Characterization of smoke/dust episode over West Africa: comparison of MERRA-2 modeling with multiwavelength Mie-Raman lidar observations

Observations of multiwavelength Mie-Raman lidar taken during the SHADOW field campaign are used to analyze a smoke/dust episode over West Africa on 24–27 December 2015. For the case considered, the dust layer extended from the ground up to approximately 2000 m while the elevated smoke layer occurred in the 2500 m–4000 m range. The profiles of lidar measured backscattering, extinction coefficients and depolarization ratios are compared with the vertical distribution of aerosol parameters provided by the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). The MERRA-2 model simulated the correct location of the near–surface dust and elevated smoke layers. The values of modeled and observed extinctions at both 355 nm and 532 nm are also rather close. Good coherence between measured and modeled extinction profiles provides an opportunity to test how well the model reproduces backscattering of dust particles at different wavelengths. The comparison shows good agreement of modeled and measured backscattering coefficients at 355 nm, meaning that the modeled dust lidar ratio of 65 sr in the near-surface layer is close to the observed value. At 532 nm however, the simulated lidar ratio is lower than measurements (about 40 sr and 50 sr respectively). The reason for this disagreement could be that the assumed imaginary part of the refractive index for dust (0.0025 at 532 nm) is too low, or that the particle size distribution in the model is too much weighted toward fine mode dust. The model predicts significant concentration of dust particles inside the smoke layer. This is supported by a high depolarization ratio of 15 % observed in the center of this layer. The backscattering Ångström exponent at 355/532 nm as well as both lidar ratios have a minimum in the center of the elevated layer, which can also be explained by the presence of dust.