In September 2015 one of the severest and most unusual dust events on record occurred in the Eastern Mediterranean. Surprisingly, operational dust transport models were unable to forecast the event. This study details the reasons for this failure and presents simulations of the event at convection-permitting resolution using the modelling system ICON-ART. The results allow for an in-depth analysis of the influence of the synoptic situation, the complex interaction of multiple driving atmospheric systems and the mineral dust radiative effect on the dust event. A comparison of the results with observations reveals the quality of the simulation results with respect to structure and timing of the dust transport. The forecast of the dust event is improved decisively. The event is triggered by the unusually early occurrence of an active Red Sea trough situation with an easterly axis over Mesopotamia. The connected sustained organized mesoscale convection produces multiple cold-pool outflows responsible for intense dust emissions. Complexity is added by the interaction with an intense heat low, the inland-penetrating Eastern Mediterranean sea breeze and the widespread occurrence of supercritical flow conditions and subsequent hydraulic jumps in the vicinity of the Dead Sea Rift Valley. The newly implemented mineral dust radiation interaction leads to systematically more intense and faster propagating cold-pool outflows.
Mineral dust aerosol plays an important role for the environment. The
transport of mineral dust within the atmosphere has an effect on physical
processes, chemical composition and biological systems on various temporal
and spatial scales
MODIS VIS satellite images of the EM region, at
In the Eastern Mediterranean the impact of mineral dust on the environment,
human population and traffic is important due to the unique environmental
setting and great population density. Dust events in the Eastern
Mediterranean (EM) are usually associated with strong south-westerly and
southerly flows in the region, although events have been reported under
easterly flow conditions
In September 2015 an exceptional dust event occurred in the EM. The impact of
the event on the EM region was severe, with five people reported to have
died, hundreds hospitalized and daily life as well as traffic in the region
disrupted
Satellite images for the visible part of the electromagnetic spectrum (VIS)
from MODIS illustrate the evolution of the dust storm
(Fig.
Adding to the extraordinariness, operational global dust transport models
were unable to forecast the event as also noted by
From EUMETSAT (European Organisation for the Exploitation of Meteorological
Satellites) SEVIRI (Spinning Enhanced Visual and Infrared Imager) satellite
observations, the development of organized mesoscale convective systems (MCSs)
which produced cold-pool outflows (CPOs) over Mesopotamia is detectable. The
CPOs and their interaction with a heat low have been suggested as important
drivers for the observed dust emissions by
A number of studies have shown that the inability to represent organized
mesoscale convection and the related CPOs in models with parametrized
convection can lead to a substantial underestimation of dust emissions.
Analysing simulations for summertime West Africa,
Dust radiative effects in general have been shown to be of great importance
for atmospheric processes
In this paper, we present a detailed analysis of the September 2015 severe
dust event in the EM. We combine observations from satellites and the ground with
new high-resolution simulations using the ICON-ART (Icosahedral
Nonhydrostatic – Aerosol and Reactive Trace gases) global modelling system
ICON is a non-hydrostatic modelling system developed jointly by the German
Weather Service (DWD) and the Max Planck Institute for Meteorology
The size distribution of mineral dust is represented by three modes in ART.
For each mode the integral values of specific number and mass are the
prognostic variables. The distribution of specific number and mass with
particle size during transport is described using log-normal distributions
for each mode with the diagnostic median diameter of the mass distribution
and constant geometric standard deviation as parameters (Mode A,
As a part of this study the on-line dust radiative effect has been
implemented in ICON-ART. It is now possible to include the radiative effect
of the current, local dust concentration from ART at every grid point and
time step in ICON instead of the previously used dust climatology. Through
its feedback to radiative fluxes the dust influences atmospheric state,
thereby providing a feedback loop back to dust processes again
Without ART, ICON uses a climatological distribution of aerosols (e.g. mineral dust, sea salt, stratospheric aerosol) to include their radiative effect. When using ART, any of these aerosol species can be calculated on-line, and therefore its radiative effect can be included with much better accuracy. For aerosol species not simulated by ART the climatological values are still used and taken from ICON. Therefore, the radiative transfer parameters provided by ART to the RRTM are combined values from the local ART aerosol concentration plus the ICON climatology, which is used only for the aerosol species not simulated. For example, in our study we simulated mineral dust using ART and therefore can include the on-line mineral dust radiative feedback. For the sea salt and stratospheric aerosol radiative effect, however, the climatological values from ICON are used.
The radiative transfer parameters needed consist of the optical depth,
single scattering albedo and asymmetry parameter. In order to obtain the
on-line mineral dust radiative feedback, the local radiative transfer
parameters are calculated using the dust optical properties and the local
dust mass concentration at every grid point and for every level as detailed
in
For consideration of the on-line dust radiative effect, the dust optical
properties need to be determined. The mineral dust optical properties are
computed with the help of Mie calculations using the complex refractive index
of mineral dust
The spatially invariant mineral composition of dust in ICON-ART means we
assume similarity to Saharan dust everywhere. Studies have shown that mineral
dust optical properties can depend on the source region
Results from Mie calculations as used by ICON-ART. Points are
normalized with respect to the value at 550 nm; horizontal lines show absolute
values. Shown are
The values of the refractive index used to conduct the Mie calculations are
the same ones used by
For the Mie calculations a code developed by
The mineral dust optical properties are calculated for three modes and 30
RRTM radiation wavebands. Results of Mie calculations for the
ART mineral dust modes are shown in Fig.
The optical properties of mineral dust are highly dependent on its particle size distribution represented through three log-normal modes in ART. From the six prognostic variables in ART, the specific dust mass and number concentration for each of the three modes, a median diameter can be diagnosed for each mode. Due to different processes such as gravitational settling acting differently on the specific dust mass and number concentrations in ART, the diagnostic median diameter of each mode changes during transport (the standard deviation of each mode is kept constant). In a physical sense, the most important effect is large particles settling out faster due to sedimentation, which results in mostly smaller particles being transported to distant regions within each mode. Therefore, the median diameter of each mode is expected to decrease during transport. Consequently, a new polynomial parametrization of the optical properties per waveband was introduced to account for the change in median diameter during the transport processes. In a post-processing step a third-order polynomial is fitted to the results of multiple Mie calculations for median diameters between 0.25 and 1.25 times the initial median diameter of the count number distribution. This is done for every mode and every RRTM waveband. The polynomial fit parameters are initialized in ICON-ART and used for the determination of optical properties at every grid point with the diagnosed median diameter being the independent variable.
The specifics of the mineral dust radiative effect implementation are
available upon request and detailed in
In this study ICON-ART is run in a set-up with one global domain and four
nests, with two-way feedback for the meteorological parameters enabled for
all domains. Each domain presents a stand-alone model run which obtains its
lateral boundary conditions from the coarser domain. For the global domain, an
R2B6 grid is used. This corresponds to an effective grid spacing of 40 km
In the global domain the model consists of
For the cloud microphysical processes, the two-moment cloud scheme is used
ICON-ART is initialized with analysis fields from the Integrated Forecasting
System (IFS) of the European Centre for Medium-Range Weather Forecasts
(ECMWF). A limitation with initializing from the IFS analysis datasets is
that the IFS has a horizontal grid spacing of approximately 13 km and is
therefore non-convection-permitting. Thus, when re-initializing ICON-ART with
the current meteorological fields any previously existing organized
convection is terminated. The IFS initialization data for soil moisture were
modified in a region along the Syrian–Iraqi border which showed high soil
moisture values and spatial inhomogeneities without preceding rain or changes
in soil properties. Therefore, in a region from 37.5–41.5
The ICON-ART mineral dust concentrations are passed on to the next run whenever a re-initialization of the meteorological fields from an IFS analysis is performed. No assimilation of mineral dust concentrations from observations takes place. Hence, the runs are performed as free runs for the mineral dust concentrations with a frequent update of the meteorological background conditions from IFS analysis in order to benefit from the data assimilation performed therein. A 2-week spin-up simulation is performed starting on 23 August 2015 in order to achieve a realistic background concentration for mineral dust on the global domain. Another re-initialization from an IFS analysis is performed at 00:00 UTC, 4 September, to obtain a realistic background concentration of mineral dust in the finer domains for the simulations starting at 18:00 UTC, 5 September. From 00:00 UTC, 6 September, onwards, a multitude of partially overlapping events takes place, which need to be simulated correctly in order to obtain a realistic dust distribution in the EM. Therefore, the time chosen for initialization is of crucial importance due to the aforementioned termination of organized convection. Various options were examined, the times chosen for initialization are 18:00 UTC, 5 September, and 12:00 UTC, 6 September.
For the investigation of the dust radiative effect, two simulations are performed. The first is the simulation including the on-line radiation interaction with mineral dust from ART which is called “ARI” (aerosol radiation interaction) in the following. The second is a simulation in which the mineral dust concentrations are multiplied by 0 in the ART routine calculating the radiative effect of dust. This simulation is called “CTRL” (control) in the following; it contains no mineral dust influence on radiation at all. In this study only the mineral dust radiation interaction is simulated on-line by ART; for all other aerosols the default climatologies are used in all runs.
For the global grid, ICON-ART produces results comparable to those from other global models. However, due to its flexible nesting capability, it allows for convection-permitting simulations for the finest resolution. As is shown in this section, ICON-ART is thereby able to resolve the meteorological drivers of the event in great detail. The results show that the dust event consists of multiple stages and is created by the interaction of different meteorological systems. A comparison of model results to available satellite observations highlights the ICON-ART simulation quality.
For the event simulated, the synoptic conditions in the Middle East are
distinctively different from the normal summer situation as the ICON-ART
model results show. An overview of the modelled synoptic situation is
provided in Fig.
Synoptic situation on 6 September at 18:00 UTC as simulated by
ICON-ART for the global domain. Shown are the
The usually dominant Persian trough
As discussed in Sect.
In the following, a detailed analysis of the development stages and
responsible atmospheric drivers, which lead to the severe dust event, is
provided. We focus on the results from the convection-permitting domain, as
it yields remarkable improvements compared to the global domain using
convective parametrization (not shown), consistent with the work by
Schematic depiction the main dust event stages and atmospheric features. The topography as used by the finest domain is colour-coded. Coloured lines represent average geopotential height of pressure levels. Labels refer to the description used in the text. Arrow size corresponds to event magnitude and arrowhead size to event speed.
During the night from 5 to 6 September 2015, a convective system exists over
the Turkey–Syria border region. It is fuelled by the inflow along the eastern
side of the RST. The system moves towards the north-east along with the mean
flow direction above 500 hPa. Due to the favourable position of the
convective system in front of the mid-tropospheric through axis, it
intensifies during the course of the night. However, in contrast to
subsequent systems, it lacks the full mesoscale organization of convection,
possibly due to less favourable wind-shear conditions. During the early
morning hours of 6 September 2015, the convective system produces a first,
weak cold-pool outflow (termed CPO1 in the following) which remains decoupled
from the surface due to the stable nocturnal boundary layer. As soon as the
sun rises, the downward mixing of momentum increases and dust is picked up (a
comparison of ICON-ART modelling results with satellite observations is shown
in Fig.
During the course of the day, the flow structure created by the CPO1 in
combination with the heat low above Syria interacts with the inland-penetrating sea breeze from the Mediterranean Sea, creating strong southward
transport of dust towards Jordan. From 10:00 UTC onwards, the atmospheric
instability created by boundary layer heating and upper-level cold air
advection is released and deep convection starts to develop over the
Syria–Iraq border region and Zagros mountain range in the RST inflow region.
From SEVIRI Meteosat satellite images a second, convective cold-pool outflow
(CPO2) which travels west from the Zagros mountain range is detectable at
12:00 UTC. This CPO2 travels fast towards the west in the RST flow structure
and is supported by the heat low. The re-initialization of ICON-ART with IFS at
12:00 UTC impairs the CPO2 development due to the termination of convective
structures (see Sect.
ICON-ART model results at 10:00 UTC, 7 September. Displayed are
The lifting caused by the gust front of the CPO2 triggers the initiation of
deep convection over the Syria–Iraq border region which organizes into
an MCS around 18:00 UTC.
By this time the CPO2 has already travelled far into Syria. The MCS starts to
develop a marked, third cold-pool outflow (CPO3) from 20:00 UTC onwards. The
CPO3 travels in the wake of the CPO2, but in a more southerly direction. The
strong third cold-pool flow towards the south counters the inflow from
southerly directions along the eastern flank of the RST, thereby lifting the
warm and moist air masses. This evolution differs markedly from the
simulations by
Past midnight on 7 September, explosive intensification of the MCS takes
places due to the favourable atmospheric conditions. It develops a sharply
defined, curved rainfall pattern in front of which the CPO3 is strongly
intensified. In connection with the only slowly advancing upper-atmospheric
trough, which causes quasi-geostrophic forced ascent, and the orographic
support from the Zagros mountain range, the MCS remains quasi-stationary over
the next 12 h. Due to the long duration and separation of the up- and
downdraught region, the MCS is able to produce an enormous amount of cool,
moist air and a mighty CPO3 downstream of the line-shaped rainfall pattern. During
the course of the night, the southerly direction of the CPO3 is deflected
into a westerly flow direction by the RST flow structure and its inflowing
air masses. The night-time spread of the CPO3 towards the west is crucial due
to its subsequent interaction with the developing boundary layer mixing
during daytime. During night-time the CPO is confined to a shallow layer of
approximately 1 km close to the surface with wind speeds above
20 m s
ICON-ART model results and satellite observations for 10:00 UTC on
7 September. Displayed are
Vertical cross section of ICON-ART model results for 10:00 UTC on
7 September and CALIPSO satellite observations from south to north along the ground track in Fig.
An in-depth analysis of model results and a comparison to satellite
observations is conducted at 10:00 UTC, 7 September
(Fig.
Towards the north, the DOD exhibits a sharp gradient (northern boundary of
the CPO3 dust plume in Fig.
Comparing ICON-ART model results to the corresponding satellite observations
shows that all modelled features are confirmed by the observations
(Fig.
The structure of the MCS determines the shape of the outflow. Therefore,
differences in outflow structure can be identified as well. Again, the CPO3
is more sharply confined between 34 and 36
The aged CPO2 spreading towards the north is detected by all satellite
instruments; it is modelled with an offset towards the north-east by ICON-ART
(northernmost solid black line in Fig.
The vertical structure of the dust plume can be investigated at this point in
time with the help of a CALIPSO overpass which occurred at 10:35 UTC.
Results along the flight track are shown from south to north in
Fig.
The main CPO3 dust plume is distinct in both figures, with ICON-ART simulating
an elevated nose of the CPO3. The differences in CPO3
structure discussed above are identifiable again. ICON-ART simulates a wider and more shallow
outflow, whereas in reality it was more confined. In addition, the
observations show an approximately 1 km higher main dust plume
compared to ICON-ART. Altitudes below 2 km are marked as no-signal regions
in the CALIPSO feature mask due to the attenuation of the lidar signal (not
shown). On the northern side of the plume beyond 36
In summary, a northward deviation of the flow structure in ICON-ART can be identified both for the CPO2 and the CPO3 although the overall intensity and characteristics are simulated well. The combination of the northern deviation towards less dust-emitting soil types and the wider, less well-defined main outflow region reduces the amount of DOD over western Syria due to a less intense channelling of the CPO3. Furthermore, the re-intensification of the aged CPO2 is modelled by ICON-ART; however, it was observed with greater magnitude in reality. The reduced amount of dust in the atmosphere in ICON-ART leads to a reduction in long-range transport of dust towards the EM over night.
When comparing our results to those of
From 10:00 UTC onwards on 7 September, the MCS starts to dissipate and is, in
the course of this, advected east along with the mean flow conditions above 500 hPa.
The dust transport to the EM is complex; the previously identified plumes are
advected in different directions at different altitudes, as visible in the
SEVIRI RGB dust product animation
With nightfall on 7 September the CPO2 and CPO3 merge. As the merged CPO is
still located in the western, downstream flank of the RST, the air mass and
dust contained within it are advected fast towards the south-west over the
course of the night. The Dead Sea Rift Valley is passed by the dust mass
after midnight on 8 September, with the dust plume interacting with the
complex orography. A comparison of simulated DOD with satellite observations
for 11:00 UTC, 8 September, shows that the model represents the observed dust
plume structure in the northern part of the EM
(Fig.
For the dust concentration, a strong gradient on small spatial scales exists
in the region of Cyprus. On 8 September
Dust transport into the southern EM is not simulated with the correct
magnitude by ICON-ART despite the overall dust plume structure being
captured, even when accounting for the MODIS AOD retrieval bias
Overall, a significant dust forecast improvement is achieved through convection-permitting simulations with ICON-ART at 47 h forecast time. During daytime on 8 September the dust plume is mostly stationary in the EM and influenced by the local circulation systems. In visible satellite pictures, the dust can be seen to remain in the EM at high concentrations over the course of the next 4 days. This period is not investigated as a part of this study as the scope of this work is the analysis of the generating mechanisms.
Due to the problems in simulating dust transport towards the southern EM with the correct magnitude, the next section investigates the timing and structure of the dust plume and CPO arrival in Israel in reality and simulation.
In this section the simulated dust concentrations are compared to
measurements from three stations in Israel. The selected stations are
Afula, Jerusalem (Bar
Ilan) and Beer-Sheva for PM
The stations all show individual dust concentration characteristics during the event, and the discussed features are present at other stations with similar characteristics as well. Beer-Sheva is chosen for its close proximity to Sedé Boqer at 45 km distance, in a similar arid desert environment. Clearly four stations are not enough for a complete validation of the complex dust distribution in the region. The comparison shown here is meant to highlight differences between the model and reality as well as to investigate dust transport features.
Observations and model results for three stations in Israel for
7 and 8 September. Points denote observations, and solid lines denote model
results. Please note the logarithmic
Due to its northern location the modelled main dust plume reaches Afula first
at 00:00 UTC, 8 September. Here, the highest values of DOD of up to 1.5 are
modelled (Fig.
A comparison of modelled DOD and AERONET AOD measurements shows a similar development of the optical depth for 7 September, although with an offset of 0.3. The offset is explainable by AERONET measuring the optical depth due to all aerosol species, whereas we only display DOD from ICON-ART, as well as a possible underestimation of dust background concentration in the model. Nevertheless, the main signal appears to be shaped by mineral dust processes captured by ICON-ART. The maximum modelled DOD for Sedé Boqer is 1.0 on 8 September, compared to 4.1 measured by AERONET. MODIS is in good agreement with the AERONET measurements. Therefore, we assume that the AERONET measurements are representative for a larger region. Thus, ICON-ART shows an underestimation of DOD by a factor of 4.
When comparing PM
Despite the large difference in absolute values, some insight into dust
transport processes can be gained by comparing the course of the measured and
modelled PM
ICON-ART model results at 03:00 UTC, 8 September. Displayed are
Highest overall PM
PM
ICON-ART fails to reproduce the measured low values of global radiation because for the radiative impact, the absolute values of the dust concentration are of importance. The differences visible in the DOD above the stations manifest themselves in differences between the modelled amount of global radiation at the stations.
In order to understand the problems of the model to forecast dust in Israel with the correct magnitude the transport processes upstream have to be investigated in more detail.
In Fig.
ICON-ART model results along a vertical cross section from
32
There is a remarkable wave structure in the extinction coefficient on the lee side
of the Golan Heights (Fig.
Hydraulic jumps are connected to flows going from a subcritical flow stage
with Froude numbers
Observations and model results for three stations in the Golan
Heights for 7 and 8 September. Points denote observations and solid
lines/crosses model results. From top to bottom:
Hydraulic jumps have been documented penetrating the Dead Sea valley for flow
conditions from the west a continuous decrease in mixing layer height detectable in the virtual
potential temperature and extinction coefficient fields leading to
compression of the subcritical flow from east to west. a sharp decrease in the vertical flow depth, connected to a rise in flow
speed, after passing the orographic Golan Heights crest. This denotes the
transformation of the flow state from sub- to supercritical. a subsequent sudden increase in flow depth when the flow state reverts to
the subcritical stage again inside the Dead Sea Rift Valley. This is
accompanied by a sharp decrease in flow speed and connected to an increase in
atmospheric turbulence.
An approximate calculation of the Froude number upstream and downstream of the
Golan Heights crest height with average flow conditions as simulated by
ICON-ART confirms the conversion of the flow state:
Hydraulic jumps have been investigated and suggested as dust-generating
mechanisms
The existence of hydraulic jumps is modelled by ICON-ART and detectable along
many cross sections and at different times in the region. Some of them occur
on the lee side of much less pronounced orographic features such as the
mountain structure visible at 33.5
The validation is done with the help of three meteorological stations
approximately along the model cross section through the Golan Heights. The
locations and names of the stations used are marked in
Fig.
The measurements of three meteorological stations are displayed together with
the respective model results for their location in
Fig.
The development of the supercritical flow regime penetrating the Dead Sea
Rift Valley is observed at two meteorological stations. It is detectable by
marked, high wind speeds, a change in wind direction towards east and a
sudden increase in temperature by approximately 4
The magnitude and timing of the supercritical flow regime is not captured correctly by ICON-ART, as a comparison of observations with model results shows. Although model results for Merom Golan show an increase in wind speed and a change in wind direction, the results are far from the observed intensity and 2 h late. For the lower Gamla station ICON-ART simulates the arrival of the supercritical flow with a delay of 6 h and only a third of the observed intensity. As in the observations, the arrival is detectable in changes in wind speed, direction and surface temperature. The Zemah station and valley floor remains unaffected in ICON-ART as in reality. The possible reasons for the deviation of the simulation from reality include an unrealistic night-time boundary layer regime, incorrect atmospheric conditions upstream of the valley and a delayed arrival of the CPO3. As a result, possible mineral dust emissions due to the supercritical flow cannot be captured by ICON-ART. Through its destabilizing night-time boundary layer effect, mineral dust itself can provide a positive feedback mechanism to higher near-surface wind speeds and again higher dust emissions in the supercritical flow region. This is visible in the Zemah measurements, where the observations show much higher night-time surface temperature values than the model. As there is a reduced amount of dust in ICON-ART compared to reality, it underestimates the mineral dust radiative night-time warming effect.
As a result of the underestimated dust concentrations, ICON-ART is unable to
capture the correct magnitude of reduction in global radiation due to mineral
dust during the daytime. Reductions in maximum global radiation for Zemah are
modelled to be 125 W m
In summary, the existence of supercritical flow conditions in the region with connected hydraulic jumps is assumed to cause widespread and strong dust emissions on the eastern side of the Dead Sea Rift Valley. This contributes to the exceptional amount of dust in the southern part of the EM on 8 September. ICON-ART captures the special flow phenomena, albeit not with the correct magnitude and timing. The lack of a sufficiently developed supercritical flow and resulting high near-surface wind speeds prevents dust emission in Jordan and Israel in the model.
ICON-ART model results at 03:00 UTC, 7 September. All results show
ARI-CTRL. Displayed are
In combination with already underestimated dust emissions due to the recent
land cover changes and soil degradation in the Mesopotamia region (see
Sects.
The validation of the mineral dust distribution and transport characteristics with satellite and station measurements show overall good agreement between ICON-ART and the observations, especially during the early stages of the event. The simulation therefore allows the investigation of the mineral dust radiative effect. Using ICON-ART, the radiative effect of mineral dust can be studied in detail through the differences between the simulation including mineral dust radiation interaction (ARI) and without it (CTRL). An analysis of the mineral dust radiative effect on surface conditions is conducted at two characteristic points in time during the dust storm development.
ICON-ART model results at 10:00 UTC, 7 September; layout and colour-coding as in Fig.
The first analysis is performed at 03:00 UTC, 7 September, as it shows
interesting night-time features due to mineral dust
(Fig.
The magnitude of the temperature response to mineral dust radiative forcing
is large when compared to other studies with values of up to 5 K
The DOD difference shows regions with an increased DOD at the leading edges
of the CPOs in ARI (Fig.
Inside the CPO3 region, a 2 m temperature cooling of more than 2 K can be
observed for ARI (Fig.
A more intense and faster spreading CPO can have multiple reasons, and further
research is necessary in order to quantify the different contributions. The reasons may be the following:
more intense convection leading to more rainfall which can evaporate,
this in turn cooling the CPO more. The intensity of the convection can be
increased due to a warmer inflowing air mass because of the mineral dust
radiative surface heating at night. more potential for evaporation due to a warmer surface boundary layer,
also creating a cooler CPO. a more stable stratified and thereby less turbulent CPO, preventing the
loss of energy due to turbulent friction. travel of the CPO into a less stable night-time boundary layer due to
mineral dust radiative surface heating. Therefore, less potential and kinetic
energy of the CPO needs to be invested in order to lift the stable night-time
boundary layer in front. Due to the reduced resistance, propagation speed can
increase.
The second analysis investigates the day-time radiative effect of mineral
dust and is done at 10:00 UTC on 7 September
(Fig.
The reductions in surface temperature have a stabilizing effect on the
boundary layer
This study presents a large-domain, convection-permitting simulation of the
September 2015 severe dust storm that allows the reproduction of the main
atmospheric processes during this event. An active RST situation and the
related MCSs and CPOs are shown to be responsible for the severe dust event
in the EM. In addition, the interaction with an intense heat low, the inland-penetrating EM inflow, and the widespread occurrence of supercritical flow
conditions and subsequent hydraulic jumps are suggested as important drivers
for dust emission. While the importance of the heat low and convection for
this event have been suggested elsewhere
The convection-permitting simulation of
the dust event with ICON-ART improves the forecast quality decisively. The
driving meteorological systems and resulting dust emissions are captured in
their horizontal, vertical and temporal structure as is shown by a comparison
with satellite observations. The simulated DOD over Syria and Iraq is of a realistic magnitude with values above 2 throughout the main dust event region
and maximum values above 6. The transport to the northern part of the EM and
Cyprus is modelled with DOD values above 2, however with a 2
For the transport to the southern EM, a hydraulic jump is demonstrated to be
of importance for dust emission in addition to the advection of the dense
dust plumes into the region. It is captured by ICON-ART, albeit with reduced
intensity compared to observations. Due to the out-of-date soil conditions in
the Mesopotamia dust source region and an underdeveloped hydraulic jump
phenomenon, dust transport into the southern EM is underestimated by 1 order
of magnitude by ICON-ART. Modelled DODs are in the range of 0.5–1.5 over
Israel, and PM
The event is triggered by an active RST situation. The occurrence of the active RST situation at the beginning of September is unusually early, thereby explaining the extraordinariness of the event with respect to timing. Furthermore, the RST situation enables the interaction of multiple dust-emitting meteorological systems over the course of 3 days, which explains the extraordinariness of the event with respect to magnitude and spatial extent. In particular, the active RST situation favours a period of convectively active days with MCSs and associated CPOs. In addition, the formation of an intense heat low above Syria is facilitated. The cyclonic flow around the RST provides the basis for the transport of dust in southern and westerly directions on its downstream flank, which explains the exceptional transport direction of the dust plume into the EM from the east.
During the early morning hours of 6 September, a sharply defined CPO (CPO1) from an MCS over the Taurus mountain range interacts with a shallow but strong heat low forming in the boundary layer above Syria. Increased dust emissions occur as soon as turbulent mixing of the boundary layer sets in. Subsequently, the flow and dust plume created interact with the EM sea breeze penetrating inland. Downstream of the upper-tropospheric trough and with orographic support from the Zagros mountain range, a second MCS develops over the Turkey–Iraq–Iran border region from noon onwards. The MCS rapidly produces a CPO (CPO2) which travels west in the wake of the CPO1 and the heat low, again producing substantial dust emission over central Syria. The lifting caused by the gust front of the CPO2 triggers the initiation of deep convection over the Syria–Iraq border region, which becomes organized into an MCS around 18:00 UTC. The MCS is again located in a dynamically favourable position downstream of the quasi-stationary upper-tropospheric trough. It produces another CPO (CPO3) from 20:00 UTC onwards. The CPO3 subsequently counters and lifts the inflow from the Persian Gulf along the RST flank, thereby fuelling the MCS and enabling its sustained lifetime of more than 12 h. During night-time, the CPO3 gains momentum and spreads towards west. With sunrise and the onset of boundary layer mixing, intense dust pickup occurs in the CPO2 and CPO3 region. The aged plume from the CPO1 and the HL is transported westward and south-westward along the EM coast, leading to a first arrival of dust in the region. During daytime on 7 September, the MCS dissipates. The dust plume connected to the CPO3 travels into a south-westerly direction supported by the flow on the downstream flank of the RST. The Dead Sea Rift Valley is passed by the merged dust plumes of the CPO2 and CPO3 after midnight on 8 September. During the night, the flow interacts with the complex orography. As a result, widespread supercritical flow conditions, and the subsequent hydraulic jumps, occur in the lee of orographic features. This flow phenomenon and the related dust emissions add to the extreme dust concentrations in the southern EM on 8 September. During daytime on 8 September, the dust plume is mostly stationary in the EM and influenced by the local circulation systems.
Besides the feedbacks of the mineral dust radiation interaction which have
been identified in the literature before, a previously undocumented effect is
found inside the CPO regions. Systematically more intense CPOs and a faster
propagation of the CPOs in the mineral dust radiation interaction run are
modelled, posing interesting questions for further research to quantify the
contributions of different physical processes to this effect.In conclusion, this comprehensive case study underlines the need to
explicitly represent deep moist convection in dust storm forecasting in
accordance with the studies by
The underlying data and model code can be obtained from the authors upon request.
ICON-ART model results and satellite observations at 08:00 UTC, 6 September. The solid black line marks the CPO1, the dashed black line the EM inflow from
5 September and the dotted black line the frontal structure of inflow from the Persian
Gulf. Displayed are
ICON-ART model results and EUMETSAT satellite observations at
00:00 UTC, 7 September. The axis and label bars for ICON-ART DOD and MODIS
AOD as well as column-integrated hydrometeor content are equal to the ones
used in Fig.
Vertical cross section of ICON-ART model results at 00:00 UTC, 7 September, and CALIPSO satellite observations from south to north along the ground track in Fig.
Vertical cross sections through ICON-ART model results along the
transects in Fig.
Vertical cross sections through ICON-ART model results; same as
Fig.
ICON-ART model results and satellite observations at 11:00 UTC,
8 September. The chain dotted black lines mark the locations of the cross
sections displayed in Fig.
The ICON-ART simulations were conducted and analysed by PG with the help of DR and CW. BV supervised the work and contributed to the discussion and design of the experiment. The Mie calculations for the mineral dust radiation interaction were conducted by PG and implemented in ART together with DR and CW, based on input by BV. The Israeli station data were provided by PK and YL, who also aided the dust transport discussion as well as synoptic situation classification. PK contributed to the discussion of meteorological drivers and event stages and the mentoring of the work leading up to this publication. PG prepared the figures and the manuscript, which was improved by all authors.
The author Bernhard Vogel is a co-editor of ACP.
This article is part of the special issue “Environmental changes and hazards in the Dead Sea region (NHESS/ACP/HESS/SE inter-journal SI)”. It is not associated with a conference.
A special thanks goes to Christoph Kottmeier for his efforts in mentoring the
work leading up to this publication. EUMETSAT and its training group
(Jochen Kerkmann, Hans-Peter Roesli and Sancha Lancaster) are gratefully
acknowledged for granting the right to use the MSG animation of the dust
event. We would like to thank Tami Bond and Christoph Maetzler for making
their MatLab-based Mie code publicly available. We thank Arnon Karnieli for
his effort in establishing and maintaining the Sedé Boqer AERONET site. The
MODIS aerosol optical depth datasets were acquired from the Level-1
Atmosphere Archive and Distribution System (LAADS) Distributed Active Archive
Center (DAAC), located in the Goddard Space Flight Center in Greenbelt,
Maryland (