Microphysical Evolution in Mixed-Phase Mid-Latitude Marine Cold-Air Outbreaks

Cold-air outbreaks off of the eastern US seaboard provide dramatic visual examples of cloud morphological transitions from closed-cell to more open-celled circulations. The cold-air outbreak clouds are typically mixed-phase, with increased ice production thought to hasten the transitions. Here the microphysics and environmental context of five cold-air outbreaks are interrogated using NASA ACTIVATE aircraft campaign data from March 2020 and January-March of 2021. Flight paths aligned with the cloud-layer flow span cloud-top temperatures of -5 to -12 C, in situ liquid water paths of up to 700 g m , while in situ cloud droplet number concentrations (N_d) of up to 1000 cm help explain cloud-top effective radii that remain below 10 micron. In the four coldest cases, rimed ice was present at the initial cloud development, indicating efficient primary ice production at temperatures between -4 to -8 C. Farther downstream, ice particle number concentrations of 0.1-2.5 10 cm exceed those predicted by primary nucleation processes alone . Secondary ice production is enhanced near cloud top, speculated to reflect collisional breakup, and near cloud base, where strong updrafts near 0 C may support more intense aggregation. Ice enhancement also occurs within the Hallett-Mossop temperature regime (-3 - -8 C) in one case. The highest ice water contents coincide with dendritic growth. The cloud evolution follows a predictable pattern. Buoyancy fluxes reach 400-600 W m near the eastern edge of the Gulf Stream, sustaining updrafts reaching five m/s that support closely-spaced convective cells. Updrafts can pierce the capping inversion, detraining to form a thin overcast stratiform cloud under a new, higher, inversion level. The updrafts can activate smaller Aitken mode aerosols, helping to explain the high N_d values. Cloud morphological transitions can occur through both precipitation, and through entrainment of warmer, drier air from above. The findings will be placed in the context of mixed-phase cloud transitions documented within other regions of the globe.