New Observations of Ocean Response to a Hurricane

Transcription

1 New Observations of Ocean Response to a Hurricane Thomas B. Sanford and James B. Girton Applied Physics Laboratory and School of Oceanography University of Washington In collaboration with: Eric A. D Asaro Applied Physics Laboratory, University of Washington Seattle, WA James F. Price Woods Hole Oceanographic Institution Woods Hole, MA Douglas C. Webb Webb Research Corporation East Falmouth, MA 02536

2 EDDIES Experiment Future Directions CLIMODE; DIMES Hurricanes and strongly forced situationss ARGO float program Outline Introduction to EM-APEX: A new autonomous profiler measuring velocity through motional induction CBLAST Hurricane Experiment Upper ocean observations Wind stress and currents Mixing and surface cooling Near-inertial wave radiation Sub-Inertial currents Surface waves Model comparison Dynamics of mixing and wave propagation Drag coefficient evaluation

3 Velocity Profiles Based on the Physics of Motional Electromagnetic Induction The horizontal components of the motionally induced ocean electric field as measured by sensors moving with the horizontal flow are (Sanford, JGR, 1971): h φ a - F z (v(z) - v*) k where h φ a is the measured (apparent) potential gradient at a sensor moving with velocity v, F z is the vertical component of the Earth s magnetic field, σ is electrical conductivity, v* is the vertically-integrated, conductivity-weighted ocean velocity and k is the vertical unit vector. Hence the measured ocean voltage gradient can be converted to a relative velocity profile.

4 AXCP Velocity Profiles in N. Pacific during Ocean Storms P3 Research Flights SGW Signals on 3 AXCPs dropped within minutes; center AXCP was slow-fall version

5 AXCP and Numerical Model Comparisons of SML Transports in E. Pacific and N. Atlantic Hurricanes Ref: Price, Sanford and Forristall, 1994: Forced Stage Response to a Moving Hurricane, JPO,

6 EM-APEX: Merger of Mature Technologies + = Webb Argo float (APEX) AVP: electromagnetic velocity profiler 6

7 EM-APEX Above: Air-launch packaging Left: Engineers Jim Carlson and John Dunlap

8 Block Diagram of EM -APEX

9 Hurricane Frances in Early Sept 2004 V,T, and S needed for dynamical ocean response studies APEX float commercially produced EM velocity well suited for float CBLAST air deployment capability

10 EM-APEXs Dropped Ahead of H. Frances from C-130 Aircraft 3 EM-APEX floats deployed from USAF Reserve C day ahead of H. Frances

11 Float Trajectories and Near-inertial KE

12 Half Inertial Period Pairs Before and After H. Frances for Float 1633 Pre-hurricane, half-inertial period pair Post-hurricane, half-inertial period pair

13 Left: Float 1633: 50 km to Right (Highest Wind) Right: Float 1634: 100 km to Right (Lower Wind)

14 Float 1636: Nearly Under Eye

15 Float Timeseries (animation by Jim Price) QuickTime and a YUV420 codec decompressor are needed to see this picture.

16 Float 1633: Early Stage at Highest Wind from 3D-PWP model by Jim Price (documented in Price, Sanford and Forristall, JPO 1994)

17 Richardson Number (Ri) remains near 1/4 long after mixing has ceased Measured Ri Model Ri

18 Nearly Under Eye vs. High Wind Floats 1636 & 1633: Eye (L) and H. Wind (R) Applied Physics Laboratory-University of Washington

19 Ocean Response at Float 1633: u & v components h( v/ t + f v) = τ s / ρ -vw e - gh η

20 Drag coefficient parameterizations

21 Ocean Response at Float 1633: u & v components h( v/ t + f v) = τ s / ρ -vw e - gh η

22 SST cooling sensitive to Cd less so to Cq: : advection and vertical mixing >> air-sea exchange Analysis and figure by Jim Price

23 Surface Gravity Wave Fit to EM-APEX u rms = u(0) rms e -z/δ, δ = g/ω 2 T = 2π δ/g & η = 4 u(0) rms δ/g

24 Float 1636 (Hurricane Eye): Near-Inertial Wave Wake (Half of differences of two profiles 1/2 IP apart)

25 Float 1636 (Hurricane Eye): Sub-Inertial Wave Wake (Half of sum of two profiles 1/2 IP apart)

26 Float 1633 (High Wind): Sub-Inertial Wave Wake (Half of sum of two profiles 1/2 IP apart)

27 Tracks of 4 EM-APEX Floats Deployed in EDDIES Experimennt

28 Float 1633: Refurbished and Deployed in EDDIES Experiment

29 Float 1633: Refurbished and Deployed in EDDIES Experiment

30 EM-APEX CBLAST Conclusions Upper ocean response to a hurricane can now be observed in unprecedented detail: Acceleration of wind-driven currents (inertial motions > 1.2 m/s) Surface layer modification (deepens by 80 m and cools by 2.5 C) Shear instability (Richardson number) dominant process Vertical internal wave energy flux (including rapid inertial pumping) Surface waves (significant wave height and dominant period) Inertial waves and subinertial response in and after cyclone Comparison with an upper-ocean model (3D-PWP) allows the direct evaluation of model dynamics and extends interpretation of sparse data: Decreasing drag coefficient at high wind speeds is supported by mixed layer acceleration and comparison of model SST cooling

31 Future Directions More hurricane observations for acceleration and mixing statistics in different storms Spatial arrays for ship-based and Lagrangian process studies; e.g. air-sea interaction and mixing studies Southern Ocean finestructure and mixing (DIMES) Internal tide and near-inertial energy: geographical distribution and fluxes Mode water formation (CLIMODE) Mesoscale eddy fluxes: heat, salt, thickness Global profiling in ARGO float program for velocity, shear, strain, mixing (e.g., Ri, Kρ), model verification

32 Future Directions: : Half-IP Profiles to Resolve Mean and IP Velocity Structure Half-IP profiles taken by EM-APEX floats every 10 days could enhance ARGO program

33 The End

34 Float 1633 (Highest Winds): Near Inertial Wave Wake