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- ItemSatellite retrievals of dust aerosol over the Red Sea and the Persian Gulf (2005-2015)(Katlenburg-Lindau : EGU, 2017) Banks, Jamie R.; Brindley, Helen E.; Stenchikov, Georgiy; Schepanski, KerstinThe inter-annual variability of the dust aerosol presence over the Red Sea and the Persian Gulf is analysed over the period 2005-2015. Particular attention is paid to the variation in loading across the Red Sea, which has previously been shown to have a strong, seasonally dependent latitudinal gradient. Over the 11 years considered, the July mean 630 nm aerosol optical depth (AOD) derived from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) varies between 0.48 and 1.45 in the southern half of the Red Sea. In the north, the equivalent variation is between 0.22 and 0.66. The temporal and spatial pattern of variability captured by SEVIRI is also seen in AOD retrievals from the MODerate Imaging Spectroradiometer (MODIS), but there is a systematic offset between the two records. Comparisons of both sets of retrievals with ship-and land-based AERONET measurements show a high degree of correlation with biases of <0.08. However, these comparisons typically only sample relatively low aerosol loadings. When both records are stratified by AOD retrievals from the Multi-angle Imaging SpectroRadiometer (MISR), opposing behaviour is revealed at high MISR AODs (>1), with offsets of C0.19 for MODIS and 0.06 for SEVIRI. Similar behaviour is also seen over the Persian Gulf. Analysis of the scattering angles at which retrievals from the SEVIRI and MODIS measurements are typically performed in these regions suggests that assumptions concerning particle sphericity may be responsible for the differences seen.
- ItemProfiling of Saharan dust from the Caribbean to western Africa-Part 1: Layering structures and optical properties from shipborne polarization/Raman lidar observations(Katlenburg-Lindau : EGU, 2017) Rittmeister, Franziska; Ansmann, Albert; Engelmann, Ronny; Skupin, Annett; Baars, Holger; Kanitz, Thomas; Kinne, StefanWe present final and quality-assured results of multiwavelength polarization/Raman lidar observations of the Saharan air layer (SAL) over the tropical Atlantic. Observations were performed aboard the German research vessel R/V Meteor during the 1-month transatlantic cruise from Guadeloupe to Cabo Verde over 4500 km from 61.5 to 20 W at 14-15 N in April-May 2013. First results of the shipborne lidar measurements, conducted in the framework of SALTRACE (Saharan Aerosol Long-range Transport and Aerosol-Cloud Interaction Experiment), were reported by Kanitz et al. (2014). Here, we present four observational cases representing key stages of the SAL evolution between Africa and the Caribbean in detail in terms of layering structures and optical properties of the mixture of predominantly dust and aged smoke in the SAL. We discuss to what extent the lidar results confirm the validity of the SAL conceptual model which describes the dust long-range transport and removal processes over the tropical Atlantic. Our observations of a clean marine aerosol layer (MAL, layer from the surface to the SAL base) confirm the conceptual model and suggest that the removal of dust from the MAL, below the SAL, is very efficient. However, the removal of dust from the SAL assumed in the conceptual model to be caused by gravitational settling in combination with large-scale subsidence is weaker than expected. To explain the observed homogenous (heightindependent) dust optical properties from the SAL base to the SAL top, from the African coast to the Caribbean, we have to assume that the particle sedimentation strength is reduced and dust vertical mixing and upward transport mechanisms must be active in the SAL. Based on lidar observations on 20 nights at different longitudes in May 2013, we found, on average, MAL and SAL layer mean values (at 532 nm) of the extinction-to-backscatter ratio (lidar ratio) of 17-5 sr (MAL) and 43±8 sr (SAL), of the particle linear depolarization ratio of 0:025±0:015 (MAL) and 0:19±0:09 (SAL), and of the particle extinction coefficient of 67±45Mm..1 (MAL) and 68±37Mm..1 (SAL). The 532 nm optical depth of the lofted SAL was found to be, on average, 0:15±0:13 during the ship cruise. The comparably low values of the SAL mean lidar ratio and depolarization ratio (compared to typical pure dust values of 50-60 sr and 0.3, respectively) in combination with backward trajectories indicate a smoke contribution to light extinction of the order of 20% during May 2013, at the end of the burning season in central-western Africa. 1.
- ItemThree-dimensional evolution of Saharan dust transport towards Europe based on a 9-year EARLINET-optimized CALIPSO dataset(Katlenburg-Lindau : EGU, 2017) Marinou, Eleni; Amiridis, Vassilis; Binietoglou, Ioannis; Tsikerdekis, Athanasios; Solomos, Stavros; Proestakis, Emannouil; Konsta, Dimitra; Papagiannopoulos, Nikolaos; Tsekeri, Alexandra; Vlastou, Georgia; Zanis, Prodromos; Balis, Dimitrios; Wandinger, Ulla; Ansmann, AlbertIn this study we use a new dust product developed using CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) observations and EARLINET (European Aerosol Research Lidar Network) measurements and methods to provide a 3-D multiyear analysis on the evolution of Saharan dust over North Africa and Europe. The product uses a CALIPSO L2 backscatter product corrected with a depolarization-based method to separate pure dust in external aerosol mixtures and a Saharan dust lidar ratio (LR) based on long-term EARLINET measurements to calculate the dust extinction profiles. The methodology is applied on a 9-year CALIPSO dataset (2007-2015) and the results are analyzed here to reveal for the first time the 3-D dust evolution and the seasonal patterns of dust over its transportation paths from the Sahara towards the Mediterranean and Continental Europe. During spring, the spatial distribution of dust shows a uniform pattern over the Sahara desert. The dust transport over the Mediterranean Sea results in mean dust optical depth (DOD) values up to 0.1. During summer, the dust activity is mostly shifted to the western part of the desert where mean DOD near the source is up to 0.6. Elevated dust plumes with mean extinction values between 10 and 75 Mm-1 are observed throughout the year at various heights between 2 and 6 km, extending up to latitudes of 40° N. Dust advection is identified even at latitudes of about 60° N, but this is due to rare events of episodic nature. Dust plumes of high DOD are also observed above the Balkans during the winter period and above northwest Europe during autumn at heights between 2 and 4 km, reaching mean extinction values up to 50 Mm-1. The dataset is considered unique with respect to its potential applications, including the evaluation of dust transport models and the estimation of cloud condensation nuclei (CCN) and ice nuclei (IN) concentration profiles. Finally, the product can be used to study dust dynamics during transportation, since it is capable of revealing even fine dynamical features such as the particle uplifting and deposition on European mountainous ridges such as the Alps and Carpathian Mountains.
- ItemSun photometer retrievals of Saharan dust properties over Barbados during SALTRACE(Katlenburg-Lindau : EGU, 2019) Toledano, Carlos; Torres, Benjamín; Velasco-Merino, Cristian; Althausen, Dietrich; Groß, Silke; Wiegner, Matthias; Weinzierl, Bernadett; Gasteiger, Josef; Ansmann, Albert; González, Ramiro; Mateos, David; Farrel, David; Müller, Thomas; Haarig, Moritz; Cachorro, Victoria E.The Saharan Aerosol Long-Range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE) was devoted to the investigation of Saharan dust properties over the Caribbean. The campaign took place in June-July 2013. A wide set of ground-based and airborne aerosol instrumentation was deployed at the island of Barbados for a comprehensive experiment. Several sun photometers performed measurements during this campaign: two AERONET (Aerosol Robotic Network) Cimel sun photometers and the Sun and Sky Automatic Radiometer (SSARA). The sun photometers were co-located with the ground-based multi-wavelength lidars BERTHA (Backscatter Extinction lidar Ratio Temperature Humidity profiling Apparatus) and POLIS (Portable Lidar System). Aerosol properties derived from direct sun and sky radiance observations are analyzed, and a comparison with the co-located lidar and in situ data is provided. The time series of aerosol optical depth (AOD) allows identifying successive dust events with short periods in between in which the marine background conditions were observed. The moderate aerosol optical depth in the range of 0.3 to 0.6 was found during the dust periods. The sun photometer infrared channel at the 1640nm wavelength was used in the retrieval to investigate possible improvements to aerosol size retrievals, and it was expected to have a larger sensitivity to coarse particles. The comparison between column (aerosol optical depth) and surface (dust concentration) data demonstrates the connection between the Saharan Air Layer and the boundary layer in the Caribbean region, as is shown by the synchronized detection of the successive dust events in both datasets. However the differences of size distributions derived from sun photometer data and in situ observations reveal the difficulties in carrying out a column closure study. © 2019 All rights reserved.
- ItemFennec dust forecast intercomparison over the Sahara in June 2011(Katlenburg-Lindau : EGU, 2016) Chaboureau, Jean-Pierre; Flamant, Cyrille; Dauhut, Thibaut; Kocha, Cécile; Lafore, Jean-Philippe; Lavaysse, Chistophe; Marnas, Fabien; Mokhtari, Mohamed; Pelon, Jacques; Reinares Martínez, Irene; Schepanski, Kerstin; Tulet, PierreIn the framework of the Fennec international programme, a field campaign was conducted in June 2011 over the western Sahara. It led to the first observational data set ever obtained that documents the dynamics, thermodynamics and composition of the Saharan atmospheric boundary layer (SABL) under the influence of the heat low. In support to the aircraft operation, four dust forecasts were run daily at low and high resolutions with convection-parameterizing and convection-permitting models, respectively. The unique airborne and ground-based data sets allowed the first ever intercomparison of dust forecasts over the western Sahara. At monthly scale, large aerosol optical depths (AODs) were forecast over the Sahara, a feature observed by satellite retrievals but with different magnitudes. The AOD intensity was correctly predicted by the high-resolution models, while it was underestimated by the low-resolution models. This was partly because of the generation of strong near-surface wind associated with thunderstorm-related density currents that could only be reproduced by models representing convection explicitly. Such models yield emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and the Aïr Mountains in the high-resolution forecasts. Over the western Sahara, where the harmattan contributes up to 80 % of dust emission, all the models were successful in forecasting the deep well-mixed SABL. Some of them, however, missed the large near-surface dust concentration generated by density currents and low-level winds. This feature, observed repeatedly by the airborne lidar, was partly forecast by one high-resolution model only.