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.
Highlights
The formation of hydrometeors in clouds is of great importance for the prediction of weather and cloud electrification, as well as for the hydrological cycle and, eventually for the evolution of climate
In this study we have performed numerical simulations of momentum, heat and mass transfer around a warm hydrometeor in order to assess the distribution of supersaturation in its wake and, the implications on ice nucleation enhancement
The hydrometeor is assumed to be of spherical shape and to possess a uniform surface temperature of 0 ◦C, while the ambient temperature has been varied in a range between −15 and 0 ◦C
Summary
The formation of hydrometeors in clouds is of great importance for the prediction of weather and cloud electrification, as well as for the hydrological cycle and, eventually for the evolution of climate. Despite its relevance and great research effort over the past decades, many aspects remain poorly understood One such puzzle is the discrepancy between the concentration of ice particles and that of available ice nuclei (IN) in airborne observations by several orders of magnitude (Pruppacher and Klett, 2010), which has been observed for various cloud types (Koenig, 1963; Auer et al, 1969; Hobbs, 1969; Hobbs and Rangno, 1985; Mossop, 1985; Hogan et al, 2002). Accepted SIP includes rime splintering (Hallett and Mossop, 1974), fragmentation of ice (Vardiman, 1978; Takahashi et al, 1995; Bacon et al, 1998) and freezing drops (Hobbs and Alkezweeny, 1968) Most of these mechanisms have been implemented into cloud models with explicit ice microphysics (see Field et al, 2017, for a recent overview), which, are not capable of satisfactorily explaining the large amount of ice particles in observations. Further mechanisms have been proposed in the past whose relative importance is still to be evaluated
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