Experimental studies on the freezing of moderate-sized raindrops
Loading...
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
Reuse License
Abstract
Freezing of raindrops does not merely refer to the change in the physical phase from liquid to solid ice, rather plays a pivotal role by influencing multiscale atmospheric processes. It influences precipitation formation and cloud dynamics, contributes to charge separation during thunderstorms, affects atmospheric chemistry and vertical redistribution of trace gases, collectively impacting cloud radiative effects and climate feedback processes. This study probes into the microphysical and chemical aspects governing the vertical redistribution and retention or scavenging of trace gases during the freezing process, alongside a descriptive analysis of freezing timescales. An essential aspect of this study is the implementation of heterogeneous freezing in the immersion mode, wherein freezing is initiated through the activation of ice nucleating particles (INPs), immersed in the drops. Freezing experiments were performed using acoustic levitation of the raindrops inside a walk-in cold room facility.
In the preliminary stage, experiments were carried out to characterize the INPs namely silver iodide (AgI) and chemically treated montmorillonite (MMT) clay powder. These experiments performed at different temperatures and concentrations with AgI as INP, provided a strong foundation for subsequent investigation of chemical retention of trace gases during freezing in the proceeding stage.
Chemical retention during freezing is essentially important to understand the fate of trace gases dissolved in aqueous phase, when they are vertically transported into the upper tropospheric regions during deep convections. Previous studies with micrometer sized (µm) cloud droplets established a strong dependence on the solubility and dissociation of the chemical species, characterized by the effective Henry's law constant (H*). For moderate sized (mm) raindrops, nitric, acetic and formic acids, and 2-nitrophenol were investigated, and no such dependencies on H* have been observed. Rather, physical aspects such as drop size and fast ice-shell formation (within milliseconds) during freezing had a dominant role in the chemical retention of the investigated substances. Freezing timescale analysis via a parameter called retention indicator (RI) provided comparable results with smaller cloud droplets. An updated parameterization for relating RI and retention coefficient R is provided, which now includes both clouds droplets and raindrops.
High chemical retention majorly due to either fast or slow ice-shell formation time provided a concrete path to further investigate the freezing dynamics of a raindrop for the final stage of this dissertation. This goal was realized through modification of the existing acoustic levitator setup with high-speed cameras. Ice-shell formation times, as well as diabatic and adiabatic freezing times were recorded and measured from freezing experiments with different INPs – AgI, illite-NX and feldspar, alongside AgI with different concentrations of dissolved NaOH. Ice-shell formation time was found to be positively correlated with drop freezing temperatures and molar concentration of INPs (at fixed ambient temperatures), and negatively correlated with dissolved NaOH concentrations.
This study contributes to ongoing research on understanding the transport, retention and redistribution of trace gases in chemistry coupled atmospheric models. It further sheds light on the freezing timescales, specifically on ice-shell formation times and its associated dependencies. These findings lay the groundwork to better understand the freezing dynamics, as well as provides a diverse experimental database to refine theoretical studies concerning raindrop freezing.