The concept of inferring time-evolving processes from static snapshots stems from the characteristics of an ergodic system. Ergodicity applies only when there is sufficient scale separation between the processes of interest and the overturning of boundary conditions. During marine cold-air outbreaks (MCAOs), spatial cloud morphological transitions are embedded in rather persistent gradients of large-scale meteorological conditions, enabling a space-time exchange. This allows the directionality of traces in geophysical variable spaces to reveal fingerprints of cloud microphysical processes governing cloud transitions.

The Science Clouds strongly influence Earth’s energy balance by reflecting sunlight and trapping heat. This study focuses on marine cold-air outbreaks over the northwest Atlantic, events where frigid continental air moves over the warm ocean, producing striking transitions from uniform, overcast cloud decks to scattered, puffy clouds downstream. These transitions greatly affect how much sunlight the atmosphere reflects and therefore the regional climate. Using a novel ‘space–time exchange’ approach, we combined satellite snapshots from GOES-16 with reanalysis wind data to trace how clouds evolve along their natural flow paths. 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.

Analysis of a cold air outbreak demonstrating the ability to infer processes from GOES retrievals in a single satellite image together with information on the airflow through the image from ERA-5 reanalysis. (a) Snapshot of a cold-air outbreak scene sampled by GOES with superimposed instantaneous trajectories based on ERA-5 reanalysis; (b) Trace of the family of instantaneous trajectories through the scene in LWP-Nd space; (c) A diagram identifying how individual processes drive the system in LWP-Nd space (1: activation, 2: condensation, 3: collision-coalescence, 4: precipitation, 5: entrainment (5.1: homogeneous, 5.2: inhomogeneous)). Figure is reproduced from Feingold et al. 2025 ACP.

The Impact This study provides new observational evidence linking microphysical ice processes to large-scale cloud transitions and radiative effects. The ‘space-time exchange’ framework offers a powerful, satellite-based way to infer cloud processes and benchmark weather and climate models’ representation of mixed-phase microphysics and its role in Earth’s energy budget.

  • J. Zhang, D. Painemal, B. Cairns, T. Dror et al. (2026): Inferring processes governing cloud transition during mid-latitude marine cold-air outbreaks from satellite. Atmos. Chem. Phys., to be submitted (under NOAA internal review).

  • G. Feingold, F. Glassmeier, J. Zhang, and F. Hoffmann (2025): Opinion: Inferring process from snapshots of cloud systems. Atmos. Chem. Phys., 25(18), 10869–10885. doi:10.5194/acp-25-10869-2025