EGU21-14659, updated on 12 May 2021
https://doi.org/10.5194/egusphere-egu21-14659
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

The relative contribution of secondary ice processes in Alpine mixed-phase clouds

Paraskevi Georgakaki1, Georgia Sotiropoulou1,2, Etienne Vignon3, Alexis Berne4, and Athanasios Nenes1,5
Paraskevi Georgakaki et al.
  • 1Laboratory of Atmospheric Processes and their Impacts (LAPI), School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland (paraskevi.georgakaki@epfl.ch)
  • 2Department of Meteorology, Stockholm University & Bolin Center for Climate Research, Stockholm, Sweden (georgia.sotiropoulou@epfl.ch)
  • 3Laboratoire de Météorologie Dynamique (LMD), CNRS, Paris, France (etienne.vignon@lmd.ipsl.fr)
  • 4Environmental Remote Sensing Laboratory (LTE), School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland (alexis.berne@epfl.ch)
  • 5Center for Studies of Air Quality and Climate Change, Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Patras, Greece (athanasios.nenes@epfl.ch)

In-situ observations of mixed-phase clouds (MPCs) forming over mountain tops regularly reveal that ice crystal number concentrations (ICNCs) are orders of magnitude higher than ice-nucleating particle concentrations. This discrepancy has often been attributed to the influence of surface processes such as blowing snow and airborne hoar frost. Ιn-cloud secondary ice production (SIP) processes may also explain this discrepancy, but their contribution has received less attention.
Here we explore the potential role of SIP processes on orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the mountain-top site of Jungfraujoch in the Swiss Alps using the Weather Research and Forecasting model (WRF). The Hallett-Mossop (H-M) mechanism, included in the default version of the Morrison scheme in WRF, is ruled out since the simulated clouds were outside the active temperature range for this process. This study investigates if the implementation of two additional SIP mechanisms in WRF, namely collisional break-up (BR) between ice hydrometeors and frozen droplet shattering (DS), can bridge the gap between observed and modeled ICNCs. DS is inefficient in the examined conditions due to a lack of sufficiently large raindrops to trigger this process. The BR mechanism is likely important in Alpine MPCs, but the process is activated only within seeder-feeder situations, when precipitation particles are seeding the low-level MPCs inducing their glaciation. At times when a cloud exists near the ground, blowing snow ice particles may be mixed among supercooled liquid droplets and thus contribute significantly to ice growth, but they cannot account for the observed ICNCs. Our findings indicate that outside the H-M temperature range, ice-seeding and blowing snow can initiate ice multiplication in the Alps through the BR mechanism, which is found to elevate the modeled ICNCs up to 3 orders of magnitude, providing a better agreement with in-situ measurements. This highlights the importance of considering both SIP and surface-based processes in weather-prediction and climate models.

How to cite: Georgakaki, P., Sotiropoulou, G., Vignon, E., Berne, A., and Nenes, A.: The relative contribution of secondary ice processes in Alpine mixed-phase clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14659, https://doi.org/10.5194/egusphere-egu21-14659, 2021.

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