Abstract
Abstract. Disparities between the measured concentrations of ice-nucleating particles (INPs) and in-cloud ice crystal number concentrations (ICNCs) have led to the hypothesis that mechanisms other than primary nucleation form ice in the atmosphere. Here, we model three of these secondary production mechanisms – rime splintering, frozen droplet shattering, and ice–ice collisional breakup – with a six-hydrometeor-class parcel model. We perform three sets of simulations to understand temporal evolution of ice hydrometeor number (Nice), thermodynamic limitations, and the impact of parametric uncertainty when secondary production is active. Output is assessed in terms of the number of primarily nucleated ice crystals that must exist before secondary production initiates (NINP(lim)) as well as the ICNC enhancement from secondary production and the timing of a 100-fold enhancement. Nice evolution can be understood in terms of collision-based nonlinearity and the “phasedness” of the process, i.e., whether it involves ice hydrometeors, liquid ones, or both. Ice–ice collisional breakup is the only process for which a meaningful NINP(lim) exists (0.002 up to 0.15 L−1). For droplet shattering and rime splintering, a warm enough cloud base temperature and modest updraft are the more important criteria for initiation. The low values of NINP(lim) here suggest that, under appropriate thermodynamic conditions for secondary ice production, perturbations in cloud concentration nuclei concentrations are more influential in mixed-phase partitioning than those in INP concentrations.
Highlights
Number concentrations of ice-nucleating particles (INPs, NINP) in the atmosphere span orders of magnitude from a few per cubic meter up to hundreds per liter (e.g., DeMott et al, 2010)
Even when INP concentrations are low at warm subzero temperatures, incloud ice crystal number concentrations (ICNCs) can be orders of magnitude higher (e.g., Hallett and Mossop, 1974; Heymsfield and Willis, 2014; Lasher-Trapp et al, 2016; Taylor et al, 2016; Ladino et al, 2017), in tropical maritime clouds (Koenig, 1963, 1965; Hobbs and Rangno, 1990)
A basic representation of large droplet coalescence is employed at temperatures above 273 K, given the importance of droplet size distribution broadening to droplet shattering (Lawson et al, 2017): Nd is reduced by 5 % every minute due to coalescence, and the mass is redistributed among the remaining large droplets
Summary
Number concentrations of ice-nucleating particles (INPs, NINP) in the atmosphere span orders of magnitude from a few per cubic meter up to hundreds per liter (e.g., DeMott et al, 2010). Even when INP concentrations are low at warm subzero temperatures, incloud ice crystal number concentrations (ICNCs) can be orders of magnitude higher (e.g., Hallett and Mossop, 1974; Heymsfield and Willis, 2014; Lasher-Trapp et al, 2016; Taylor et al, 2016; Ladino et al, 2017), in tropical maritime clouds (Koenig, 1963, 1965; Hobbs and Rangno, 1990) This discrepancy may be explained in some cases by shattering upon cloud probe tips (Field et al, 2003; Heymsfield, 2007; McFarquhar et al, 2007), but even as instrumentation and algorithms have been developed to minimize these artifacts (Korolev et al, 2013; Korolev and Field, 2015), the disparity has remained, supporting several hypothesized secondary ice production processes. We provide more comprehensive estimates of NI(NliPm) here for three secondary production processes over a range of thermodynamic conditions and fragment numbers
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