Quantifying ice crystal growth rates in natural clouds from glaciogenic cloud seeding experiments

Ice crystals in mixed-phase clouds (MPCs) can grow rapidly to large sizes by vapor deposition via the Wegener-Bergeron-Findeisen (WBF) process, i.e., growth of ice crystals at the expense of cloud droplets. This rapid growth can trigger subsequent processes such as riming and aggregation, which often initiate precipitation, making MPCs the major source of precipitation over continents. The growth of ice crystals has been thoroughly studied in the laboratory for many decades and several theoretical models were developed on their basis. However, in situ measurements of growth rates to confirm laboratory studies are still sparse due to the lack of controllability of experiments in natural clouds.

In the CLOUDLAB project, we conducted confined, controlled, and repeatable glaciogenic cloud seeding experiments to study ice crystal growth in natural clouds. A drone released seeding particles in supercooled low stratus clouds to initiate the formation of ice crystals. The freshly formed ice crystals were observed 5-10 minutes downwind of the seeding location using cloud radars and holographic imager for in situ observations. The holographic imager obtains phase-resolved information on cloud droplets > 6 µm and ice crystals > 25 µm with a high spatio-temporal resolution, which allows us to quantify accurate ice crystal growth rates.

In this study, we present ice crystal growth rates obtained from in-situ observations from 14 seeding experiments in the temperature range between -5.1°C and -8.3°C. During the seeding experiments, ice crystal number concentrations (ICNC) increased by several orders of magnitude, accompanied by a strong reduction in cloud droplet number concentration, a clear indicator of the WBF process.  We also observed that high ICNCs limit or inhibit ice growth due to the competition of the ice crystals for the available water vapor.  The obtained ice crystal growth rates are compared with laboratory studies and show on average slightly lower values. We also observed the expected variation in growth rates across our temperature range agreeing with laboratory findings. These findings can connect laboratory studies and in situ observations and provide valuable insights into vapor depositional growth of ice crystals in natural clouds.