Observations of Tropical Cyclone Vortex Rossby Waves and Their Role in Inner Core Rainbands

Abstract

Vortex Rossby Waves (VRWs) are hypothesized to exist in tropical cyclones (TCs), acting as an axisymmetrizing mechanism to the inner core TC flow and restored by the radial gradient of storm vorticity. In addition, they have been hypothesized to be a leading mechanism of inner core spiral rainband formation and a mechanism by which the symmetric flow may be enhanced. The majority of the knowledge of VRWs comes from numerical modeling. Only two observational studies performed at coarse temporal and spatial resolutions have previously been published. Since VRWs may influence the structure of the inner core of TCs, additional observations are essential to their understanding. The Shared Mobile Atmospheric Research and Teaching (SMART) radars have sampled multiple landfalling TCs in the United States including Hurricanes Isabel (2003), Frances (2004), and Irene (2011), among others. The data collected over the inner core environments of these three TCs provided the opportunity to examine the inner core structure of TCs outside of the eyewall, including VRW-induced spiral rainbands. Three-dimensional radar wind retrievals indicate that the structure of spiral rainbands was similar to that of numerically simulated VRWs. In Hurricane Isabel, which was particularly well sampled, the measured azimuthal and radial phase speeds were compared to that of VRW theory and were found to be consistent with the theoretical phase speeds. While other mechanisms may produce rainband-like features, the rainbands observed by the SMART radars in three TCs were induced by VRWs. Moreover, the VRW-induced bands were key in producing a significant portion of rainfall within the inner core of TCs. Thus, the current stratiform conceptual model of the inner core does not adequately identify the roles of VRWs. A new conceptual model of the inner core and its associated rainbands is presented to reflect these new observations of the inner core.