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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. Aircraft Observations of the Hurricane Boundary Layer Structure. Hurricane intensity is related to:

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Aircraft Observations of the Hurricane Boundary Layer Structure

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  1. Jun Zhang Collaborators: William Drennan, Peter Black, Jeffrey French, Frank Marks, Kristina Katsaros, and Susanne Lehner National Hurricane Center, Feb. 6th, 2009 Aircraft Observations of the Hurricane Boundary Layer Structure

  2. 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

  3. 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.

  4. Depiction of the ABL processes ------- Boundary layer height http://www.esrl.noaa.gov/research/themes/pbl/

  5. Why is the boundary layer so important in hurricanes? • The boundary layer provides a powerful coupling between theprimary circulation(the azimuthal component) and thesecondary 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

  6. 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

  7. Surface Fluxes and Parameterizations

  8. 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.

  9. 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 —

  10. Moss (1978) 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. Zi

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

  12. 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

  13. 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

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

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

  16. Spectral Analysis

  17. 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(+)

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

  19. Stanton Numbers Δ CBLAST X HEXOS

  20. 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. Δ CBLAST X HEXOS -------- COARE 3.0 -------- Emanuel’s threshold CK/CD = 0.63 < 0.75

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

  22. Vertical Structure of Momentum flux –— Moss (1978)

  23. Profiles of humidity and sensible heat fluxes

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

  25. Turbulent Kinetic Energy Budget Lenschow et al. (1980) Nicholls (1985) Zhang et al. 2009 JAS accepted

  26. Hurricane Boundary Layer Rolls Morrison et al., 2005

  27. RADARSAT SAR imagery during Hurricane Isidore Zhang et al. 2008 BLM

  28. Boundary Layer Flight in Hurricane Isidore

  29. Wavelet Analysis

  30. Momentum Flux Zhang et al. 2008 BLM ─── leg A --------- legs B C D leg E Wavelength ~ 950 m ----- alongwind leg ─── crosswind leg

  31. Sensible Heat Flux Zhang et al. 2008 BLM ─── leg A --------- legs B C D leg E

  32. Hurricane Hugo (1989) Marks et al. 2008 MWR

  33. hi – BL height of the idealized slab model h – BL height defined from the flux profile hinflow – height of the inflow layer zi – BL height (mixed layer depth) defined from theta profile - Scaling depth intheoretical models A schematic of the hurricane boundary layer height z hinflow Vr=0 h Vr hi zi Max wind radius θ Eye radius Vr= -10 m s-1 re rmw r r=150 km

  34. Mean Wind Speed Profiles WL150 is the mean wind of the lowest 150 m data Height of maximum wind speed 230 237 238 170 157 146

  35. Mean Potential Temperature Profiles mixed layer height zi Stable layer Transition Layer Mixed Layer Δθ (zi) ~ 1K 230 237 238 170 157 146

  36. Mean Radial Wind Profiles Inflow layer height Vr = 0 230 237 238 170 157 146

  37. Where is the top of the hurricane boundary layer? Entrainment ? ? hinflow Vr=0 ? ? hVmax z zi Max wind radius Eye radius 800 Wm-2 rmw re r r=500 km r=150 km

  38. 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.

  39. 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

  40. End Thanks!

  41. 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)

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