Chain Aggregate Particles in Upper-Tropospheric Clouds

Elongated, chain-like aggregates comprised of ice crystals and frozen droplets have been observed in cirrus cloud anvils produced by electrified, summertime thunderstorms. Ice crystal chain aggregates have mainly been found in tropical and sub-tropical convection induced cirrus anvils, while frozen droplet chain aggregates have mainly been observed in mid-latitude continental convection induced cirrus anvils. Not having chain aggregate formation processes parameterized in cloud models causes inaccuracies in cloud radiative transfer properties. Furthermore, chain aggregates are important to consider for nose cone erosion during supersonic flight. While it is believed that electric fields are important for chain aggregate formation, exactly where and how the chain aggregation process occurs within thunderstorms is not well understood. During the CapeEx19 field campaign in the summer of 2019 near Melbourne, Florida, the North Dakota Citation II Research Aircraft utilized state-of-the-art instrumentation and sampled cirrus anvils over multiple days in order to gain a better understanding into the chain aggregation process. Results from the case study on the 3 August 2019 flight during CapeEx19 suggests that chain aggregation may be occurring in multiple regions of the thunderstorm including convective cores and cirrus anvils. In-situ electric field observations in the cirrus anvil region during the 3 August 2019 flight are seldom as high as to the electric field strengths utilized in the cloud chamber experiments to generate ice crystal chain aggregates. Until now there have been no reports of chain aggregates in wintertime storms. However, during the recent Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign, chain aggregates comprised of ice crystals were observed by airborne microphysical probes in the upper levels of nor'easters on multiple research flights. These storms were not strongly electrified. An analysis is performed to investigate the presence of chain aggregates in upper-level clouds for summertime and wintertime storms to provide insight into the necessary conditions that support chain aggregation.