Tropical Cyclone Kinematic Structure Retrieved from Single-Doppler Radar Observations. Part I: Interpretation of Doppler Velocity Patterns and the GBVTD Technique

Abstract

Deducing the three-dimensional primary circulation of landfalling tropical cyclones (TCs) from single ground-based Doppler radar data remains a difficult task. The evolution and structure of landfalling TCs and their interactions with terrain are left uncharted due to the lack of dual-Doppler radar observations. Existing ground-based single-Doppler radar TC algorithms provide only qualitative information on axisymmetric TC center location and intensity. In order to improve understanding of the wind structures of landfalling TCs using the widely available WSR-88D data along the U.S. coastal region, a single ground-based radar TC wind retrieval technique, the ground-based Velocity Track Display (GBVTD) technique, is developed. Part I of this paper presents 1) single-Doppler velocity patterns of analytic, asymmetric TCs, 2) derivation of the GBVTD technique, and 3) evaluation of the GBVTD-retrieved winds using analytic TCs. The Doppler velocity patterns of asymmetric TCs display more complex structure than their axisymmetric counterparts. The asymmetric structure of TCs can be inferred qualitatively from the pattern (or curvature) of the zero Doppler velocity line and the position and shape of the Doppler velocity dipole. However, without knowing the axisymmetric portion of the TC circulation, it is extremely difficult to extract quantitative information from these similar Doppler velocity patterns. Systematic evaluations on the GBVTD-retrieved winds show good agreement compared with the original analytic wind fields for axisymmetric flows plus mean wind and/or angular wavenumber 1, 2, and 3 asymmetry. It is also shown that the GBVTD technique retrieves wind maxima that are not directly observed (perpendicular to the radar beams) because the GBVTD technique uses the Doppler velocity gradient, not the observed maxima, to retrieve wind maxima. The success of the GBVTD-retrieved winds and understanding their characteristics provide the theoretical basis to nowcast TC kinematic structure.

The Gradient Velocity Track Display (GrVTD) Technique for Retrieving Tropical Cyclone Primary Circulation from Aliased Velocities Measured by Single-Doppler Radar

“…Previous studies (Roux and Marks 1996;Lee and Marks 2000) have shown that the quality of the GBVTDretrieved winds critically depends on accurately knowing the circulation center. Since GrVTD is based on the same geometric relations as GBVTD, its performance also depends on the accuracy of TC center position.…”

“…In practice, how to objectively determine a TC circulation center from V d becomes an important issue for GrVTD. Lee and Marks (2000) combined the ''simplex'' method (Nelder and Mead 1965) and the GBVTD-retrieved tangential circulations to form an elaborate center finding algorithm, named ''GBVTD-simplex,'' that can identify the TC center with reasonably good accuracy for both analytical and real TCs. They also noted that the estimated center discrepancy needs to be less than 5% RMW to keep the nominal error of the apparent wavenumber 1 less than 20%.…”

“…As described in Lee and Marks (2000), the GrVTDsimplex method begins with an initial TC center and RMW estimated from the single-Doppler velocity dipole signature (Wood and Brown 1992). To search for a global maximum of the mean tangential winds, an array of 16 initial guess centers surrounding this initial center is conducted for subsequent GrVTD-simplex algorithm and yields 16 different simplex center estimates for a given radius within a range of likely RMWs.…”

“…By using all available Doppler velocities in a constant altitude PPI (CAPPI), the velocity track display (VTD) technique (Lee et al 1994) formulates a more complete mathematical representation between the TC primary circulation and the observed Doppler velocities, enabling the reconstruction of three-dimensional (3D) primary circulations of TCs from data collected by a single airborne Doppler radar. A series of VTD extensions (referred to as the ''VTD family of techniques'')-including extended VTD (EVTD; Roux and Marks 1996), the ground-based VTD (GBVTD; Lee et al 1999, hereafter LJCD), ground-based extended VTD (GB-EVTD; Roux et al 2004), extended GBVTD (EGBVTD; Liou et al 2006), and generalized VTD (GVTD; Jou et al 2008)-expanded the VTD analysis into multiple flight legs for airborne Doppler radar and multiple ground-based Doppler radars (e.g., Roux and Marks 1996;Lee et al 2000;Harasti et al 2004;Liou et al 2006; Lee and Bell 2007;Zhao et al 2008).…”

“…A series of VTD extensions (referred to as the ''VTD family of techniques'')-including extended VTD (EVTD; Roux and Marks 1996), the ground-based VTD (GBVTD; Lee et al 1999, hereafter LJCD), ground-based extended VTD (GB-EVTD; Roux et al 2004), extended GBVTD (EGBVTD; Liou et al 2006), and generalized VTD (GVTD; Jou et al 2008)-expanded the VTD analysis into multiple flight legs for airborne Doppler radar and multiple ground-based Doppler radars (e.g., Roux and Marks 1996;Lee et al 2000;Harasti et al 2004;Liou et al 2006; Lee and Bell 2007;Zhao et al 2008). Recently, the GBVTD technique has been incorporated into the Vortex Objective Radar Tracking and Circulation (VORTRAC) software for real-time analysis of TCs at the National Hurricane Center (NHC) of the United States.…”