The Impact of Potential Vorticity Dipoles on the Life Cycle of Snow Multibands

The life cycle of multibanded precipitation structures is closely examined using an idealized baroclinic wave simulation down to 4-km grid spacing. The model develops a wedge-shaped region of multibanded precipitation east of the near-surface low center within a region of 700–500-hPa potential instability and 600–500-hPa southwesterly vertical wind shear. Cells that develop near the southern tip of the wedge elongate into southwest–northeast-oriented bands as they move northward with the mean flow and then dissipate several hours later as they move further northward within the cyclone comma head. Using a band-following framework and a potential vorticity (PV) budget, the processes resulting in band genesis, growth, and decay are investigated. First, the cell’s moist updraft from below 600 hPa redistributes the horizontal vorticity within the 600–500-hPa layer into the vertical, which combined with latent heat release results in a horizontal PV dipole around the cell. This PV is advected northeastward at midlevels, causing the dipole to extend from the cell. Flow perturbations between the two PV anomalies result in 600–500-hPa divergence northeast of the cell and an elongated region of upward motion and the genesis of the precipitation band. The PV dipole and band continue to intensify primarily from latent heating. As the band moves northward away from the 700–500-hPa potential instability, diffusion and turbulent mixing weaken the PV dipole and the circulations maintaining the band. As the updraft subsequently weakens, snow fallout persists for about 1–2 h before the band fully dissipates.