Investigating the contribution of secondary ice production to in‐cloud ice crystal numbers

Abstract

In‐cloud measurements of ice crystal number concentration can be orders of magnitude higher than the precloud ice nucleating particle number concentration. This disparity may be explained with secondary ice production processes. Several such processes have been proposed, but their relative importance and even the exact physics are not well known. In this work, a six‐hydrometeor‐class parcel model is developed to investigate the ice crystal number enhancement, both its bounds and its value for different cloud states, from rime splintering and breakup upon graupel‐graupel collision. The model also includes ice aggregation and droplet coalescence, ice hydrometeor nonsphericity, and a time delay formulation for hydrometeor growth. Conditions to maximize the breakup contribution, as well as the effects of nonsphericity and turbulence, are discussed. We find that the largest enhancement of ice crystal number occurs for “intermediate” conditions, characterized by moderate updrafts and activation and nucleation rates. In this case, vertical motion is strong enough, and new hydrometeor formation limited enough, to sustain supersaturation as hydrometeors grow to larger sizes. After these larger hydrometeors form at sufficient number concentrations, the ice crystal number can be enhanced by a factor of 104 in some cases relative to the number generated by primary ice nucleation alone. Excluding ice hydrometeor nonsphericity limits secondary production significantly, and the parcel updraft can modulate it by about an order of magnitude.

A representation of the collisional ice break-up process in the two-moment microphysics LIMA v1.0 scheme of Meso-NH

“…Using an analytical model, they showed that CIBU could lead to an explosive growth of small ice crystal concentrations. Afterwards, Sullivan et al (2017) tried to include CIBU in a six-hydrometeorclass parcel model, in which hydrometeors were assumed to be monodispersed, in an attempt to investigate the ice crystal number enhancement. However, intriguingly, and in contrast to the Hallett-Mossop ice multiplication mechanism 1 (hereafter H-M) (Hallett and Mossop, 1974), the vast majority of microphysics schemes do not include the CIBU process.…”

Initiation of secondary ice production in clouds

“…S1 in the Supplement, and parameter values and sources are given in Table 1. Model assumptions, thermodynamic tendencies and correlations, and collection kernels are more thoroughly described in Sullivan et al (2017) and their effects more thoroughly discussed below in Sect. 4.…”

The effect of secondary ice production parameterization on the simulation of a cold frontal rainband

“…Sullivan et al (2017) found that the dominance of rime splintering versus collisional breakup was determined by timing of large hydrometeor formation in the liquid versus ice phase. Given the relatively low graupel and hail numbers for this cold frontal rainband (Crosier et al, 2014), it is to be expected then that the rime splintering parameterization is most 25 influential on P tot (Section 4.2).…”

“…where the number of fragments generated ℵ BR is based upon the laboratory data of Takahashi et al (1995) as in Sullivan et al (2017). Within Equation 5, T min is a minimum temperature below which no breakup occurs and γ BR is the decay rate of fragment number at warmer subzero temperatures.…”