Chain Aggregate Particles in Mid-to-upper Tropospheric Clouds during IMPACTS - 15 January 2023 Case Study

Elongated, chain-like aggregates comprised of ice crystals and frozen droplets have been observed (in-situ) in mid- to upper-level clouds produced by tropical (Northern Australia), sub-tropical (Florida), and mid-latitude (Colorado) summertime thunderstorms. Similarly, chain aggregates have also been generated in several cloud chamber experiments using high electric field strengths, varying ice particle concentrations, and temperatures. Results from the various cloud chamber experiments are consistent in that chain aggregation is temperature dependent and is enhanced in an electric field greater than approximately 50 kV m^-1. However, there are several nuances when relating the experiments to atmospheric processes and the in-situ observations. While it is believed that electric fields are important for chain aggregate formation, exactly where and how the chain aggregation process occurs within storms is not well understood. The inability to parameterize the formation processes of chain aggregates in cloud models causes inaccuracies in cloud radiative transfer properties.

Recently during the NASA-funded Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign, the “storm penetrating” NASA P-3 Orion and “satellite simulating” ER-2 research aircraft were used to acquire microphysical and remote sensed observations, respectively. These observations support the project’s objective to enhance our understanding of winter storms. Interestingly, chain aggregates comprised of ice crystals were observed by airborne microphysical probes during multiple flights in the mid-to-upper levels of nor’easter and extra-tropical wintertime storms. These are very unique observations made with state-of-the-art cloud probes.

During the IMPACTS 15 January 2023 research flight, imaging probes on the P-3 Orion observed chain aggregates on multiple occurrences at different altitudes. The chain aggregates observed during this flight are strikingly similar to those observed in summertime thunderstorms. Coincidentally, between 16:04-16:24 and 17:16-17:26 UTC chain aggregates were observed while the P-3 and ER-2 research aircraft were co-located. This analysis relates the in-situ microphysical properties during these periods to radar (reflectivity and linear depolarization ratio) and lidar (backscatter and depolarization ratio) measurements taken from the ER-2. Further insight into the conditions conducive to support the formation of chain aggregates is also discussed.