Daily dust aerosol properties (AOD and altitude) from infrared sounders. Application to IASI from July 2007 to day-1.
During the past decades, determination of atmospheric aerosol characteristics from space has been extensively done using instruments measuring in the visible part of the spectrum. This has greatly contributed enhancing knowledge of the aerosol impact on the Earth radiation balance (direct effect) as well as on the clouds (albedo, lifetime) (indirect effect). However, these processes are complex as they involve the aerosol distribution (spatial, in particular vertical, and temporal), and their microphysical and optical properties (size, shape, composition, etc.). Moreover, the accuracy obtained on the atmospheric radiative effect also depends on surface characteristics (albedo, temperature). This complexity still leads to large uncertainties in the estimation of aerosols impact on climate ( IPCC, 2013). In this context, satellite observations may offer a global and continuous observation at high resolution (spatial, spectral, or both).
After a long period of relative lack of interest in aerosol remote sensing in the infrared, a marked growing interest in the infrared is now observed with the emergence of hyperspectral instruments as AIRS and IASI (Pierangelo et al., 2004, 2005; De Souza-Machado et al., 2006; Peyridieu et al., 2010, 2013; Capelle et al., 2014; Klüser et al., 2011, 2012).
This domain offers some unique opportunities such as nighttime aerosol observation, the determination of the aerosol layer mean altitude, or the aerosol characterization over deserts. Mineral dust is a major contributor to total aerosol loading and has been the subject of an increasing number of studies due, in particular, to its potentially large contribution to atmospheric radiative forcing. Visible wavelengths are sensitive to both fine and coarse mode particles when infrared wavelengths are essentially sensitive to the coarse mode. Associating these two spectral domains should help improving our knowledge of the impact of aerosols on climate, its variability and evolution.
Dust transport over the Atlantic during June 2013 :
Example of dust transported from Africa to Caribbean from 10 to 16th June 2013 (observed during the SALTRACE campaign, an initiative to investigate the long-range transport of Saharan mineral dust across the Atlantic Ocean into the Caribbean. ):
The September 22th-24th 2009 Australian dust storm seen by IASI:
also known as the Eastern Australian dust storm, it swept across the Australian states of New South Wales and Queensland from 22 to 24 September 2009
IASI 10µm AOD restitution for the 22th September 2009-23:45UTC. The black window corresponds to the MODIS region.
Credit : http://en.wikipedia.org/wiki/2009_Australian_dust_storm ; MODIS NASA-Terra 250m. 23th September 2009-00:05UTC.
The method developed at LMD to derive dust characteristics from IASI (or AIRS) observations is a three-step algorithm based on a “Look-Up-Table” (LUT) approach (Pierangelo et al., 2004, 2005; Peyridieu et al., 2010, 2013, Capelle et al., 2014). The first step determines the atmospheric state observed; the second step determines simultaneously the 10 µm AOD and the aerosol layer mean altitude while the dust coarse mode effective radius is determined in the third step. Look-Up-Tables of simulated brightness temperatures are calculated using the forward coupled radiative transfer model 4A/OP-DISORT (Scott and Chédin, 1981; Stamnes et al., 1988; http://4aop.noveltis.com) for a large selection of atmospheric situations, of observing conditions, of surface characteristics (in particular the surface emissivity and temperature from Capelle et al., 2012), and different aerosol refractive index models.
The method is applied over sea and over land, at daily scale, daytime and nighttime, and at the satellite pixel resolution (12 km at nadir), for the latitude band 60S-60N. Both IASI overpasses are processed, providing two measurements at 9:30AM and 9:30PM (equator local time) each day, opening the way to the analysis of the aerosol properties diurnal cycle.
Validating remotely sensed infrared aerosol properties requires using well recognized, accurate and independent, measurements of these variables, as well as understanding possible differences brought by such comparison.
For the AOD, comparisons are made with AERONET ground-based data, when available, in order to evaluate the results, and show the importance of a better knowledge of the aerosol diurnal cycle, especially close to the sources. It is worth pointing out that, for the AOD, such an evaluation raises the problem of the difference between the two spectral domains used: infrared for IASI and visible for AERONET. Consequently, the two measurements do not share the same metrics: translating visible coarse mode AOD into infrared AOD requires accurate knowledge of variables such as the effective radius of the particles, their size distribution, and the infrared refractive index.
IASI time series of the daily AOD at 10 µm and of the AERONET AOD for several AERONET sites over Sahara, Atlantic or Mediterranean and a few months. Here, the AERONET AOD is given at 10 µm using the theoretical infrared/visible ratio). A good correlation between the two sources of data is observed.
For the mean aerosol layer altitude obtained from IASI, comparisons are made, at local scale, with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP/CALIPSO) aerosol altitude. This has first been done at the monthly scale and results were reported in Capelle et al., 2014). This is now done at daily scale, and at the IASI spot resolution.
Comparisons between IASI and CALIOP mean dust layer altitude (June 18, 2011). Top: Profiles of attenuated backscatter coefficient measured by CALIOP; In black : IASI mean altitude and standard deviation. Bottom left: IASI AOD, bottom right: IASI altitude. Note the difference in time (9:30 PM for IASI and 1:44 AM for CALIOP) and the (great) difference in space resolution.
Gridded daily or monthly 10µm Aerosol Optical Depth and mean aerosol layer altitude retrieved from IASI (Metop-A) data from July 2007 to now (today-2) can be downloaded here after registration. Database is in netcdf format and covers the region 180W-180E in longitude and about 60S-60N in latitude with a resolution of 0.5°x0.5°, including ocean and land. Day-time and night-time daily results are stored in separate files. Monthly average corresponds to the average of the whole day and night daily data. Detailed description of the method will come soon. Results can be visualized in the section Data visualization. Note that the "spot by spot " resolution (around 12km per pixel) database may be obtained upon request.
Capelle V., Chédin A., Siméon M., Tsamalis C., Pierangelo C., Pondrom M., Armante R., Crevoisier C., Crépeau L. and Scott. N.A., Evaluation of IASI derived dust aerosols characteristics over the tropical belt. Atmos. Chem. Phys., 14, 9343-9362 doi:10.5194/acp-14-9343-2014 (2014).
Peyridieu S., Chédin A., Capelle V., Tsamalis C., Pierangelo C., Armante R., Crevoisier C., Crépeau L., Siméon M., Ducos F., Scott N.A. Characterization of dust aerosols in the infrared from IASI and comparison with PARASOL, MODIS, MISR, CALIOP, and AERONET observations. Atmos. Chem. Phys., 13, 6065-6082 doi:10.5194/acp-13-6065-2013 (2013).
Capelle V., Chédin A., Péquignot E., Schluessel P., Newman S.M. and Scott N.A. Infrared continental surface emissivity spectra and skin temperature retrieved from IASI observations over the tropics. J. Appl. Meteor. Climatol., 51, 1164–1179. http://dx.doi.org/10.1175/JAMC-D-11-0145.1 (2012)
Peyridieu S., Chédin A., Tanré D., Capelle V., Pierangelo C., Lamquin N. and Armante R. Saharan dust infrared optical depth and altitude retrieved from AIRS : a focus over North Atlantic - Comparison to MODIS and CALIPSO. Atmos. Chem. Phys. 10, 1953-1967 (2010).
Pierangelo C., Chédin A., Heilliette S., Jacquinet-Husson N., Armante R. Dust altitude and infrared optical depth from AIRS Atmospheric Chemistry and Physics , 2004-4-1823 (2004).