On Characteristics and Microphysical Pathways for Widespread Substantial Ice Water Content in Deep Convection Outflow

Abstract:

Occurrences of jet engine malfunction in recent years have been increasingly associated with extended flight through widespread deep convection anvils and attributable to exposure to fully glaciated icing conditions rather than supercooled water. Malfunctions commonly occur with radar reflectivity less than 30 or even 20 dBZ at flight level and with heavy precipitation below. Airbus airborne measurements in several tropical regions were obtained with the sole objective of encountering and characterizing regions of substantial ice water content (in excess of ~3 g m^-3) in the absence of radar reflectivity exceeding 30 dBZ in mesoscale convective systems. Measurements of particle size distributions (PSDs) and particle shapes reveal signatures of ice microphysical pathways associated with the targeted conditions. First we show that column simulations with size-resolved microphysics of quasi-steady, heavy stratiform rain indicate that the measured PSDs are generally consistent with a range of ground-based radar measurements and retrievals obtained under similar, steady stratiform rain conditions in the tropics. Second we consider the convective updraft microphysical pathways that may lead to the observed stratiform ice PSD features using parcel simulations with size-resolved microphysics that include heterogeneous and homogeneous ice nucleation, in which aerosol and ice microphysical properties and microphysical processes in updrafts are informed by relevant measurements. We find that a partially resolved smaller mode in the observed PSDs is likely attributable to homogeneous droplet freezing whereas the dominant mode in the observed mass distributions is consistent with an ice multiplication process active at warmer temperatures, such as Hallett-Mossop splintering.