Aerosol data assimilation in the chemical-transport model MOCAGE during the TRAQA/ChArMEx campaign: Aerosol optical depth

摘要

Abstract. In this study, we describe the development of the aerosol optical depth (AOD) assimilation module in the chemistry-transport model (CTM) MOCAGE (Modèle de Chimie Atmosphérique à Grande Echelle). Our goal is to assimilate the 2D column AOD data from the National Aeronautics and Space Administration (NASA) Moderate-resolution Imaging Spectroradiometer (MODIS) instrument and to estimate improvements in a 3D CTM assimilation run compared to a direct model run. Our assimilation system uses 3D-FGAT (First Guess at Appropriate Time) as an assimilation method and the total 3D aerosol concentration as a control variable. In order to have an extensive validation data set, we set our experiment in the northern summer of 2012 when the pre-ChArMEx (CHemistry and AeRosol MEditerranean EXperiment) field campaign TRAQA (TRAnsport à longue distance et Qualité de l’Air dans le bassin méditerranéen) took place in the western Mediterranean basin. The assimilated model run is evaluated independently against a range of aerosol properties (2D and 3D) measured by in-situ instruments (the TRAQA size-resolved balloon and aircraft measurements), the satellite Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instrument and ground-based instruments from the Aerosol Robotic Network (AERONET) network. The evaluation demonstrates that the AOD assimilation greatly improves aerosol representation in the model. For example, the comparison of the direct and the assimilated model run with AERONET data shows that the assimilation reduced the bias in the AOD (from 0.050 to 0.006) and increased the correlation (from 0.74 to 0.88). When compared to the 3D concentration data obtained by the in-situ aircraft and balloon measurements, the assimilation consistently improves the model output. The best results as expected occur when the shape of the vertical profile is correctly simulated by the direct model. We also examine how the assimilation can influence the modelled aerosol vertical distribution. The results show that a 2D continuous AOD assimilation can improve the 3D vertical profile, as a result of differential horizontal transport of aerosols in the model.