Mesoscale Structure of Tropical Cyclones

Principal Investigator:
Frank Marks

Other Scientists:
John Gamache
Peter Dodge

Objective

Earlier studies of Hurricane's Norbert and Gloria have shown that the airborne radar data are one of the most effective tools for describing the structure and organization of mesoscale wind and precipitation features in tropical cyclones. In particular, the airborne Doppler radars are essential for studying how the changes in organization and structure of the precipitation features relate t o changes in the storms wind field and track.

Rationale

Analysis of the airborne Doppler observations have revealed a number of common features of the three dimensional kinematic structure of the inner core of tropical cyclones. The inner core circulation is dominated by three components that vary on a time scale of 6-24 h; (1)the mean azimuthal flow; (2) the mean horizontal wind; and (3) a wave number one asymmetry. The mean horizontal wind is a measure of the environmental flow in the vicinity of the cyclone, and is comparable in magnitude to the secondary circulation. The vertical variatio n of this component provides an estimate of the vertical wind shear or baroclinity of the environmental flow. A vertical mass-weighted mean of the mean horizontal wind provides an estimate of the mean cyclone motion. The temporal variation of this compone nt is a measure of the changing environmental flow in which the cyclone is embedded and is related to the cyclone motion.

The wave number one asymmetry, characterized by cyclonic and anticyclonic gyres centered on the radius of maximum wind, is comparable in magnitude to the secondary circulation and mean horizontal wind. Observations of this as ymmetry indicate that the orientation of the gyres is related to the shear of the mean horizontal wind, suggesting that the gyres are related to changes in the environmental flow.

Method

The Velocity Track Display (VTD) algorithm is similar to the Velocity Azimuth Display (VAD) technique developed for ground-based radars. The VTD technique utilizes a harmonic analyses of t he Doppler velocities collected along a single flight leg in a series of concentric rings about the storm center at a number of altitudes. In this way, the cross-track component of the mean flow, the mean tangential and radial velocity components can be d educed for each ring when enough Doppler velocities are available. An improved version of the VTD algorithm, Extended Velocity Track Display (EVTD) is proposed based on a different mathematical formulation of the problem and on the simultaneous consideration of Doppler data collected during successive legs crossing the storm center. The improved algorithm permits the solution of some uncertainties associated with VTD: i.e., non-uniqueness of the values for the mean flow o n successive rings; lack of wavenumbers higher than wavenumber 0 for the radial wind component; and random variations of the different coefficients for successive rings.

Accomplishments

During the last 5 years, the two algorithms have been developed to analyze airborne Doppler radar hurricane wind data on a workstation on one of the NOAA P-3s. The VTD algorithm derives a radius-height mean tangential wind field from data collected during a single pass of the aircraft through the center of a tropical cyclone. The EVTD method uses Doppler velocities from two or more flight legs to retrieve the horizontal mean wind over the vortex core, the axisymmetric and wave number one radial wind field, and the temporal evolution of the tangential and radial components.

During 1993-95, Doppler wind sets were obtained in Hurricanes Emily (93), Olivia (94), Erin (95), Iris (95), Luis (95), and Marilyn (95). VTD and EVTD analyses of these data depict the evolution of the horizontal mean wind and the mean tangential wind as the storms evolved. The efforts will focus on four activities:

  1. Continued testing of the real-time VTD and EVTD airborne algorithms during hurricane research flights, culminating with a plan to transmit data to NHC during the 1996 season.

  2. Compare the EVTD wind analyses from Emily, Erin, and Marilyn with the mesoscale surface wind analyses to determine the usefulness of the EVTD analyses in estimating the low level wind structure.

  3. Use the EVTD wind analyses over the two days in Olivia together with dual-Doppler analyses to study the storm structure and motion.

  4. Compare the vertical profile of the mean horizontal wind produced by the EVTD wind analyses to that determined from the ODW observations gathered in Iris and Luis to ascertain whether the EVTD winds are useful in studies of storm motion.

References

Burpee, R.W., S.D. Aberson, P.G. Black. M. DeMaria, J.L. Franklin, J.S. Griffin, S.H. Houston, J. Kaplan, S.J. Lord, F.D. Marks, M.D. Powell, and H.E. Willoughby, 1994: Real-time guidance provided by NOAA's Hurricane Research Div ision to forecasters during Hurricane Emily of 1993. Bull. Amer. Meteor. Soc., 75, 1765-1783.

Franklin, J.L., S.J. Lord, S.E. Feuer, and F.D. Marks, 1993: The kinematic structure of Hurricane Gloria (1985) determined from nested analyses of dropwindsonde and Doppler radar data. Mon. Wea. Rev., 121, 2433-2 451.

Griffin, J.S., R.W. Burpee, F.D. Marks, And J.L. Franklin, 1992: Real-time airborne analysis of aircraft data supporting operational hurricane forecasting. Wea. Forecasting, 7, 480-490.

Lee, W.-C., F.D. Marks, and R.E. Carbone, 1994: Velocity Track Display, A technique to extract real-time tropical cyclone circulations using a single airborne Doppler radar. J. Atmos. and Ocean.Tech., 11, 337-356.

Marks, F.D., R.A. Houze, and J.F. Gamache, 1992: Dual-Aircraft investigation of the inner core of Hurricane Norbert: Part I: Kinematic structure. J. Atmos. Sci., 49, 919-942.

Roux, F. and F.D. Marks, 1995: Extended velocity track display (EVTD): An improved processing method for Doppler radar observation of tropical cyclones. accepted for J. Atmos. Ocean. Tech..

Conference Papers

Dodge, P., F. Marks, and J. Gamache, 1995: The evolution of the inner core of Hurricane Olivia (1994) from EVTD Doppler radar analysis. Preprints of the 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, AMS, 463-465.

Marks, F., 1992: Kinematic structure of the hurricane inner core as revealed by airborne Doppler radar. Preprints of Fifth Conference on Mesoscale Processes. Atlanta, GA, AMS, 127-132.
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marks@aoml.noaa.gov
Updated Tue, Nov 14, 1995