Abstract
Abstract. Radiative forcing (RF) of black carbon (BC) in the atmosphere is estimated using radiative transfer codes of various complexities. Here we show that the two-stream radiative transfer codes used most in climate models give too strong forward scattering, leading to enhanced absorption at the surface and too weak absorption by BC in the atmosphere. Such calculations are found to underestimate the positive RF of BC by 10% for global mean, all sky conditions, relative to the more sophisticated multi-stream models. The underestimation occurs primarily for low surface albedo, even though BC is more efficient for absorption of solar radiation over high surface albedo.
Highlights
Black carbon (BC) in the atmosphere has been investigated over many decades (Novakov and Rosen, 2013)
In the present study we further investigate this potential underestimation of BC radiative forcing (RF) in many of the global climate models, and develop a physical understanding for why it occurs
The underestimation in the two-stream simulation is shown here to be largest over ocean, with low surface albedo, whereas over regions with high surface albedo the two-stream more closely reproduces the eight-stream simulation
Summary
Black carbon (BC) in the atmosphere has been investigated over many decades (Novakov and Rosen, 2013). Accurate results can be achieved by using multi-stream line-by-line codes These calculations are computationally demanding and are usually not applied for global scale simulations. One of the radiative transfer codes was run both as a multi-stream code resembling the benchmark codes, as well as run as a two-stream code resembling the simpler codes used in climate models. These two codes were denoted as numbers 3 and 4, respectively in Randles et al (2013) and used in the current work. In the present study we further investigate this potential underestimation of BC RF in many of the global climate models, and develop a physical understanding for why it occurs
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