Improved Cloud Parameterization in Global Climate Model : Aerosol effects and secondary ice production mechanisms

Sammanfattning: The response of clouds to the changes in climate is uncertain, and the representation of the cloud-climate feedback is a key challenge in the global circulation models (GCM) for future climate projections. Factors contributing to this uncertainty include processes that involve particles of various sizes and phases, as well as the interactions between these particles and the surrounding atmosphere. The deep convective and stratiform cloud parameterizations in the Community Earth System Model (CESM) were improved and validated against the observations from the field campaign. These parameterizations explicitly resolve all known modes of heterogeneous cloud droplet activation, ice nucleation, secondary ice production (SIP) mechanisms, and growth processes, including collision/coagulation. The focus of the thesis is to improve the aerosols-cloud interaction and include the overlooked SIP mechanisms. A new detrainment scheme from a recent formulation based on high-resolution cloud modelling provides a better linkage between the convective and stratiform clouds. The mesoscale convective system (MCS) observed over Oklahoma, USA, during the Midlatitude Continental Convective Cloud Experiment (MC3E) was simulated with a single-column model, version 6 (SCAM6). The results are validated with ground measurements and in-situ aircraft observations. The analysis shows that the new microphysical parameterization provides a good representation of the ice initiation processes by including the empirically quantified primary ice processes and SIP mechanisms. Furthermore, the addition of SIP mechanisms has significantly improved the ice number concentrations in the mixed-phaseregions of simulated MCS. Sensitivity experiments show that breakup in ice-ice collisions is predicted to be the most significant SIP mechanism in the simulations. Increasing the soluble aerosols resulted in reduced cloud particle sizes, inhibiting the growth of droplets, and thus enhancing the homogeneous cloud dropletfreezing.