Observational Validation of Boundary‐Layer Cloud Regimes in Arctic Cold Air Outbreaks

Abstract

Abstract Arctic cold air outbreaks (CAOs) produce low‐level cloud patterns influencing the surface radiative balance. The stability parameter (boundary‐layer height over Monin‐Obukhov length) is theorized to distinguish shear‐driven roll convection from buoyancy‐driven cellular convection, but observational evidence remains limited. Using satellite‐based cloud classification and over 400 dropsonde profiles from four aircraft campaigns, we provide the first large‐sample assessment of and the marine CAO ‐index as joint discriminators of convection regimes. Cloud street and open cell regimes separate completely along an empirical boundary in the (, ) space. Classical free‐roll convection occurs for and ‐indices of 5–8 K, whereas cloud streets span a much wider range of instability, with 96% occurring at wind speeds above 10 . The boundary's increase with ‐index shows organized rolls persist to under strong thermodynamic forcing, interpreted as forced roll convection potentially triggered by marginal‐ice‐zone surface heterogeneities.