Re-Organization of Snowfall Beneath Cloud-top Within the Comma Head Region of Two Extreme U.S. East Coast Winter Cyclones

This paper examines ice particle reorganization by three-dimensional horizontal kinematic fl ows within the comma head regions of two U.S. East Coast winter storms and the effect of reorganization on particle concentrations within snowbands in each storm. In these simplified fi ed experiments, the kinematic fl ows are from the initialization of the HRRR model. Ice particles falling through the comma head were started from either 9-, 8-, or 7-km altitude, spaced every 200 m, and were transported north or northwest, arriving within the north or northwest half of the primary snowband in each storm. The greatest particle concentration enhancement within each band was a factor of 2.32-3.84 for the 16-17 December 2020 storm and 1.76-2.32 for the 29-30 January 2022 storm. Trajectory analyses for particles originating at 4 km on the southeast side of the comma head beneath the dry slot showed that this region supplied particles to the south side of the band with particle enhancements of factor of 1.36-2.08 for the 16-17 December 2020 storm and 1.04-2.16 for the 29-30 January 2022 storm. Snowfall within the bands had two source regions: 1) on the north/northwestern side, from ice particles falling from the comma head, and 2) on the southeastern side, from particles forming at or below 4-km altitude and transported northwestward by low-level fl ow off the Atlantic. While the fi ndings give information on the source of particles in the bands, they do not definitively fi nitively determine the cause of precipitation banding since other factors, such as large-scale ascent and embedded convection, also contribute to snow growth.