Microphysical Evolution in Mixed-Phase Midlatitude Marine Cold-Air Outbreaks

Five cold-air outbreaks are investigated with aircraft offshore of continental northeast America. Flight paths aligned with the cloud-layer flow from January through March span cloud-top temperatures from 258 to 2128C, in situ liquid water paths of up to 500 g m²², while in situ cloud droplet number concentrations exceeding 500 cm²³ maintain effective radii below 10 mm. Rimed ice is detected in the four colder cases within the first cloud pass. After further fetch, ice particle number concentrations reaching 2.5 L²¹ support an interpretation that secondary ice production is occurring. Rime splintering is clearly evident, with dendritic growth increasing ice water contents within deeper clouds with colder cloud-top temperatures. Buoyancy fluxes reach 400–600 W m²² near the Gulf Stream’s western edge, with 1-s updrafts reaching 5 m s²¹ supporting closely spaced convective cells. Near-surface rainfall rates of the three more intense cold-air outbreaks are a maximum near the Gulf Stream’s eastern edge, just before the clouds transition to more open-celled structures. The milder two cold-air outbreaks transition to lower-albedo cumulus with little or no precipitation. The clouds thin through cloud-top entrainment.