Microphysical evolution in mixed-phase mid-latitude marine cold-air outbreaks

Abstract

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 of -5 to -12 °C, in situ liquid water paths of up to 500 g m-2, while in situ cloud droplet number concentrations exceeding 500 cm-3 maintain effective radii below 10 µm. Rimed ice is detected in the 4 colder cases within the first cloud pass. After further fetch, ice particle number concentrations reaching 2.5 L-1 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 m-2 near the Gulf Stream’s western edge, with 1-second updrafts reaching 5 m s-1 supporting closely-spaced convective cells. Near-surface rainfall rates of the 3 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 2 cold-air outbreaks transition to lower-albedo cumulus with little or no precipitation. The clouds thin through cloud-top entrainment.