Characterizing the Shape of Cloud Particle Size Distributions in High‐Latitude Marine Cold‐Air Outbreaks

Abstract Marine cold‐air outbreaks (MCAOs) drive significant evolutions in marine boundary layer clouds and play a crucial role in high‐latitude climate systems. This study examines the variability of cloud particle size distributions (PSDs) in high‐latitude MCAOs and how well their spectral shapes are represented by the gamma shape parameter μ used in model bulk microphysics parameterizations. Aircraft in situ measurements from 20 flights in stratocumulus and cumulus cloud regimes within MCAO conditions were collected during two recent field campaigns: Arctic cold‐air outbreak conducted over the Arctic‐Nordic seas in March 2022 and the M‐phase conducted over the sub‐Arctic Labrador Sea in October–November 2022. Results show that high‐latitude MCAO clouds in the Northern Hemisphere exhibit narrow PSDs, characterized by higher μ (mean μ = 20) that imply more reflective clouds than the fixed μ = 2.5 assumption in some bulk microphysics schemes. Cloud PSDs narrow and μ increase with height in near‐adiabatic stratocumulus clouds, while there is more vertical variability in broken cumulus clouds. Liquid water content correlates more strongly with μ variability than cloud number concentrations, suggesting its better predictability as a bulk prognostic variable for PSD variability in these cloud systems. A higher μ and its derived relation with cloud liquid water content could better represent the microphysical and radiative properties of high‐latitude MCAO clouds in bulk microphysics parameterizations, particularly at the typical horizontal resolutions of numerical weather prediction and regional climate models.