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

Hannah M Horowitz,Marcus Thatcher,Willem A Landman,Rebecca M Garland,Zane Dedekind,Jacobus Van Der Merwe,Francois A Engelbrecht

doi:10.5194/acp-17-13999-2017

Abstract

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.

Highlights

Africa contains the largest individual sources of biomass burning emissions and dust globally (Crutzen and Andreae, 1990; van der Werf et al, 2010; Schütz et al, 1981; Prospero et al, 2002)
An accurate representation of African aerosols is critical in climate models to understand the regional and global radiative forcing and climate impacts of dust and biomass burning aerosols, at present and under future climate change, and is currently a major challenge
We evaluate Cubic Atmospheric Model (CCAM) using the CMIP5 emissions inventory against long-term aerosol optical depth (AOD) retrievals across Africa and outflow regions off the coast from the Aerosol Robotic Network (AERONET) (Holben et al, 1998)

Summary

Introduction

Africa contains the largest individual sources of biomass burning emissions and dust globally (Crutzen and Andreae, 1990; van der Werf et al, 2010; Schütz et al, 1981; Prospero et al, 2002). Dust aerosols and carbonaceous aerosols produced from biomass burning are known to impact climate through direct scattering and absorption of radiation, and indirectly through their effects on cloud formation and properties. Black carbon is estimated to be second only to CO2. Horowitz et al.: Evaluation of climate model aerosol seasonal and spatial variability over Africa in contributing to warming globally (Bond et al, 2013). The largest uncertainty in climate models is the impact of aerosols on the radiative balance of the Earth (Boucher et al, 2013)

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