On the ice-nucleating potential of warm hydrometeors in mixed-phase clouds

Abstract

Abstract. The question as to whether or not the presence of warm hydrometeors in clouds may play a significant role in the nucleation of new ice particles has been debated for several decades. While the early works of Fukuta and Lee (1986) and Baker (1991) indicated that it might be irrelevant, the more recent study of Prabhakaran et al. (2020) suggested otherwise. In this work, we attempt to quantify the ice-nucleating potential using high-fidelity flow simulation techniques around a single hydrometeor and use favorable considerations to upscale the effects to a collective of ice particles in clouds. While we find that ice nucleation may be significantly enhanced in the vicinity of a warm hydrometeor and that the affected volume of air is much larger than previously estimated, it is unlikely that this effect alone causes the rapid enhancement of ice nucleation observed in some types of clouds, mainly due to the low initial volumetric ice concentration. Furthermore, it is demonstrated that the excess nucleation rate does not primarily depend on the rate at which cloud volume is sampled by the meteors' wakes but is rather limited by the exposure time of ice-nucleating particles to the wake, which is estimated to be of the order of few microseconds. It is suggested to further investigate this phenomenon by tracking the trajectories of ice-nucleating particles in order to obtain a parametrization which can be implemented into existing cloud models to investigate second-order effects such as ice enhancement after the onset of glaciation.

Population Distribution in the Wake of a Sphere

“…Quantification of scalar transport in the wake of spherical objects is important for understanding various physical phenomena. For example, Bhowmick et al (2020) [27], and also in Chouippe et al (2019) [23] and Krayer et al (2020) [28], scalar transport in the wake is used to understand the spatial distribution of supersaturation in the wake of precipitating cloud hydrometeors. By scaling the passive scalars as the temperature and the water vapor density fields around the droplets, we used three dimensional population distribution of supersaturation also in Bhowmick et al (2020) [27] to quantify the supersaturated volume produced in the wake of precipitating cloud hydrometeors in presence of a sufficient temperature gradient in a slightly subsaturated cloudy ambient, which can activate cloud aerosols and thus contribute to the cloud life cycle.…”

“…Descriptive statistics on the spatial structure of the wake is of primary importance if the extent of the wake with certain properties needs to be quantified. Supersaturation in the wake of a precipitating cloud water droplets [27,28] for example requires a detailed analysis of the wake population. In this paper, we present a comprehensive numerical study on the details of the momentum and scalar transport in the wake of a sphere using a population density distribution for the steady axisymmetric and oblique wake regimes.…”