The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. XI: Comparisons between Observational and Theoretical Aspects of Rainbands

Abstract

Comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale. The warm-sector flow ahead of the cold front and above the cold-frontal zone aloft was unstable to conditional symmetric instability, and theoretical predictions for this mechanism am consistent with several aspects of the warm-sector and wide cold-frontal rainbands. In the case of the warm-sector rainbands, other mechanisms (e.g., wave-CISK and mixed dynamic/convective instabilities) may have also played a role. The core structure of the narrow cold-frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front. Also, many aspects of the narrow cold-frontal rainband were similar to a density current. Shear-induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold-frontal zone aloft. The orientation of the postfrontal rainbands suggests that energy from the mean flow was responsible for their organization. Convection, in the presence of horizontal temperature gradients and vertical shear, could explain the existence of the postfrontal rainbands through either wave-CISK or a mixed dynamic/convective instability. Since the postfrontal rainbands are often aligned along the thermal gradient, the symmetric instabilities may also play a role in their formation. Buoyant vertical motions under relatively uniform conditions can explain the hexagonally-shaped convective cells observed well behind the cold front.