Using aircraft measurements to estimate the magnitude and uncertainty of the shortwave direct radiative forcing of southern African biomass burning aerosol

Abstract

We estimate the shortwave, diurnally averaged direct radiative forcing (RF) in cloud‐free conditions of the biomass burning aerosol characterized by measurements made from the University of Washington (UW) research aircraft during the Southern African Regional Science Initiative in August and September 2000 (SAFARI‐2000). We describe the methodology used to arrive at the best estimates of the measurement‐based RF and discuss the confidence intervals of the estimates of RF that arise from uncertainties in measurements and assumptions necessary to describe the aerosol optical properties. We apply the methodology to the UW aircraft vertical profiles and estimate that the top of the atmosphere RF (RFtoa) ranges from −1.5 ± 3.2 to −14.4 ± 3.5 W m−2, while the surface RF (RFsfc) ranges from −10.5 ± 2.4 to −81.3 ± 7.5 W m−2. These estimates imply that the aerosol RF of the atmosphere (RFatm) ranges from 5.0 ± 2.3 to 73.3 ± 11.0 W m−2. We compare some of our estimates to RF estimated using Aerosol Robotic Network (AERONET) aerosol optical properties and show that the agreement is good for RFtoa, but poor for RFsfc. We also show that linear models accurately describe the relationship of RF with the aerosol optical depth at a wavelength of 550 nm (τ550). This relationship is known as the radiative forcing efficiency (RFE) and we find that RFtoa (unlike RFatm and RFsfc) depends not only on variations in τ550, but that the linear model itself is dependent on the magnitude of τ550. We then apply the models for RFE to daily τ550 derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite to estimate the RF over southern Africa from March 2000 to December 2006. Using the combination of UW and MODIS data, we find that the annual RFtoa, RFatm, and RFsfc over the region is −4.7 ± 2.7 W m−2, 11.4 ± 5.7 W m−2, and −18.3 ± 5.8 W m−2, respectively.