Inferring processes governing cloud transition during mid-latitude marine cold-air outbreaks from satellite

Abstract. Cloud morphological transitions strongly influence radiative effects and the regional radiation budget. Marine cold-air outbreaks (MCAOs) over the northwestern Atlantic feature such transitions, from overcast stratiform to broken cumuliform cloud fields downwind. Characterizing these transitions requires an understanding of the thermodynamic and dynamical evolution of the marine boundary layer and the interplay between warm- and cold-phase processes. Using a novel 'space–time exchange' approach, we construct instantaneous trajectories using reanalysis winds and extract geophysical variable traces along these trajectories from GOES-16 satellite snapshots for five MCAO events sampled during the NASA ACTIVATE campaign (2020–2022). Clear directionality of traces in liquid water path (LWP)–droplet number (Nd) space reveals sequential dominance of drop activation, condensational growth, and collision–coalescence during cloud thickening. Patterns of traces in domain-LWP versus domain-IWP (ice water path) suggest fingerprints of two distinct mixed-phase processes: (i) gradual liquid depletion via vapor deposition and (ii) rapid depletion via riming, preceded by co-growth of liquid and ice. Elevated Nd suppresses peak LWP and delays cloud breakup. A large spread in shortwave albedo is found during cloud transition, reflecting mixed-phase processes. Metrics denoting cloud organization converge towards the end of the transition, despite differences in cloud micro- and macro-physical properties among cases. These results underscore the central role of frozen hydrometeors in governing cloud transitions and demonstrate a powerful framework for process inference based on satellite snapshots using the 'space-time exchange' approach. This framework offers a new pathway to benchmarking model representations of mixed-phase microphysics and advancing model-observation synergy.