Three-Dimensional Precipitation Structure of Tropical Cyclones

AMS Hurricane and Tropical Meteorology ConferenceMay 2nd, 2008

Deanna A. Hence and Robert A. Houze, Jr.University of Washington

Willoughby (1988)

What is the typical vertical structure of these mesoscale features?

Idealized vertical structures suggested by past studies

Inner rainband

Outer rainband

Outer rainband

Inner rainband

DeepDeep Restricted

Objectives of this study

• Determine statistically the vertical structures of the mesoscale precipitation features of hurricanes

• Determine how these structures vary from eyewall to inner rainband to outer rainband regions

TRMM Precipitation Radar (PR)

• 13.8 GHz (2.17 cm) radar

• 17º (from nadir) scanning angle

• 215 km swath width• 4.3 km horizontal

resolution• 250 m vertical resolution

From NASA TRMM website (http://trmm.gsfc.nasa.gov/)

Annuli and Quadrant Analysis

TRMM PR Reflectivity at 03:36 on 28 August 2005

79°W

51

45

39

33

27

21

dBZ

15

9

3

808190 89 88 87 86 85 84 83 82

RR

LR

25

24 LF

RF

28°N

26

27Storm motion

TRMM PR Reflectivity at 03:36 on 28 August 2005

79°W

51

45

39

33

27

21

dBZ

15

9

3

808190 89 88 87 86 85 84 83 82

RR

LR

25

24 LF

RF

28°N

26

27Storm motion

• Center location, eye diameter and storm motion provided in the National Hurricane Center (NHC) best track data

Primary Analysis Tools

• Contoured Frequency by Altitude Diagrams (CFADS, Yuter and Houze 1995)

• NCAR Zebra software

Re

lati

ve

fre

qu

en

cy

of

oc

cu

rre

nc

e

Ordinary Deep Convection CFAD

Convective component

Stratiform component

Total

• Analyze CFADs of all overpasses of hurricanes that reached Category 4 or 5.

• Atlantic/Gulf of Mexico/Caribbean basin storms from 1998-2007

• Determine how structures of eyewall, rainbands, and outer bands vary with radius and storm quadrant

Vertical Structure Analysis with CFADS

CFADS by Annulus-Outer Region

• Resembles ordinary buoyant convection

– Broad distribution of reflectivities below melting level

– Bright band signature– Relatively broad

distribution above melting level reaching to 11 km

Outer Region

Annulus 5

CFADS by Annulus -- Rainband Region

• Strong brightband signature

• Sharp dropoff of reflectivity above melting level reaching to just below 10 km

• But… Narrower distribution of reflectivity below melting level…not like ordinary convection

• Looks more like an eyewall CFAD

Rainbands

Annulus 3

CFADs by Annulus– Eyewall Region

• Relatively narrow but intense distribution below melting level

• Weak if any brightband signature

• Mostly narrow & sharp distribution above melting level

• Outliers suggest occasional intense convective towers reaching over 12 km

• Not like convective or stratiform CFADs of ordinary convection

Eyewall

Annulus 1

Eyewall CFADs by Quadrant — All Overpasses

Sto

rm m

otio

n

Quad 1 Quad 4

Quad 2 Quad 3

LF

Quad 1

RF

Quad 4

RR

Quad 3

LR

Quad 2

Rainbands CFADs by Quadrant — All Overpasses

Sto

rm m

otio

n

RF

Quad 4

RR

Quad 3

LR

Quad 2

LF

Quad 1

Conclusions

• Outer region CFADs – convective-like distribution

• Rainband CFADs– Vertically limited– Narrow distribution– CFADs similar in all quadrants

• Eyewall CFADs– Deep– Intense reflectivities– Relatively narrow distribution with extreme outliers– Right side stronger than left side

Ongoing and Future Work

• Continue expansion of database to all Atlantic Basin hurricanes, and eventually to all basins

• Stratify overpasses based on intensity, track speed, wind shear, sea surface temperature, and other factors known to influence storm structure and intensity

• Perform EOF analysis to objectively sort CFADs

• Use technique to analyze model simulations