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)