Characterisation of dust aerosols in the infrared from IASI and comparison with PARASOL, MODIS, MISR, CALIOP, and AERONET observations

Abstract

Infrared Atmospheric Sounder Interferometer (IASI) observations covering the period from July 2007 to December 2011 are interpreted in terms of monthly mean, 1°×1°, 10 μm dust Aerosol Optical Depth (AOD), mean altitude and coarse mode effective radius. The geographical study area includes the northern tropical Atlantic and the northwest Arabian Sea, both characterised by strong, regular dust events. The method developed relies on the construction of Look-Up-Tables computed for a large selection of atmospheric situations and observing conditions. At a regional scale, a good agreement is found between IASI-retrieved 10 μm AOD and total visible optical depth at 550 nm from either the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua or Terra), or the Multi-angle Imaging SpectroRadiometer (MISR), or the Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar (PARASOL). Taking into account the ratio existing between infrared and visible AODs, the diversity between the different 550 nm AODs is similar to the difference between these and the IASI AODs. The infrared AOD to visible AOD ratio, partly reflecting the varying distribution of the dust layer between the dust coarse mode particles seen by IASI, and the fine mode seen by the other instruments, is found to vary with the region observed with values close to already published values. Comparisons between the climatologies of the 10 μm IASI AOD and of the PARASOL non-spherical coarse mode AOD at 865 nm, both expected to be representative of the dust coarse mode, lead to conclusions differing according to the region considered. These differences are discussed in the light of the MODIS Angström exponent (865–550 nm). At local scale, around six Aerosol Robotic Network (AERONET) sites, close or far from the dust sources, a similar satisfactory agreement is found between IASI and the visible AODs and the differences between these products are shown and analysed. IASI-retrieved dust layer mean altitudes also compare well with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP/CALIPSO) aerosol mean layer altitude, both in terms of climatology and of zonal evolution throughout the Atlantic. Comparisons between the IASI-retrieved dust coarse mode effective radius and retrievals from AERONET at the six sites brings into evidence an almost systematic bias of about +0.35 μm (IASI-AERONET). Removing this bias leads to a satisfactory agreement between the climatologies of these two products. Overall, these results illustrate the dust westward transport characterised by a fast decrease of the dust optical depth, a somewhat slower decrease of the altitude, and an effective radius remaining almost constant during summer throughout the northern tropical Atlantic. They also demonstrate the capability of high resolution infrared sounders to contribute improving our understanding of processes related to the aerosols (transport, sources, cycles, effect of aerosols on the terrestrial radiation, etc.)

Comparison of dust-layer heights from active and passive satellite sensors

“…Peyridieu et al (2013) reported similar values for the same data set. In this study for LMD over ocean, a bias of −0.922 km (−0.501 km) against the CALIOP geometric mean (cumulative extinction) height is found for data recorded during the day overpasses with a standard deviation of 1.142 km (1.409 km).…”

“…For the night overpasses the differences are −0.674 km (0.352 km) and the standard deviation 0.878 (1.180) ( Table 3). One reason for the larger spread in this study may be the use of monthly and spatially averaged data by Peyridieu et al (2013) and Capelle et al (2014), while here the comparison is made on a pixelby-pixel basis. Hence, extreme values are not averaged out.…”

“…The various dust height retrieval algorithms used in this study are summarized in Table 1 and described in more detail below. Peyridieu et al (2010Peyridieu et al ( , 2013, from Volz (1972Volz ( , 1973 layer height Capelle et al (2014) The Mineral Aerosol Profiling from Thermal Infrared (MAPIR) retrieval algorithm is an extensive technical and scientific improvement of the algorithm first published by Vandenbussche et al (2013). Version 3.5 of the algorithm, fully described in Vandenbussche and De Mazière (2017), is used in this study.…”

“…The LMD (Laboratoire de Météorologie Dynamique) method for the retrieval of dust characteristics from IASI observations was originally developed for application to the Atmospheric Infrared Sounder (AIRS) (Pierangelo et al, 2004(Pierangelo et al, , 2005 and then slightly modified as described in detail in Peyridieu et al (2010Peyridieu et al ( , 2013 and in Capelle et al (2014) for application to IASI. The method used to derive dust characteristics from IASI observations is a three-step physical algorithm based on a look-up table (LUT) approach.…”

“…Earlier studies (Capelle et al, 2014;Peyridieu et al, 2010Peyridieu et al, , 2013 compared monthly averaged and gridded data, or investigated a specific episode (Vandenbussche et al, 2013;Cuesta et al, 2015). We perform a point-by-point comparison for selected episodes.…”