Observed Strong Ice Generation at Temperatures Warmer than -8°C and Potential Contribution of Biological Ice Nuclei

Ice generation at temperatures warmer than - 8°C has a strong control on cloud evolution by influencing liquid/ice partition and precipitation development. However, it is poorly understood. This study provides evidences of strong ice generation at this warm temperature range. During the NSF ICE-T field campaign, convective and stratiform clouds with temperatures above -15°C are sampled by NSF/NCAR C-130 aircraft, which carried a variety of in situ cloud probes and aerosol instruments and up-down lidar and cloud radar measurements. During ICE-T, up to 5/L ice concentrations are observed in several stratiform and pileus clouds with top temperature around -5°C. The spatial extensiveness of this warm ice generation is conformed by lidar depolarization measurements. Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) measurements of cloud particle residues indicate high occurrences of biological aerosols in these clouds. Considering known warmer activation temperatures of biological ice nuclei (IN) than the other types of INs, we believe that biological INs are mainly responsible for these warm ice generation. Combined CloudSat and CALIPSO measurements of middle level stratiform mixed-phase clouds confirmed that ice generation at temperatures warmer than - 8°C are occurring globally. These results clearly indicate that weather and climate models need to include biological INs in their simulations and measurements are needed to better quantify the sources of biological INs.