Turbulence and Ice Nucleation in Mixed-Phase Altocumulus Clouds in the Mid-Latitudes

Mixed-phase layer clouds are common throughout the globe, from the tropics where detrainment from convection forms long-lived altocumulus layers, to the mid-latitudes where humidity is brought to the mid-troposphere by cyclonic activity, and in the Arctic regions where low-level mixed-phase stratocumulus clouds persist. Supercooled water is common in these clouds and so they have a strong impact on the radiative balance of the planet.

Global climate and numerical weather prediction models fail to predict sufficient mid-level cloud and this leads to deficiencies in the representation of incoming solar radiation at the surface and thus large biases in surface temperature, notably in the Southern Ocean. High resolution and cloud resolving models do not perform significantly better, and in part this is due to large uncertainties in the nature of ice nucleation and the phase transition from liquid to ice.

This study exploits new observations of mixed-phase cloud to attempt to better understand the processes that control their evolution. Observations of altocumulus clouds from an instrumented aircraft are presented that probe the nature of liquid and ice cloud particles, and the underlying aerosol population. The performance of cloud microphysics probes in measuring ice particles smaller than 100 microns when liquid cloud drops are present is assessed. New characterisation of SID2 (Small Ice Detector 2) and CIP15 (Cloud Imaging Probe, 15 microns) is presented. Calculations are performed that assess the ice nucleating particle budget in altocumulus, and the ice production rate in mixed-phase altocumulus and cumulus clouds.