Hurricane structure and Hurricane structure and intensity change : Effects of intensity change : Effects of

wind shear and Air-Sea wind shear and Air-Sea InteractionInteraction

Mélicie DesflotsMélicie DesflotsRosenstiel School of Marine & Atmospheric Science Rosenstiel School of Marine & Atmospheric Science

4600 Rickenbacker Causeway, Miami, FL, 33149-10984600 Rickenbacker Causeway, Miami, FL, 33149-1098

What controls Hurricane Intensity ?

• Inner core (eye and eyewall) dynamics and rainbands (Theoretical and modeling work, e.g., Montgomery& Kallenbach 1997, Schubert et al. 1999, and recent field program – RAINEX, Houze et al. 2006, Chen 2006)

• Environmental conditions-vertical wind shear (e.g., Frank&Ritchie 1999, Black et al. 2002, Rogers et al. 2003, Chen et al. 2006, Desflots and Chen 2006)-moisture distribution-sea surface temperature (upper ocean heat content), surface properties, etc. (Theoretical, observational, and modeling work, e.g., Emanuel 1995, Bao et al. 2000, and recent results from CBLAST, Black et al. 2006, Chen et al. 2006)

OutlineOutline

Model and data Effect of vertical wind shear on

hurricane intensity Importance of air-sea interaction for

hurricane intensity (sensitivity to sea-spray parameterization)

Conclusions

ModelModel

ATMOS. MODEL

(MM5)

OCEAN MODEL(3DPWP)

WAVE MODEL

(WAVEWATCH III)

Wave-induced stress

Surface wind

Heat & Moisture fluxes

SST

Cur

rent

vel

ocity

Wav

e-In

duce

d st

ress

DataData

Storm Name

Storm Year

Number of match

Erika 1997 10/17

Bonnie 1998 23/25

Floyd 1999 6/6

Fabian 2003 4/4

Frances 2004 2/2

Jeanne 2004 3/3

Dennis 2005 5/5

Rita 2005 15/17

79 co-located dropsondes

+AXBTData68

Effect of vertical wind shear on hurricane intensity

Effect of Vertical Wind Shear on TCs’ Effect of Vertical Wind Shear on TCs’ intensity intensity

e.g. Simpson and Riehl 1958; Gray 1968; Willoughby 1984; Marks et al 1992; Franklin et al. 1993; Jones 1995,2000a, 2000b; DeMaria 1996; Gamache et al. 1997; Frank and Ritchie 2001; Black et al. 2002; Corbosiero and Molinary 2002; Rogers et al 2003; Wong and Chan 2004 ; Lonfat 2004; Chen et al. 2006

Mechanisms: 1) vortex tilt and 2) shear-induced secondary circulation

Low shear High shear

Shear-Induced Rainfall Asymmetry (Chen et al. 2006)

(Global composite from TRMM and SHIPS data)

MM5: the 5th generation high resolution, non- hydrostatic PSU/NCAR mesoscale model

Multi-nested, vortex-following domains with grid resolution of 15,5,1.67, 0.55 km, respectively

28 vertical sigma-levels

Model descriptionModel description

Model initializationModel initialization

Initialized at 0000 UTC, 10/01/02, integrated for 72 hrs.

NCEP global 1°x1° analysis is used as initial and lateral boundary conditions

Time varying SST from SSMI/satellite (1/4°)

A vortex relocation procedure, similar to Liu et al. (1997), is used at the initial time.

TrackTrack

IntensityIntensitylandfallBest Track

5 km1.67 km 0.55 km

Evolution of the Intensity of Evolution of the Intensity of Hurricane Lili (2002)Hurricane Lili (2002)

Time

landfall

Sea Level Pressure

Radius Radius

Tangential Velocity in m/s Rain Rate in mm/h

Shear Analysis from MM5Shear Analysis from MM5

(1)

(2)

(1) (2)

Vertical wind shear vectorStorm motion

Vortex Tilt/ Temperature perturbations

T/W

at

550

hP

a R

ain

rate

10/02 21Z 10/03 03Z10/02 13Z

Combine effect of the increasing vertical wind shear and the vertical wind shear direction created a rainfall asymmetry

The vertical wind shear weakened the storm’s intensity

Effect of Vertical Wind Shear on Effect of Vertical Wind Shear on Hurricane LiliHurricane Lili

Importance of the of air-sea interactions for hurricane

intensity

Hurricanes intensity and Air-Sea Interaction

• Ocean coupling by reducing the amount of available heat fluxes reduces the storm intensity (e.g., Chen et al. 2006)

• Sensitivity to sea spray parameterization in a coupled model (e.g., Bao et al. 2000, Kepert, 2001)

Importance of Air-Sea Fluxes in Hurricane Intensity

• Energy source (enthalpy) and sink momentum/dissipation) for hurricanes

• The balance between the two can potentially affect hurricane intensity

• They are a good constraint to develop and evaluate coupled air-sea models for hurricane intensity forecasting

Air-Sea Fluxes formulation

• Momentum flux

• Sensible Heat flux

• Latent Heat flux

In a turbulent boundary layer the turbulent fluxes can be approximated to bulk fluxes :

''uwa

''TwcHs paa

''qwLHl ea

UUCda

TUCc hpaa

qUCL eea

Behavior of the exchange coefficients at high Behavior of the exchange coefficients at high wind speedwind speed

The drag coefficient levels off at high wind speed (Powell et al. 2003; Donelan et al. 2004)

103CE

CBLAST Observations: Black et al. (2006), Drennan et al. (2006), French et al. (2006)

CD

Total Surface Heat Fluxes

Uncoupled model

Coupled model

Bulk fluxes estimates from GPS dropsondes and AXBTs

Air-Sea temperatures

Coupled Model Observations

SST

Sea-

air

tem

per

atu

redi

ffer

ence

Are the total heat fluxes correlated to hurricane intensity or local wind speed ?

Wind speed in m/s

Erika 970908Bonnie 980821Bonnie 980824Bonnie 980825Bonnie 980826Floyd 990913Floyd 990914Fabian 030902Fabian 030903Fabian 030904Frances 040901Jeanne 040925Dennis 050709Dennis 050710Rita 050922Rita 050923

***

Xx

Tropical Storm

Cat. 1-2Major hurricanes

FRFLRLRR

Air-sea Fluxes in the eyewall

Floyd (1999)

Frances (2004)

Bonnie (1998)

Lili (2002)

Heat fluxes

SLP

Air-sea Fluxes within 200km radius

Lili (2002)

Floyd (1999)

Frances (2004)

Bonnie (1998)

Heat fluxes

SLP

Effect of Sea Spray on surface heat fluxes

• Fairall et al. 1994 : The effect of sea spray on the surface energy transports over the ocean. Global Atmos. Ocean Syst., 2, 121-142• Bao et al. 2000 : Numerical Simulations of Air-Sea Interaction under High Wind Conditions Using aCoupled Model : A study of Hurricane Development, Mon. Wea. Rev. ,128, 2190-2210

• On the sensible heat flux : -The sea spray droplet cools from the sea temperature to the air temperature (or to Tw’) and gives up sensible heat TO the atmosphere - to evaporate the sea spray some sensible heat flux FROM the atmosphere is required- frictional terms due to waves

• On the latent heat flux : - when the sea spray evaporates it gives up some latent heat flux TO the atmosphere

Sensitivity to sea spray

Frances without Sea Spray Frances with Sea Spray

Air

-sea

Flu

xes

in

the

eyew

all

Air

-sea

Flu

xes

wit

hin

200

km

ra

diu

s

SLP

Heat fluxes

Conclusions• Strong wind shear-induced asymmetry in rainfall (convective

heating) and vortex circulation is a major limiting factor on hurricane intensity

• Observed surface enthalpy flux has a large storm-to-storm variability as well as spatial variability within each storms

• Storm-averaged total surface enthalpy flux is not a good predictor of hurricane intensity or intensity change

• Inner-core (eye and eyewall) structure and dynamics dominate the rapid intensification process, which is sensitive to the surface enthalpy flux in the eyewall region

• Coupled atmosphere-wave-ocean model can produce the general characteristics of observed surface fluxes. However, parameterizations of the air-sea fluxes remain to be a challenge, especially sea spray due to the lack of observations in high winds.

• Part of this work was supported by a research grant from the Office of Naval Research (N00014-01-1-0156)

• My advisor : Dr. Shuyi Chen• My committee members : Drs. David Nolan, Mark

Donelan, James Price, Frank Roux and Will Drennan• Drs. Wei Zhao, Jian-Wien Bao • Mike Black for help to process the dropsondes• HRD and CBLAST program for collecting precious

data• Jun Zhang for helpful discussion• Thank to all the people who helped me one way or

another

Acknowledgements