Principal Rainband of Hurricane Katrina
as observed in RAINEX
Willoughby 1988
Anthony C. Didlake, Jr.
28th Conference on Hurricanes and Tropical Meteorology
April 29, 2008
Barnes et al. 1983


Low-level radial inflow
overturns inside of
leaning reflectivity tower
Downdraft within
reflectivity tower
continues as radial inflow
Hurricane Katrina (2005)
ELDORA radar
• Sampling resolution ~0.4 km
Similarities to Barnes et
al. 1983
Hence and Houze 2008
Inner-edge downdraft



What causes it?
How often does it occur?
What are the effects of it?
Convective/stratiform
classification
Convective

Similar to Steiner et al. 1995,
TRMM satellite data classification
Stratiform
Weak echo
No echo
Rainband cross sections

Radial cross sections at regular angular intervals
• 0.375°  109 cross sections

Cross section coordinates based on classification
dBZ



Two downdraft regimes
Inner-edge downdrafts
are slightly weaker and
more localized
Updrafts are strong and
broad
Vertical velocity (m/s) (plan view)
dBZ
Buoyant air parcel
dBZ
Conclusions


Overturning updraft, low-level downdraft,
inner-edge downdraft
Inner-edge downdraft:
• Convective scale feature, ~5 km
• Creates sharp reflectivity gradient
• Forced by rainband updrafts?
Questions?
Buoyancy pressure-gradient acceleration field
H
H
Buoyant air parcel
L
L
Idealized structure of a tropical cyclone
downwind


Primary and
Secondary eyewalls
Stationary Band
Complex (SBC)
• principal band
• secondary band
upwind
Willoughby 1988
Autocorrelation along rainband
D
C
B
A
Strong inner-edge downdrafts
Low-level tangential wind max on
inner-side of rainband
References



Willoughby, H.E., 1988: The dynamics of the
tropical cyclone core. Aust. Met. Mag., 36, 183191.
Barnes, G.M., E.J. Zipser, D. Jorgensen, and F.
Marks, Jr., 1983: Mesoscale and convective
structure of a hurricane rainband. J. Atmos. Sci.,
40, 2125-2137.
Hence, D.A. and R.A. Houze, Jr., 2008: Kinematic
structure of convective-scale rainband features in
Hurricanes Katrina and Rita (2005). J. Geophys.
Res., accepted.


“Strong” inner-edge
downdrafts occur less
frequently than “strong”
updrafts
Inner-edge downdrafts
occur right along the
reflectivity gradient
Convective/stratiform classification



Technique used in Steiner et al. 1995, Yuter
and Houze 1997, Yuter et al. 2005
Algorithm separates convective regions from
stratiform regions by comparing local
reflectivity to background reflectivity
Tuning of algorithm required to recognize
convective regions; the rest is designated as
stratiform
Classification Algorithm

Convective center if:
• Z  Zti
• Z-Zbg  Zcc(Zbg)

Classified convective within R(Zbg)
from convective center, remaining is
classified stratiform (unless Z < Zwe)
Zcc
 1 Z bg 

Z cc  a cos
b
2


Zti = 45 dbZ
Zwe = 20 dbZ
R = 0.5+.23(Zbg-20)
Rbg = 11 km
a=9, b=45
Zbg
Statistics of reflectivity data
Yuter and Houze 1995
Beyond Barnes et al. (1983)
x 10 -3
K1
- Deanna Hence
Strong downdrafts where
horizontal velocity decreases
with height
Tilting of vorticity tubes creates
negative vertical vorticity
Negative vertical vorticity is
stretched in region of convergence
and advected downward. It is
confined to the lower layers by
divergence at the ocean surface.
Strong lower-level vertical
vorticity is manifested in a local
tangential wind maximum
Future work




Explore the roll of fluctuating updrafts and
downdrafts in strength of storm circulation
Compare dynamics with other convective
regimes: eyewall, secondary eyewall,
outer rainbands
Analyze more ELDORA data volumes, N43
data, Rita rainbands
Compare observations with model
simulations, analyze evolution of
rainbands
altitude (km)
Average tangential wind (m/s)
distance (km)
Aircraft and instruments


NOAA P3 (N42, N43)
Dropsondes and Doppler
radar (dual-Doppler analysis)


US Naval Research
Laboratory P3 (NRL)
ELDORA radar
• Sampling resolution ~0.4 km
Motivation for RAINEX
How do interactions of
environment, eyewalls, and
rainbands in the mature storm…
Max Wind Speed (knots)
…lead to intensity changes like
these?
Intensity of Katrina (2005)
24
25
26
27
August
28
29