Sensitivity of precipitation formation to secondary ice production in winter orographic mixed-phase clouds

The discrepancy between the observed concentration of ice-nucleating particles (INPs) and ice crystal number concentration (ICNC) remains unresolved and limits our understanding of ice formation and hence precipitation amount, location and intensity. Enhanced ice formation through secondary ice production (SIP) could be accounting for this discrepancy. Here, we present the results from a sensitivity model study in the Eastern Swiss Alps with additional simulated in-cloud SIP on precipitation formation and consequently on surface precipitation. The SIP processes considered include rime splintering, droplet shattering during freezing and breakup through ice-ice collisions. We simulated the passage of a cold front at Gotschnagrat, a peak at 2281 m above sea level (a.s.l.), on 7 March 2019 with COSMO, at a 1 km horizontal resolution, as part of the RACLETS field campaign in the Davos region in Switzerland. The largest simulated difference in ICNC at the surface originated from the breakup simulations. Indeed, breakup caused a 1 to 3 order of magnitude increase in ICNC compared to SIP from rime splintering or without SIP processes in the control simulations. The ICNCs from the collisional breakup simulations at Gotschnagrat were in better agreement with the ICNC concentrations measured on a gondola near the surface. However, these simulations were not able to reproduce the ice crystal habits near the surface. Enhanced ICNCs from collisional breakup reduced localized regions of higher precipitation and thereby improving the model performance in terms of surface precipitation over the domain.