Prediction of Atlantic Tropical Cyclones with the Advanced Hurricane WRF (AHW) Model

1. Prediction of Atlantic Tropical Cyclones with the Advanced Hurricane WRF (AHW) Model Jimy Dudhia James Done, Wei Wang, Yongsheng Chen, Qingnong Xiao, Chris Davis, Greg Holland, Rich Rotunno, and Ryan Torn MMM Division, NCAR • Outline • AHW forecast system in 2007 • Sensitivity to surface exchange coefficients • Verification comparison with operational models

2. Advanced Hurricane WRF in 2007 • Features • 3 domains (12 km, 4 km, 1.33 km) • 00Z start time, 5 days (domain 3 starts at 12 h) • Domains 2 and 3 move with storm using vortex following algorithm • 1d ocean mixed-layer model • New surface flux formulations for hurricane conditions (described later)

3. Dean track forecasts

4. Hurricane Dean • Reached category 5 in Caribbean • Minimum central pressure - 906 hPa • Maximum wind - 165 mph • Made landfall as category 5 in Belize and Mexican Yucatan • Redeveloped over Gulf • Second landfall in Mexico as category 2

5. Hurricane Dean (2007) Note that forecasts underestimate maximum windspeed

6. Hurricane Dean (2007) Forecasts also underestimate pressure drop

7. Roughness Lengths • Roughness lengths are a measure of the “initial” length scale of surface eddies, and generally differ for velocity and scalars • In 2006 AHW z0h=z0q are calculated based on Carlson-Boland (~10-4 m for water surfaces, weak variation with wind speed) • z0 for momentum is a function of wind speed following tank experiments of Donelan (this replaces the Charnock relation in WRF). This represents the effect of wave heights in a simple way.

8. Enthalpy Exchange Coefficient • CE10is the 10 m moisture exchange coefficient, defined such that It is related to the roughness lengths (assuming neutral conditions) by Often it is assumed that CH=CE=Ck where Ck is the enthalpy exchange coefficient. However, since 90% of the enthalpy flux is latent heat, the coefficient for sensible heat (CH) matters less than that for moisture (CE)

9. CDand Ck • From the works of Emanuel (1986), Braun and Tao (2001) and others the ratio of Ckto CD is an important factor in hurricane intensity • Observations give some idea of how these coefficients vary with wind speed but generally stop before hurricane intensity

10. 27th AMS Hurricane conference Black et al. (2006)

11. Modification to Ck in AHW • Commonly z0q is taken as a constant for all wind speeds • However for winds greater than 25 m/s there is justification for increasing this to allow for sea-spray effects that may enhance the eddy length scales • We modify z0q in AHW to increase at wind speeds > ~25 m/s • This impacts Ck as shown next

12. Modification to Ck in AHW Cd - red Old CB - green New Ck - blue dashed Z0q const - blue solid

15. Impact on Dean forecasts • General improvement in both wind and pressure intensity measures • This is consistent with enhanced ratio of Ck to CD as expected from previous theoretical and modeling studies • This new formulation was used for the 2007 season verifications • Impact of changing Ck in the case of Dean was greater than impact from increasing ocean mixed-layer depth

16. 2007 Season Verification From Weather Underground

17. 7 8 11 16 17 20 22

18. Model Intensity Comparison • AHW seems to improve relative to other models at longer ranges (similar result was found in previous seasons) • Resolution must be a major factor in this

19. Summary • Hurricane forecasts are sensitive to surface flux treatment • A change that may represent sea-spray effects has a positive impact on intensity for strong hurricanes • Intensity verification shows the benefits of AHW at high resolution compared to other models