Cirrus Cloud Microphysical and Optical Properties at Southern and Northern Midlatitudes during the INCA Experiment

Abstract

Microphysical and optical measurements were performed in midlatitude cirrus clouds at temperatures between −33°C and −60°C during southern and northern Interhemispheric Differences in Cirrus Properties From Anthropogenic Emissions (INCA) field experiments carried out at equivalent latitudes (53°S and 55°N) from Punta Arenas in the Southern Hemisphere (SH, Chile) and Prestwick in the Northern Hemisphere (NH, Scotland). The aim of this paper is to analyze the microphysical and optical properties of cirrus clouds and to compare the results of the two campaigns. Compared with the mean properties of cirrus clouds in SH, the cirrus sampled in NH were characterized by a greater concentration of ice crystals (2.2 cm^-3 versus 1.4 cm^-3, respectively) with a lower effective diameter (36 μm versus 42 μm, respectively). A significant contrast in extinction coefficient was also evidenced with larger values in NH than in SH (0.61 km^-1 versus 0.49 km¹, respectively), whereas no significant differences in the ice water content were observed (8 mg m^-3). The first measurements of the asymmetry parameter obtained in midlatitude cirrus clouds revealed rather uniform particle scattering properties (median g values between 0.76 and 0.78) with small differences between SH and NH (0.770 versus 0.767, respectively). No significant differences in crystal shape were evidenced between the two data sets. For similar environmental conditions (i.e., over a similar range of temperature and vertical velocity) and for given values of the relative humidity, the comparisons clearly show distinct differences between the microphysical and optical properties of cirrus clouds sampled in the SH and NH field experiments. These differences may be related to the contrasts in cirrus freezing thresholds in terms of relative humidity over ice for onset of clouds, which is about 20% lower in NH than in SH, as reported in previous INCA data analyses. Nevertheless, definite conclusions are hampered by the large natural variability of cirrus cloud properties and the limited number of flights performed, not forgetting both the actual freezing mechanisms and potential differences in small-scale dynamical variability, which are still insufficiently understood.