Shear-Driven Instabilities as the Origin of Multi-Banded Cloud and Precipitation Structures in an Extratropical Cyclone

This paper investigates the dynamics governing multi-banded cloud and precipitation in extratropical cyclones through a case study from the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. On 1 February 2020, a low-pressure system emerged off the North Carolina coast at 1200 UTC, deepening by 7 hPa in six hours as it accelerated northeast over the Atlantic. High-resolution GOES visible imagery revealed multiple bands of high-reflectance cloud to the north/northeast of the center, along with clusters of convective cells closer to the core. Wavelet analysis identified a dominant multi-band wavelength of 30 km and a secondary peak at 15-20 km. Airborne radar measurements from IMPACTS flights showed deep convection near the center and narrow, elevated reflectivity bands linked to the multi-band features farther out. Numerical simulations reproduced the multi-bands, enabling exploration of their dynamical origin. Intrinsic phase speed calculations revealed that, contrary to several previous studies, the dominant multi-bands were not gravity waves. Instead, the features were identified as dynamic instabilities (Kelvin–Helmholtz instability) arising from vertical wind shear and low Richardson numbers near the upper-level outflow. Gravity waves were present in the low to mid levels (0–6 km) generated by convection, but they did not account for the strong perturbations in the mid to upper levels (6–10 km). This study presents new scientific insight into the governing dynamics of multi-banded structures in extratropical cyclones that highlights the role of shear-driven instabilities.