The Development of Snow Multi-Bands in High-Resolution Idealized Baroclinic Wave Simulations

Single-banded precipitation structures have been extensively studied in the comma head of winter storms, but there is less understanding of multi-bands. In particular, the mechanisms by which multi-bands initiate, grow, and maintain themselves needs to be better understood. This study uses the idealized baroclinic wave setup from the Weather Research and Forecasting (WRF; Skamarock et al. 2008) model (version 3.4.1), with the same model physics as Norris et al. (2017) and a baroclinically unstable zonal jet derived by Rotunno et al. (1994). Inner nests covering the comma-head region of the developing surface cyclone are added after 108 hours of simulation time, with horizontal grid spacings going down to 800 m.

The 4-km WRF will be the focus of the analysis since this grid generates multi-bands with characteristics similar to the 800-m grid. This talk will highlight the life cycle of these bands as the environmental conditions change, and the band processes. The multi-bands develop northeast of the low center by 120 h as the baroclinic wave amplifies. Leading up to this period, both 700-600-hPa potential instability and 600-500-hPa vertical wind shear increase east of the surface low center. The cellular convection triggered by low-level frontogenesis organizes into segments parallel to the vertical shear from southwest-to-northeast. Once developed, these banded segments propagate northward with the mean flow, bounded by a trowel frontal structure to the east and drier more stable air to the west. This band activity slopes up the frontal zone to ~550 hPa, accompanied by weak frontogenesis. The bands weaken 12-18 h after genesis (by 138 h), as the ambient potential instability is depleted by convection.

A potential vorticity (PV) budget is used to show how the individual bands elongate to the northeast due to interactions between their moist updrafts and the horizontal relative vorticity from the vertical shear. The updrafts tilt the horizontal vorticity, creating a positive vertical vorticity anomaly to the right of the shear vector and a negative anomaly to the left. Meanwhile, the updraft releases latent heat in between the two vorticity anomalies, thus corresponding to potential vorticity (PV) dipole. This band of PV is advected northeastward by the southwesterly winds. Meanwhile, the PV dipoles perturbs the flow such that there is divergence at mid-levels to the northeast of the band, resulting in additional upward motion and growth and maintenance of the band as it moves northward into the cyclone comma head.

References:

Norris, J., G. Vaughan, and D. M. Schultz, 2017: Variability of Precipitation along Cold Fronts in Idealized Baroclinic Waves. Mon. Wea. Rev., 145, 2971-2992, doi: 10.1175/MWR-D-16-0409.1

Rotunno, R., W. C. Skamarock, and C. Snyder, 1994: An Analysis of Frontogenesis in Numerical Simulations of Baroclinic Waves. J. Atmos. Sci., 51, 3373–3398, doi:10.1175/1520-0469(1994)051,3373:AAOFIN.2.0.CO;2.