1. Introduction

The growth of ice crystals under fixed ambient temperature and relative humidity conditions is well established. Libbrecht (2017) summarizes these growth behaviors in the ice crystal habit diagram. For example, at temperatures between $-10^\circ\,\text{C}$ and $-22^\circ\,\text{C}$, ice crystals predominantly form plate-like structures. At low supersaturations with respect to ice, simple plates develop, while increasing supersaturation promotes branching, leading to dendritic structures. At warmer temperatures, between $-4^\circ\,\text{C}$ and $-10^\circ\,\text{C}$, columnar growth becomes dominant: low supersaturation produces solid columns, higher supersaturation yields hollow columns, and very high supersaturation results in needle-like crystals.

In the atmosphere, however, ice particles do not grow under constant conditions. As they sediment toward the ground, they pass through multiple growth regimes. The transition from columnar to plate-like growth, resulting in capped columns, is well documented (e.g., Libbrecht 2017 and references therein). In contrast, the inverse process—when a plate enters the columnar growth regime—is less well understood. Pasquier et al. (2023) observed complex particles where plate-like structures form on the six corners of an initial plate.

To date, to the best of our knowledge, no studies have investigated the growth of a dendrite that subsequently enters the needle growth regime. It remains unclear how needle structures develop on a pre-existing dendrite. This uncertainty poses challenges for models that incorporate habit-dependent depositional growth (for example, the 1D super particle model McSnow used in the PROM project FRAGILE), especially given that dendrites play a crucial role in atmospheric processes such as aggregation and secondary ice production (e.g., von Terzi et al., 2022).

Understanding how a dendrite evolves as it sediments through the needle growth regime is therefore essential for improving our knowledge of these processes. During our research stay at the cold chamber of the University of Mainz, we investigated the growth behavior of dendritic crystals subjected to needle growth conditions.

2. Experimental setup

The experiments were conducted over the course of one week in the cold chamber at the University of Mainz. Ice particles were grown above an aquarium containing approximately a 1 cm high liquid water layer. The water was maintained at a temperature slightly above freezing using a heating mat placed beneath the aquarium. Water vapor evaporating from the surface provided the necessary humidity for ice crystal growth above the water.

To better control the vapor flow, a styrofoam lid with a small central opening was positioned over the aquarium. This configuration channelled the rising water vapor through the opening, creating a localized region suitable for controlled crystal growth. Ice crystals formed within this opening on a 100 μm nylon string.

The temperature and relative humidity were continuously monitored at the same height as the nylon string to ensure accurate characterization of the growth environment. A schematic of the experimental setup is shown in Figure 1.

3. Experiments

3.1. The growth of dendritic structures

Dendritic structures were observed to grow rapidly at temperatures colder than $-10^\circ\,\text{C}$ on various surfaces, provided that the supersaturation with respect to ice exceeded approximately 20%. Even at warmer temperatures, above $-10^\circ\,\text{C}$, branched morphologies appeared under high supersaturation conditions, consistent with the findings of Libbrecht (2017).

During the experiments, it became evident that producing a single isolated crystal was challenging. Therefore, we adopted an alternative approach in which dendritic branches were allowed to grow directly on the nylon string.

3.2. Placing a dendritic particle into the needle growth regime

A total of 13 experiments were conducted in which dendritic structures were first grown on the nylon string at temperatures between $-12^\circ\,\text{C}$ and $-17^\circ\,\text{C}$. After initial growth, the cold chamber temperature was gradually increased until $-7^\circ\,\text{C}$ was reached above the aquarium, corresponding to the needle growth regime. Each experiment was recorded on video, and following growth at $-7^\circ\,\text{C}$, the particles were examined under a microscope. The videos are available here: http://leonie.von-terzi.de/research/fun_snow/. The results from Experiments 4 and 8 are presented here as representative examples.

Experiment 4

Dendritic structures were grown at temperatures between $-12^\circ\,\text{C}$ and $-17^\circ\,\text{C}$ for approximately 20 minutes to produce large, well-developed dendrites. The chamber was then slowly warmed, reaching the needle growth regime (around $-10^\circ\,\text{C}$) after approximately 10 minutes. The particles were subsequently maintained in this regime for 20 minutes to observe further development.

Figure 3 shows the dendritic particles at $-15^\circ\,\text{C}$ and at $-7^\circ\,\text{C}$. Upon entering the needle regime, the dendrites appeared to become denser and more compact, with the individual branches becoming less distinct. The overall structure appeared more clumped.

Experiment 8

Following the same procedure as in Experiment 4, dendrites were initially grown at $-15^\circ\,\text{C}$ for approximately 20 minutes, after which the temperature was gradually increased, reaching the needle growth regime after about 10 minutes. The crystals were then held under these conditions for an additional 20 minutes.

A similar loss of distinct dendritic features was observed. The particles became more compact, and the branch structure less pronounced. Figure 4 presents microscope images of the dendrites taken at $-12^\circ\,\text{C}$, prior to warming, and again after 20 minutes of growth at $-7^\circ\,\text{C}$. These images show the formation of columnar structures growing on the existing dendritic branches. The added columnar growth appears to fill the space between branches, resulting in a denser, clumpier overall morphology.

3.3 Sequential Growth at $-15^\circ\,\text{C}$, $-7^\circ\,\text{C}$, and $-15^\circ\,\text{C}$

In the 14th experiment, a dendritic crystal was first grown at $-15^\circ\,\text{C}$. The chamber temperature was then increased to $-7^\circ\,\text{C}$, allowing needle-like structures to develop on the dendrite. Subsequently, the particle was carefully transferred onto tweezers, and close-up images were taken as the chamber was cooled back down to $-15^\circ\,\text{C}$.

Figure 5 shows the particle at the end of the experiment, following the second growth phase at $-15^\circ\,\text{C}$. Remarkably, the overall dendritic framework remained largely intact. Needle-like extensions were visible, with slightly broadened tips indicating continued deposition.

However, close-up images (Figure 6) revealed the detailed evolution of the structure: during the $-7^\circ\,\text{C}$ phase, needle-like “fingers” had formed along the dendritic branches. As the temperature decreased again to $-15^\circ\,\text{C}$, plate-like structures began to grow at the tips of these needles, resulting in capped needles.

4. Conclusions

The experiments have shown that when a dendrite falls into the needle growth regime, columnar particles start to grow at each branch of the dendrite, thereby effectively filling in the space between the arms. This effect probably increases the density of the particle, and in extreme cases could change the appearance of the dendrite. After these experiments, the question arises, if this fill-in effect could also happen for aggregates. We suspect that the frequent observations of no distinct monomers inside aggregates collected at the ground might be partially explained by our results of “monomer metamorphosis” when growing in different temperature regimes. More important, the habit metamorphosis might also explain the too low densities predicted by aggregation models, such as Leinonen and Moisseev 2015, and measured aggregates at the ground.

Further laboratory studies on this topic should be undertaken, with a focus on precise control of the temperature and humidity as well as mass and size measurements of the grown crystals.

All videos of the growth of ice particles can be found here: http://leonie.von-terzi.de/research/fun_snow/.