Principal Rainband of Hurricane Katrina as observed in RAINEX Anthony C. Didlake, Jr. 28 th...
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Principal Rainband of Hurricane Katrina Principal Rainband of Hurricane Katrina as observed in RAINEXas observed in RAINEX
Anthony C. Didlake, Jr.Anthony C. Didlake, Jr.2828thth Conference on Hurricanes and Tropical Meteorology Conference on Hurricanes and Tropical Meteorology
April 29, 2008April 29, 2008
Willoughby 1988
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Barnes et al. 1983Barnes et al. 1983
Low-level radial inflow Low-level radial inflow overturns inside of overturns inside of leaning reflectivity towerleaning reflectivity tower
Downdraft within Downdraft within reflectivity tower reflectivity tower continues as radial inflowcontinues as radial inflow
Cross-band structureCross-band structure
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Hurricane Katrina (2005)Hurricane Katrina (2005)
ELDORA radarELDORA radar• Sampling resolution ~0.4 kmSampling resolution ~0.4 km
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Similarities to Barnes et al. 1983
Hence and Houze 2008
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Inner-edge downdraftInner-edge downdraft
What causes it?What causes it? How often does it occur?How often does it occur? What are the effects of it?What are the effects of it?
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Convective/stratiform Convective/stratiform classificationclassification
Convective
Stratiform
Weak echo
No echo
Similar to Steiner et al. 1995, Similar to Steiner et al. 1995, TRMM satellite data classificationTRMM satellite data classification
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Radial cross sections at regular angular intervalsRadial cross sections at regular angular intervals• 0.375° 0.375° 109 cross sections 109 cross sections
Cross section coordinates based on classificationCross section coordinates based on classification
Rainband cross sectionsRainband cross sections
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dBZ
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Updrafts are strong and Updrafts are strong and broadbroad
Two downdraft regimesTwo downdraft regimes Inner-edge downdrafts Inner-edge downdrafts
are slightly weaker and are slightly weaker and more localizedmore localized
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Vertical velocity (m/s) (plan view)
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Buoyant air parcel
dBZ
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dBZ
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ConclusionsConclusions
Overturning updraft, low-level downdraft, Overturning updraft, low-level downdraft, inner-edge downdraftinner-edge downdraft
Inner-edge downdraft:Inner-edge downdraft:• Convective scale feature, ~5 kmConvective scale feature, ~5 km• Creates sharp reflectivity gradientCreates sharp reflectivity gradient• Forced by rainband updrafts?Forced by rainband updrafts?
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Questions?Questions?
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Buoyancy pressure-gradient acceleration fieldBuoyancy pressure-gradient acceleration field
H H
L L
Buoyant air parcel
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Idealized structure of a tropical cycloneIdealized structure of a tropical cyclone
Primary and Primary and Secondary eyewallsSecondary eyewalls
Stationary Band Stationary Band Complex (SBC)Complex (SBC)• principal bandprincipal band• secondary bandsecondary band
Willoughby 1988
upwind
downwind
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B
A
D
C
Autocorrelation along rainband
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Strong inner-edge downdrafts
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Low-level tangential wind max on inner-side of rainband
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ReferencesReferences Willoughby, H.E., 1988: The dynamics of the Willoughby, H.E., 1988: The dynamics of the
tropical cyclone core. tropical cyclone core. Aust. Met. Mag.Aust. Met. Mag., , 3636, 183-, 183-191.191.
Barnes, G.M., E.J. Zipser, D. Jorgensen, and F. Barnes, G.M., E.J. Zipser, D. Jorgensen, and F. Marks, Jr., 1983: Mesoscale and convective Marks, Jr., 1983: Mesoscale and convective structure of a hurricane rainband. structure of a hurricane rainband. J. Atmos. Sci., J. Atmos. Sci., 4040, 2125-2137., 2125-2137.
Hence, D.A. and R.A. Houze, Jr., 2008: Kinematic Hence, D.A. and R.A. Houze, Jr., 2008: Kinematic structure of convective-scale rainband features in structure of convective-scale rainband features in Hurricanes Katrina and Rita (2005). Hurricanes Katrina and Rita (2005). J. Geophys. J. Geophys. Res.Res., accepted., accepted.
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““Strong” inner-edge Strong” inner-edge downdrafts occur less downdrafts occur less frequently than “strong” frequently than “strong” updraftsupdrafts
Inner-edge downdrafts Inner-edge downdrafts occur right along the occur right along the reflectivity gradientreflectivity gradient
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Convective/stratiform classificationConvective/stratiform classification
Technique used in Steiner et al. 1995, Yuter Technique used in Steiner et al. 1995, Yuter and Houze 1997, Yuter et al. 2005and Houze 1997, Yuter et al. 2005
Algorithm separates convective regions from Algorithm separates convective regions from stratiform regions by comparing local stratiform regions by comparing local reflectivity to background reflectivityreflectivity to background reflectivity
Tuning of algorithm required to recognize Tuning of algorithm required to recognize convective regions; the rest is designated as convective regions; the rest is designated as stratiformstratiform
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Classification AlgorithmClassification Algorithm Convective center if:Convective center if:
• Z Z Z Ztiti
• Z-ZZ-Zbgbg Z Zcccc(Z(Zbgbg))
Classified convective within R(ZClassified convective within R(Zbgbg) ) from convective center, remaining is from convective center, remaining is classified stratiform (unless Z < Zclassified stratiform (unless Z < Zwewe))
Zcc
2
1cos bg
cc
Z
baZ
Zbg
Zti = 45 dbZ
Zwe = 20 dbZ
R = 0.5+.23(Zbg-20)
Rbg = 11 km
a=9, b=45
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Statistics of reflectivity dataStatistics of reflectivity data
2D frequency distributions (in % of height total)2D frequency distributions (in % of height total)
Convective pixels Stratiform pixels Convective pixels Stratiform pixels
Heig
ht
(km
) H
eig
ht
(km
)
dBZ dBZdBZ dBZ
Yuter and Houze 1995
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Beyond Barnes et al. (1983)Beyond Barnes et al. (1983)
K1x
10 -3
- Deanna Hence
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Strong downdrafts where horizontal velocity decreases with height
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Tilting of vorticity tubes creates negative vertical vorticity
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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.
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Strong lower-level vertical vorticity is manifested in a local tangential wind maximum
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Future workFuture work
Explore the roll of fluctuating updrafts and Explore the roll of fluctuating updrafts and downdrafts in strength of storm circulationdowndrafts in strength of storm circulation
Compare dynamics with other convective Compare dynamics with other convective regimes: eyewall, secondary eyewall, regimes: eyewall, secondary eyewall, outer rainbandsouter rainbands
Analyze more ELDORA data volumes, N43 Analyze more ELDORA data volumes, N43 data, Rita rainbandsdata, Rita rainbands
Compare observations with model Compare observations with model simulations, analyze evolution of simulations, analyze evolution of rainbandsrainbands
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Average tangential wind (m/s)Average tangential wind (m/s)
distance (km)
alt
itude (
km
)
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Aircraft and instrumentsAircraft and instruments
US Naval Research US Naval Research Laboratory P3 (NRL)Laboratory P3 (NRL)
ELDORA radarELDORA radar• Sampling resolution ~0.4 kmSampling resolution ~0.4 km
NOAA P3 (N42, N43)NOAA P3 (N42, N43) Dropsondes and Doppler Dropsondes and Doppler
radar (dual-Doppler analysis)radar (dual-Doppler analysis)
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Motivation for RAINEXMotivation for RAINEX
How do interactions of environment, eyewalls, and rainbands in the mature storm… …lead to intensity changes like
these?
Intensity of Katrina (2005)
Max W
ind S
peed (
knots
)
24 25 2826 2927August