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Hurricane structure and intensity change : Effects of wind shear and Air-Sea Interaction

Hurricane structure and intensity change : Effects of wind shear and Air-Sea Interaction. M é licie Desflots Rosenstiel School of Marine & Atmospheric Science 4600 Rickenbacker Causeway, Miami, FL, 33149-1098. What controls Hurricane Intensity ?.

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Hurricane structure and intensity change : Effects of wind shear and Air-Sea Interaction

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  1. Hurricane structure and intensity change : Effects of wind shear and Air-Sea Interaction Mélicie Desflots Rosenstiel School of Marine & Atmospheric Science 4600 Rickenbacker Causeway, Miami, FL, 33149-1098

  2. What controls Hurricane Intensity ? • Inner core (eye and eyewall) dynamics and rainbands(Theoretical and modeling work, e.g., Montgomery& Kallenbach 1997, Schubert et al. 1999, and recent field program – RAINEX, Houze et al. 2006, Chen 2006) • Environmental conditions • vertical wind shear(e.g., Frank&Ritchie 1999, Black et al. 2002, Rogers et al. 2003, Chen et al. 2006, Desflots and Chen 2006) • moisture distribution • sea surface temperature (upper ocean heat content),surface properties, etc. (Theoretical, observational, and modeling work, e.g., Emanuel 1995, Bao et al. 2000, and recent results from CBLAST, Black et al. 2006, Chen et al. 2006)

  3. Outline • Model and data • Effect of vertical wind shear on hurricane intensity • Importance of air-sea interaction for hurricane intensity (sensitivity to sea-spray parameterization) • Conclusions

  4. Model ATMOS. MODEL (MM5) Heat & Moisture fluxes OCEAN MODEL (3DPWP) SST Surface wind Wave-induced stress Current velocity Wave-Induced stress WAVE MODEL (WAVEWATCH III)

  5. Data 79 co-located dropsondes +AXBT Data 68

  6. Effect of vertical wind shear on hurricane intensity

  7. Low shear High shear Effect of Vertical Wind Shear on TCs’ intensity Shear-Induced Rainfall Asymmetry (Chen et al. 2006) (Global composite from TRMM and SHIPS data) Mechanisms: 1) vortex tilt and 2) shear-induced secondary circulation e.g. Simpson and Riehl 1958; Gray 1968; Willoughby 1984; Marks et al 1992; Franklin et al. 1993; Jones 1995,2000a, 2000b; DeMaria 1996; Gamache et al. 1997; Frank and Ritchie 2001; Black et al. 2002; Corbosiero and Molinary 2002; Rogers et al 2003; Wong and Chan 2004 ; Lonfat 2004; Chen et al. 2006

  8. Model description • MM5: the 5th generation high resolution, non- hydrostatic PSU/NCAR mesoscale model • Multi-nested, vortex-following domains with grid resolution of 15,5,1.67, 0.55 km, respectively • 28 vertical sigma-levels

  9. Model initialization • Initialized at 0000 UTC, 10/01/02, integrated for 72 hrs. • NCEP global 1°x1° analysis is used as initial and lateral boundary conditions • Time varying SST from SSMI/satellite (1/4°) • A vortex relocation procedure, similar to Liu et al. (1997), is used at the initial time.

  10. Track

  11. Best Track 5 km 1.67 km 0.55 km Intensity landfall

  12. Evolution of the Intensity of Hurricane Lili (2002) landfall Time Radius Radius Sea Level Pressure Rain Rate in mm/h Tangential Velocity in m/s

  13. Shear Analysis from MM5 (1) (2) (1) (2) Vertical wind shear vector Storm motion

  14. 10/02 13Z 10/02 21Z 10/03 03Z Vortex Tilt/ Temperature perturbations T/W at 550 hPa Rain rate

  15. Effect of Vertical Wind Shear on Hurricane Lili • Combine effect of the increasing vertical wind shear and the vertical wind shear direction created a rainfall asymmetry • The vertical wind shear weakened the storm’s intensity

  16. Importance of the of air-sea interactions for hurricane intensity

  17. Hurricanes intensity and Air-Sea Interaction • Ocean coupling by reducing the amount of available heat fluxes reduces the storm intensity (e.g., Chen et al. 2006) • Sensitivity to sea spray parameterization in a coupled model (e.g., Bao et al. 2000, Kepert, 2001)

  18. Importance of Air-Sea Fluxes in Hurricane Intensity • Energy source (enthalpy) and sink momentum/dissipation) for hurricanes • The balance between the two can potentially affect hurricane intensity • They are a good constraint to develop and evaluate coupled air-sea models for hurricane intensity forecasting

  19. Air-Sea Fluxes formulation In a turbulent boundary layer the turbulent fluxes can be approximated to bulk fluxes : • Momentum flux • Sensible Heat flux • Latent Heat flux

  20. 103CE C D Behavior of the exchange coefficients at high wind speed The drag coefficient levels off at high wind speed (Powell et al. 2003; Donelan et al. 2004) CBLAST Observations: Black et al. (2006), Drennan et al. (2006), French et al. (2006)

  21. Total Surface Heat Fluxes Uncoupled model Bulk fluxes estimates from GPS dropsondes and AXBTs Coupled model

  22. Air-Sea temperatures SST Sea-air temperature difference Coupled Model Observations

  23. Erika 970908 Bonnie 980821 Bonnie 980824 Bonnie 980825 Bonnie 980826 Floyd 990913 Floyd 990914 Fabian 030902 Fabian 030903 Fabian 030904 Frances 040901 Jeanne 040925 Dennis 050709 Dennis 050710 Rita 050922 Rita 050923 * * * X x Are the total heat fluxes correlated to hurricane intensity or local wind speed ? FR FL RL RR Tropical Storm Wind speed in m/s Cat. 1-2 Major hurricanes

  24. Air-sea Fluxes in the eyewall Bonnie (1998) SLP Heat fluxes Floyd (1999) Lili (2002) Frances (2004)

  25. Air-sea Fluxes within 200km radius Bonnie (1998) Floyd (1999) SLP Heat fluxes Frances (2004) Lili (2002)

  26. Effect of Sea Spray on surface heat fluxes • On the sensible heat flux : • The sea spray droplet cools from the sea temperature to the air temperature (or to Tw’) and gives up sensible heat TO the atmosphere • to evaporate the sea spray some sensible heat flux FROM the atmosphere is required • frictional terms due to waves • On the latent heat flux : • when the sea spray evaporates it gives up some latent heat flux TO the atmosphere • Fairall et al. 1994 : The effect of sea spray on the surface energy transports over the ocean. • Global Atmos. Ocean Syst.,2, 121-142 • Bao et al. 2000 : Numerical Simulations of Air-Sea Interaction under High Wind Conditions Using a • Coupled Model : A study of Hurricane Development, Mon. Wea. Rev. ,128, 2190-2210

  27. Sensitivity to sea spray Air-sea Fluxes in the eyewall Heat fluxes SLP Air-sea Fluxes within 200km radius Frances without Sea Spray Frances with Sea Spray

  28. Conclusions • Strong wind shear-induced asymmetry in rainfall (convective heating) and vortex circulation is a major limiting factor on hurricane intensity • Observed surface enthalpy flux has a large storm-to-storm variability as well as spatial variability within each storms • Storm-averaged total surface enthalpy flux is not a good predictor of hurricane intensity or intensity change • Inner-core (eye and eyewall) structure and dynamics dominate the rapid intensification process, which is sensitive to the surface enthalpy flux in the eyewall region • Coupled atmosphere-wave-ocean model can produce the general characteristics of observed surface fluxes. However, parameterizations of the air-sea fluxes remain to be a challenge, especially sea spray due to the lack of observations in high winds.

  29. Acknowledgements • Part of this work was supported by a research grant from the Office of Naval Research (N00014-01-1-0156) • My advisor : Dr. Shuyi Chen • My committee members : Drs. David Nolan, Mark Donelan, James Price, Frank Roux and Will Drennan • Drs. Wei Zhao, Jian-Wien Bao • Mike Black for help to process the dropsondes • HRD and CBLAST program for collecting precious data • Jun Zhang for helpful discussion • Thank to all the people who helped me one way or another

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