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

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

Hurricane Katrina (2005)Hurricane Katrina (2005)

ELDORA radarELDORA radar• Sampling resolution ~0.4 kmSampling resolution ~0.4 km

Similarities to Barnes et al. 1983

Hence and Houze 2008

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?

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

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

dBZ

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

Vertical velocity (m/s) (plan view)

Buoyant air parcel

dBZ

dBZ

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?

Questions?Questions?

Buoyancy pressure-gradient acceleration fieldBuoyancy pressure-gradient acceleration field

H H

L L

Buoyant air parcel

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

B

A

D

C

Autocorrelation along rainband

Strong inner-edge downdrafts

Low-level tangential wind max on inner-side of rainband

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.

““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

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

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

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

Beyond Barnes et al. (1983)Beyond Barnes et al. (1983)

K1x

10 -3

- 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 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

Average tangential wind (m/s)Average tangential wind (m/s)

distance (km)

alt

itude (

km

)

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)

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