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

Abstract

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 (Nd‐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 Nd‐bg < 50 cm−3, which was almost neutralized by a positive CRF for the 40% of the cases with Nd‐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 Nd‐core was at maximum for the lowest Nd‐bg and disappeared near 60 cm−3. The albedo susceptibility to Nd‐core reached its theoretical limit of 1/3 for constant LWP when Nd‐bg <40 cm−3, and diminishes to 0.2 when Nd‐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 Nd < 50 cm−3, thus possessing the indicated large susceptibility of negative radiative forcing to anthropogenic aerosols.

Observational evidence of strong forcing from aerosol effect on low cloud coverage

“…Once background clouds are cleaner than the threshold, i.e., N d less than 60 cm −3 or mean R e greater than around 14 μm, the sensitivity of Cf to N d , dCf / dN d , increases substantially with decreasing background N d , or increasing R e . Our results agree with similar N d threshold behavior previously reported using different approaches and data ( 21 ). The 14-μm threshold is physically meaningful because low clouds start to produce substantial drizzle that depletes cloud water once R e exceeds the 14-μm threshold ( 28 ).…”

“…1). Previous analyses of ship-tracks and similar opportunistic experiments suggest variable LWP adjustments to aerosols (10,(19)(20)(21). Toll et al (10) show a slight LWP increase in shiptracks on average and the magnitude partially results from the balance between entrainment drying and moistening by precipitation suppression.…”

Opportunistic experiments to constrain aerosol effective radiative forcing

“…Another potential caveat to consider is that the presence of black carbon may lead to cloud burn-off and affect cloud properties in this shipping corridor off the coast of South Africa ( Hu et al, 2021 ), although attempts to quantify this effect suggest its magnitude may be insignificant ( Diamond et al, 2020 ). Also, ship emissions may also be important for mixed-phase cloud properties, although studies have suggested that the effect on cloud brightness is more muted than in warm clouds ( Christensen et al, 2014 ; Possner et al, 2017 ).…”

Emission Reductions Significantly Reduce the Hemispheric Contrast in Cloud Droplet Number Concentration in Recent Two Decades

“…The impacts of shipping emissions and volcanic emissions of Kilauea were clearly shown in N d illustrated in Figure 2 for the 2.1 μm channel and Figure S1 in Supporting Information for the 3.7 μm channel, respectively. The signal of shipping emissions over the southern tropical Indian Ocean was reflected in the N d of convective cores and the grid‐mean N d , indicating that ship emissions have changed marine boundary layer clouds (Diamond et al., 2020; Gryspeerdt et al., 2019; Hu et al., 2021). However, no unambiguous shipping signals are evident in Figure 2b when focusing just on homogeneous clouds.…”