 0.6 and 1.2 while
the Ångström exponent was below 0.4. This AOD increase was retrieved from
MODIS and forecast rather well by the models. AERONET AOD peaked at 1.2 on 21
and 23 June. AROME, MNH20 and MNH05 regularly forecast peaks of AOD, but for
different days and with larger magnitude than observed with AERONET (up to
1.9, 1.9 and 1.3, respectively). MODIS and ALADIN presented much less
temporal variation, with AOD being limited to a maximum of 1.0 and 0.8,
respectively.
At Bordj Mokhtar (Fig. b), the AOD was characterized by an
enhanced variability in AOD with respect to Zouerate. The Ångström
exponent remained below 0.4, indicating that dust contributed the most to the
AOD. Before 8 June, the centre of the heat low was close to the station, to
its west. Consequently, the station was affected by the moist south-westerly
monsoon flow and the AOD was observed to exceed twice the value of 1
. The high-resolution forecasts presented a similar change
in AOD, but not at the right time. Between 8 and 12 June, the heat low moved
to its easternmost position and did not significantly influence the AOD
observed in Bordj Mokhtar. In the absence of any significant meteorological
disturbance affecting the station, the AOD decreased below 0.4. The low value
of AOD was well forecast by all the models. From 13 June onward, the
station was inside the heat low as it settled in its Saharan location for the
year 2011. Two dust emission processes were predominant: cold pools
associated with deep convection initiating in the southerly monsoon flow and
the north-easterly harmattan on the western flank of the heat low
. The AOD was frequently observed to exceed the background
value of 0.4. It sometimes reached values as large as 2. Five episodes of AOD
larger than 1 and lasting for about 3 days were distinguished on 13, 17,
21, 25 and 29 June associated with propagating AEWs. AEWs favour dust
emission by increasing the low-level flows and conditions, creating conditions
where deep convection tends to occur. The occurrence of these high AODs was
better simulated in the high-resolution forecasts. This shows the predominant
role of deep convection at the origin of these AOD peaks.
At Tamanrasset (Fig. c), the AOD derived from the AERONET
station was always above 0.4 and the Ångström exponent less below 0.4
until 20 June. There were four episodes with AODs above 1 associated with
near-zero values of the Ångström exponent. These days corresponded to
episodes of long-range transport of dust from eastern sources, some of which
were associated with the same AEWs as those travelling across Bordj
Mokhtar, rather than local emissions. It is worth noting that most of the
Hoggar Mountains is covered with silt loam in which the low percentage of
sand does not favour saltation and hence dust emission. The increase in
Ångström exponent seen in late June suggests that dust was mixed with
other aerosol species. All the forecasts gave an AOD temporal variability
that was lower than observed. However, two dust episodes (on 15 and 19 June)
out of four were forecast correctly in time, but not in magnitude, with the
forecast AODs being biased towards low values. The low bias AOD was largely
explained by an underestimation of the transport from eastern sources as well
as the lack of remobilization of dust from the eastern sources deposited on
the surface, which is not represented by the DEAD scheme but is frequently
observed in this region.
The comparison of AOD time series is summarized in a quantitative way by
means of a Taylor diagram in Fig. , where the AERONET AOD
values at the three stations are taken as references. Overall, the forecasts
show rather similar skills for all AERONET stations. In most of the cases,
they underestimate AODs with a bias between -40 and -60 %. A
notable exception is the bias at Zouerate, which is smaller than
-20 % for ALADIN and AROME and +8 and +29 % for MNH05
and MNH20, respectively. Relatively good scores in terms of correlation were
achieved at Zouerate and Tamanrasset, the stations that bordered the heat low
in the second half of June 2011 (up to 0.84 for MNH05 at Zouerate). The
lowest correlation coefficients (between 0.25 and 0.41) were obtained in
Bordj Mokhtar, where the AOD variation found was the largest, and thus the most
difficult to forecast, especially regarding deep convection and related dust
uplifts at the leading edge of the cold pools. There was no a strong contrast
in scores between forecasts initialized with ARPEGE and ECMWF, nor was there in
forecasts at low and high resolutions.
MODIS outperformed the forecasts in presenting correlation coefficient values
larger than 0.75 regardless of the station. At Tamanrasset and Bordj Mokhtar,
MODIS was biased particularly low compared to AERONET, with biases on the order
of -63 and -53 %, respectively. The difficulty of retrieving AOD
from MODIS under heavy dust loading was noticed by . This
makes the use of MODIS retrieval as a reference for model validation in the
source regions questionable. It also points out the great advantage of
installing two AERONET stations, specifically at Zouerate and Bordj Mokhtar,
during the Fennec campaign.
Wind regimes controlling dust emission
Dust emission probably explains most of the differences in AOD among the
models. In addition to the changes made in the revised version of DEAD, the
difference in emission was due to differences in initial conditions (from
either ARPEGE or ECMWF) and model characteristics (advection scheme and grid
mesh). Here, dust emission is examined first by looking at the source areas
and the wind regimes they experienced and second by focusing on the western
Sahara, the area where the Fennec aircraft operation took place. Because of
operational constraints, dust emission fields calculated from AROME were not
saved and therefore are not included in this analysis.
Taylor diagram showing normalized standard
deviations (radius) and correlation (cosine of angle) of AOD with respect to
those of the AERONET observations. The size of symbols varies with the bias
relative to AERONET AOD values averaged at each station. For each model, the
station number in which the bias is minimum is set in a box.
Fields at 21:00 UTC 20 June 2011. Top row: brightness temperature
(a) from SEVIRI observation and (b) from MNH05. Bottom row:
(c) 3-hourly dust emission and (d) attributed wind regime
from MNH05. HRM, ATL, MSN and CPL stand for harmattan, Atlantic flow, monsoon
flow and cold pool, respectively. In (c), the line represents the
mixing ratio of water vapour at 2 m equal to 10 gkg-1
and the arrows represent the wind field at 2 m. In (c, d), the black
rectangle shows the domain of analysis.
Three-hourly dust emission in June 2011 from (a) ALADIN,
(b) MNH20 and (c) MNH05. The dust emission in ALADIN was
multiplied by a factor of 3. The colours represent the dominant wind regime in
emission and the colour intensity the magnitude of the dust emission. HRM,
ATL, MSN and CPL stand for harmattan, Atlantic flow, monsoon flow, and cold
pool, respectively. Arrows indicate the amplitude and phase of the diurnal
variation of dust emission. The length of an arrow represents the magnitude
of the maximum 3-hourly dust emission and its direction the time of the day
when this maximum occurred with respect to a 24 h clock. Arrows pointing
upward indicate a peak at 00:00 UTC, those toward the right indicate a peak at
06:00 UTC, etc. Arrows representing less than 2 gm-2 have been
omitted. The solid line shows the 600 m altitude. The locations of the three
AERONET stations used in this study are shown with stars.
Attribution of dust emission to wind regimes
Dust emission occurs when wind friction speed exceeds a threshold that
depends on the surface roughness and soil moisture. High winds are associated
with different wind regimes at synoptic, regional and local scales. Atlantic
inflow associated with synoptic systems affects the western coast of northern
Africa . Over the continent, the wind regime in June depends
on the location of the intertropical discontinuity (ITD). The ITD is the
near-surface convergence zone between the moist, cold monsoon flow and the dry,
warm harmattan. To the north of the ITD, the harmattan wind blows from the
north-east or east in the western Sahara. To the south of the ITD, the monsoon
flow is a south-westerly wind. These low-level winds can strengthen during the
night, when the decoupling between surface and atmosphere occurs. Last but not
least, density currents associated with thunderstorms lead to dust emission
see the review by.
The attribution of wind regimes to dust emission is a two-step method. In
order to be easily applicable to models with different horizontal and
vertical resolutions, it is based on wind at 10 m only. In the first
step, we define dust emissions as surface objects. This allows us to describe
each individual dust emission cluster with the average and standard deviation
of their wind speed and direction. A dust emission grid point is part of the
same cluster as its neighbours if they share a common face in the horizontal
cardinal direction. In other words, diagonal connections are ruled out. In
the second step, we attribute the dust emission of a cluster to a wind regime
depending on its mean wind direction and its wind speed standard deviation.
In the case of a standard deviation of wind speed larger than
3 ms-1, the emission is attributed to a cold pool. Cold pools
associated with thunderstorm emission are local processes that show a much
larger spatial variability within a dust emission cluster than large-scale
winds. Otherwise, emission was attributed to the monsoon flow or the Atlantic
flow or the harmattan. To attribute the emission to one of these three wind
regimes, we divided the domain of analysis with respect to the position of
the ITD. Here, the ITD was defined as the southern limit where the mixing
ratio of water vapour at 2 m equals 10 gkg-1. This value
corresponds to the 14∘ C dew-point temperature criterion used by
and , among many others. To the south of
the ITD, the emission was attributed to the monsoon flow. To the north of the
ITD, the dust emission associated with north-westerly winds west of
10∘ W was attributed to the Atlantic flow. Otherwise, the emission
process was attributed to the harmattan.
Probability density function of the 10 m wind speed in
June 2011 from (a) ALADIN, (b) AROME, (c) MNH20
and (d) MNH05.
An example of dust attribution to wind regime is shown for 21:00 UTC on
20 June (Fig. ), the evening before the morning flight F22
(discussed in the next section). Low values of brightness temperatures at
10.8 µm (blue and white colours) were observed by SEVIRI over the
Atlas Mountains, southern Mali, between Mauritania and Mali, and at the
southern tip of the Hoggar Mountains (Fig. a). These low values
corresponded to high clouds, typical of anvils of mesoscale convective
systems (MCSs). Similar features were simulated by MNH05 (Fig. b;
see , for further details of the brightness temperature
calculation). The MCSs were forecast at about the same locations as observed,
showing the good skill of MNH05. Dust emission was associated with the MCSs
over Niger and the Atlas Mountains (Fig. c). Because the standard
deviation of the 10 m wind speed exceeded 3 ms-1, it was
attributed to cold pools (Fig. d). In other places, dust emission
was attributed to some large-scale wind regimes: to the Atlantic inflow over
Mauritania, to the monsoon flow over Senegal and to the harmattan elsewhere.
Diurnal cycle of dust emission in June 2011 from ALADIN, MNH20 and
MNH05 over the western Sahara, the area of interest shown in
Fig. .
Aerosol optical depth from MISR and wind at 925 hPa from
MNH20 during the heat low phase (13–30 June 2011). The white rectangle shows
the area of interest and the black lines the location of the selected legs of
the Falcon flight tracks shown in Fig. . The positions of the
dropsondes are shown with dots for F16, F21, F22, and F24, open circles for
F17 and F25, and squares for F19.
Dust emission sources and prevailing wind regimes
The magnitude of the dust emission and the dominant wind regimes leading to
emission in June 2011 are shown in Fig. . Arrows indicate the
amplitude and phase of the diurnal variation of dust emission. For the sake
of visibility, ALADIN emission was multiplied by 3.
Profiles of (top) potential temperature, (middle) relative humidity
and (bottom) dust extinction for the selected legs of the Falcon flights F16,
F17, F19, F21, F22, F23, F24 and F25. The locations of the legs are shown in
Fig. .
Emission due to the harmattan was the main mechanism for the three forecasts.
It occurred in specific areas, between the Atlas and Hoggar Mountains in
Algeria, over Libya and in the Bodélé depression. Over Libya, in the
Bodélé depression and some other places, the maximum in the 3-hourly
accumulated emission occurred at 09:00 UTC. This morning maximum
corresponded to the breakdown of the nocturnal low-level jet and the
downward transfer of momentum leading to high near-surface winds. Over
central Algeria and northern Mauritania, the emission was maximum between
15:00 and 18:00 UTC in connection with particularly strong harmattan winds
and the surface warming, which is maximum over the central Sahara in that
time frame. Emission due to the Atlantic inflow impacted western Africa. Over
the coasts, it was maximum at 18:00 UTC. By construction, emission due to
monsoon flow was found over the Sahelian band south of 20∘ N. The
time of the maximum differed among the models. It occurred at 12:00 UTC for
ALADIN and MNH20 and 00:00 UTC for MNH05. The last mentioned early
night-time maximum of emission agreed well with the dust emission observed at
the leading edge of the nocturnal monsoon flow . Finally,
emission with cold pools as the dominant mechanism was found over the western
fringes of the Adrar des Iforas and the Aïr Mountains for MNH05 only. This
agrees well with the location of deep convective systems close to mountains,
a well-known feature of the African monsoon
e.g..
In the DEAD scheme used for all the forecasts, dust emission depends on the
horizontal dust flux, which is a function of several parameters, including
the cube of the friction velocity. The latter is derived from the
10 m wind speed. It was therefore expected that the largest AOD and
emission seen for the high-resolution model forecasts would be explained by a
change in wind speed. Note, however, that the sandblasting efficiency, which
gave the dust vertical flux from its horizontal counterpart, varies greatly
between the models.
The frequency of the 10 m wind speed is shown in Fig. for
the four forecasts (including AROME) at every output time. Whatever the model
forecast, the frequency of 10 m wind speed drops rapidly to near
7 ms-1 below frequencies of less than 5 %. For all the
models, the frequency of strong winds is maximum in the afternoon, around
18:00 UTC, and minimum around 09:00 UTC. With the exception of ALADIN, the
slope for large wind speed diminishes with time. While wind speed is limited
to values around 15 ms-1 in ALADIN, it can reach
20 ms-1 in MNH20 and above 25 ms-1 in AROME and
MNH05. Over 1 ‰ of the domain, the wind speed is over
12 ms-1 for the low-resolution models, while it exceeds
15 ms-1 in the afternoon for the two high-resolution models. The
larger wind speed produced at high resolution was related to a more detailed
representation of orography and physical processes. Because this increase
occurred in the afternoon, it was related to strong harmattan winds evidenced
in Fig. and discussed previously. In the case of the
high-resolution models, a fraction of the high wind speeds at 18:00 UTC were
also probably due to density currents produced by thunderstorms described
explicitly.
Aerosol optical depth on 16 June 2011 from (a) MODIS,
(b) MISR, (c) ALADIN, (d) AROME,
(e) MNH20 and (f) MNH05. The black line shows the location
of the Falcon flight F19 track (leg between 15:18 and 16:00 UTC). The
vectors show the wind at 925 hPa for speeds higher than
10 ms-1 forecast by the models.
Vertical cross section of the extinction on 16 June 2011 along the
F19 track shown in Fig. from (a) LNG,
(c) ALADIN, (d) AROME, (e) MNH20 and
(f) MNH05. LNG observations were taken between 15:18 and 16:00 UTC
and forecasts are at 15:00 UTC. The black lines show the potential
temperature every 5 K in (c–e). Vertical hatched
areas in the LNG observations are missing data due to the presence of clouds
at the top of the SABL. (b) Evolution of the AOD along the F19 track
from LNG observations (thin black solid line) and the four forecasts (see
legend).
Diurnal cycle of dust emission over the western Sahara
Dust emission was further examined by looking at its diurnal cycle over
the area of the western Sahara where the aircraft operation took place (Fig. ).
ALADIN presented the lowest dust emission, with a daily average of
0.6 gm-2. Emission occurred on average between 09:00 and 18:00 UTC,
peaking at around noon, while it was very low during the night and early
morning. MNH20 showed a much stronger daily average of 4.6 gm-2.
Dust emission also peaked around noon, but the period of large emission
started earlier, i.e. extending from 06:00 to 18:00 UTC. Despite stronger
near-surface winds than in MNH20, MNH05 emitted 74 % less dust (an
average of 3.4 gm-2) mainly because the sandblasting efficiency
was reduced by 62 %. Compared to the low-resolution forecasts, the
emission peak in MNH05 was delayed in the afternoon, i.e. 15:00 UTC instead
of 12:00 UTC for MNH20 and ALADIN. The evening emission was also
significantly higher than the early morning one for both MNH05 and MNH20,
unlike for ALADIN. In MNH05, the evening dust emissions were larger than in
MNH20, due to the contribution of cold pools related to evening
thunderstorms.
For all the models, and independent of the time considered, the dominant
wind regime for dust emission over the western Sahara was the harmattan. In
total, it accounted for 80 % in ALADIN, 78 % in MNH20 and
74 % in MNH05. This regime occurred mainly between 09:00 and
18:00 UTC. The second most important wind regime was the Atlantic flow,
accounting for 17 % in ALADIN and MNH20 and 15 % in MNH05.
The third wind regime was the monsoon flow, accounting for 5 % in MNH20 and
MNH05 and 4 % in ALADIN, with the strongest values between 09:00 and
12:00 UTC. Finally, emission due to cold pools was identified between 15:00
and 24:00 UTC for MNH05 only. It accounted for 6 % of the total
dust emission but occurred for 13 % of days in June (on 16, 19, 23 and
24 June). It is worth emphasizing that this value was calculated for the
western Sahara (i.e. the domain delimited by the box in Fig. ).
Higher values were obtained over other areas, such as the western fringes of
the Adrar des Iforas and the Aïr Mountains.
Assessment in the heat low region
The vertical structure of the atmosphere is now assessed in the heat low
region using observations obtained from the Falcon 20. In the following,
profiles of temperature, moisture and dust extinction from forecasts are
first compared with dropsonde and LNG observations during the so-called heat
low phase, when the Saharan heat low was moving towards western Sahara. Three
flights representative of the meteorological conditions of the heat low phase
over the western Sahara are then presented.
Vertical structure of the Saharan atmosphere
We selected the eight long-range flights operated between 14 and 22 June
(flights F16, F17, F19, F21, F22, F23, F24, and F25). The objective of F16,
F17 and F19 was to document the boundary layer. F21 was dedicated to the
survey of dust associated with the ITD and the heat low. F22 was designed to
survey dust associated with a Mediterranean surge and density currents from
the Atlas Mountains. The purpose of F24 and F25 was to survey the Saharan
heat low. For each flight, we only analysed data from the longest leg because
they provided the most comprehensive view of the Saharan atmosphere
(Fig. ). During the heat low phase, the averaged wind at
925 hPa (from MNH20) exhibited an almost closed cyclonic circulation
over eastern Mauritania, and the MISR AOD was larger than 0.7 over a zonal
band around 18–20∘ N. For the sake of concision, the profiles
of potential temperature, relative humidity and dust extinction coefficient
were averaged along the flight tracks (Fig. ). The forecast
outputs closest to the time of the flight tracks were selected.
In the afternoon, most of the observed temperature profiles showed a
well-mixed, deep SABL (Fig. top). The potential temperature was
quasi-uniform with values of 318–320 K in the first
4.5–5.5 km. F19 differed from the other afternoon flights by showing
a 1 km layer that was cooler than aloft. This air was advected from
the Atlantic, as detailed in the next subsection. The morning flight F22
showed a cold-air layer associated with north-easterlies. A more detailed
analysis of F22 is given in Sect. 5.3.
At the surface, the air was very dry with a relative humidity of 5 %
(Fig. middle). The air became more moist with altitude, reaching
a maximum between 50 and 70 % at the SABL top. The SABL top reached
an altitude of around 5.5 km on 14 and 15 June and then moved lower to
4 km on 16 June due to cool, moist near-surface conditions related to
the inland penetration of the Atlantic inflow, which delayed the SABL
development. The SABL depth then rose again to 6 km from 21 June
onwards. These changes in the moisture distribution at the SABL top were
caused by the passage of AEWs. The latter enhances the northward propagation
of the monsoon flow in the low levels, with this moisture then being mixed
vertically by the strong updrafts in the SABL. In particular, moisture events
were reported on 13, 17 and 21 June at Bordj Mokhtar . This
matches the increase in moisture observed further west on 15, 20 and 21 June
(F17, F21, and F23, respectively).
The models represented the lower part of the temperature profile rather well,
with maximum errors of less than 4 K (Fig. top). They
reproduced the profile of 14 June (F16) and the afternoon of 22 June (F25)
remarkably well, with maximum differences of less than 2 K below
4 km (with the exception of ALADIN in the first kilometre on
14 June). The difference in the SABL temperature was the largest on 16 and
20 June (F19 and F21), two flights during which cool near-surface conditions
were observed in relation to an Atlantic outflow and the monsoon flow.
The forecasts were successful in capturing the dry air in the first
kilometres (Fig. middle). An exception was F21, which
documented the northern fringes of the monsoon flow. The forecasts of
relative humidity at the SABL top were more erroneous. Meso-NH followed the
observations rather well with maximum errors often limited to less than
20 %, while ALADIN and AROME were either too dry (F17, F21, F22 and
F23) or too moist (F19).
Dust extinction was observed within the SABL only (Fig.
bottom). It decreased with altitude, with the largest amount being found close to
the surface. Two major dust events were reported with extinction larger than
0.5 km-1 in the first kilometre (flights F22 and F24). These
corresponded to dust uplifts associated with a density current and a
harmattan outbreak, respectively (see Sect. 5.3 on F22 below). The mean
vertical profiles of dust extinction in the forecasts showed systematic
errors. For most of the flights, dust was correctly forecast below the SABL
top. There were, however, some exceptions (F21 for Meso-NH and F17, F19, and
F21 for AROME). Another drawback was excessively large values of extinction
in the upper part of the SABL. This can be seen for F17 for all the
forecasts. In the lowest levels, ALADIN and AROME forecast reduced dust
extinction incorrectly. The structure was much better forecast by Meso-NH.
However, some peaks were strongly underestimated (e.g. F24) or overestimated
(F21 by MNH05). This shows the great difficulty in forecasting dust events
accurately at the mesoscale.
Dust mobilization at the migration of the heat low over the western Sahara
A first example of the vertical section of dust extinction is taken from the
flight conducted on 16 June 2011 at the time of the migration of the heat
low in its Saharan location. Figure shows the longest leg of
the flight F19, which took place between 15:18 and 16:00 UTC superimposed on
the AOD from MODIS, MISR and the dust forecasts at 15:00 UTC. On the western
part of the MISR overpass, the satellite retrievals were in agreement. In
particular, they both retrieved with the Zouerate AERONET value of 0.3. The
relatively low AOD values over Western Sahara were associated with the cool
north-easterly Atlantic inflow. On the eastern part, MISR presented a strip of
AOD over 0.6 that was absent in MODIS. The forecasts all showed this strip of
enhanced AOD, in better agreement with MISR than with MODIS. In the morning,
the south-westerlies mobilized dust over Western Sahara, while the
north-easterlies activated the sources of the Grand Erg Occidental further
north. The AOD strip marked the convergence of these two wind flows at low
level. The north-easterlies at 925 hPa were stronger in Meso-NH than
in ALADIN and AROME, which explains the difference in the magnitude of the
forecast AOD.
Same as in Fig. but for the F22 flight on 21 June 2011
(leg between 07:58 and 08:50 UTC).
Same as in Fig. but for the F22 flight on
21 June 2011. LNG observations were taken between 07:58 and 08:50 UTC and
forecasts are at 09:00 UTC.
Consistently with MISR, the LNG observation showed a strong gradient of dust
extinction in the boundary layer across the track (Fig. a). Dust
extinction larger than 0.05 km-1 was found in a layer for which
the depth increased from 2 km over northern Mauritania due to the
cooler temperatures) to 6 km in central Mauritania. It resulted in
AODs varying from 0.3 to 1.1 as derived from LNG (Fig. b). This
increase was reproduced by all models, with the exception of ALADIN, which
exhibited a nearly constant AOD across the domain, as seen in
Fig. c. AROME and Meso-NH exhibited an AOD maximum at a distance
between 300 and 400 km, while LNG observed a continuously increasing
AOD. Over central Mauritania, dust extinction larger than 0.3 km-1
observed in the first kilometre (at a distance between 400 and
500 km) corresponded to a dust uplift associated to the flow coming
the Atlantic and merging with the monsoon flow in the south-eastern part of
the aircraft leg (Fig. f). The increase in extinction with the
distance from the north-eastern position of the leg was forecast by all the
models (Fig. c–f). All models show the intrusion of colder air
in the lower 2 km associated with the cool Atlantic inflow. However,
they all tend to forecast too much dust in the residual layer between 2 and
6 km altitude over the Atlantic inflow. Nevertheless, the MNH05
forecast evidenced the slanted dust filaments as in the LNG observations
(Fig. f), while AROME forecast an almost dust-free air in the
whole atmospheric column (Fig. d), thereby also mimicking the
observations. Over central Mauritania (within distances between 400 and
500 km), Meso-NH was the only model able to forecast the dust uplift
associated with the strong south-westerlies. AROME lacked any significant
extinction at the first levels as already seen in the vertical profile for
the F19 flight (Fig. bottom).
Dust mobilization due to a Mediterranean surge and overnight density current
A large part of dust emission observed in the second half of June 2011 in the
area of interest was due to strong harmattan wind (see Fig. ). An
example is taken here from the leg of flight F22 between 07:58 and 08:50 UTC
on 21 June. This case is particularly interesting as additional emission of
dust was due to an overnight density current originating from deep convection
over the Moroccan Atlas Mountains .
Along the F22 flight track, the MODIS AOD showed values greater than 0.8
(Fig. a). A maximum of AOD larger than 1.2 was even reached in
northern Mauritania. These values matched those obtained from LNG rather well
(Fig. b). Over Western Sahara, AODs retrieved from both MODIS and
MISR aboard Terra differed (less than 0.8 and in excess of 0.8,
respectively, Fig. a, b). In the absence of any delay in time of
observation, this discrepancy illustrates well the difficulty in retrieving AOD
accurately from space.
The AOD forecast by ALADIN at 09:00 UTC was the most different from the
two satellite observations (Fig. c). It varied between 0.2 and
0.4 over most of the area. In the south-western corner, AODs larger than 0.6
were forecast to be associated with southerly winds. This area of large AOD
expanded further north-east in AROME, which forecast south-westerlies
(Fig. d). The difference in wind direction might explain the
difference in AOD. In Meso-NH, the south-western area with large AOD spread
considerably northward (Fig. e, f). It even reached Zouerate,
where the AERONET station recorded AOD values of 1 in the morning. It also
spread into the western Sahara, providing the AOD forecasts closest to MISR
observations. Along the F22 track and to its north-east, AROME and Meso-NH
showed large AOD values that were lacking in ALADIN. The MODIS maximum was
well forecast by Meso-NH but was missed by AROME and ALADIN. The MODIS secondary
maximum at 6∘ W, 22∘ N was forecast by AROME but was missed
(or delayed) by Meso-NH and ALADIN.
When the vertical section of dust extinction was examined
(Fig. ), the LNG observation revealed a near-surface dust
aerosol layer with extinction larger than 0.4 km-1 in the first
kilometre (Fig. a). This layer resulted from a southward
Mediterranean surge enriched with an overnight current from thunderstorms
triggered over the Atlas Mountains . ALADIN forecast an
increase in dust near the surface, but with too low a magnitude
(Fig. c). In the western part of the leg, the dusty layer was
forecast by Meso-NH (Fig. e, f), with an AOD reaching almost 1,
though less than the observed values of 1.2–1.5. The dusty layer forecast by
Meso-NH was indeed too thin. In the eastern part of the leg, all the models
failed to represent the near-surface dust layer. AROME did a better job by
forecasting a dust extinction above 1 km, the closest to the
observations. AROME was initialized at 18:00 UTC the previous day and was
able to forecast the development of the thunderstorms over the Atlas
Mountains and the associated cold pools (not shown). ALADIN did not forecast
any density current, which explains why the magnitude of dust extinction was
too low. The MNH05 simulation on 20 June started at 00:00 UTC forecast
thunderstorms over the Atlas Mountains (Fig. ). However, the
meteorological imprint of the associated density currents was removed when
initializing the Meso-NH forecasts with ECMWF 00:00 UTC analysis on 21 June.
Afternoon planetary boundary layer in the heat low region
A last example is taken from the afternoon survey of the SABL across the heat
low on 22 June. analysed the meteorological situation
for that day in some detail. They showed that the monsoon flow encircled the
heat low from its eastern flank, at both low- and mid-level altitudes.
Same as in Fig. but for the F25 flight on 22 June 2011
(leg between 16:01 and 17:00 UTC). White lines show mean sea level pressure (MSLP) in
(c–f).
Same as in Fig. but for the F25 flight on
22 June 2011. LNG observations were taken between 16:01 and 17:00 UTC and
forecasts are at 18:00 UTC.
The leg of the flight F25 track between 16:01 and 17:00 UTC overlaid on the
AOD field from the satellite retrievals and the forecasts is shown in
Fig. . According to the mean sea level pressure (MSLP)
forecast by Meso-NH, the heat low exhibited an elongated pattern with a
NE-SW orientation. Its centre, located around 18∘ N, 6∘ W,
was sampled by the aircraft in the southern part of the leg. In the Meso-NH
forecasts, the location of the MSLP minimum was much better defined than in
the other two forecasts. The forecasts agreed on the broad structure of the
wind flow at 925 hPa, characterized by a Mediterranean surge from the
north-east and the monsoon flow from the south-west. It resulted in an AOD
larger than 1 over central Mauritania (around 20∘ N, 12∘ W)
for all the forecasts. These values contrasted with the AOD of less than 0.8
from MODIS. The forecasts also matched the AERONET value of 0.9 at Zouerate.
The vertical distribution of extinction observed by LNG showed a deep, dusty
SABL (Fig. ), extending up to an altitude of 6 km with
extinction larger than 0.4 km-1 in the first kilometre. In the
northernmost 450 km, the upper boundary layer was cloudy with little
dust extinction, less than 0.1 km-1. In the southernmost
300 km, which crossed the centre of the heat low, there was no cloud
and the dust extinction was larger than 0.1 km-1 up to an altitude
of 6 km. AROME overestimated the dust extinction in most parts of the
leg. This drawback was linked to the mislocation of the heat low centre.
ALADIN, MNH20 and MNH05 forecast the dust increase towards the heat low
centre well. However, ALADIN forecast too low an extinction, while Meso-NH
overestimated the northern limit of the deep, dusty layer at a distance of
about 350 km (compared to the observed 450 km).
Conclusions
During the 2011 Fennec field campaign, the Saharan atmosphere was probed
using ground-based and airborne observations. For the purpose of aircraft
guidance, dust forecasts were produced specifically for Fennec using three
limited-area models: ALADIN, AROME and Meso-NH. Among the four sets of
forecasts, two were made with a horizontal grid spacing of 5 km,
permitting the deep convection to be represented explicitly. The unique data
set allowed the first ever intercomparison of dust forecasts over the western
Sahara.
At the monthly timescale, less AOD was forecast by the low-resolution models. By
construction, these models generated lower near-surface wind speeds, which
resulted in a weaker dust emission. This contrasted with the high-resolution
models that forecast a much stronger variability and intensity of AOD. This
was partly due to the generation of density currents that mobilized dust.
This effect was well illustrated by the change in AOD and emission between
the two sets of the Meso-NH forecasts. Over the western Sahara, the
high-resolution forecast emitted more dust in the evening than its low-resolution
counterpart while using a reduced sandblasting efficiency. Another striking
difference was the AOD forecast over the Grand Erg Occidental. The harmattan
was more prevalent there in the Meso-NH forecasts starting with ECMWF
analysis than in ALADIN and AROME, which used the ARPEGE forecasts.
The agreement found with the MISR retrieval in the strip of large AOD around
18∘ N suggests that the high-resolution models performed better than
the low-resolution ones. This is an indication that these models are doing a
better job at correctly pin-pointing dust emissions in both time and space,
in spite of their potential for producing highly variable and more spurious
fields. However, the MISR mean AOD differed from that of MODIS with regard to the AOD
magnitude, with a maximum of 1 and 0.6, respectively. The larger number of
observations obtained by MODIS (about 28 compared to 5 for MISR) should give more
confidence on the MODIS product. Although the MODIS AOD retrievals and the
AERONET observations were in agreement over Tamanrasset, they differed
significantly over Zouerate. Too large an AOD was retrieved there, while the
Atlantic inflow characterized by a low aerosol load led to near-zero AOD
values. The fact that this synoptic-scale feature was forecast well suggests
that the MODIS retrieval was not reliable at Zouerate and certainly over some
other areas, such as the source regions.
At daily timescales, the vertical structure of temperature and humidity was
forecast well, with maximum errors often limited to less than 4 K and
20 %, respectively. Both the mixed layer air in the SABL and the
dryness of the Saharan air near the surface were captured successfully. All
the models correctly forecast the decrease in dust extinction with altitude within the
SABL. Because the dust emission was too low, the magnitude of dust
extinction forecast by ALADIN was also too low. AROME missed the larger
amount of dust extinction in the first kilometre. In contrast, the Meso-NH
model provided a much more realistic vertical distribution of dust
extinction. The high resolution also showed fine vertical gradients of dust
that appeared very realistic when compared with lidar observations.
This intercomparison underlined the importance of density currents for dust
emission, which was observed in a few cases with LNG. While cold pools
contributed to 6 % of the total dust emission in MNH05 over the
western Sahara only, they dominated the dust emission over the western
fringes of the Adrar des Iforas and the Aïr Mountains. On the one hand, the
study emphasizes the need for the convection-parameterizing models to
represent this process. On the other hand, the convection-permitting models
can overestimate the wind speed strongly, and hence the dust emission. This issue
raises some questions on the representation of subgrid eddies and associated
drawbacks in the turbulence scheme. Tests of sensitivity to model resolution
for some dedicated case studies should be carried out in order to further
investigate this issue.