ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-17655-2018Unexpected vertical structure of the Saharan Air Layer
and giant dust particles during AER-DUnexpected vertical structure of the Saharan Air Layer
and giant dust particlesMarencoFrancofranco.marenco@metoffice.gov.ukhttps://orcid.org/0000-0002-1833-1102RyderClairehttps://orcid.org/0000-0002-9892-6113EstellésVictorO'SullivanDebbieBrookeJenniferhttps://orcid.org/0000-0001-5752-5877OrgillLukeLloydGaryGallagherMartinhttps://orcid.org/0000-0002-4968-6088Met Office, Fizroy Road, Exeter, EX1 3PB, UKDepartment of Meteorology, University of Reading, Reading, RG6 6BB, UKDepartment de Física de la Terra i Termodinàmica, Universitat de València, 46100 Burjassot, SpainCollege of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, UKSchool of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UKFranco Marenco (franco.marenco@metoffice.gov.uk)12December20181823176551766824July201829August201828November20184December2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/17655/2018/acp-18-17655-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/17655/2018/acp-18-17655-2018.pdf
The Saharan Air Layer (SAL) in the summertime eastern Atlantic is
typically well mixed and 3–4 km deep, overlying the marine boundary layer
(MBL). In this paper, we show experimental evidence that at times a very
different structure can be observed. During the AERosol properties – Dust (AER-D) airborne campaign in
August 2015, the typical structure described above was observed most of the
time, and was associated with a moderate dust content yielding an aerosol
optical depth (AOD) of 0.3–0.4 at 355 nm. In an intense event, however, an
unprecedented vertical structure was observed close to the eastern boundary
of the basin, displaying an uneven vertical distribution and a very large AOD
(1.5–2), with most of the dust in a much lower level than usual (0.3–2 km).
Estimated dust concentrations and column loadings for all flights during the
campaign spanned 300–5500 and 0.8–7.5 g m-2,
respectively. The shortwave direct radiative impact of the intense dust event
has been evaluated to be as large as -260±30 and -120±15 W m-2 at the surface and top of atmosphere (TOA), respectively. We also
report the correlation of this event with anomalous lightning activity in the
Canary Islands.
In all cases, our measurements detected a broad distribution of
aerosol sizes, ranging from ∼0.1 to ∼80µm (diameter), thus highlighting the presence of giant particles.
Giant dust particles were also found in the MBL.
We note that most aerosol models may miss the giant particles
due to the fact that they use size bins up to 10–25 µm.
The unusual vertical structure and the giant particles may have
implications for dust transport over the Atlantic during intense
events and may affect the estimate of dust deposited to the ocean.
We believe that future campaigns could focus more on events with
high aerosol load and that instrumentation capable
of detecting giant particles will be key to dust observations in
this part of the world.
Introduction
The Saharan Air Layer (SAL) is a deep, hot and dry layer of air
transported over the tropical Atlantic from the western African
coast, above a cool and moist marine boundary layer (MBL)
.
It is generally associated with a mid-level easterly jet, and it
often displays a large content of mineral dust from northern Africa.
Using observational data and chemical transport model simulations
of dust lifetime, found that atmospheric dust
is substantially coarser than represented in current global
climate models, i.e. that the particle size distribution (PSD) in
the models has too many fine particles and too few coarse
particles.
As coarse dust warms the climate, they also found that the global
dust direct radiative effect is likely to be less cooling than
estimated by models in a current global aerosol model ensemble and
that following this a net warming of the planet by dust cannot be
ruled out.
A number of experiments have been carried out to characterise
the properties of mineral dust and the associated meteorology,
targeting the African continent
and the Atlantic region .
According to the conceptual model by , dust is
lifted in the Saharan boundary layer in the source region and then
transported westward.
This transport mechanism results in a typical vertical structure
which sees a dust-laden SAL between ∼850 hPa (∼1.5 km)
and ∼500 hPa (∼6 km) in the eastern Atlantic, characterised
by a uniform potential temperature.
There is then a reduction of the SAL top height, as dust travels
across to the Americas, accompanied by a rise of its base.
The quantity and size distribution of dust are however not completely
understood .
Moreover, the dust vertical distribution affects its lifetime, as
free tropospheric particles are generally long lived compared to
boundary layer particles which are turbulently mixed down to the
surface.
Using airborne dust as a tracer, the main properties of the SAL have been
characterised using the spaceborne lidar on-board CALIPSO.
report that African dust is transported across the
Atlantic all year long, with strong seasonal variations in the transport
pathways (mainly in the free troposphere in summer, and at the low
altitudes in winter).
They also highlight that transatlantic dust transported at low altitudes
is important for all seasons, especially further across the ocean.
The Atlantic dusty zones were found to be shifted southward
from summer to winter, with a similar southward shift of dust-generating
areas over the continent.
present a systematic study of the SAL using
CALIPSO over a 5-year period, showing that the SAL can be identified
all year round, with a marked seasonal cycle.
They found that the SAL occurs higher in altitude and at northern
latitudes during summer (5–30∘ N and 1–5 km near Africa) than during
winter (5∘ S–15∘ N and 0–3 km near Africa), following the Intertropical
Convergence Zone (which forms its southern boundary) and the
mid-latitude westerlies (which prevent its further northward development).
also found that the vertical structure of the
SAL can be described as a single layer, of which the mean altitude and
geometric depth decrease towards the Americas.
The spatial distribution of dust over the Atlantic is also addressed
in , using lidar observations obtained during
a transatlantic cruise from Guadeloupe to Cabo Verde.
The properties of the SAL were found to be homogeneous from base to top,
a result which suggests that particle sedimentation is reduced and/or
that upward transport mechanisms are active.
These results are further investigated in , where
the removal of dust in three atmospheric models was found to be too
strong and accompanied with an excess of fine particles.
have also studied the spatial and seasonal
distribution of mineral aerosols using SeaWiFS, OMI and CALIPSO
and have highlighted a significant change in the vertical distribution
at the western African coastal transition during summer.
This transition can be summarised as an uplift of the aerosol over
the ocean above an altitude of 1–2 km, as opposed to a profile
down to the surface over the continent, and it is not observed
in winter.
The formation of this elevated dust layer is also studied in
, using WRF-Chem simulations at high resolution,
together with the observations of the SAMUM-1 campaign: they
highlighted the effect of orographic lifting and the interaction
of the continental outflow with the sea breeze as key factors.
have presented nearly a decade of lidar observations
at M'Bour, Senegal, a site located on the western African coast
at a low latitude (14∘ N): their study also highlights a vertical
distribution up to ∼5 km during summertime dust transport
events.
Lidar measurements at M'Bour are also described by
, and a vertical distribution up to
∼5 km during summer was also observed at Cabo Verde in
May–June 2008 .
The vertical distribution of dust is paired with its particle
size distribution, as larger particles are thought to be
preferentially removed with gravitational settling during transport
across the Atlantic.
Using results from the SALTRACE field experiment,
investigate the possibility that vertical mixing may occur in the
SAL, driven by the absorption of sunlight by dust in daytime.
This hypothesis is driven, in particular, by the observed lack of
vertical change in the PSD and the presence of large particles
near the top of the SAL.
Here we present results from the AERosol properties – Dust (AER-D)
airborne campaign, carried out during August 2015 over the eastern tropical Atlantic.
This campaign was aimed at characterising mineral dust, for the
benefit of validating modelling and remote sensing (RS) methods, and for
improving the knowledge on optical and microphysical properties.
Our measurements of the particle size distribution in the coarse
and giant mode during summer as well as the characterisation of an
outbreak with large aerosol optical depth (AOD) contribute
with a new perspective to the knowledge of airborne mineral dust.
The AER-D observations bring an important insight on mineral dust
transported over the basin and provide
a wealth of data that are further analysed in a variety of studies
.
a RS: airborne remote sensing (with limited in situ sampling); SAVEX-D:
Sunphotometer Airborne Validation Experiment in
Dust; CATS: underflight of the CATS lidar on the ISS. b B923: from
Praia (14∘57′ N, 23∘29′ W) to Fuerteventura
(28∘27′ N, 13∘52′ W); B924: from Fuerteventura to Praia.
Research flights
Between 6 and 25 August 2015, the Facility for Airborne Atmospheric
Measurements (FAAM) research aircraft was based in Praia, Cabo Verde,
in the island of Santiago (14∘57′ N, 23∘29′ W).
AER-D was conducted simultaneously with ICE-D (Ice in Clouds Experiment
– Dust), aimed at studying dust–cloud interactions
and the evolution of towering cumulus clouds due to the role of dust
as ice nuclei
.
In addition, the Sunphotometer Airborne Validation Experiment in
Dust (SAVEX-D) was also carried out and is treated here as a component
of AER-D.
Sixteen research flights were carried out between AER-D and ICE-D.
Here we present data from six AER-D flights (see Table ),
representing nearly 28 h of flight time.
All flights were above the Atlantic Ocean, off the west coast of Africa,
and they targeted studies on the mineral dust transported within the Saharan
Air Layer.
The range of conditions was varied, and dust loadings from moderate
to very high were encountered.
The area covered is between Cabo Verde and the Canary Islands.
The aircraft was equipped with in situ and remote sensing equipment,
as described in the following sections.
Forecasts of Saharan dust and cloud cover from the Met Office Unified
Model , as well as from the
Copernicus Atmosphere Monitoring Service based at the European Centre
for Medium-Range Weather Forecasts , were used to plan the research flights.
Dust outbreak events could be predicted several days in advance,
and detailed guidance was provided in real time using Meteosat Second Generation (MSG)-SEVIRI satellite
imagery products, enabling the flight plans to be adapted just before
or during each mission.
The meteorology of the campaign has been analysed in ,
where two regimes have been identified.
In the first part of the campaign, until 14 August, winds in the
SAL above Cabo Verde were found to be slack (<10 m s-1 and
with an oscillating direction between NE and SE).
Instead, from 15 August onwards the winds in the SAL above Cabo
Verde were stronger (>10 m s-1 and from a marked E direction).
This change in regime was associated with the migration of a high
at 700 hPa towards the SE, from western to central
Algeria, leading to a more intense easterly jet.
A slacker NE flow (∼5 m s-1) was found in the MBL, controlled
by the trade winds.
Three types of research flights have been carried out:
Remote sensing (RS) flights, designed mainly to provide mapping of
the dust layer using the airborne lidar (three flights), and
accompanied with additional measurements using dropsondes
and radiometers.
These flights typically aimed for the heavy dust outbreaks off the
African coast and travelled a long distance at high altitude;
limited in situ sampling at lower altitudes was also conducted.
The goal of these flights was to provide a small but valuable
dataset, useful for the validation of satellite retrievals,
model forecasts and dust data assimilation schemes.
Validation of the retrievals of aerosol microphysical properties
from two types of ground-based sun photometers (PREDE/SKYNET and
CIMEL/AERONET) under the SAVEX-D project (two flights). For these
flights, we sampled a limited geographic area, as near as possible
to a ground-based site equipped with sun photometers, and we aimed
to fully characterise the atmospheric column by sampling at several
different altitude levels.
The validation of products from the Cloud-Aerosol Transport
System lidar (CATS; ) on-board the
International Space Station (ISS) (one flight).
This type of flight was coordinated with the predicted track of
the ISS, and a flight pattern similar to SAVEX-D was adopted
(limited geographical area and sampling at several different
altitudes).
Note that all times reported in the current paper are UTC.
Vertical profiles of the aerosol extinction coefficient measured
by airborne lidar,
identified by flight number and time (UTC).
The number in parentheses is the AOD resulting from the lidar profile.
For flights B923 and B924, a letter (a–h) identifies the transects
in Fig. used to compute the
profiles shown in this figure.
Panel (a) refers to the moderate dust conditions, most often
encountered during the campaign, whereas panel (b) refers to
the heavy dust loads encountered on 12 August.
Vertical structure
Figure displays the vertical profiles of the aerosol
extinction coefficient at 355 nm during the high-altitude transects.
They have been obtained with the on-board elastic backscatter lidar,
and they have been evaluated using previously published methods
.
The vertical resolution of the processed dataset is 45 m and the
integration time is 1 min, corresponding to a ∼9 km footprint.
The AER-D lidar measurements and their uncertainties are described
in more detail in .
In brief, the lidar inversion is based on a double iteration.
In the first iteration, a campaign-mean lidar ratio (extinction-to-backscatter
ratio) is computed, which ensures consistency of the lidar signals
with the layers identified as aerosol-free.
Then, the data analysis is reiterated using this lidar ratio (54±8 sr),
and using the slope-Fernald
approach, based on the use of a far-end
reference within the bottom portion of the aerosol layer (see
, for details).
In most cases, a deep dust layer is identified, with base at 1–2 km and top at 5–6 km altitude, above a MBL also
displaying a significant aerosol content (see Fig. a).
As expected with aerosol fields, there are day-to-day variations;
however the main properties are consistent: the AOD is in the 0.3–0.4
range, the aerosol extinction coefficient is
of the order of 100–200 Mm-1 and the depth of the elevated
dust layer is 3–4 km.
This observed structure is in agreement with expectations from the
conventional model for Saharan dust transport over the Atlantic
.
We shall classify this typical condition as “moderate dust”.
During flights B923 and B924, however, a different vertical
structure was observed near the western African coast
(Fig. b).
The AOD was 1.5–2, the MBL was compressed with a top at ∼0.3 km
and the dust extended from the top of the MBL up to a layer top at
∼5.3 km.
The highest loadings were found at low altitudes
(1–2 km), with extinction coefficients in excess of 1000 Mm-1,
and we shall denote this condition as “heavy dust” and/or “anomalous
structure”.
Temperature, moisture and wind profiles (not shown here) suggest
that the upper dust layer (2–5 km) was well mixed (constant potential
temperature, θ) and characterised by a moderate ESE flow.
On the other hand, the lower layer (0.3–2 km), where the intense dust
concentration was found, was stable (increasing θ with height
and constant temperature) and displayed a northeasterly wind
.
We believe that this observed vertical distribution of dust, with a
double layer and a large concentration very low above a compressed MBL,
reveals a surprising exceptional structure, not previously encountered
during measurements of the SAL over the Atlantic Ocean, and it is
particularly interesting since it coincides with a large AOD event.
This observation shows that the anomalous structure can exist in the
near range for Saharan dust transport across the Atlantic (100–300 km from the coast); however nothing can be inferred for longer transport
distances.
Moreover, nothing can be inferred concerning the frequency of occurrence
of the anomalous structure, but the fact that it was not reported before
may suggest that it is sporadic and limited to heavy dust outbreaks.
Particle size distribution
The layers
with the largest concentrations, identified by lidar or during aircraft
ascent and descent, have been sampled in situ with the aircraft
instruments.
Figure shows the particle size distributions (PSDs)
obtained during 19 straight and level runs (SLRs) sampled during
the AER-D flights.
We have subdivided the PSDs into marine boundary layer, moderate
dust loading and heavy dust loading, based on the concentrations observed.
The heavy dust PSD was collected in flight B924 (run 5 at 1000 m
altitude), at the same location where the lidar highlighted the
heavy dust condition, whereas the moderate dust PSD refers to all
the other samples and hence corresponds to the cases clustered as moderate
dust in the previous section.
The PSDs shown here have been obtained using three wing-mounted
probes which, when combined together, are capable of sampling the
spectrum between 0.1 and 1000 µm (diameter): Passive Cavity Aerosol
Spectrometer Probe (PCASP), Cloud Droplet Probe (CDP) and
two-dimensional stereo probe (2DS) .
Calibration of the PCASP was done before and after the campaign,
whereas the CDP was also calibrated before most flights.
The CDP size resolution was enhanced at the smaller end of
the spectrum by using custom settings and making use of the
particle-by-particle data.
For both probes, particle spectra have been processed for an
assumed refractive index of dust of 1.53-0.001i, thus
correcting for the bin ranges calibrated using polystyrene
latex spheres, and the first bin has been discarded due to
its undefined lower edge.
The 2DS is a shadowing probe with 10 µm resolution, and
it does not rely on refractive index to infer particle size.
The AER-D particle size distributions and the instruments
are described in detail in .
Particles size distributions obtained during in situ
straight-level runs using the PCASP (0.1–3 µm),
CDP (4–20 µm) and 2DS (>20µm).
Blue: MBL;
red: SAL, moderate dust loading;
dark green: SAL, heavy dust loading (flight B924 run 5);
dashed blue: MBL under heavy dust layer (flight B924 run 4).
We note that authors in the geological sciences often consider that
62.5–2000 µm particles are “sand” as opposed to “dust.”
Here, however, we will use the term “dust” for the particles that
we observed, adhering
to a terminology in use in the atmospheric sciences, where “dust”
is considered to be suspended material transported by the wind
.
The volume PSDs are dominated by a very broad coarse mode centred at
5–6 µm (diameter), which is also where the volume distributions peak.
Giant particles (diameter 20–80 µm) were detected for 75 % of the samples.
Runs in the MBL exhibit a clearly pronounced fine
mode, peaking at ∼0.2µm diameter, whereas for the dust layers
the fine mode peaks at 0.25–0.3 µm diameter and is less marked.
For 25% of the in-dust SLRs, a distinct fine mode is not observed.
As expected, the largest concentrations were encountered in the
heavy dust case (flight B924).
Somewhat surprisingly, however, some of the SLRs in the
MBL exhibit a concentration of giant particles similar to the
heavy dust case.
For the samples taken in dust, giant
particles do not represent a separate mode of the size distribution,
but rather an extension and broadening of the coarse mode.
Contrastingly, for two of the MBL samples the giant particles appear as a
separate mode with diameter between ∼15 and ∼80µm,
showing a broad peak at 20–40 µm.
Table displays the effective diameter
(Deff), derived from the PSDs;
the SLRs in dust exhibited a Deff of 3.6–4.7 µm,
whereas in the MBL Deff was 3.4–5.5 µm, thus
exhibiting a larger variability from flight to flight.
The in situ measurements from AER-D are discussed in more detail in
.
Moreover, describes the measurements of hematite
content of dust during ICE-D and AER-D, as well as their dependency on
dust age.
Exceptional dust event
On 12 August a major outbreak of dust occurred west of the African
continent: see the Meteosat Second Generation (MSG) image shown in
Fig. .
From model predictions, significant dust concentrations were expected
between Western Sahara and the Canary Islands.
A targeted mission was planned, consisting of a double flight on a
single day.
Flight B923 was a 3 h high-altitude mapping flight
held in the morning, from Praia to Fuerteventura, and it was followed
by flight B924 in the afternoon.
The latter was a full 5 h scientific flight, and in addition
to dust mapping it also allowed for a 1.5 h long descent in the dust
layers, enabling the in situ sampling at two
altitude levels, 30 m and 1000 m above sea level. Figure shows the flight tracks, and a red circle is used
to highlight the area of the in situ sampling: it
can be seen from the underlying satellite image that the latter area
is ideally located at the leading edge of the “dust front”.
Meteosat Second Generation “321 RGB” image for 12 August
at 16:30 UTC, showing the extent of the dust plume advected off
the west coast of Africa.
The track of flights B923 and B924 is overplotted.
A colour scale depicts the lidar-derived AOD, and a red
circle indicates the location where the aircraft descended
to sample the heavy dust in situ.
A SLR at 30 m altitude was performed between 15:52 and 16:04 UTC
(run 4), and a SLR at 1000 m took place between 16:09 and 16:29 UTC
(run 5).
Bi-dimensional structure of the atmosphere on 12 August, for
the high-altitude transects: (a) flight B923 from 15 to 28.5∘ N;
(b) first part of B924, from 28.5 to 23.5∘ N; second part of
B924, from 24 to 15∘ N.
This curtain plot is coloured according to the aerosol extinction
coefficient measured by lidar at 1 min integration time
(∼9 km footprint).
The green dots indicate cloud tops detected with the lidar at
2 s integration time (∼300 m horizontal resolution).
The red boxes and letters identify the transect portions that
have been used to derive the profiles displayed in
Fig. .
The arrows indicate the direction of travel of the aircraft.
Figure shows the vertical structure of the atmosphere,
as revealed by the lidar during the three high-altitude transects.
The plots reveal both the aerosol and the cloud fields, and the
location of the vertical profiles shown in Fig.
is indicated with red boxes.
This figure clearly shows the contrast between the pre-front SAL
(moderate dust conditions) and the much more intense dust loading
in the post-front SAL (heavy dust).
The pre-front SAL is on the left/south and the post-front
on the right/north.
It also shows well that the front advanced towards the south and that
its leading edge featured a very intense dust layer at low altitude (1–2 km).
Figure a represents the first flight, heading northwards.
We initially observed
a shallow SAL at 16∘ N, identified through the presence of dust and positioned
above the marine stratocumulus deck.
As we continued, the SAL deepened until we reached 20∘ N:
there, we overflew a high cloud deck, situated at the top of the SAL and
obscuring the layer below.
The band of clouds is clearly identified in the 10:30 UTC MSG imagery (not
shown here), and it runs in a southeast–northwest direction, crossing
our track orthogonally.
The image in Fig. refers to the afternoon (16:30 UTC), when the
band of clouds had mainly disappeared, although some scattered clouds remain
(it still suggests the location of the band we encountered in the morning).
At 21.2∘ N, beyond this band of clouds, we continued
overflying a SAL extending between 1–2 and ∼5 km, exhibiting a
moderate AOD (∼0.34) and a moderate extinction coefficient
(∼100 Mm-1).
No further clouds were encountered during this flight.
As we reached a point at 24∘ N, 17∘ W, the aerosol
load and its vertical profile changed: the AOD was observed to
increase to ∼1.5 and the intensity of the extinction
coefficient increased markedly at all altitudes.
At its leading (southward) edge, this dust front exhibited a very
intense extinction coefficient, especially in its lower layers
(≳1000 Mm-1 at around ∼1 km and of the order of
200–400 Mm-1 up to altitudes of ∼5 km).
On the return flight (Fig. b, c), the dust front
boundary was crossed again, and similar features were observed once more.
The extinction coefficient at its peak intensity had
risen to ∼1500 Mm-1, and the AOD reached 2.
The boundary between the heavy dust and the moderate dust had
advanced south and west: this southward motion is evident when
considering the succession of Fig. a, b and c.
Between 15:26 and 16:48 UTC a descent of the aircraft into the front was
made, at around 24∘ N, 18.2∘ W.
The in situ sampling levels were initially identified during flight at
high altitude, using the real-time lidar display, and refined during the
descent using our nephelometer.
The nephelometer showed that the dust layer extended from
0.3 to 5.2 km, with a sharp boundary at its base; we consider
the air below this base to be in the MBL.
An intense layer was observed between 0.85 and 1.4 km, with a 550 nm
extinction coefficient of 2100 Mm-1 as determined by the
nephelometer in combination with a
particle soot absorption photometer, whereas above 2.6 km the
extinction coefficient was only 140 Mm-1.
Extinction in the MBL was 100–150 Mm-1.
During the SLR at 1000 m, particles with diameter up to 80 µm were
observed (green line, Fig. ), with an increased particle
concentration for all sizes.
In comparison, the measurements in the MBL (dashed blue line in
Fig. ) showed a much smaller number
of particles in the 0.25–40 µm diameter range, whereas similar
particle numbers were observed below ∼0.25µm and above
∼40µm.
The most striking feature is the fact that the size and concentration
of giant particles in the MBL is very similar to the ones observed in
the dust layer.
MSG “dust RGB” imagery permitted us to track the origin of this dust
(see ).
The dust that we sampled at 24∘ N, 18.2∘ W, at the edge of the dust front,
seems to have been uplifted in northern Mali 2 days earlier, during
a haboob generated by a mesoscale convective system, and to have travelled
2000 km towards the WNW towards the location where we sampled it.
However, we also found two other dust uplifting events that may have
contributed to the wider dust outbreak off the African coast on that
day: one of them happened in Central Algeria (2000 km, 2 days transport)
and the other one in northern Niger (3000 km, 3 days transport).
The CATS spaceborne lidar detected dust that can be ascribed to these
events, between 00:54 and 00:59 UTC on 11 August (not shown here).
The 1064 nm total attenuated backscatter image from CATS also
revealed a peculiar vertical distribution, with a layer in the
2–6 km altitude range and another closer to the surface, not too
dissimilar from our anomalous structure.
Shortwave radiative effect of the exceptional dust event.
Upward- and downward-facing broadband pyranometers measured the
downwelling and upwelling solar irradiance between 0.3 and 3 µm
during straight and level runs.
They have been used in conjunction
with a radiative transfer model to calculate the surface and top-of-atmosphere (TOA) solar direct radiative effect
of the exceptional dust event measured during flight B924.
We used the established methodology of .
The shortwave upwelling and downwelling irradiance with aerosol
present, SWUaer and SWDaer,
were measured by the pyranometers.
The shortwave upwelling and downwelling irradiance with no aerosol
present (clear sky), SWUcs and SWDcs,
were calculated from a radiative transfer model.
The direct radiative effect on SWU at the TOA
has been estimated as follows, using measurements during a high-level,
straight-level run above the dust layer:
DRESW,TOA=SWUcs-SWUaer.
At the surface, pyranometer irradiance measurements from a low-level, straight-level run below the layer were similarly used
to determine the direct solar radiative effect of the dust at
the surface, as follows:
DRESW,SURF=SWDaer-SWDcs.
For model calculations, the Suite Of Community RAdiative Transfer
codes based on Edwards and Slingo (SOCRATES) model was used ,
configured to use two streams and six spectral bands to represent the
spectral range of the aircraft pyranometers.
Surface albedo values were derived from pyranometer observations during the
low-level run, and solar properties were accounted for.
Vertical profiles of atmospheric temperature and composition were taken
from a standard tropical atmosphere , but replaced
by aircraft in situ measurements for temperature, water vapour and ozone
for the layers sampled during ascent and descent (19–6000 m).
A range of irradiances was calculated,
where surface albedo was varied between 0.03 and 0.06 (the range measured
during the low-level run), solar zenith angle was varied in a range
corresponding to the times of each relevant run, and the meteorology was
varied between the values measured during aircraft descent and ascent.
In this way, we account for the uncertainty due to spatial
variability in the input parameters.
Pyranometer data have been corrected to account for the
pitch and roll of the aircraft as a function of time, using offsets
of the instruments relative to the aircraft fuselage of
-4.6∘ and +0.9∘, respectively, determined from
a series of dedicated “box pattern” and “pirouette” manoeuvres during
the campaign (see ).
An uncertainty of 5.5 % was adopted for the pyranometer measurements
based on , and this includes a contribution of the
levelling corrections.
Figure shows the low-level (30 m) and high-altitude (6–6.5 km)
irradiance measurements, as well as the AOD from the aircraft lidar
(the latter was determined during the high-level runs).
The southern end of these plots corresponds to moderate dust conditions,
whereas the northern end to heavy dust conditions.
Note that the two high-level runs considered here overflew the same
segment of the low-level measurements, respectively, beforehand and
afterwards.
Solar irradiance and instantaneous direct radiative effect
of dust ahead of and within the dust front during flight
B924.
Observations are the thin lines, model clear sky calculations
are the bold lines, uncertainties are displayed through dashed lines
and lidar AODs are indicated with diamonds.
(a) Low-level (run 4, 30 m) measured and modelled shortwave
downwelling irradiance;
(b) surface direct radiative effect on the shortwave downwelling
irradiance;
(c) high-altitude shortwave upwelling irradiance for two aircraft
legs (run 3, blue; and run 6, red);
(d) high-altitude (TOA) direct radiative effect on the shortwave
upwelling irradiance for runs 3 and 6.
In Fig. a, it can be seen that the SWDaer
measured at low level (run 4) dropped
by 90 W m-2, from around 610 to 520 W m-2 at the
northern end of the run, i.e. underneath the dust front (note
the rise in AOD from <0.7 to 1–1.5).
The resulting surface DRESW,SURF is shown
in Fig. b, and it changes from -170 W m-2 in the
south to the larger negative value of -260±30 W m-2 in the
north, under the dust front.
SWU irradiance measurements above the dust layer are shown in
Fig. c, for runs 3 and 6, before and after the descent into
the dust layer, respectively.
During the time between these two runs (1.5 h), the solar zenith angle
increased from 30∘ to 51∘, and the dust front moved
southwards.
The increase in SWU in the location of the larger AOD
can be seen in Fig. c, with SWU increasing by around
50 and 35 W m-2 for runs 3 and 6, respectively.
Figure d shows the TOA DRESW,TOA, which
was more negative
over the intense dust loading, changing from around -50 to
-120 W m-2 (run 3) and from -65 to -100 W m-2 (run 6).
For the larger solar zenith angle (run 6), the DRESW,TOA is
more negative due to the greater backscatter fraction, although the lower
incoming solar radiation means that the change in DRESW,TOA
from ahead of the dust front to over it is smaller in magnitude.
DRESW,TOA values have been combined with the AOD measurements to
determine the dust shortwave radiative efficiency.
Values computed for the TOA vary between -50 and
-95 W m-2τ-1.
Run 3 shows no latitudinal dependence, while run 6 shows a clear trend
of most negative values, with -88 W m-2τ-1 in the south and
-51 W m-2τ-1 in the north.
This difference between the two runs can be attributed to the complexity
in relating the pyranometer measurements, with a near-hemispherical view,
to the lidar, measuring vertically downwards, and how this relationship
may change with varying solar zenith angle due the phase function of the dust.
Surface radiative efficiencies have also been estimated (but note that
the AOD was not measured concurrently to the irradiance):
taking AOD and DRESW,SURF values at the beginning and
end of the low-level run gives an approximation of -230 and
-170 W m-2τ-1 ahead of and under the dust front, respectively.
Conclusions
The AER-D campaign
encountered some unprecedented conditions associated with an outbreak
of dust slightly downstream of continental Africa.
Typical and exceptional properties of the layer have been documented
during the field deployment,
and the concentration of giant particles has been measured.
We believe that these results contribute to reinforce the evidence
in favour of a presence of giant and coarse dust particles in
the eastern Atlantic, which may
currently be missed in most aerosol models (see e.g. Table 1 of
).
The dust size distribution strongly controls the radiative impact of
the aerosols, as well as their interactions with clouds.
The size of particles also controls how far downwind they travel and
thus their ability to impact biogeochemistry downwind of the source region
.
The magnitude of the dust direct and indirect radiative effects
is still uncertain, and it remains unclear whether atmospheric
dust has a net warming or cooling effect on global climate.
The dust particle size distribution has a role to play in this
because fine particles predominantly scatter solar radiation
(cooling effect), whereas for coarse particles absorption of
solar and thermal radiation plays a larger role (warming).
By applying experimental constrains to the global dust abundance and
particle size, conclude that the global dust direct radiative
effect is likely to be less cooling than estimated currently by models,
and that a net warming effect is not to be ruled out.
Our finding of very large particles in the eastern Atlantic,
both in the SAL and in the MBL, supports these conclusions.
Our observations of the SAL vertical structure and particle size
distribution have been classified into two clusters: moderate dust
(encountered most of the time) and heavy dust (encountered on
12 August, north of 24∘ N).
In general, the SAL geometry in the eastern Atlantic in the season
considered was one of a deep layer between 1–2 km and 5–6 km, with
an extinction coefficient at 100–200 Mm-1 and an AOD of
the order of 0.3–0.4 (moderate dust),
in agreement with the findings of previous studies such as
and .
Using the specific extinction estimated by
(0.27–0.35 m2 g-1), these extinctions and AODs translate
into estimated dust concentrations around 300–700 µg m-3
and column loadings of 0.8–1.5 g m-2.
The latter figure is comparable with the estimate obtained from the
in situ data of about 1 g m-2.
In situ measurements displayed comparable particle sizes with the
ones reported for the SAMUM-2 campaign
and highlighted a significant contribution from giant
particles.
On 12 August 2015 we observed a heavy dust outbreak, uplifted
in northwestern Africa 2–3 days earlier, which featured an
AOD of 1.5–2, and a particularly intense layer at
1–2 km, where the extinction coefficient was 1000–1500 Mm-1.
This corresponds to estimated concentrations between 3000 and
5500 µg m-3 and column dust loadings of 4–7.5 g m-2.
The latter figure is slightly larger than the estimate from the in situ
measurements (3–6 g m-2, ).
The vertical distribution for this event is unprecedented
over the Atlantic, and our combination of remote sensing and
in situ measurements yields a unique insight into the properties
of this event.
Note also that, when observing this layer, we are pushing the
observation by lidar to its limits, as extinction of the signal
is large.
The PSD for this intense layer showed
the largest numbers and the largest particles, and it exhibited only
one very broad mode.
Giant particles up to ∼80µm (diameter) were detected, and
giant particles with the same PSD signature were also
found in
the underlying shallow MBL, despite a much less intense coarse mode.
In principle, the giant particles in the MBL could be either dust particles
depositing from above or sea spray (or a combination of both).
However, as documented in , the analysis of filter
samples collected during the research flights suggests that
giant particles observed in the MBL during the AER-D flights
are mineral dust, thus highlighting what is likely the effect
of dry deposition.
The unusual heavy dust event was associated with a large
direct solar radiative effect, reaching -260±30
and -120±15 W m-2
at the surface and top of atmosphere, respectively.
Moreover, we registered a rapid spatial variation across the dust
front, of the order of 90 and 35–50 W m-2, respectively.
Such perturbations to the radiative budget are significant, and to the
authors' knowledge it is the first time that such large values have been
measured over ocean, in combination with dust in situ, vertically
resolved properties.
documented a major dust storm over Niger, with
extremely high AOD peaking 3–4 and a subsequent midday solar
direct radiative effect of -100 W m-2 at the TOA and
-250 W m-2 at the surface.
Their surface and TOA flux changes were of a similar magnitude to those
measured over the dust front discussed here;
however, the change per AOD unit that we observed is
larger, and this can be explained with the low ocean albedo.
performed similar estimates, for a moderate AOD
of 0.26 over the tropical eastern Atlantic, finding a smaller
radiative effect of -47 and -33 W m-2 at the surface and at
the TOA, respectively.
Moreover, the intense dust event was followed by anomalous electric
discharges.
On 12 and 13 August 2015, the Spanish Meteorological State Agency's
Lightning Detection Network (Red de Detección de Rayos de AEMET, REDRA)
recorded nearly 6000 lightning strikes in the Canary Islands region
.
The lightning activity began at around 16:00 on 12 August and reached its
maximum intensity between 19:00 and 02:00 during the night, followed
by additional strikes on the second day.
Meteosat Second Generation imagery shows the formation of convective
cells over the area during the night, which are likely connected to
the electrical activity.
This lightning density was described as extraordinary, because in 12 years
of operations the network has only observed a limited number of events
(23 days) of a comparable intensity, all of which were during the autumn
and winter seasons.
It is beyond the scope of this paper to establish whether there is a causal
link between the dust event and the electric discharges.
Note that the association of aerosols with lightning is still a relatively
new field of science, where the understanding of the underlying processes
is still weak.
, showed an increased lightning activity
east of the Philippines, associated with secondary sulfates that formed
following the eruption of Mt Anatahan, Mariana Islands,
and demonstrated that aerosols increase lightning activity in tropical
regions through modification of cloud microphysics.
Moreover, showed that lightning density can be
twice as large over shipping lanes, thus reinforcing the hypothesis
that aerosols may lead to a microphysical enhancement of convection
and of storm electrification.
Similar observations where also reported over the Mediterranean region,
where moreover a correlation was found with the aerosol extinction
coefficient between ∼1 and ∼3 km, and the aerosols mostly
associated to lightning events over this area were identified as being
dust and smoke .
As a word of caution, however, we have to mention that observations
over the Amazon basin seem to cast doubt on a primary role for
aerosols in enhancing cloud electrification .
Our results trigger some questions and the need for further research.
(1) How frequent is an anomalous vertical structure like the one we
encountered?
We are not aware of other measurements in intense outbreaks
100–300 km off the coast of West Africa, and we believe that further
field deployments targeting this region could offer insights.
An analysis of past satellite observations may also help assess this.
(2) How long lived are giant particles like the ones we observed?
Only few airborne campaigns have been able to measure the full particle
size spectra, and little information is available: once again, this may
be addressed in future campaigns.
(3) What may have happened subsequently to the considerable amount of dust
that we encountered at an altitude of 1–2 km during our intense case? Has it mixed through the
SAL, was it deposited rapidly into the ocean (as could be suggested by
the finding of giant particles in the underlying MBL) or did it
keep its distinct identity downstream?
(4) Does dust play a role in triggering convection and lightning events
over the Atlantic, like the one that affected the Canary Islands on
12–13 August 2015?
Answering such questions may be vital for a better understanding of
the aerosol processes driving deposition, transport, and aerosol–cloud
interactions and hence for
the improvement of models and assessing the climate impact of dust.
Further observations are needed, focusing on the evolution of
aerosol properties during transport across the Atlantic, and a
coordinated experiment on both sides of the basin could be a means
to achieve this.
The FAAM aircraft datasets collected during the ICE-D and
AER-D campaigns are available from the British Atmospheric Data Centre,
Centre for Environmental Data Analysis (2018, http://www.ceda.ac.uk/),
at the following URL:
http://catalogue.ceda.ac.uk/uuid/d7e02c75191a4515a28a208c8a069e70
(Bennett, 2018).
FM proposed and coordinated
the AER-D campaign, interpreted the results discussed here and wrote this
article. VE led the SAVEX-D experiment. FM, CR, VE and JB worked as the
AER-D/SAVEX-D mission science team, implementing the airborne sampling
strategy for aerosol science objectives. DOS and FM carried out the data
analysis of the lidar dataset. CR, GL and MG analysed the in situ
measurements. CR and LO processed the pyranometer measurements and evaluated
the dust radiative effect. All authors read the paper and provided
constructive comments.
The authors declare that they have no conflict of
interest.
Acknowledgements
Airborne data were obtained using the BAe-146-301 Atmospheric Research
Aircraft operated by Directflight Ltd and managed by the FAAM, which
is a joint entity of NERC and the Met Office.
The staff of the Met Office, the University of Leeds, Manchester
and Hertsfordshire, FAAM, Directflight Ltd,
Avalon Engineering and BAE
Systems are thanked for their dedication in making the ICE-D and AER-D
campaigns a success.
Claire Ryder was funded by NERC grant NE/M018288/1.
SAVEX-D was possible thanks to EUFAR TNA (European Union Seventh
Framework Programme grant agreement 312609) and projects
PROMETEUII/2014/058 and GV/2014/046 from the Valencia Autonomous
Government, as well as CGL2015-70432-R from the Spanish Ministry of Economy
and Competitiveness – European Regional Development Fund.
We thank CAMS/ECMWF for providing model products in support of the
ICE-D and AER-D campaigns, as well as EUMETSAT for providing imagery from
MSG-SEVIRI in real time.
Edited by: Yinon Rudich
Reviewed by: two anonymous referees
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