Distinction of the Winter-Time Ice Cloud Fractions between Eastern and Western Eurasia Viewed from CALIPSO

We examined the contrast in the fraction of ice (or water) cloud layers between eastern and western parts of Eurasia using the products from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data in January 2007. Through the analyses, we found that the fraction of ice cloud layers between −25 and 0°C was larger in eastern than in western Eurasia in January 2007. Eastern Eurasia had a lower cloud fraction than western Eurasia, which resulted in strong radiative cooling. The atmospheric temperature decreased due to radiative cooling from the ground surface. Therefore, the freezing layer is close to the ground surface in this region. We considered that the freezing layer was strongly affected by ice nuclei from the ground surface, caused by the changing altitude of the freezing layer, and thus the fraction of ice clouds increased. This indicates that the process of ice particle formation might be changed under the influence of atmospheric conditions.

The fraction of ice-containing clouds in eastern Eurasia was higher than in western Eurasia and globe with a peak difference of about 30% around -15 degrees Celsius. The fraction of ice cloud layers between −20 and −5 degrees Celsius was about 20% higher in eastern Eurasia than in western Eurasia. In this temperature range, ice particles are formed by the contact freezing and immersion freezing processes, and our results indicate that these formation processes were promoted in eastern Eurasia.

The fractions of ice and water cloud layers in the lower troposphere (below 3 km) were significantly distinct between eastern and western parts of Eurasia. The fraction of ice cloud layers below 3 km was higher in eastern Eurasia than in western Eurasia. This suggests that the formation of ice particles in eastern Eurasia is initiated at a lower altitude, closer to the ground surface, than in western Eurasia. We also considered that the impact of the ground surface on ice particle nuclei was larger in eastern Eurasia than in western Eurasia. The supersaturation and dynamics are important factors for ice particle formation and the subsequent evolution of the ice phase in the cloud layer.