Volcano and ship tracks lend causal relationship to large aerosols effects on marine boundary layer clouds cover and radiation

Abstract:

The attribution of the observed relationships between aerosols on cloud drop concentrations (Nd), cloud fraction (Cf) and cloud radiative effect (CRE) was tested by quantifying the effects of added aerosols in volcanic plumes and shipping lanes on cloud properties. The volcanoes are Ambryn in the Southwest Pacific and Sierra Fernandina volcano in the Galapagos Islands. The shipping lane is located in the southeast Atlantic. The results show that clouds in the shipping lane had an average CRE of +13% compared to adjacent background areas when background Nd is less than 60 cm^-3. The relative changes in Cf and albedo contributed to relative change in CRE are 42% and 58% respectively when compare shipping lane to background clouds. The CRE of the clouds in the volcanic plumes was larger by +24% and +59% with respect to the background clouds for Ambryn and Fernandina respectively. The increase in CRE was contributed by additions of 63% and 37% to Cf and albedo, respectively. The added Cf and albedo were 71% and 29% for Fernandina, respectively. These two volcanoes had different response of cloud properties to the volcanic plumes due to the different meteorological conditions. The shape of increase in Cf is indicative to be caused mainly by rain suppression. The susceptibilities of cloud albedo and Cf to N_d show high similarities, when compare the global data when constraining the global data to the same range of meteorological condition as in the volcanic plume affected areas. This result indicates that the changes in clouds Cf, albedo and CRE are caused by the volcanic aerosols, and that the same pattern is repeated globally by other types of aerosols. It supports the notion that changes in aerosols drive the changes in cloud properties as quantified by the relationships in Rosenfeld et al. (Science, 2019).