Understanding the seasonality and climatology of aerosols in Africa through evaluation of CCAM aerosol simulations against AERONET measurements

摘要

Abstract. The sensitivity of climate models to the characterization of African aerosol particles is poorly understood. Africa is a major source of dust and biomass burning aerosols and so this represents an important research gap in understanding the impact of aerosols on radiative forcing of the climate system. Here we evaluate the current representation of aerosol particles in the Conformal Cubic Atmospheric Model (CCAM) with ground-based observations across Africa, and additionally provide an analysis of aerosol optical depth at 550 nm (AOD550nm) and Ångström exponent data from thirty-four Aerosol Robotic Network (AERONET) sites. Analysis of the 34 long-term AERONET sites confirms the importance of dust and biomass burning emissions to the seasonal cycle and magnitude of AOD550nm across the continent and the transport of these emissions to regions outside of the continent. Western African sites had the largest AOD550nm values, on average, with the timing and magnitude of AOD550nm maxima dominated by desert dust. The impact of dust on aerosol loading is also apparent at northern African sites, with peak AOD550nm occurring later than the western sites. The seasonal variation in the location of the intertropical convergence zone and associated northward shift in dust transport may be responsible for the shift in timing of maximum AOD550nm between the western and northern African sites. Southern African sites have the lowest AOD550nm values on average, and peak during the biomass burning period. The outflow of these aerosol particles was observed at Ascension Island and Reunion Island AERONET stations. In general, CCAM captures well the seasonality of the AERONET data across the continent. The magnitude of modeled and observed multi-year monthly average AOD550nm overlap within ±1 standard deviation of each other for at least 7 months at all sites except Reunion Island. The timing of peak AOD550nm in southern Africa in the model occurs one month prior to the observed peak, which does not align with the timing of maximum fire counts in the region. For the western and northern African sites, it is evident that CCAM currently overestimates dust in some regions while others (e.g., the Arabian Peninsula) are better characterized. This may be due to overestimated dust lifetime, or that the characterization of the soil for these areas needs to be updated with local information. The CCAM simulated AOD550nm for the global domain is within the spread of previously published results from CMIP5 and AeroCom experiments for black carbon, organic carbon and sulfate aerosols. The model’s performance provides confidence for using the model to estimate large-scale regional impacts of African aerosols on radiative forcing, but local feedbacks between dust aerosols and climate over northern Africa and the Mediterranean may be overestimated.