Aircraft Observations of the Hurricane Boundary Layer Structure

Aircraft Observations of the Hurricane Boundary Layer

Structure

Jun Zhang

Collaborators: William Drennan, Peter Black, Jeffrey French, Frank Marks, Kristina Katsaros, and Susanne

Lehner

National Hurricane Center, Feb. 6th, 2009

Hurricane intensity is related to:

Environmental control;Ocean feedback;Cloud Microphysics;……

Physical processes near the air-sea interface across the boundary layer

The Hurricane boundary

layer is the least well observed part in a storm till now

Outline

1. Background and Motiviations;

2. Experiment and Instrumentation;

3. Results: 1) turbulent fluxes and parameterization

2) vertical structure of turbulence

3) Turbulent Kinetic Energy (TKE) budget

4) Boundary layer rolls

5) Hurricane boundary layer height

4. Summary and Future Work.

Depiction of the ABL processes

http://www.esrl.noaa.gov/research/themes/pbl/

------- Boundary layer height

Why is the boundary layer so important in hurricanes?

The boundary layer provides a powerful coupling between the primary circulation (the azimuthal component) and the secondary circulation (the radial-vertical, or “in-up-and-out” component).

Moisture enters a hurricane from the sea surface and its radial distribution is strongly influenced by that of the boundary layer winds.

The boundary layer dynamics and thermodynamics determine the vertical transport of moisture and angular momentum out of the boundary layer.

The radial distribution of these quantities on leaving the layer exerts a strong constraint on the radial distribution of buoyancy.

Courtesy of Roger Smith

Emanuel (1995):

•Axisymmetric model

•Slab boundary layer

•Use gradient wind

•Bulk BL parameterization CD drag coefficient (momentum) CK enthalpy coefficient

)( *0

0

02

max kkT

TT

C

CV s

D

k

qLTqcqck vlpd ])1([

CK/CD ~ 1.2 – 1.5

CK/CD > 0.75

2)0()( UzUC zD

)0()())0()(( TzTUzUCcH zHps

)0()())0()(( qzqUzUCLH zEve

Surface Fluxes and Parameterizations

z

uKuwvwu m

2

*2/122

)''''(

zcKtucTwcH phpps

**''

z

qKquLqwLH qvve

**''

MM5 simulation of Hurricane Bob (1991)

Braun and Tao, 2000

Sensitivity to boundary-layer

parameterization

Skillful prediction of intensity change requires an accurate representation of the boundary layer and parameterization of surface fluxes.

O AGILE (Donelan & Drennan 1995) X HEXOS (DeCosmo et al 1996) ◊ GASEX (McGillis et al 2004) SOWEX (Banner et al 1999)□ SWADE (Katsaros et al 1993)

COARE-3 ---COARE 2.5 —

EC Data from 8 field experiments : AGILE, AWE, ETCH,GASEX,HEXOS,RASEX, SHOWEX, SWADE, WAVES (4322 pts).

— Smith (1980)

Powell et al. 2003

Donelan et al. 2004

Prior to 2003, the only boundary layer in-situ turbulence structure measurement was conducted by Moss (1978) in the periphery of marginal hurricane Eloise (1975) at surface wind speed of about 20 m/s.

Moss (1978)

Zi

2002: 3 Test flights in Hurricanes Edouard, Isidore, and Lili

2003: 6 flights in Hurricanes Fabian and Isabel

2004: Flights at top of boundary layer, only 2 flux flights in Hurricanes Frances and Jeanne

Black et al. 2007 BAMSDrennan et al. 2007 JASFrench et al. 2007 JASZhang et al. 2008 GRL

The Coupled Boundary Layer Air-sea Transfer Experiment (CBLAST)

N43RF flux instrumentation - BAT (“Best Aircraft Turbulence”) probe on boom - Rosemount Gust probes in radome and fuselage - Inertial navigation, GPS systems in fuselage

- LICOR LI-7500 hygrometer (modified)

- Rosemount temperature sensors - PRT5 radiometer for sea surface temperature - Stepped Frequency Microwave Radiometer (SFMR)

←LICOR head

↓ BAT

CBLAST STEPPED DESCENTSCBLAST STEPPED DESCENTS

108 flux runs for momentum flux measurement

42 flux runs for enthalpy flux measurement

Black lines represent the flux runs

Typical length of a flux run is 24 km

Vertical profiles of Mean Flow(Data are from measurements during Sept. 12th 2003)

zi

To Eye

pitch

Time series for a typical flux run

ualtitude

roll

heading humidity, q

w

vpitch

(40 Hz data)

Spectral Analysis

Drag Coefficients

)ˆ''ˆ''(ˆ 2* jvwiuwu

)/( 21010 NND UC

Smith (1992) ------Large and Pond (1980) ------ Smith (1980) -------COARE 3.0 — CBLAST LOW (o)Powell et al. (2003) −∙−−∙Donelan et al. (2004) −−∙−∙−

CBLAST DataLF (◊) RF (□)LR (X) RR(+)

Dalton Numbers

CE10N = <w'q'>/U10N(qsat-q10N)

O AGILE Δ CBLAST X HEXOS ◊ GASEX ▼ SOWEX □ SWADE

CBLAST - - - HEXOS —

Stanton Numbers

Δ CBLAST X HEXOS

''wkFK

)]([/ 10010 NNKK kkUFC

-------- COARE 3.0-------- Emanuel’s threshold

CK/CD = 0.63 < 0.75

Δ CBLAST X HEXOS

Zhang, Black, French and Drennan, 2008: First direct measurements of enthalpy flux in the hurricane boundary layer: the CBLAST results. Geophys. Res. Lett., 35, L14813, oi:10.1029/2008GL034374.

Fk (W m-2)u* (m s-1)

Cdx1000Ckx1000

Vertical Structure of Momentum flux

–— Moss (1978)

Profiles of humidity and sensible heat fluxes

TKE Budget

Z

pw

Z

ewqwgwg

Z

vwv

Z

uwu

Dt

De ''1'''61.0'')/(''''

2/32/3 )]([2

ffSU

fuuu

I : Shear production

II: Buoyancy

III: Turbulent transport

IV: Pressure transport

V: Rate of dissipation

2/1222 )'''( wvue I II III IV V

TKE:

Turbulent Kinetic Energy Budget

Nicholls (1985)

Lenschow et al. (1980)

Zhang et al. 2009 JAS accepted

Hurricane Boundary Layer Rolls

Morrison et al., 2005

RADARSAT SAR imageryduring Hurricane Isidore

Zhang et al. 2008 BLM

Boundary Layer Flight in Hurricane Isidore

Wavelet Analysis

Momentum Flux

----- alongwind leg

─── crosswind leg

Wavelength ~ 950 m

─── leg A --------- legs B C D leg E

Zhang et al. 2008 BLM

Sensible Heat Flux

─── leg A --------- legs B C D leg E

Zhang et al. 2008 BLM

Hurricane Hugo (1989)Marks et al. 2008 MWR

rmwre r

z

hi

θ

Vr

A schematic of the hurricane boundary layer height

0'' wu

0''wu

Vr=0

Vr= -10 m s-1

r=150 km

h

hinflow

zi

hi – BL height of the idealized slab modelh – BL height defined from the flux profilehinflow – height of the inflow layerzi – BL height (mixed layer depth) defined from theta profile

2/1)/2( IK)//)(/2(2 rVrVfrVfI

- Scaling depth in theoretical models

Eye

rad

ius

Max

win

d ra

diu

s

Mean Wind Speed Profiles

170

230237238

157146

WL150 is the mean wind of the lowest 150 m data

Height of maximum

wind speed

Mean Potential Temperature Profiles

Stable layer

Transition Layer

Mixed Layer

170

230237238

157146

mixed layer height zi

Δθ (zi) ~ 1K

Mean Radial Wind Profiles

170

230237238

157146

Inflow layer height Vr = 0

rmwre r

z 0'' wu

Vr=0

hinflow

r=150 km

Entrainment ?

r=500 km

zi

? ?

?E

ye r

adiu

s

Max

win

d ra

diu

s

Where is the top of the hurricane boundary layer?

hVmax

800 Wm-2

1. Bulk parameterizations of momentum and enthalpy fluxes were extended up to near hurricane force.

2. The vertical structure of turbulence and fluxes are presented for the boundary layer between the outer rainbands.

3. The boundary layer height defined from the turbulent flux profiles is nearly twice the height of the mixed layer defined from the thermodynamic profiles.

4. Turbulent kinetic energy budget indicates that the advection term is important.

5. Boundary layer rolls rescale the sensible heat flux transport and enhance the momentum flux.

Summary

Future Possible Hurricane Boundary Layer Turbulence and flux Observations

• P3 aircraft flying low again?

• GPS dropsonde

• Remote sensing (Radar, Lidar, etc.)

• Aerosonde with turbulence instrumentation

• Buoy designed to sustain hurricane force

End

Thanks!

Hurricane boundary layer depth

• Deardorff (1972) : zi = c u*/f

• Anthes and Chang (1978) :

zi =

• Kepert (2001) :

2/1)]//(2[ rVfK tm

2/1)/2( IKzi

)//)(/2(2 rVrVfrVfI

zuK * is Inertial instability parameter

I

Consistent with Anthes and Chang (1978) and Kepert (2001)