Characteristics and radiative impact of the aerosol generated by the Canberra firestorm of January 2003

[1] The extensive fires in southeastern Australia in January 2003 culminated in the Canberra firestorm of 18 January. The fires generated a pall of smoke that blanketed the Canberra area for the following week and affected the region for most of the following month. Analysis of Sun photometer data shows that aerosol optical depths tau(500) > 7 were observed during the week following the firestorm. During the subsequent month the smoke aerosol dispersed, although background conditions (tau(500) < 0.1) did not return until after significant rain in late February. Analysis of aerosol optical properties retrieved from sky radiance scans shows remarkably high single scattering albedo (<(omega)over bar>(440) similar to 0.96) with only a small depression evident in the week following the firestorm. This indicates that the aerosols were significantly less absorbing than would be expected from smoke freshly generated by flaming combustion, possibly a result of inefficient combustion due to the extreme intensity of the fire, and marks them out as quite distinct from the more absorbing biomass burning aerosols encountered in the savannas of tropical Australia. The high single scattering albedo gave rise to mean radiative forcings of - 50 W m(-2) at the top of the atmosphere and - 172 W m(-2) at the surface during the week following the firestorm. Maximum heating rates are depressed by a factor of two at (omega) over bar (440) = 0.96 compared with (omega) over bar (440) = 0.88. Nevertheless, for plausible atmospheric profiles the large optical depths caused heating rates peaking at more than 10 K/day near the top of the smoke layer. The layer stabilized the lower troposphere through a combination of reduced surface heating and a positive gradient in heating rate through most of the layer. The enhanced stability suppressed surface temperatures and winds during the week following the firestorm, reducing the risk of renewed fire danger, and suggesting a useful role for aerosol effects in weather prediction models.