Aircraft Observations of the Hurricane Boundary Layer Structure

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• 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
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• 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

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Outline

• Background and Motiviations
• Experiment and Instrumentation
• 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.

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Depiction of the ABL processes
------- Boundary layer height
http//www.esrl.noaa.gov/research/themes/pbl/
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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
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• 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 gt 0.75
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Surface Fluxes and Parameterizations
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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.
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EC Data from 8 field experiments AGILE, AWE,
ETCH,GASEX,HEXOS,RASEX, SHOWEX, SWADE, WAVES
(4322 pts). Smith (1980)
Donelan et al. 2004
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)
Powell et al. 2003
COARE-3 --- COARE 2.5
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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
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2002 3 Test flights in Hurricanes Edouard,
Isidore, and Lili2003 6 flights in Hurricanes
Fabian and Isabel2004 Flights at top of
boundary layer, only 2 flux flights in
Hurricanes Frances and JeanneBlack 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)
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• 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
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CBLAST STEPPED DESCENTS
Black lines represent the flux runs Typical
length of a flux run is 24 km
108 flux runs for momentum flux measurement 42
flux runs for enthalpy flux measurement
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Vertical profiles of Mean Flow(Data are from
measurements during Sept. 12th 2003)
To Eye
zi
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Time series for a typical flux run
pitch
(40 Hz data)
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Spectral Analysis
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Drag Coefficients
Smith (1992) ------ Large and Pond (1980) ------
Smith (1980) ------- COARE 3.0 CBLAST LOW
(o) Powell et al. (2003) --- Donelan et al.
(2004) ----
CBLAST Data LF (?) RF (?) LR (X) RR()
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Dalton Numbers
CE10N ltw'q'gt/U10N(qsat-q10N)
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Stanton Numbers
? CBLAST X HEXOS
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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,
oi10.1029/2008GL034374.
? CBLAST X HEXOS
-------- COARE 3.0 -------- Emanuels threshold
CK/CD 0.63 lt 0.75
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Fk (W m-2)
u (m s-1)
Cdx1000
Ckx1000
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Vertical Structure of Momentum flux
Moss (1978)
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Profiles of humidity and sensible heat fluxes
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TKE Budget
I II
III IV V
TKE
I Shear production II Buoyancy III
Turbulent transport IV Pressure transport V
Rate of dissipation
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Turbulent Kinetic Energy Budget
Lenschow et al. (1980)
Nicholls (1985)
Zhang et al. 2009 JAS accepted
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Hurricane Boundary Layer Rolls
Morrison et al., 2005
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RADARSAT SAR imagery during Hurricane Isidore
Zhang et al. 2008 BLM
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• Boundary Layer Flight in Hurricane Isidore

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Wavelet Analysis
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Momentum Flux
Zhang et al. 2008 BLM
--- leg A --------- legs B C D
leg E
Wavelength 950 m
----- alongwind leg --- crosswind leg
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Sensible Heat Flux
Zhang et al. 2008 BLM
--- leg A --------- legs B C D
leg E
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Hurricane Hugo (1989)
Marks et al. 2008 MWR
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A schematic of the hurricane boundary layer
height
z
Vr0
hinflow
h
Vr
hi
zi
Max wind radius
?
Eye radius
Vr -10 m s-1
rmw
re
r
r150 km
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Mean Wind Speed Profiles
WL150 is the mean wind of the lowest 150 m data
Height of maximum wind speed
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Mean Potential Temperature Profiles
mixed layer height zi
Stable layer Transition Layer Mixed Layer
?? (zi) 1K
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Mean Radial Wind Profiles
Inflow layer height Vr 0
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Where is the top of the hurricane boundary layer?
Entrainment ?
?
hinflow
Vr0
?
?
hVmax
z
zi
Max wind radius
Eye radius
800 Wm-2
rmw
re
r
r500 km
r150 km
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Summary

• 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.
• The boundary layer height defined from the
  turbulent flux profiles is nearly twice the
  height of the mixed layer defined from the
  thermodynamic profiles.
• Turbulent kinetic energy budget indicates that
  the advection term is important.
• Boundary layer rolls rescale the sensible heat
  flux transport and enhance the momentum flux.

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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

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End
Thanks!
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Hurricane boundary layer depth

• Deardorff (1972) zi c u/f
• Anthes and Chang (1978)
• zi
• Kepert (2001)

is Inertial instability parameter
Consistent with Anthes and Chang (1978) and
Kepert (2001)