Local Interactions by Diffusion between Mixed-Phase Hydrometeors: Insights from Model Simulations

Abstract

Di usion of water vapor is the dominant growth mechanism for small water droplets and ice crystals in clouds. In current cloud models, Maxwell's theory is used for describing growth of cloud particles. In this approach the local interaction between particles is neglected; the particles can only grow due to changes in environmental conditions, which are assumed as boundary conditions at in nity. This assumption is meaningful if the particles are well separated and far away from each other. However, turbulent motions might change the distances between cloud particles and thus these particles are no longer well separated leading to direct local interactions. In this study we develop a reference model for investigating the direct interaction of cloud particles in mixed-phase clouds as driven by di usion processes. The model is numerically integrated using nite elements. Additionally, we develop a numerical method based on generalized nite elements for including moving particles and their direct interactions with respect to di usional growth and evaporation. Several idealized simulations are carried out for investigating direct interactions of liquid droplets and ice particles in a mixed-phase cloud. The results show that local interaction between cloud particles might enhance life times of droplets and ice particles and thus lead to changes in mixed-phase cloud life time and properties.

Towards a bulk approach to local interactions of hydrometeors

“…is incorporated, and the remaining water vapor is released to the atmosphere. In this study, we use a value n = 1.8 obtained with the help of direct numerical simulations using the reference model described in Baumgartner and Spichtinger (2017), since the authors are not aware of any direct measurements. We compare the temporal evolution of the ice crystal and droplet mass obtained from the reference model with results from the model presented in Sect.…”

“…In Baumgartner and Spichtinger (2017), the authors documented the largest effect of surrounding droplets on ice growth in an ice subsaturated environment because the evaporating droplets can deliver enough water vapor towards the ice crystal to produce a local supersaturation with respect to ice, allowing the crystal to grow instead of evaporate. With the new model, we also observe a similar behavior.…”

“…according to the environmental rather than the local conditions. In Baumgartner and Spichtinger (2017), we investigated the impacts of local interactions by diffusion between an ice crystal and surrounding droplets qualitatively using a reference model that resolves the hydrometeors as well as the vapor and temperature fields. For convenience, we put the results from Baumgartner and Spichtinger (2017) into a "guiding schematic" illustrating the local water vapor field configurations; see Fig.…”

“…In both cases, Maxwellian growth theory is not applicable. Apart from numerical simulations, the question of the solvability of the underlying governing equations is addressed in Baumgartner and Spichtinger (2018); for a reduced model problem existence and uniqueness of solutions could be proven. In this study, we focus on a theoretical description of local interactions between an ice crystal and nearby droplets suited to be incorporated into a bulk-microphysical formulation.…”