Effects of marine biology and air-mass trajectories on cloud brightness

The role of marine phytoplankton emissions in aerosol-cloud interactions is still a cause of large uncertainties in climate modelling. We investigate the effects of DMS and sea spray aerosol, which are both affected by marine phytoplankton, on the droplet concentrations of liquid clouds. To do this we examine MODIS satellite data for an area of liquid clouds in the Southern Ocean during the southern hemispheric summer. Backwards trajectories of air masses from the clouds were simulated with the FLEXPART Lagrangian particle dispersion model for a duration of 9 days.

Clouds whose trajectories were not influenced by continental air masses could be split into two groups based on their trajectory history in the two days before encountering cloud: group 1 spent most of their time in the free troposphere, whereas group 2 spent their last two days within the boundary layer. 

Group 1 cloud droplet concentrations were positively correlated with exposure to sea surface chlorophyll that took place prior to them entering the free troposphere (i.e., at least 2 days before encountering their clouds), whereas droplet concentrations were negatively correlated with wind speed in the last day before encountering clouds. Group 2 clouds did not show the same correlations. Instead, wind speeds in the last two days before a cloud encounter were positively correlated with cloud droplet number concentration, with the relationship between the two having a steeper slope for higher chlorophyll values.

These results give an insight into the factors controlling the changes in Southern Ocean cloud properties with associated climate impacts via cloud brightening effects.