Role of INP activation schemes and concentrations in controlling ice formation pathways in Warm Conveyor Belts

Abstract. Warm conveyor belts (WCBs) in extratropical cyclones transport moist boundary-layer air into the upper troposphere and form extensive mixed-phase cloud bands and cirrus cloud shields. The microphysical pathways governing ice formation within these systems remain poorly quantified, particularly with respect to heterogeneous ice nucleation and secondary ice production. We investigate the impact of uncertainty in the ice formation representation on clouds in a WCB using the ICON model with an ice modes two-moment scheme that explicitly tracks ice formed by immersion freezing (IMM), deposition nucleation (DEP), homogeneous droplet freezing (FRZ), homogeneous solution freezing (HOM), and secondary ice (SEC). We test the sensitivity of ice formation pathways to three dust INP activation schemes, perturbations of INP concentration, and three secondary ice mechanisms (rime splintering, collisional breakup, droplet fragmentation).

Ice number concentrations in the cirrus regime respond strongly to both INP abundance and activation scheme. Enhanced INP concentrations strengthen heterogeneous modes, particularly DEP, leading to DEP-dominated cirrus and suppression of homogeneous nucleation. Reduced INP concentrations promote compensating increases in HOM and FRZ. Secondary ice from rime splintering and collisional breakup dominates number concentrations in low level mixed-phase clouds but is negligible in cirrus. Importantly, altering the INP activation scheme produces changes comparable in magnitude to variations in INP concentration with scaling coefficients between 0.1 and 15, highlighting substantial uncertainty in heterogeneous ice nucleation assumptions. Overall, the partitioning among ice modes is highly sensitive to microphysical assumptions, but the balance between liquid-origin and in-situ cirrus remains primarily controlled by WCB dynamics and sedimentation.