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 northwestern 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 radiation budget. Using a novel ‘space-time exchange’ approach, we construct instantaneous trajectories from reanalysis winds and extract geophysical variable traces along them from GOES-16 satellite snapshots for five MCAO events.

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. Traces in domain-mean LWP versus IWP (ice water path) space exhibit two distinct couplings between liquid and ice, consistent with different mixed-phase process fingerprints: (i) gradual liquid depletion dominated by vapor deposition and (ii) rapid liquid depletion driven by collisional freezing, aided by precipitation and dynamical feedbacks. In-situ measurements from the NASA ACTIVATE campaign provide independent evidence supporting these process interpretations. Delayed cloud breakup during the 29 March 2022 event is consistent with a shift from precipitation- to entrainment-driven breakup under high Nd conditions. As the cloud field breaks up, two distinct scalings between shortwave albedo and cloud fraction emerge, consistent with the identified mixed-phase fingerprints, with the degree of cloud organization converging toward the end of the transition.

Why It Matters The results demonstrate an effective ‘space-time exchange’ framework for process inference from satellite snapshots, enabling a new pathway for the synergistic characterization of mixed-phase microphysics in models and observations.

References:

  • J. Zhang, D. Painemal, T. Dror, J.-S. Lim, A. Sorooshian, and G. Feingold (2026): Inferring processes governing cloud transition during mid-latitude marine cold-air outbreaks from satellite. Atmos. Chem. Phys., 26(9), 6015–6034. doi:10.5194/acp-26-6015-2026

  • 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