Modeling the upper ocean response to Hurricane Igor

1. Modeling the upper ocean response to Hurricane Igor Zhimin Ma 1, Guoqi Han 2, Brad deYoung 11 Memorial University2 Fisheries and Oceans Canada

2. Objective and Methodology Extreme events like hurricane can induce strong storm surge along the coastline and lead to erosion or flooding to the coastal areas. • To establish a 3-D baroclinic ocean model for simulating the upper ocean response to hurricane. • The Holland hurricane model is combined into the numerical ocean model to improve the accuracy of simulating the storm surge. We use hurricane Igor in 2010 as an example. Hurricane Igor passed through Newfoundland between Sep 21 and 22.

3. FVCOM (3.1.4) and GOTM(Chen et al., 2003, 2006) • Atmospheric static forcing is added into the momentum equation. • GOTM is hooked into the FVCOM model: so the turbulent model is second-order k-epsilon model with dynamic dissipation rate equation.

4. Holland hurricane model (1) • Where is the radial distance from the hurricane center, is the wind speed, and MCP are the ambient and minimum central atmospheric pressures, respectively, RMW is the radius of maximum winds, is the maximum sustained wind speed, and B determine the shape of the storm. (2) (3)

5. Slns4 node: 33863 Element:65601 High resolution

6. Transect

7. Model Forcing • External atmospheric forcing: Three hourly NOAA north Atlantic regional reanalyzed model results ( high resolution NCEP Eta Model (32km/45 layer) together with the regional Data Assimilation System(RDAS)) • Hurricane wind and air pressure forcing: Holland hurricane wind and air pressure field is constructed based on the NOAA IGOR 3 hourly center tracking points and forecasting report. These wind and pressure fields are combined with NOAA wind and pressure fields. • Open boundary sea level: monthly climatology sea level (Han et., al, 2008)

8. 64 kt wind radius from forecasting report Center • Based on Equation (2) , we estimate the B is around 1.0 and keep constant. http://www.nhc.noaa.gov/archive/2010/al11/al112010.fstadv.051.shtml? Based on the equation (1), we can get the maximum wind radius for four directions. The averaged radius is used in calculation of the whole wind field. Averaged maximum wind radius

9. Model Setup and Initial Condition (1)Initial temperature and salinity: Monthly temperature and salinity (Geshelin et al., 1999) (2)Time Step: Internal time step is 1 second and external time step is 10 seconds. Model running: (1)First period, July climatology running for 15 days with climatology wind and hydrographic condition. (2)Second period, real forcing from August 1st to October 15th The results between September 1st and October 15th are analyzed.

10. Sea level Comparison RMSE 7.2 cm RMSE 6.8 cm RMSE 7.6 cm RMSE 6.9 cm Year Day

11. Temperature comparison RMSE 1.6 RMSE 1.9

12. Area Averaged Mixed layer depth

13. Surface current 10 days before Sep-21 18:00 UTC

14. Bottom current

15. Current Transect -40 -80 -120 10 days before Sep-21 18:00 UTC -160 U U V V -200

16. Conclusion • Model well simulates the sea level variability and storm surge during hurricane Igor. • Simulated sea surface temperature decrease agrees with the buoy observations. Mixed layer depth deepened significantly. • The storm can strongly impact not only the surface current but also the bottom flow over the Grand Banks.

17. Further Improvement • Wind and air pressure field during hurricane can be improved by assimilation the observed wind filed into the Holland model if there are enough observed wind stations. • Wind field time resolution can be improved using the hourly storm center locations. • Remote traveling wave induced by hurricane can be obtained from a large scale barotropic model and added into Newfoundland shelf model (slns4).

18. Thanks