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Volume 16, issue 13
Atmos. Chem. Phys., 16, 8609-8642, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

Atmos. Chem. Phys., 16, 8609-8642, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 15 Jul 2016

Research article | 15 Jul 2016

Mediterranean intense desert dust outbreaks and their vertical structure based on remote sensing data

Antonis Gkikas1, Sara Basart1, Nikos Hatzianastassiou2, Eleni Marinou3,9, Vassilis Amiridis3, Stelios Kazadzis4,5, Jorge Pey6, Xavier Querol7, Oriol Jorba1, Santiago Gassó8, and José Maria Baldasano1,8 Antonis Gkikas et al.
  • 1Earth Sciences Department, Barcelona Supercomputing Center, Barcelona, Spain
  • 2Laboratory of Meteorology, Department of Physics, University of Ioannina, Ioannina, Greece
  • 3Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Athens, 15236, Greece
  • 4Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Switzerland
  • 5Institute of Environmental Research and Sustainable Development, National Observatory of Athens, Athens, Greece
  • 6Geological Survey of Spain (IGME), Zaragoza, Spain
  • 7Institute of Environmental Assessment and Water Research, IDÆA-CSIC C/Jordi Girona, 18–26, 08034 Barcelona, Spain
  • 8Environmental Modelling Laboratory, Technical University of Catalonia, Barcelona, Spain
  • 9Laboratory of Atmospheric Physics, Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece

Abstract. The main aim of the present study is to describe the vertical structure of the intense Mediterranean dust outbreaks, based on the use of satellite and surface-based retrievals/measurements. Strong and extreme desert dust (DD) episodes are identified at 1° × 1° spatial resolution, over the period March 2000–February 2013, through the implementation of an updated objective and dynamic algorithm. According to the algorithm, strong DD episodes occurring at a specific place correspond to cases in which the daily aerosol optical depth at 550nm (AOD550nm) exceeds or equals the long-term mean AOD550nm (Mean) plus two standard deviations (SD), which is also smaller than Mean+4 × SD. Extreme DD episodes correspond to cases in which the daily AOD550nm value equals or exceeds Mean+4 × SD. For the identification of DD episodes, additional optical properties (Ångström exponent, fine fraction, effective radius and aerosol index) derived by the MODIS-Terra & Aqua (also AOD retrievals), OMI-Aura and EP-TOMS databases are used as inputs. According to the algorithm using MODIS-Terra data, over the period March 2000–February 2013, strong DD episodes occur more frequently (up to 9.9 episodesyear−1) over the western Mediterranean, while the corresponding frequencies for the extreme ones are smaller (up to 3.3 episodesyear−1, central Mediterranean Sea). In contrast to their frequency, dust episodes are more intense (AODs up to 4.1), over the central and eastern Mediterranean Sea, off the northern African coasts. Slightly lower frequencies and higher intensities are found when the satellite algorithm operates based on MODIS-Aqua retrievals, for the period 2003–2012. The consistency of the algorithm is successfully tested through the application of an alternative methodology for the determination of DD episodes, which produced similar features of the episodes' frequency and intensity, with just slightly higher frequencies and lower intensities. The performance of the satellite algorithm is assessed against surface-based daily data from 109 sun-photometric (AERONET) and 22 PM10 stations. The agreement between AERONET and MODIS AOD is satisfactory (R = 0.505 − 0.750) and improves considerably when MODIS level 3 retrievals with higher sub-grid spatial representativeness and homogeneity are considered. Through the comparison against PM10 concentrations, it is found that the presence of dust is justified in all ground stations with success scores ranging from 68 to 97%. However, poor agreement is evident between satellite and ground PM10 observations in the western parts of the Mediterranean, which is attributed to the desert dust outbreaks' vertical extension and the high altitude of dust presence. The CALIOP vertical profiles of pure and polluted dust observations and the associated total backscatter coefficient at 532nm (β532nm), indicate that dust particles are mainly detected between 0.5 and 6km, though they can reach 8km between the parallels 32 and 38°N in warm seasons. An increased number of CALIOP dust records at higher altitudes is observed with increased latitude, northwards to 40°N, revealing an ascending mode of the dust transport. However, the overall intensity of DD episodes is maximum (up to 0.006km−1sr−1) below 2km and at the southern parts of the study region (30–34°N). Additionally, the average thickness of dust layers gradually decreases from 4 to 2km, moving from south to north. In spring, dust layers of moderate-to-high β532nm values ( ∼ 0.004km−1sr−1) are detected over the Mediterranean (35–42°N), extending from 2 to 4km. Over the western Mediterranean, dust layers are observed between 2 and 6km, while their base height is decreased down to 0.5km for increasing longitudes underlying the role of topography and thermal convection. The vertical profiles of CALIOP β532nm confirm the multilayered structure of the Mediterranean desert dust outbreaks on both annual and seasonal bases, with several dust layers of variable geometrical characteristics and intensities. A detailed analysis of the vertical structure of specific DD episodes using CALIOP profiles reveals that the consideration of the dust vertical structure is necessary when attempting comparisons between columnar MODIS AOD retrievals and ground PM10 concentrations.

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Short summary
This study presents the 3-D structures of intense Mediterranean desert dust outbreaks, over the period Mar 2000–Feb 2013. The desert dust (DD) episodes are identified through an objective and dynamic algorithm, which utilizes satellite retrievals (MODIS, TOMS and OMI) as inputs. The performance of the satellite algorithm is evaluated vs. AERONET and PM10 data. The geometrical characteristics of the identified DD episodes are analyzed using the collocated CALIOP profiles as a complementary tool.
This study presents the 3-D structures of intense Mediterranean desert dust outbreaks, over the...