Articles | Volume 17, issue 22
Atmos. Chem. Phys., 17, 13999–14023, 2017
Atmos. Chem. Phys., 17, 13999–14023, 2017

Research article 24 Nov 2017

Research article | 24 Nov 2017

Evaluation of climate model aerosol seasonal and spatial variability over Africa using AERONET

Hannah M. Horowitz1, Rebecca M. Garland2,3, Marcus Thatcher4, Willem A. Landman2,5, Zane Dedekind2, Jacobus van der Merwe2, and Francois A. Engelbrecht2,6 Hannah M. Horowitz et al.
  • 1Department of Earth & Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
  • 2Natural Resources and the Environment Unit, Council for Scientific and Industrial Research, Pretoria 0001, South Africa
  • 3Climatology Research Group, North West University, Potchefstroom 2520, South Africa
  • 4Marine and Atmospheric Research, Commonwealth Scientific and Industrial Research Organisation, Melbourne 3195, Australia
  • 5Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Hatfield 0028, South Africa
  • 6School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg 2000, South Africa

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 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 remote retrievals across Africa, and additionally provide an analysis of observed aerosol optical depth at 550 nm (AOD550 nm) and Ångström exponent data from 34 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 AOD550 nm across the continent and the transport of these emissions to regions outside of the continent. In general, CCAM captures the seasonality of the AERONET data across the continent. The magnitude of modeled and observed multiyear monthly average AOD550 nm overlap within ±1 standard deviation of each other for at least 7 months at all sites except the Réunion St Denis Island site (Réunion St. Denis). The timing of modeled peak AOD550 nm in southern Africa occurs 1 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 AOD550 nm 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.

Short summary
Africa is a major source of particles (or aerosols) from dust and fires, which impact climate. Models used to predict impacts of future climate change have not been well tested for aerosols over Africa. In this study we evaluate aerosols in the CCAM climate model against observations across Africa and surrounding regions. We find the model generally captures observed variability but overestimates dust in northern Africa, which has implications for its representation of climate feedbacks.
Final-revised paper