Thermodynamic phase profiles of optically thin midlatitude clouds and their relation to temperature

Abstract

The relationship between cloud thermodynamic phase and temperature in some aircraft measurements conducted in midlatitude frontal clouds suggests that significant liquid does not exist at temperatures colder than 258 K. This data set is often used to verify parameterizations of cloud phase in general circulation models. However, other aircraft campaigns and different instruments suggest a different relationship. Here we examine the temperature-phase relationship for midlatitude optically thin winter clouds. Cloud phase and temperature profiles derived from 5 years of ground-based lidar depolarization and radiosonde measurements are analyzed for two midlatitude locations: the U. S. Atmospheric Radiation Measurement Program Southern Great Plains site and the Site Instrumental de Recherche par Tldtection Atmosphrique in France. Because lidars are attenuated in optically thick clouds, the data set only includes clouds with optical thickness of <3. Cloud phase is obtained by using the classical method based on a depolarization ratio threshold of 11% for differentiating liquid from ice. The frequency of occurrence of clouds either completely liquid or completely glaciated in the temperature range from 233 to 273 K is similar to previous observations in the midlatitudes but somewhat greater than in the Arctic. The relationship between ice phase occurrence and temperature only slightly changes between cloud base and top. At both sites, liquid is more prevalent at colder temperatures than has been found previously in some thicker frontal clouds, suggesting different processes for glaciation in nonfrontal optically thin clouds. Copyright 2010 by the American Geophysical Union.