Multi-bands in the Comma Head of the 26-27 December 2010 Northeast U.S. Winter Storm

Mesoscale precipitation structures within Northeast U.S. winter storms result in heterogeneous spatial and temporal snowfall throughout the region during any one particular storm. There have been many studies of single-banded snowbands in the comma head, and several successful modeling studies of these bands, but fewer studies of multi-banded events. Multi-bands are defined as > 3 finescale (5–20 km width) bands with periodic spacing and similar spatial orientation maintained for at least 1 h. While multi-bands have been observed to be more transient and shorter-lived structures than single bands, they are capable of producing similar enhanced snowfall rates and wind speeds. The Northeast U.S. blizzard of 26-27 December 2010, also known as the “Boxing Day Storm,” was an exemplary case of multi-banding.

This poster will examine the processes resulting in the genesis, maturity, and decay of the multi-bands by employing available observations and 444-m horizontal grid spacing output from a representative Weather Research and Forecasting (WRF) simulation. Snowband lifecycle was reproduced within the model simulation with a ~2 h timing error. Prior to the formation of multi-bands, the genesis area is dry after the passage and pivot of the single band to the northwest and at the edge of the intrusion of dry air from a cyclonically-curved upper-level jet core. By 2300 UTC, cells are triggered along this moisture gradient from shear induced circulations indicated by 900-hPa Bulk Richardson Number (BRN) < 0.25 in a region of ~35 kts of 700-900 hPa wind shear. At the same time, PV dipoles arise from pre-existing vertical wind shear and the diabatic conversion of latent vorticity to relative vorticity. These PV dipoles take on a linear form oriented parallel to the 700-900-hPa wind shear. By 0045 UTC, there is a pulsing yet quasi-stationary 700 hPa frontogenesis maxima in the multi-band genesis region beneath a ~200 hPa layer of conditional symmetric instability (CSI). Each band that forms is steered towards the northwest by the 700-hPa wind and is collocated with a strong linear PV dipole. New bands are generated at the boundary. Cells and bands formed initially as rain but as they move into the cold air, snow and ice is more dominant and the bands are slow to decay potentially from the slower fallout of these hydrometeors. Some multi-bands retain a strong diabatic signature which can cause localized circulations which may increase the duration of a band. Some multi-bands enter favorable environments and are intensified such as differential cyclonic vorticity advection (CVA) and last long enough to merge with the primary band. The primary band was consistently situated near a linear 700-hPa frontogenesis maximum and did not exhibit a PV dipole signature. By 0500 UTC, dynamical support is lost for both the primary and multi-bands that are situated upstream of the mid-level trough as the system tracks to the northeast.