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) and the representation of the Gamma shape parameter μ for high-latitude MCAO clouds in the Northern Hemisphere, focusing on the initial stratocumulus stages and cumulus regime transitions. Aircraft in-situ measurements from 20 flights with identified MCAO conditions were collected during two recent field campaigns: M-Phase conducted over the Labrador Sea in March 2022, and Arctic Cold-Air Outbreak (ACAO) conducted over the Nordic Seas in October to November 2022. Results show that high-latitude MCAO clouds in the Northern Hemisphere exhibit narrow PSDs, characterized by higher μ values (mean μ=20) that imply more reflective clouds compared to the fixed μ=2.5 assumption in some bulk microphysics schemes. Cloud PSDs narrow and μ values increase with height in near-adiabatic stratocumulus clouds, while their patterns are more variable in broken cumulus clouds. Liquid water content correlates more strongly with μ variability than cloud number concentrations, suggesting its better predictability as a prognostic variable for PSD variability in these cloud systems. Both the μ=20 and its derived relation with cloud liquid water content can be applied in bulk microphysics schemes to better represent the microphysical and radiative properties of high-latitude MCAO clouds. The proposed high μ values for MCAO clouds are applicable mainly to typical horizontal resolutions of numerical weather prediction and regional climate models.