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

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

Jimy Dudhia

Wei Wang

James Done

Chris Davis

MMM Division, NCAR

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)

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

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

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)

1D Ocean Mixed-Layer Model

Heat fluxes

wind

Surface stress

SST

Mixed

layer

depth

Mixed layer

current

mixing

Stable layer

T profile

1D Ocean Mixed-Layer Model

Heat fluxes

wind

Surface stress

SST

Mixed

layer

depth

Mixed layer

current

mixing

Stable layer

T profile

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

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.

Drag Coefficient

- CD10 is the 10 m drag coefficient, defined such that

It is related to the roughness length by (in neutral conditions)

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)

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

Hurricane Dean (2007)

Note that forecasts underestimate maximum windspeed

Hurricane Dean (2007)

Forecasts also underestimate pressure drop

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

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

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

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

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

2007 Season Verification

From Weather Underground

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)

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

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

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

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