Shear Instability Wave along a Snowband: Instability Structure, Evolution, and Energetics Derived from Dual-Doppler Radar Data

Abstract

This article presents a detailed analysis of a meso-?-scale (?17 km wavelength) shear instability wave along a snowband using a series of dual-Doppler radar data. The wave developed along a low-level shear line that formed under the strain wind field caused by an adjacent mesoscale vortex. The horizontal wind shear across the line was largest at lower levels, and the eddy-component horizontal winds and the retrieved pressure anomaly showed a bottom-intensified structure as well. The resultant vertical pressure gradient force was found to be responsible for the enhancement of alternating updrafts and downdrafts that were subsequently related to the formation of the reflectivity core/gap structure of the wave. Eddy kinetic energy (EKE) budgets of the evolving disturbance were investigated using time series of retrieved kinematic and thermodynamic data. The wave grew at an approximately constant growth rate for about 40 min from its onset. The EKE in this quasi-linear growth period was primarily generated by the horizontal shear that decreased with height. The pressure work was found to remove about two-thirds of this generation in the layer below 1 km, while in the upper layer it was constructive to EKE generation and comparable to the generation of EKE by horizontal shear. These results indicate that the source of EKE was basically located at low levels and the energy was transported upward mainly by the pressure work. After the quasi-linear growth period, horizontal shear generation rapidly decreased and EKE peaked. The buoyancy generation of EKE was small but positive in the quasi-linear growth period, then became negative because of the development of thermally indirect circulations.