Snow Bands and Velocity Waves Within Coastal Northeast U.S. Snow Storms

The spatial pattern of snowfall accumulation is difficult to forecast accurately. Many disparate microphysical, thermodynamic, and kinematic factors can influence the snow that reaches the ground. The residence times of individual precipitation-sized ice particles are usually an hour or more. Hence, the observed microphysics at any one time represents the slower time-integrated result of the faster responding kinematics. Snow bands tend to form toward the middle of the snow storm life cycle after several hours of more uniform, weaker precipitation. A key source of snow mass within these storms are small scale (few km) over-turning generating cells which are nearly ubiquitous in the upper levels of snow storm radar echoes. Generating cells are usually too small to be discerned with scanning operational radar and require observations obtained at smaller spatial scales.

Locally intense snowfall within mesoscale snow bands can have large impacts on snow accumulation. Previous work has shown that long snow bands that occur one at a time (> 250 km long, > 20 km wide), termed single bands, are associated with mid-level frontogenesis and relatively weak stability. We have examined over 100 coastal northeast U.S. snow storms from Delaware to Maine using a combination of operational WSR-88D scanning radar data, soundings, vertically-pointing radar (MicroRainRadar), and reanalysis. Smaller snow bands (< 250 km long, 10-20 km wide) that occur in sets, termed multi-bands, are neither consistently associated with frontogenesis nor clear, sustained convergence signatures which would be required for sustained updrafts at the scale of individual multi-bands. In contrast, rain bands typically do have associated, sustained convergence signatures.

Placing the storms into a Lagrangian framework relative to the motion of the low pressure center reveals that sets of multiple snow bands can be relatively stationary or move radially away from the low. Multi-bands that are stationary relative to the low pressure center move parallel to the motion of the low. Multi-bands that move radially away from the low pressure center move perpendicular to the motion of the low. When single bands are present with radially moving multi-bands, the multi-bands can converge with the single band, appearing to fuel the single band.

Moving bands of velocity change within snow storms were identified using the temporal difference between Doppler radar velocity fields. We refer to these features as waves in the generic sense. These waves are consistent across adjacent radar domains and appear to originate outside of the precipitation echo. The waves often move radially outward from the vicinity of the low but typically several m/s faster than the motion of radially moving multi-bands in the same storm. Seventy percent of cases with multi-bands (with or without coexisting single bands) are associated with waves. The waves may be gravity waves or shear instabilities. Comparison of waves within multiple elevation angles indicates vertical depths of 2 to 10 km with a mean of 4.5 km. Most of the time the wave depth is much larger than the expected depth of the shear instability or inversion (wave duct).

The mechanisms responsible for movement of snow bands radially away from the low are unclear since synoptic air flows at the altitudes where precipitation-sized particles are present do not typically move in that direction. The frequent co-occurrence, similar orientations, and direction of motion of the velocity waves and multi-band snow bands are intriguing and suggest new avenues for exploration of the origination and maintenance of mesoscale snow bands which are not associated with frontogenesis. While appearing similar in radar reflectivity, multi-bands that are stationary relative to the low versus those that move radially away may be subject to different origination and maintenance mechanisms.