A microphysical classification of mixed-phase clouds in the liquid-ice Coexistence and Wegener-Bergeron-Findeisen regime

The degree of glaciation and the sizes and habits of cloud particles formed in mixed-phase clouds are not yet fully understood. Since the radiative feedback of this cloud type in the Earth's climate system is crucially dependent on these properties, it is important to accurately represent mixed-phase clouds in global climate models.

This study focuses on the occurrence and characteristics of both types of mixed-phase clouds: 'Coexistence' clouds where liquid drops and ice crystals coexist, as well as fully glaciated clouds (Wegener-Bergeron-Findeisen -WBF- regime).

Previous airborne cloud particle investigations have often been technically limited by adequate humidity measurements, thus in these studies the differentiation between Coexistence clouds (RHw and RHi > 100%; where RHw/i are the relative humidity wrt water/ice respectively) and WBF clouds (RHi > 100%, RHw < 100%) cannot be based on saturation data that would require those parameters. However, in mixed-phase clouds formed under controlled conditions at the AIDA cloud chamber, RH can be measured with high precision (using the APicT-TDL) and thus both mixed-phase cloud regimes can be identified. The size distributions of the clouds, measured with the NIXE-CAPS (Cloud and Aerosol Particle Spectrometer), can be divided in two types: In the Coexistence regime the distribution shows a high number of small liquid cloud drops (diameter < 50 µm) and a low number of larger ice crystals (diameter > 50 µm). As soon as the conditions turn to the WBF-regime, the size distribution changes: The mass in the liquid mode shifts to the large ice crystals as the smaller drops quickly evaporate.

Based on the assumptions that ice particles grow rapidly under WBF conditions and that water droplets only persist when the RHw is 100% or higher, the microphysical classification of the two cloud types found in the AIDA cloud chamber can be applied to mixed clouds detected during airborne field experiments.

We have applied this approach to field measurements during the mixed-phase clouds experiments at mid-latitudes (COALESC 2011, UK) and in the Arctic (VERDI 2012, Canada). We are able to clearly identify the two cloud types and confirm the dependence of the Coexistence and WBF clouds on RH, even with RH measurements that are less accurate than those available at the AIDA chamber. Sorting the occurrence of WBF clouds by temperature shows that they become more frequent as temperature decreases.

We further analyzed the particle shapes (NIXE-CIP: Cloud Imaging Probe, diameters > 15 µm) and the particles' depolarization signature (NIXE-CAS: Cloud and Aerosol Spectrometer, diameters < 50 µm) to estimate the aspherical fractions of the cloud particles as an indicator for glaciation. A surprising result is that in the water-subsaturated WBF regime where no liquid drops can persist, spherical ice crystals are detected, especially in the size range < 50 µm. We identified two correlations: More spherical frozen cloud particles exist (i) the smaller the particles are and (ii) the warmer the temperature is. We suggest that the reason for this is that the ice crystals tend to become spherical again when the environment becomes subsaturated with respect to water.