Weather and Climate Systems

Mobile Radar Observations of Vortex Rossby Waves in Three Landfalling Tropical Cyclones

Addison Alford

School Of Meteorology

27 April 2016, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Vortex Rossby Waves (VRWs) propagating along the radial gradient of storm vorticity are hypothesized to be an axisymmetrizing mechanism of tropical cyclone (TC) eyewalls. Their structure has been examined mostly via numerical modeling. Observational analyses of VRWs have been limited by coarse temporal and/or spatial resolutions. In this presentation, high temporal (3-10 minutes) and high spatial resolution (0.5-1 km) data from the Shared Mobile Atmospheric Research and Teaching (SMART) radars are used to document the structure and behavior of VRWs within the inner core of three landfalling TCs: Hurricane Isabel (September 2003), Hurricane Frances (September 2004), and Hurricane Irene (August 2011).

Kinematic analyses of inner core spiral rainbands reveal a wave number 1 asymmetry in the vertical velocity field associated with outward-propagating spiral rainbands. Updrafts are analyzed to be 6-8 km deep, deeper than expected in the nearly moist-neutral environment of the inner core. Azimuthally elongated vorticity maxima are apparent and found to be about a quarter-wavelength behind the vertical velocity maxima, consistent with theoretical and numerical modeling studies of VRWs. Additionally, the radial wavelength becomes smaller as the wave propagates outward due to shearing of the wave by the horizontal wind. This causes the vorticity maxima to decay in amplitude over time and slow near the stagnation radius, or the radius at which the storm vorticity gradient becomes small, reducing the ability of the wave to propagate. In addition to the kinematic structure, the observed phase speed of these waves is compared to the phase speed from VRW theory. It was found that the dispersion relation from VRW theory satisfies observations from the SMART radars to a high degree of certainty, particularly for Hurricane Isabel in which 3-minute dual-Doppler analyses are available. Given the strong similarity of SMART radar observations to theory, it seems that VRWs are responsible for at least some inner core rainbands. Additionally, simple means of differentiating stratiform and convective contributions to rainfall are used to estimate VRW-induced rainbands’ rainfall contribution to the inner core region. Based on the case studies presented, a new conceptual model of the inner core rainbands will be presented, discussing the differences from the previously understood structure of TC inner core environments.