A 4-D climatology (1979–2009) of the monthly tropospheric aerosol optical depth distribution over the Mediterranean region from a comparative evaluation and blending of remote sensing and model products

摘要

Since the 1980s several spaceborne sensors have been used to retrieve theaerosol optical depth (AOD) over the Mediterranean region. In parallel, AODclimatologies coming from different numerical model simulations are now alsoavailable, permitting to distinguish the contribution of several aerosoltypes to the total AOD. In this work, we perform a comparative analysis ofthis unique multi-year database in terms of total AOD and of itsapportionment by the five main aerosol types (soil dust, sea-salt, sulfate,black and organic carbon). We use 9 different satellite-derived monthly AODproducts: NOAA/AVHRR, SeaWiFS (2 products), TERRA/MISR, TERRA/MODIS,AQUA/MODIS, ENVISAT/MERIS, PARASOL/POLDER and MSG/SEVIRI, as well as 3 morehistorical datasets: NIMBUS7/CZCS, TOMS (onboard NIMBUS7 and Earth-Probe)and METEOSAT/MVIRI. Monthly model datasets include the aerosol climatologyfrom Tegen et al. (1997), the climate-chemistry models LMDz-OR-INCA and RegCM-4,the multi-model mean coming from the ACCMIP exercise, and the reanalyses GEMSand MACC. Ground-based Level-2 AERONET AOD observations from 47 stationsaround the basin are used here to evaluate the model and satellite data. Thesensor MODIS (on AQUA and TERRA) has the best average AOD scores over thisregion, showing a relevant spatio-temporal variability and highlighting highdust loads over Northern Africa and the sea (spring and summer), and sulfateaerosols over continental Europe (summer). The comparison also showslimitations of certain datasets (especially MERIS and SeaWiFS standardproducts). Models reproduce the main patterns of the AOD variability over thebasin. The MACC reanalysis is the closest to AERONET data, but appears tounderestimate dust over Northern Africa, where RegCM-4 is found closer toMODIS thanks to its interactive scheme for dust emissions. The verticaldimension is also investigated using the CALIOP instrument. This studyconfirms differences of vertical distribution between dust aerosols showing alarge vertical spread, and other continental and marine aerosols which areconfined in the boundary layer. From this compilation, we propose a 4-Dblended product from model and satellite data, consisting in monthly timeseries of 3-D aerosol distribution at a 50 km horizontal resolution over theEuro-Mediterranean marine and continental region for the 2003–2009 period.The product is based on the total AOD from AQUA/MODIS, apportioned intosulfates, black and organic carbon from the MACC reanalysis, and into dustand sea-salt aerosols from RegCM-4 simulations, which are distributedvertically based on CALIOP climatology. We extend the 2003–2009reconstruction to the past up to 1979 using the 2003–2009 average andapplying the decreasing trend in sulfate aerosols from LMDz-OR-INCA, whoseAOD trends over Europe and the Mediterranean are median among the ACCMIPmodels. Finally optical properties of the different aerosol types in thisregion are proposed from Mie calculations so that this reconstruction can beincluded in regional climate models for aerosol radiative forcing andaerosol-climate studies.