Articles | Volume 18, issue 6
Atmos. Chem. Phys., 18, 4377–4401, 2018

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

Atmos. Chem. Phys., 18, 4377–4401, 2018

Research article 29 Mar 2018

Research article | 29 Mar 2018

Determining the infrared radiative effects of Saharan dust: a radiative transfer modelling study based on vertically resolved measurements at Lampedusa

Daniela Meloni1, Alcide di Sarra1, Gérard Brogniez2, Cyrielle Denjean3,4, Lorenzo De Silvestri1, Tatiana Di Iorio1, Paola Formenti3, José L. Gómez-Amo5, Julian Gröbner6, Natalia Kouremeti6, Giuliano Liuzzi7,8, Marc Mallet9, Giandomenico Pace1, and Damiano M. Sferlazzo10 Daniela Meloni et al.
  • 1Laboratory for Observations and Analyses of Earth and Climate, ENEA, Rome, Via Anguillarese 301, 00123, Italy
  • 2Laboratoire d'Optique Atmosphérique, University of Lille 1, Villeneuve d'Ascq, France
  • 3Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR-CNRS 7583, Université Paris-Est-Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, France
  • 4now at Centre National de Recherches Météorologiques, UMR 3589, CNRS, Météo-France, Toulouse, France
  • 5Departament de Física de la Terra y Termodinàmica, Universitat de València, Valencia, Spain
  • 6Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center, Davos Dorf, Dorfstrasse 33,7260, Switzerland
  • 7Solar System Exploration Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
  • 8Department of Physics, American University, 4400 Massachusetts Avenue NW, Washington, DC 20016, USA
  • 9CNRM UMR 3589, Météo-France/CNRS, Toulouse, France
  • 10Laboratory for Observations and Analyses of Earth and Climate, ENEA, Lampedusa, Contrada Capo Grecale, 92010, Italy

Abstract. Detailed measurements of radiation, atmospheric and aerosol properties were carried out in summer 2013 during the Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region (ADRIMED) campaign in the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) experiment. This study focusses on the characterization of infrared (IR) optical properties and direct radiative effects of mineral dust, based on three vertical profiles of atmospheric and aerosol properties and IR broadband and narrowband radiation from airborne measurements, made in conjunction with radiosonde and ground-based observations at Lampedusa, in the central Mediterranean. Satellite IR spectra from the Infrared Atmospheric Sounder Interferometer (IASI) are also included in the analysis. The atmospheric and aerosol properties are used as input to a radiative transfer model, and various IR radiation parameters (upward and downward irradiance, nadir and zenith brightness temperature at different altitudes) are calculated and compared with observations. The model calculations are made for different sets of dust particle size distribution (PSD) and refractive index (RI), derived from observations and from the literature. The main results of the analysis are that the IR dust radiative forcing is non-negligible and strongly depends on PSD and RI. When calculations are made using the in situ measured size distribution, it is possible to identify the refractive index that produces the best match with observed IR irradiances and brightness temperatures (BTs). The most appropriate refractive indices correspond to those determined from independent measurements of mineral dust aerosols from the source regions (Tunisia, Algeria, Morocco) of dust transported over Lampedusa, suggesting that differences in the source properties should be taken into account. With the in situ size distribution and the most appropriate refractive index the estimated dust IR radiative forcing efficiency is +23.7 W m−2 at the surface, −7.9 W m−2 within the atmosphere, and +15.8 W m−2 at the top of the atmosphere. The use of column-integrated dust PSD from AERONET may also produce a good agreement with measured irradiances and BTs, but with significantly different values of the RI. This implies large differences, up to a factor of 2.5 at surface, in the estimated dust radiative forcing, and in the IR heating rate. This study shows that spectrally resolved measurements of BTs are important to better constrain the dust IR optical properties, and to obtain a reliable estimate of its radiative effects. Efforts should be directed at obtaining an improved description of the dust size distribution and its vertical distribution, as well as at including regionally resolved optical properties.

Short summary
This study examines how different aerosol optical properties determine the dust longwave radiative effects at the surface, in the atmosphere and at the top of the atmosphere, based on the combination of remote sensing and in situ observations from the ground, from airborne sensors, and from space, by means of radiative transfer modelling. The closure experiment is based on longwave irradiances and spectral brightness temperatures measured during the 2013 ChArMEx–ADRIMED campaign at Lampedusa.
Final-revised paper