PNNL

Abstract

Marine clouds undergo a transition from subtropical stratocumulus (Sc) to shallow cumulus (Cu) and eventually to deep convective (DC) systems as air masses progress from the subtropics towards the deep tropics. How aerosols modulate this Lagrangian cloud evolution remains largely uncertain. Using both 5-year long satellite observations mapped along 8-day Lagrangian trajectories and complementary large-eddy simulations from nine initiation locations across the Northeast Pacific, Southeast Pacific, and Southeast Atlantic. This framework allows us to quantify the aerosol effect and its co-variability with meteorological conditions on cloud microphysics, macrophysics, and top-of-atmosphere radiation through the full Sc-Cu-DC transition. This research reveals that increasing aerosol concentrations leads to deeper, and more reflective clouds throughout this cloud transition. Examining the thermodynamic evolution along the trajectory indicates a well-known trend: enhanced moistening near the boundary-layer top and lower free troposphere in polluted cases, suggesting that some of the co-variability between aerosol and meteorological conditions is internally driven. The agreement between model simulations and satellite data alongside the multi-basin coherence of the results indicates that aerosols systematically amplify cloud depth and reflectivity during the subtropical–to–tropical cloud transition.