1. Direct Measurements of Momentum and Enthalpy Fluxes in the Hurricane Force Wind Regime
Jun Zhang
NOAA/AOML/HRD
William Drennan
Univ. of Miami/ RSMAS
Jeffrey French
Univ. of Wyoming
Peter Black
NOAA/AOML/HRD (Currently NRL)
AMS 89th Annual Meeting, Paper 10.2 Phoenix, 14 January, 2009

2. Bulk BL parameterization
Emanuel (1995):
Axisymmetric model
Slab boundary layer
Use gradient wind
Bulk BL parameterization
CD drag coefficient (momentum)
CK enthalpy coefficient
CK/CD ~ 1.2 – 1.5
CK/CD > 0.75

3. 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.

4. - CDN increases linearly with wind speed from 5 to 20 m/s.
EC Data from 8 field experiments : AGILE, AWE, ETCH,GASEX,HEXOS,RASEX, SHOWEX, SWADE, WAVES (4322 pts).
— Smith (1980)
Consensus (pre-2003):
- CDN increases linearly with wind
speed from 5 to 20 m/s.
- Scatter due to wave age and swell
- No direct measurements of drag coefficients for U > 23 m/s;
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
COARE 2.5 —
- CEN constant (or nearly so) with
wind speed from 5 to 20 m/s.
- No direct measurements of exchange coefficient of heat flux
for U > 20 m/s;

5. Cd leveling off at wind speed higher than 30-40 m/s
Powell et al. 2003
Cd leveling off at wind speed higher than m/s
Donelan et al. 2004

6. The Coupled Boundary Layer Air-sea Transfer (CBLAST) Hurricane Experiment
Six flights in two hurricanes in 2003
3 Hurricane Fabian
3 Hurricane Isabel
Two flights in two hurricanes in 2004
1 Hurricane Frances
1 Hurricane Jeanne
Black et al BAMS Drennan et al JAS French et al JAS
Zhang et al GRL

7. ←LICOR ↓ BAT - LICOR LI-7500 hygrometer (modified)
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)
↓ BAT
←LICOR
head

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

9. Drag Coefficients Smith (1992) ------ Large and Pond (1980) ------
COARE 3.0 —
CBLAST LOW (o)
Powell et al. (2003) −∙−−∙
Donelan et al. (2004) −−∙−∙−
CBLAST Data
LF (◊) RF (□)
LR (X) RR(+)

10. CE10N = <w'q'>/U10N(qsat-q10N)
Dalton Numbers
O AGILE Δ CBLAST
X HEXOS ◊ GASEX
▼ SOWEX □ SWADE
CBLAST - - -
HEXOS —
CE10N = <w'q'>/U10N(qsat-q10N)

11. Stanton Numbers
Δ CBLAST
X HEXOS

12. CK/CD = 0.63 < 0.75
Can we explain the disagreement between the CBLAST data against Emanuel’s theory?
Budget study of Emanuel (2006 Hurricane Conference) suggests that the ratio may increase significantly for intense hurricane conditions;
Strong air-sea interaction inward from the eyewall transports extra entropy through mesovortices was a key reason to get a cat 5 hurricane in Isabel case (Montgomery et al.,2006);
Emanuel’s model ignores the dynamics of the boundary layer;
We need measurements of turbulent fluxes, especially enthalpy fluxes, at higher wind speeds.
Δ CBLAST
X HEXOS
COARE 3.0
…….. Emanuel’s threshold 12

13. u* (m s-1)
Fk (W m-2)
Cdx1000
Ckx1000

14. Summary
1. Direct measurements of turbulent fluxes were made in the boundary layer of major hurricanes; Bulk parameterizations of momentum and enthalpy fluxes were extended up to near hurricane force;
2. Drag coefficients level off at around m/s, slightly lower than the that found from previous studies;
3. Exchange coefficients of sensible heat and latent heat, as well as enthalpy are nearly constant with wind speed up to 30 m/s;
4. The ratio of Ck/CD is significantly lower than the threshold used in the hurricane potential intensity theory, indicating that the enthalpy flux required for hurricane development may come from sources other than turbulent fluxes, or it demands a re-evaluation of the theoretical models used to derive the threshold.

15. Acknowledgements: Office of Naval Research (ONR)
CBLAST Hurricane Program
NOAA Hurricane Research Division
OAR & USWRP
NOAA/OMAO
Aircraft Operations Center
Support of National Research Council Fellowship