Analysis of Snow Multi-Bands and Their Environments with High-Resolution Idealized Simulations

Nested idealized baroclinic wave simulations at 4-km and 800-m grid spacing are used to analyze the precipitation structures and their evolution in the comma head of a developing extratropical cyclone. After the cyclone spins up by hour 120, snow multi-bands develop within a wedge-shaped region east of the near-surface low center within a region of 700-500-hPa potential and conditional instability. The cells deepen and elongate northeastward as they propagate north. There is also an increase in 600-500-hPa southwesterly vertical wind shear prior to band development. The system stops producing bands 12 hours later as the differential moisture advection weakens, and the instability is depleted by the convection. Sensitivity experiments are run in which the initial stability and horizontal temperature gradient of the baroclinic wave are adjusted by 5-10%. A 10% decrease in initial instability results in less than half the control run potential instability by 120 h and the cyclone fails to produce multi-bands. Meanwhile, a 5% decrease in instability delays the development of multi-bands by 18 h. Meanwhile, decreasing the initial horizontal temperature gradient by 10% delays the growth of vertical shear and instability, corresponding to multi-bands developing 12-18 hours later. Conversely, increasing the horizontal temperature gradient by 10% corresponds to greater vertical shear, resulting in more prolific multi-band activity developing ∼12 hours earlier. Overall, the relatively large changes in band characteristics over a ∼12-hour period (120-133 h) and band evolutions for the sensitivity experiments highlight the potential predictability challenges.