The Dependence of Ship-Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

Marine low clouds of the busy shipping lane in the southeast Atlantic during the springs of 2003–2015 were analyzed to study the dependence of their properties and radiative forcing on the background cloud drop concentrations (N_d-bg). The overall average cloud radiative effect within the shipping lane was larger by only −1 Wm⁻² compared to the adjacent clouds. However, this near-zero averaged effect was composed of large negative cloud radiative forcing (CRF) for the cleanest (13%) of the cases with N_d-bg < 50 cm⁻³, which was almost neutralized by a positive CRF for the 40% of the cases with N_d-bg > 70 cm⁻³. A possible explanation for this positive forcing is the cloud burning effect of the black carbon from ship emissions. The negative forcing was composed of 45% for the cloud fraction (Cf) effect and 55% for the albedo effects, which included a small contribution of the liquid water path (LWP) effect. Positive Cf susceptibility to N_d-core was at maximum for the lowest N_d-bg and disappeared near 60 cm⁻³. The albedo susceptibility to N_d-core reached its theoretical limit of 1/3 for constant LWP when N_d-bg <40 cm⁻³, and diminishes to 0.2 when N_d-bg = 60 cm⁻³. These susceptibilities represent cause and effect relationships because the differences of aerosols caused by the ship emissions vary at a much smaller scale than meteorology. Globally, under similar environmental conditions, nearly half of the area has N_d < 50 cm⁻³, thus possessing the indicated large susceptibility of negative radiative forcing to anthropogenic aerosols.