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 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 C D drag coefficient (momentum) C K enthalpy coefficient C K /C D ~ 1.2 – 1.5 C K /C D > 0.75

Surface Fluxes and Parameterizations

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 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 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) ZiZi

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 BAMS Drennan et al JAS French et al JAS Zhang et al 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 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) zizi To Eye

pitch Time series for a typical flux run u altitude roll heading humidity, q w v pitch (40 Hz data)

Spectral Analysis

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( +)

Dalton Numbers C E10N = /U 10N (q sat -q 10N ) O AGILE Δ CBLAST X HEXOS ◊ GASEX ▼ SOWEX □ SWADE CBLAST HEXOS —

Stanton Numbers Δ CBLAST X HEXOS

COARE Emanuel’s threshold C K /C D = 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: /2008GL

F k (W m -2 )u * (m s -1 ) C d x1000 C k x1000

Vertical Structure of Momentum flux –— Moss (1978)

Profiles of humidity and sensible heat fluxes

TKE Budget I : Shear production II: Buoyancy III: Turbulent transport IV: Pressure transport V: Rate of dissipation I II III IV V TKE:

Turbulent Kinetic Energy Budget Nicholls (1985) Lenschow et al. (1980) Zhang et al JAS accepted

Hurricane Boundary Layer Rolls Morrison et al., 2005

RADARSAT SAR imagery during Hurricane Isidore Zhang et al 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 BLM

Sensible Heat Flux ─── leg A legs B C D leg E Zhang et al BLM

Hurricane Hugo (1989) Marks et al MWR

rmwrmw rere r z hihi θ VrVr A schematic of the hurricane boundary layer height V r =0 V r = -10 m s -1 r=150 km h h inflow zizi h i – BL height of the idealized slab model h – BL height defined from the flux profile h inflow – height of the inflow layer z i – BL height (mixed layer depth) defined from theta profile - Scaling depth in theoretical models Eye radiusMax wind radius

Mean Wind Speed Profiles W L150 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 mixed layer height z i Δθ (z i ) ~ 1K

Mean Radial Wind Profiles Inflow layer height Vr = 0

rmwrmw rere r z V r =0 h inflow r=150 km Entrainment ? r=500 km zizi ? ? ? Eye radiusMax wind radius Where is the top of the hurricane boundary layer? h Vmax 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) : z i = c u * /f Anthes and Chang (1978) : z i = Kepert (2001) : is Inertial instability parameter Consistent with Anthes and Chang (1978) and Kepert (2001)