Applying and Evaluating Random Forest Classification to Identify Ice Crystal Chain Aggregates During the IMPACTS Field Campaign

In-situ observations of electrically induced aggregation of cloud ice and/or frozen droplets has primarily been observed in mid- to upper-level clouds of summertime storms. These aggregates, distinguished by their elongated, quasi-linear structure, are specifically termed as chain aggregates. Cloud chamber experiments reveal that chain aggregation is temperature-dependent, and their formation is enhanced in an electric field exceeding approximately 60 kV m^-1. However, various complexities arise when connecting the laboratory experiments to in-situ observations. While there is evidence that significant electric fields are required for chain aggregate formation, the precise locations and the mechanisms for chain aggregation within storms remain poorly understood. This knowledge gap hinders the accurate parameterization of chain aggregate formation processes in cloud models, leading to inaccuracies in cloud radiative transfer properties.

During the recent Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign, airborne microphysical (imaging) probes onboard the NASA P-3 Research Aircraft observed chain aggregates across multiple research flights in the mid-to-upper levels of nor’easter and extra-tropical winter storms. A Random Forest classification scheme, that employs straightforward morphological properties of particles captured by the Cloud Particle Imager (CPI) probe, achieved 89% accuracy in identifying chain aggregates. Although this accuracy was based on a subset of in-situ flight legs (approximately 42 minutes of sampling), it demonstrates an efficient method for quantifying and detecting chain aggregate occurrences. This study improves upon the existing Random Forest classification scheme by integrating data from other in-situ instruments aboard NASA P-3, such as the magnitude of vertical velocity, for identifying the presence of chain aggregates. The improved scheme is then applied to the CPI particle images obtained from other IMPACTS flights to assess the abundance and distribution of chain aggregates in winter storms. Additionally, this analysis seeks to understand the impact of chain aggregates on corresponding collocated 'satellite-simulating' ER-2 radar and lidar observations.