Microphysical and optical properties of midlatitude cirrus clouds observed in the southern hemisphere during INCA

Abstract

Airborne microphysical and optical properties of cirrus clouds, jet-stream and wave clouds were measured at temperatures ranging from −25° C to −62° C in the southern hemisphere from Punta Arenas (53° S) in March and April 2000 during the INCA experiment (INterhemispheric differences in Cirrus properties from Anthropogenic emissions). The observations related to cirrus clouds show general decreases of the ice water content (18 mg m⁻³ to 0.05 mg m⁻³), extinction coefficient (0.70 km⁻¹ to 0.08 km⁻¹), ice particle concentration (2.2 cm⁻³ to 0.5 cm⁻³), and the effective diameter (80 μm to 17 μm) linked to the variation of ambient temperature (−25° C to −60° C). The lowest temperature at which supercooled water droplets were detected was −33° C. The asymmetry parameter shows relatively small variations, with the smallest values (0.758) observed at the lowest temperatures. High-altitude clouds which form with a rapid vertical transport, i.e. jet-stream cirrus and orographic-wave ice-cloud, are characterized by very high values of ice particle concentrations (up to 100 cm⁻³) compared to mean values as a whole (1.45 cm⁻³). These two kinds of high clouds are the subjects of detailed case-studies. Although formed at a similar range of temperatures and with similar aerosol properties, the strong differences in cloud properties and humidity fields highlight the key role of the dynamical structure in controlling the formation, evolution and subsequent radiative properties of such high-altitude clouds.