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 to 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 variation 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 component 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 asymmetry 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 the 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 deduced 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 on 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:
- 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.
- 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.
- Use the EVTD wind analyses over the two days in Olivia together with dual-Doppler analyses to study the storm structure and motion.
- 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.
Key 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 Division 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..
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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