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

Abstract. Ice crystals are essential in the evolution of mixed-phase clouds, as ice crystals can quickly grow to large sizes by vapor diffusion and thereby trigger precipitation formation. Vapor diffusional growth rates of ice crystals were quantitatively studied in the laboratory for several decades, forming the basis of various ice crystal growth models. Since field measurements generally only provide snapshots that lack information on ice crystal age or changes induced by cloud processes, significant gaps remain in quantitative field observations, impeding the validation of laboratory experiments and models. Our study addresses this gap through innovative glaciogenic cloud seeding experiments in persistent low-level stratus clouds in the CLOUDLAB project. The controllability and repeatability of our seeding experiments facilitates the quantification of diffusional ice crystal growth rates in natural clouds via in situ measurements. We report growth rates of 0.17–0.81 µm s−1 (major axis of pristine ice crystals) from 14 seeding experiments between −5.1 and −8.3 °C. We also observe how microphysical characteristics induce strong variations in the growth rates, e.g., reduced growth rates in seeding-induced regions of high ice crystal number concentrations. For better comparison to laboratory and non-seeded clouds, we developed two filtering methods to isolate growth conditions less affected by the experimental setup. The comparison shows that the temperature-dependent growth rate variations align with laboratory data, whereas absolute laboratory values are higher. Our findings provide valuable insights into the vapor diffusional growth of ice crystals in natural clouds and connect in situ observations with laboratory and modeling studies.