The role of ice multiplication processes on high ice water content clouds: Evidence from observations and simulations

Primary ice nucleation mechanisms are rarely sufficient for producing observed concentrations of ice crystals, meaning secondary ice production processes (SIP) must occur under some conditions. Data from the High Altitude Ice Crystals-High Ice Water Content (HAIC-HIWC) field campaigns and related mesoscale model simulations with a property-based microphysics scheme (P3) are used to show multiple different SIP processes must be considered to explain observed ice crystal number concentrations. In particular, a multi-modal gamma fitting routine is used to automatically determine the number of modes and fit parameters of each mode for measured size distributions. These are combined with measured total water contents (TWCs) to characterize how cloud microphysical properties vary with temperature, TWC, and convective and meteorological characteristics (e.g., strength, proximity to convection, surface characteristics, updraft velocities) to assess under what conditions SIP processes might act. Comparison against WRF simulations using existing cloud microphysical parameterization schemes show that the intensity and spatial extent of the observed airborne X-band reflectivity and measured size distributions are not well replicated. Sensitivity tests showed that without modeling a variety of SIP processes, total ice number concentrations were 2 to 3 orders of magnitude less than observed between -10C and -45C. Including only one of three SIP mechanisms separately (i.e., Hallett-Mossop mechanism, fragmentation during ice-ice collisions, and shattering of freezing droplets) cannot replicate observed concentrations, but including all three SIP processes can. An effect of SIP on convective invigoration is also exhibited. Implications for understanding SIP processes and for their representations in models are discussed.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)