Abstract
Abstract. The discrepancy between the observed concentration of ice nucleating particles (INPs) and the 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 account for this discrepancy. Here, in a region over the eastern Swiss Alps, we perform sensitivity studies of additional simulated SIP processes on precipitation formation and surface precipitation intensity. The SIP processes considered include rime splintering, droplet shattering during freezing and breakup through ice–graupel collisions. We simulated the passage of a cold front at Gotschnagrat, a peak at 2281 m a.s.l. (above sea level), on 7 March 2019 with the Consortium for Small-scale Modeling (COSMO), at a 1 km horizontal grid spacing, as part of the RACLETS (Role of Aerosols and CLouds Enhanced by Topography and Snow) field campaign in the Davos region in Switzerland. The largest simulated difference in the ICNC at the surface originated from the breakup simulations. Indeed, breakup caused a 1 to 3 orders of magnitude increase in the ICNC compared to SIP from rime splintering or without SIP processes in the control simulation. The ICNCs from the collisional breakup simulations at Gotschnagrat were in best agreement with the ICNCs 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, improved the model performance in terms of surface precipitation over the domain.
Highlights
Clouds consisting only of liquid droplets contribute only a small fraction to the overall precipitation on Earth (Heymsfield et al, 2020)
Depending on the ice crystal number concentration (ICNC) and their size, an updraft of 2 m s−1 may enable a mixed-phase clouds (MPCs) to sustain the simultaneous growth of ice particles and supercooled cloud droplets (Korolev, 2007)
How active secondary ice production (SIP) processes are in wintertime orographic MPCs, and whether they could explain the observed ICNC in a case study of the RACLETS (Role of Aerosols and CLouds Enhanced by Topography and Snow) field campaign
Summary
Clouds consisting only of liquid droplets contribute only a small fraction to the overall precipitation on Earth (Heymsfield et al, 2020). The co-existence of ice crystals and supercooled liquid water droplets in clouds plays a significant role in the formation of precipitation in the midlatitudes (Mülmenstädt et al, 2015). These clouds are known as mixed-phase clouds (MPCs) and can be found in the temperature regime between 0 to approximately −38 ◦C (Korolev and Mazin, 2003; Morrison et al, 2012; Lohmann et al, 2016a). The liquid droplets may eventually evaporate completely in the vicinity of the growing ice crystals This process is called the Wegener–Bergeron–Findeisen process (WBF; Wegener, 1911; Bergeron, 1965; Findeisen et al, 2015) and can lead to rapid partial or full glaciation of MPCs
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