Transport Efficiency of Turbulent Parcel in the Marine Boundary Layer and Its Implications for Droplet Activation in Marine Cloud Brightening

Abstract. Marine cloud brightening (MCB) has been proposed as a potential climate intervention strategy in which sea-salt aerosols are introduced into the marine boundary layer to enhance cloud albedo by increasing cloud droplet number concentrations (CDNC). However, whether turbulent motions can efficiently transport sea-surface-released aerosols to the cloud base remains a major uncertainty in evaluating MCB feasibility. This study investigates a stratocumulus case over the tropical Southeast Atlantic using large-eddy simulation (LES) coupled with the FLEXPART Lagrangian particle dispersion model to quantify turbulent parcel transport efficiency. Short-duration releases and high-LWP stages favor stronger transport of turbulent parcels to the cloud base. For parcels released at 00:00 UTC on 24 September, the peak instantaneous arrival rate reaches 3.39 % within 15 min in d02, whereas it decreases to 1.86 % and the peak arrival time is delayed to 17 min in d04; the corresponding cumulative arrival rate decreases from 94.16 % to 79.70 %. Although the mean in-cloud residence time decreases with increasing resolution, parcels with extremely long residence times become more frequent. Under the adopted activation parameterization, higher-resolution LES simulations yield higher activation fraction (AF) because they better resolve strong cloud-base updrafts, with AF ranging from 0.73–0.91 in d02 to 0.92–0.96 in d04. The AF is relatively insensitive to release duration but increases under high-LWP conditions. These results provide a process-level quantification of parcel transport, in-cloud residence time, and conditional activation, establishing a physical basis for evaluating aerosol delivery constraints in MCB applications.