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.