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 Wei Wang James Done Chris Davis MMM Division, NCAR

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 (described later) • New surface flux formulations for hurricane conditions (described later)

3. 1D Ocean Mixed-Layer Model • In 2007 a 1d model based on Pollard, Rhines and Thompson (1973) was added to the hurricane forecasts • Purpose is to represent cooling of sea-surface temperature due to deep mixing of the oceanic boundary layer with stably-stratified cooler water below • This has a negative feedback on hurricanes which helps to prevent over-prediction

4. 1D Ocean Mixed-Layer Model • Mechanism • Stress from strong surface winds generates currents in the oceanic mixed layer (typically 30-100 m deep) • Currents lead to mixing with cooler water below when Richardson number becomes low enough • This cools the mixed layer, which changes the SST and hence surface fluxes

5. 1D Ocean Mixed-Layer Model • Model • Slab mixed layer • Predicts depth, vector current components, temperature • Initial depth specified from obs or climatology • Initial temperature is from SST • Initial current is zero • hurricane induced currents are assumed to dominate over pre-existing currents • Lapse rate below mixed layer is specified from obs or climatology • Inertial turning effects from Coriolis are included • Thermal energy balance in mixed layer due to surface fluxes and radiation is included (small effect)

6. 1D Ocean Mixed-Layer Model Heat fluxes wind Surface stress SST Mixed layer depth Mixed layer current mixing Stable layer T profile

7. 1D Ocean Mixed-Layer Model Heat fluxes wind Surface stress SST Mixed layer depth Mixed layer current mixing Stable layer T profile

8. Surface Fluxes • Heat, moisture and momentum Subscript r is reference level (lowest model level, or 2 m or 10 m) z0 are the roughness lengths

9. 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 small constants (=10-4 m for water surfaces) • 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.

10. Drag Coefficient • CD10 is the 10 m drag coefficient, defined such that It is related to the roughness length by (in neutral conditions)

11. 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)

12. Dean track forecasts

13. 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

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

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

16. 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

17. 27th AMS Hurricane conference Black et al. (2006)

18. 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

19. Modification to Ck in AHW Cd - red, Ck - blue New - dashed Old - solid

22. Dean track forecasts

24. 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

25. Hurricane Felix • Reached category 5 in Caribbean • Minimum central pressure - 929 hPa • Maximum wind - 165 mph • Made landfall possibly as category 5 in Nicaragua and Honduras

26. Felix track forecasts

29. Tropical Storm Karen • Did not reach category 1 • Minimum central pressure - 990 hPa • Maximum wind - 70 mph • An example of AHW over-intensifying a weaker storm

32. 2007 Season Verification From Weather Underground

33. 2007 Atlantic Season • 15 named storms including • 5 hurricanes (Dean, Felix, Humberto, Lorenzo, and Noel) • All 5 were land-falling • Only Dean and Felix exceeded category 1 • Dean, Felix, Gabrielle, Humberto, Ingrid, Karen, Lorenzo, Melissa simulations were included in the verification against other models (Mark DeMaria)

34. 7 8 11 16 17 20 22

35. 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

36. 5 8 11 16 17 20 22

37. Model Track Comparison • AHW produces better track than GFDL, and worse than AVN, comparable with HWRF. These models all use AVN large scale. • UKMO may have a better large scale leading to better track

38. 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 • Accuracy of track may be limited by global model boundary conditions • Weaker storms may still be over-intensified