Impacts of Secondary Ice Production on Arctic Mixed-Phase Clouds based on Observations and a Global Climate Model

Abstract:

For decades, strong cloud and radiation biases over the Arctic have been reported in many global models. How ice particles are generated and evolved in mixed-phase clouds is not well understood. Furthermore, measured ice crystal number concentrations have been found to be orders of magnitude higher than observed ice nucleating particles in modest cold clouds, indicating the importance of secondary ice production (SIP) in addition to the primary ice nucleation. However, current regional and global models generally do not represent the SIP processes well. Motivated by the new understanding from observations, we improve the representation of a global climate model (GCM) by conducting a first attempt of implementing different SIP mechanisms (breakup in ice-ice collisions, raindrop-freezing fragmentation, and riming splintering) in the model. The model is run in the single column mode to facilitate comparisons with the Mixed-Phase Arctic Cloud Experiment (M-PACE) observations. We show the SIP importance in different types of clouds observed in M-PACE, with maximum enhancement of ice crystal number concentrations (ICNCs) by up to 4 orders of magnitudes in moderately cold clouds. We find that SIP is the dominant source of ice crystals near the cloud base for Arctic single-layer mixed-phase clouds. The model with SIP improves the occurrence and phase partitioning of this type of clouds; reverses the vertical distribution pattern of ICNCs; provides a better agreement with observations. The findings of this study highlight the importance of considering the SIP in GCMs.