scholarly journals Profiling of Saharan dust from the Caribbean to West Africa, Part 2: Shipborne lidar measurements versus forecasts

2017 ◽  
Author(s):  
Albert Ansmann ◽  
Franziska Rittmeister ◽  
Ronny Engelmann ◽  
Sara Basart ◽  
Angela Benedetti ◽  
...  
Keyword(s):  
Mass Concentration ◽  
Dust Emission ◽  
Saharan Dust ◽  
Light Extinction ◽  
Tropical Atlantic ◽  
Anthropogenic Aerosol ◽  
Dust Transport ◽  
Source Regions ◽  
Dust Sources ◽  
Lidar Measurements

Abstract. A unique 4-week ship cruise from Guadeloupe to Cabo Verde in April–May 2013 (see part 1, Rittmeister et al., 2017) is used for an in-depth comparison of dust profiles observed with a polarization/Raman lidar aboard the German research vessel Meteor over the remote tropical Atlantic and respective dust forecasts of a regional (SKIRON) and two global atmospheric (dust) transport models (NMMB/BSC-Dust, MACC/CAMS). New options of model-observation comparisons are presented. We analyze how well the modeled fine dust (submicrometer particles) and coarse dust contributions to light extinction and mass concentration match respective lidar observations, and to what extent models, adjusted to aerosol optical thickness observations, are able to reproduce the observed layering and mixing of dust and non-dust (mostly marine) aerosol components over the remote tropical Atlantic. Based on the coherent set of dust profiles at well defined distances from Africa (without any disturbance by anthropogenic aerosol sources over the ocean) we investigate how accurately the models handle dust removal at distances of 1500 km to more than 5000 km west of the Saharan dust source regions. It was found that (a) dust predictions are of acceptable quality for the first several days after dust emission up to 2000 km west of the African continent, (b) the removal of dust from the atmosphere is too strong for large transport paths in the global models, and (c) the simulated fine-to-coarse dust ratio (in terms of mass concentration and light extinction) is too high in the models compared to the observations. This deviation is already given close to the dust sources and then increases with distance from Africa.

scholarly journals Profiling of Saharan dust from the Caribbean to western Africa – Part 2: Shipborne lidar measurements versus forecasts

2017 ◽  
Vol 17 (24) ◽  
pp. 14987-15006 ◽  
Author(s):  
Albert Ansmann ◽  
Franziska Rittmeister ◽  
Ronny Engelmann ◽  
Sara Basart ◽  
Oriol Jorba ◽  
...  
Keyword(s):  
Mass Concentration ◽  
Dust Emission ◽  
Saharan Dust ◽  
Light Extinction ◽  
Tropical Atlantic ◽  
Fine Dust ◽  
Anthropogenic Aerosol ◽  
Dust Transport ◽  
Western Africa ◽  
Dust Sources

Abstract. A unique 4-week ship cruise from Guadeloupe to Cabo Verde in April–May 2013 see part 1, Rittmeister et al. (2017) is used for an in-depth comparison of dust profiles observed with a polarization/Raman lidar aboard the German research vessel Meteor over the remote tropical Atlantic and respective dust forecasts of a regional (SKIRON) and two global atmospheric (dust) transport models (NMMB/BSC-Dust, MACC/CAMS). New options of model–observation comparisons are presented. We analyze how well the modeled fine dust (submicrometer particles) and coarse dust contributions to light extinction and mass concentration match respective lidar observations, and to what extent models, adjusted to aerosol optical thickness observations, are able to reproduce the observed layering and mixing of dust and non-dust (mostly marine) aerosol components over the remote tropical Atlantic. Based on the coherent set of dust profiles at well-defined distances from Africa (without any disturbance by anthropogenic aerosol sources over the ocean), we investigate how accurately the models handle dust removal at distances of 1500 km to more than 5000 km west of the Saharan dust source regions. It was found that (a) dust predictions are of acceptable quality for the first several days after dust emission up to 2000 km west of the African continent, (b) the removal of dust from the atmosphere is too strong for large transport paths in the global models, and (c) the simulated fine-to-coarse dust ratio (in terms of mass concentration and light extinction) is too high in the models compared to the observations. This deviation occurs initially close to the dust sources and then increases with distance from Africa and thus points to an overestimation of fine dust emission in the models.

scholarly journals Aeolian dust sources, transport and deposition over the Chinese Loess Plateau during 1999-2019: A study using the FLEXDUST and FLEXPART models

2021 ◽  
Author(s):  
Ove Haugvaldstad ◽  
Hui Tang ◽  
Anu Kaakinen ◽  
Frode Strodal
Keyword(s):  
Atmospheric Circulation ◽  
Loess Plateau ◽  
Dust Emission ◽  
Chinese Loess Plateau ◽  
Dust Transport ◽  
Aeolian Dust ◽  
Chinese Loess ◽  
Source Regions ◽  
Dust Sources ◽  
The Chinese Loess Plateau

<p>The aeolian dust deposits in the Chinese Loess Plateau (CLP) contain valuable information about past environmental changes in Asia. Unlocking this information requires knowledge on the Asian dust sources and dust transport mechanisms, and how the different source regions contribute to the total dust loading and deposition over the CLP.  By studying the dust transport and deposition under present day conditions using the Lagrangian Particle Dispersion model,  FLEXPART,  and the FLEXDUST dust emission model, we aim to better understand the dust signal in the Chinese loess records to constrain their interpretation as paleoclimate proxies.  </p><p>Here we present results from a 20 year simulation of transport and deposition of aeolian dust over the CLP from 1999 until 2019, during the dust event season March until May. Both FLEXPART and FLEXDUST are driven by ERA5 ECMWF meteorological reanalysis data. FLEXPART is set up in a receptor oriented configuration, where many computational particles are released from the receptor points at each timestep. The computational particles are followed for 5 days backward in time probing for possible source regions. The end product is emission sensitivity, i.e. how sensitive the receptor is to emissions in possible source regions. The emission sensitivity establishes a linear relation between the source and receptor. Therefore, multiplying the emission sensitivity with the dust emission flux estimated by FLEXDUST produces a map of the source contribution for each receptor point. To investigate the difference in source regions between the fine and coarse dust, we include two particle sizes, 2 μm and 20 μm, in our simulation. The output from the model is compared against Asian polar vortex (APV) and Asian winter monsoon indices to identify how changes in the large scale atmospheric circulation affect the interannual variation of dust transport and deposition, and to determine whether the amount of deposited dust over the CLP is primarily governed by changes in the emission strength or by changes in the atmospheric circulation.  </p>

scholarly journals New developments in the representation of Saharan dust sources in the aerosol–climate model ECHAM6-HAM2

2016 ◽  
Vol 9 (2) ◽  
pp. 765-777 ◽  
Author(s):  
Bernd Heinold ◽  
Ina Tegen ◽  
Kerstin Schepanski ◽  
Jamie R. Banks
Keyword(s):  
Climate Model ◽  
Dust Emission ◽  
Source Area ◽  
Saharan Dust ◽  
Moist Convection ◽  
Dust Source ◽  
New Developments ◽  
Infrared Imager ◽  
Dust Sources ◽  
Dust Events

Abstract. In the aerosol–climate model ECHAM6-HAM2, dust source activation (DSA) observations from Meteosat Second Generation (MSG) satellite are proposed to replace the original source area parameterization over the Sahara Desert. The new setup is tested in nudged simulations for the period 2007 to 2008. The evaluation is based on comparisons to dust emission events inferred from MSG dust index imagery, Aerosol Robotic Network (AERONET) sun photometer observations, and satellite retrievals of aerosol optical thickness (AOT).The model results agree well with AERONET measurements especially in terms of seasonal variability, and a good spatial correlation was found between model results and MSG-SEVIRI (Spinning-Enhanced Visible and InfraRed Imager) dust AOT as well as Multi-angle Imaging SpectroRadiometer (MISR) AOT. ECHAM6-HAM2 computes a more realistic geographical distribution and up to 20 % higher annual Saharan dust emissions, using the MSG-based source map. The representation of dust AOT is partly improved in the southern Sahara and Sahel. In addition, the spatial variability is increased towards a better agreement with observations depending on the season. Thus, using the MSG DSA map can help to circumvent the issue of uncertain soil input parameters.An important issue remains the need to improve the model representation of moist convection and stable nighttime conditions. Compared to sub-daily DSA information from MSG-SEVIRI and results from a regional model, ECHAM6-HAM2 notably underestimates the important fraction of morning dust events by the breakdown of the nocturnal low-level jet, while a major contribution is from afternoon-to-evening emissions.

scholarly journals Seasonal provenance changes in present-day Saharan dust collected in and off Mauritania

2017 ◽  
Vol 17 (16) ◽  
pp. 10163-10193 ◽  
Author(s):  
Carmen A. Friese ◽  
Johannes A. van Hateren ◽  
Christoph Vogt ◽  
Gerhard Fischer ◽  
Jan-Berend W. Stuut
Keyword(s):  
Environmental Changes ◽  
Saharan Dust ◽  
Local Source ◽  
Trade Wind ◽  
Back Trajectory ◽  
Dust Source ◽  
Western Sahara ◽  
Source Areas ◽  
Source Regions ◽  
Dust Sources

Abstract. Saharan dust has a crucial influence on the earth climate system and its emission, transport and deposition are intimately related to, e.g., wind speed, precipitation, temperature and vegetation cover. The alteration in the physical and chemical properties of Saharan dust due to environmental changes is often used to reconstruct the climate of the past. However, to better interpret possible climate changes the dust source regions need to be known. By analysing the mineralogical composition of transported or deposited dust, potential dust source areas can be inferred. Summer dust transport off northwest Africa occurs in the Saharan air layer (SAL). In continental dust source areas, dust is also transported in the SAL; however, the predominant dust input occurs from nearby dust sources with the low-level trade winds. Hence, the source regions and related mineralogical tracers differ with season and sampling location. To test this, dust collected in traps onshore and in oceanic sediment traps off Mauritania during 2013 to 2015 was analysed. Meteorological data, particle-size distributions, back-trajectory and mineralogical analyses were compared to derive the dust provenance and dispersal. For the onshore dust samples, the source regions varied according to the seasonal changes in trade-wind direction. Gibbsite and dolomite indicated a Western Saharan and local source during summer, while chlorite, serpentine and rutile indicated a source in Mauritania and Mali during winter. In contrast, for the samples that were collected offshore, dust sources varied according to the seasonal change in the dust transporting air layer. In summer, dust was transported in the SAL from Mauritania, Mali and Libya as indicated by ferroglaucophane and zeolite. In winter, dust was transported with the trades from Western Sahara as indicated by, e.g., fluellite.

scholarly journals Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara

2020 ◽  
Author(s):  
Claire Ryder ◽  
Eleanor Highwood ◽  
Adrian Walser ◽  
Petra Walser ◽  
Anne Philipp ◽  
...  
Keyword(s):  
Mass Concentration ◽  
Size Range ◽  
State Of The Art ◽  
Chem Phys ◽  
Saharan Dust ◽  
Dust Particles ◽  
Field Campaign ◽  
Dust Transport ◽  
Coarse Mode ◽  
Significant Underestimation

<p>Mineral dust is an important component of the climate system, interacting with radiation, clouds, and biogeochemical systems and impacting atmospheric circulation, air quality, aviation, and solar energy generation. These impacts are sensitive to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (<span><em>d</em></span>) <span>>2.5</span> <span>µ</span>m) and giant (<span><em>d</em>>20</span> <span>µ</span>m) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (<span><em>d</em>=0.1</span> to <span>>100</span> <span>µ</span>m) at different stages during transport with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties using airborne observations over the Sahara (from the Fennec field campaign) and in the Saharan Air Layer (SAL) over the tropical eastern Atlantic (from the AER-D field campaign).</p><p>Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 <span>µ</span>m were observed over the Sahara compared to 4 <span>µ</span>m in the SAL. Excluding giant particles over the Sahara results in significant underestimation of mass concentration (40 %), as well as underestimates of both shortwave and longwave extinction (18 % and 26 %, respectively, from scattering calculations), while the effects in the SAL are smaller but non-negligible. The larger impact on longwave extinction compared to shortwave implies a bias towards a radiative cooling effect in dust models, which typically exclude giant particles and underestimate coarse-mode concentrations.</p><p>A compilation of the new and published effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 d, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 d, PSD barely changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational sedimentation alone. The reasons for this are unclear and warrant further investigation in order to improve dust transport schemes and the associated radiative effects of coarse and giant particles in models.</p><p>This work has been recently published in ACP (Ryder, C. L., Highwood, E. J., Walser, A., Seibert, P., Philipp, A., and Weinzierl, B.: Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara, Atmos. Chem. Phys., 19, 15353–15376, https://doi.org/10.5194/acp-19-15353-2019, 2019).</p>

Contribution of IASI to the observation of the dust aerosol diurnal cycle over Sahara

2020 ◽  
Author(s):  
virginie capelle ◽  
alain chedin ◽  
Noelle Scott ◽  
Martin Todd
Keyword(s):  
Diurnal Cycle ◽  
Deep Convection ◽  
Dust Emission ◽  
Saharan Dust ◽  
Dust Aerosols ◽  
Source Regions ◽  
Resolution Model ◽  
High Resolution Model ◽  
Dust Distribution ◽  
Realistic Information

<p>The Infrared Atmospheric Sounder Interferometer (IASI) is well suited for monitoring of dust aerosols because of its capability to determine both AOD and altitude of the dust layer, and because of the good match between the IASI times of observation (9.30 am and pm, local time) and the time of occurrence of the main Saharan dust uplift mechanisms. Here, starting from IASI-derived dust characteristics for an 11-year period, we assess the capability of IASI to bring realistic information on the dust diurnal cycle. We first show the morning and nighttime climatology of IASI-derived dust AOD for two major dust source regions of the Sahara: The Bodele Depression and the Adrar region. Compared with simulations from a high resolution model, permitting deep convection to be explicitly resolved, IASI performs well. In a second step, a Dust Emission Index specific to IASI is constructed, combining simultaneous information on dust AOD and mean altitude, with the aim of observing the main dust emission areas, daytime and nighttime. Comparisons are then made with other equivalent existing results derived from ground based or other satellite observations. Results demonstrate the capability of IASI to improve the documentation of dust distribution over Sahara over a long period of time. Associating observations of dust aerosols in the visible, on which a majority of aerosol studies are so far based, and in the infrared thus appears as a way to complement the results from other satellite instruments in view of improving our knowledge of their impact on climate.</p>

scholarly journals Saharan dust transport and deposition towards the tropical northern Atlantic

2009 ◽  
Vol 9 (4) ◽  
pp. 1173-1189 ◽  
Author(s):  
K. Schepanski ◽  
I. Tegen ◽  
A. Macke
Keyword(s):  
Dust Emission ◽  
Source Area ◽  
Saharan Dust ◽  
Dust Deposition ◽  
Deposition Rates ◽  
Dust Transport ◽  
Tropical North Atlantic ◽  
Different Seasons ◽  
Northern Atlantic ◽  
Dust Distribution

Abstract. We present a study of Saharan dust export towards the tropical North Atlantic using the regional dust emission, transport and deposition model LM-MUSCAT. Horizontal and vertical distribution of dust optical thickness, concentration, and dry and wet deposition rates are used to describe seasonality of dust export and deposition towards the eastern Atlantic for three typical months in different seasons. Deposition rates strongly depend on the vertical dust distribution, which differs with seasons. Furthermore the contribution of dust originating from the Bodélé Depression to Saharan dust over the Atlantic is investigated. A maximum contribution of Bodélé dust transported towards the Cape Verde Islands is evident in winter when the Bodélé source area is most active and dominant with regard to activation frequency and dust emission. Limitations of using satellite retrievals to estimate dust deposition are highlighted.

scholarly journals Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara

2019 ◽  
Vol 19 (24) ◽  
pp. 15353-15376 ◽  
Author(s):  
Claire L. Ryder ◽  
Eleanor J. Highwood ◽  
Adrian Walser ◽  
Petra Seibert ◽  
Anne Philipp ◽  
...  
Keyword(s):  
Mass Concentration ◽  
Size Range ◽  
State Of The Art ◽  
Saharan Dust ◽  
Dust Particles ◽  
Field Campaign ◽  
Dust Transport ◽  
Saharan Air Layer ◽  
Coarse Mode ◽  
Significant Underestimation

Abstract. Mineral dust is an important component of the climate system, interacting with radiation, clouds, and biogeochemical systems and impacting atmospheric circulation, air quality, aviation, and solar energy generation. These impacts are sensitive to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (d) >2.5 µm) and giant (d>20 µm) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (d=0.1 to >100 µm) at different stages during transport with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties using airborne observations over the Sahara (from the Fennec field campaign) and in the Saharan Air Layer (SAL) over the tropical eastern Atlantic (from the AER-D field campaign). Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 µm were observed over the Sahara compared to 4 µm in the SAL. Excluding giant particles over the Sahara results in significant underestimation of mass concentration (40 %), as well as underestimates of both shortwave and longwave extinction (18 % and 26 %, respectively, from scattering calculations), while the effects in the SAL are smaller but non-negligible. The larger impact on longwave extinction compared to shortwave implies a bias towards a radiative cooling effect in dust models, which typically exclude giant particles and underestimate coarse-mode concentrations. A compilation of the new and published effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 d, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 d, PSD barely changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational sedimentation alone. The reasons for this are unclear and warrant further investigation in order to improve dust transport schemes and the associated radiative effects of coarse and giant particles in models.

scholarly journals Discernible rhythm in the spatio/temporal distributions of transatlantic dust

2011 ◽  
Vol 11 (8) ◽  
pp. 23513-23539 ◽  
Author(s):  
Y. Ben-Ami ◽  
I. Koren ◽  
O. Altaratz ◽  
A. B. Kostinski ◽  
Y. Lehahn
Keyword(s):  
North Atlantic Ocean ◽  
Dust Emission ◽  
Low Frequency ◽  
Saharan Dust ◽  
Boreal Winter ◽  
Dust Transport ◽  
Dust Loading ◽  
Spatio Temporal ◽  
Dust Events ◽  
Southern Route

Abstract. The differences in North African dust emission regions and transport routes, between the boreal winter and summer are thoroughly documented. Here we re-examine the spatial and temporal characteristics of dust transport over the tropical and subtropical North Atlantic Ocean, using 10 years of satellite data, in order to determine better the different dust transport periods and their characteristics. We see a robust annual triplet: a discernible rhythm of "transatlantic dust weather". The proposed annual partition is composed of two heavy loading periods, associated here with a northern-route period and southern-route period, and one clean, light-loading period, accompanied by unusually low average optical depth of dust. The two dusty periods are quite different in character: their duration, transport routes, characteristic aerosol loading and frequency of pronounced dust episodes. The southern route period lasts about ~4 months, from the end of November to end of March. It is characterized by a relatively steady southern positioning, low frequency of dust events, low background values and high variance in dust loading. The northern-route period lasts ~6.5 months, from the end of March to mid October, and is associated with a steady drift northward of ~0.1 latitude day−1, reaching ~1500 km north of the southern route. The northern period is characterized by higher frequency of dust events, higher (and variable) background and smaller variance in dust loading. It is less episodic than the southern period. Transitions between the periods are brief. Separation between the southern and northern periods is marked by northward latitudinal shift in dust transport and by moderate reduction in the overall dust loading. The second transition between the northern and southern periods commences with an abrupt reduction in dust loading (thereby initiating the clean period) and rapid shift southward of ~0.2 latitude day−1, and 1300 km in total. These rates of northward advance and southern retreat of the dust transport route are in accordance with the simultaneous shift of the Inter Tropical Front. Based on cross-correlation analyses, we attribute the observed rhythm to the contrast between the northwestern and southern Saharan dust source spatial distributions. Despite the vast difference in areas, the Bodélé Depression, located in Chad, appears to modulate transatlantic dust patterns about half the time. The proposed partition captures the essence of transatlantic dust climatology and may, therefore, supply a natural temporal framework for dust analysis via models and observations.

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