Importance of Secondary Ice Production in Mixed-Phase Monsoon Clouds Over the Indian Subcontinent

The accurate representation of mixed-phase monsoon clouds and their phase distribution is of great importance for numerical models used to predict monsoon rainfall. Therefore, it is essential for these models to correctly capture the phase fraction of clouds, which includes the proportions of liquid and ice. Ice particle formation in clouds occurs through primary ice production and secondary ice production (SIP). Most weather and climate models tend to overlook secondary SIP mechanisms, often only including rime-splintering. This oversight can introduce biases in the phase partitioning of mixed-phase clouds and monsoon rainfall predictions. In this study, we investigate the roles of three major SIP mechanisms: Hallett-Mossop (HM), droplet shattering (DS), and ice-ice collision (IIC) in mixed-phase monsoon clouds. This investigation is the first of its kind and was conducted using high-resolution simulations of mixed-phase convective clouds observed during the fourth phase of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over a rain shadow region of India. The default cloud microphysical scheme, which originally included only the HM process, was modified to incorporate additional SIP mechanisms such as DS and IIC. The simulated cloud parameters, including liquid water content and ice number concentration, showed good agreement with airborne measurements. Our findings indicate that IIC is the predominant SIP mechanism, contributing 90% to the total ice production through SIP. The inclusion of the three SIP mechanisms resulted in an enhancement of ice concentration by three to four orders of magnitude at temperatures warmer than -20oC. SIP significantly influenced various cloud parameters between 0 to -20°C, including total ice number concentration, ice crystal mass, rimed mass, liquid water content, and phase fraction. The rates of several mixed-phase processes were also affected by the SIP mechanisms. Overall, SIP led to a 25% reduction in accumulated surface precipitation. The impact of pollution on SIP mechanisms was found to be insignificant.