Environments associated with large droplet, small droplet, mixed-phase icing, and glaciated conditions aloft

During the winters of 1997 and 1998, the NASA-Glenn Twin Otter research aircraft sampled a variety of icing and non-icing environments over the Great Lakes region. Conditions encountered include freezing rain, freezing drizzle, small/cloud droplets, ice crystals and mixtures thereof. Occurrences of these phenomena are objectively compared with observational data from GOES-8 satellite, METARs, and stability derived from aircraft temperature profiles.

GOES-8 infrared satellite indicated that small-droplet and "non-classical" large-droplet icing occurred most often occurred when cloud top temperatures (CTTs) were warmer than -17C, while mixed-phase icing occurred within a cooler subset of CTTs. A significant percentage of large- and small-droplet icing occurred beneath melting zones in "classical" environments, when CTTs were much colder. A GOES-8 based icing algorithm is shown to perform quite well for all but the "classical" icing encounters.

Surface observations of cloud coverage and certain precipitation types provide good information about icing and SLD encounters aloft, while other observations provide much less. In particular, surface reports of freezing drizzle, freezing rain, and ice pellets are excellent indicators of SLD environments in the lower atmosphere. Results differ significantly between non-classical and classical situations, and with location relative to melting zones.

Aircraft temperature profiles reveal that the stability in non-classical SLD environments varies greatly. Non-classical SLD was observed with in-cloud temperature structures that ranged from isothermal to moist adiabatic. SLD formed in relatively low stability tended to be isolated to situations where relatively low concentrations of cloud condensation nuclei are expected. Classical SLD primarily occurred in very stable environments beneath the melting zone.