The impact of secondary ice processes on orographic mixed-phase clouds

Ground based and airborne observations of orographic mixed-phase clouds (MPCs) forming over mountain top research stations have long reported a discrepancy between the measured ice crystal number concentrations (ICNCs) and the concentration of ice nucleating particles, the former being several orders of magnitude higher (e.g., Lloyd et al., 2015). Additionally, model simulations of Alpine clouds are frequently found to underestimate the amount of ice compared with observations (Farrington et al., 2016). Although surface-based processes such as blowing snow and hoar frost have been suggested to explain this discrepancy, the potential role of secondary ice production (SIP) processes – especially mechanical breakup of cloud ice and droplet fragmentation during freezing – has been less studied. In this study we utilize the Weather Research and Forecasting model (WRF) to explore the potential contribution of SIP processes on the orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the mountain-top site of Jungfraujoch in the Swiss Alps. The only SIP mechanism included in the default version of WRF is the Hallett–Mossop process (H-M), which is however ruled out since the recorded temperatures were generally colder than -8 ˚C. We modified the default WRF to include parameterizations of two additional SIP mechanisms, namely the collisional break-up (BR) upon collisions between ice particles and droplet shattering (DS), in order to investigate if the performance of the model is improved. Simulations suggest that the DS mechanism is not a significant source of ICNCs. The BR mechanism however is quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the BR mechanism is primarily associated with the occurrence of seeder-feeder situations, which are widespread phenomena over Switzerland (Proske et al., 2021). Including a source of ice crystals from the effect of blowing snow episodically affects cloud ICNCs; the numbers reaching cloud base is not large, but the concentrations are multiplied through the action of the BR mechanism. Our findings highlight the importance of considering both secondary ice and an “external” seeding mechanism – primarily falling ice from above and to a lesser degree blowing ice from the surface - in weather-prediction models in order to predict correctly the amount of liquid and ice in MPCs, which is in turn critical for the accurate representation of radiation processes and precipitation patterns. Farrington, R. J., Connolly, P. J., Lloyd, G., Bower, K. N., Flynn, M. J., Gallagher, M. W., Field, P. R., Dearden, C., and Choularton, T. W. (2016). Comparing model and measured ice crystal concentrations in orographic clouds during the INUPIAQ campaign. Atmos. Chem. Phys., 16, 4945–4966, https://doi.org/10.5194/acp-16-4945-2016 Lloyd, G., Choularton, T. W., Bower, K. N., Gallagher, M. W., Connolly, P. J., Flynn, M., Farrington, R., Crosier, J., Schlenczek, O., Fugal, J. and Henneberger, J. (2015). The origins of ice crystals measured in mixed-phase clouds at the high-alpine site Jungfraujoch. Atmos. Chem. Phys., 15, 12953–12969. https://doi.org/10.5194/acp-15-12953-2015 Proske, U., Bessenbacher, V., Dedekind, Z., Lohmann, U., and Neubauer, D. (2021). How frequent is natural cloud seeding from ice cloud layers ( < −35 °C) over Switzerland?. Atmos. Chem. Phys., 21, 5195–5216. https://doi.org/10.5194/acp-21-5195-2021