Prediction of Atlantic Tropical Cyclones with the

Advanced Hurricane WRF (AHW) Model

Prediction of Atlantic Tropical Cyclones with the

Advanced Hurricane WRF (AHW) ModelJimy Dudhia

James Done, Wei Wang, Yongsheng Chen, Qingnong Xiao, Chris Davis, Greg Holland,

Rich Rotunno, and Ryan Torn

MMM Division, NCAR

Jimy DudhiaJames 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

Advanced Hurricane WRF in 2007

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)

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)

Dean track forecastsDean track forecasts

Hurricane DeanHurricane 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

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)Hurricane Dean (2007)

Note that forecasts underestimate maximum windspeed

Hurricane Dean (2007)Hurricane Dean (2007)

Forecasts also underestimate pressure drop

Roughness LengthsRoughness 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.

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.

Enthalpy Exchange Coefficient

Enthalpy Exchange Coefficient

CE10 is the 10 m moisture exchange coefficient, defined such that

CE10 is the 10 m moisture exchange coefficient, defined such that

CE10 =k

ln(z10 / z0 )⎛

⎝⎜⎞

⎠⎟k

ln(z10 / z0q)

⎛

⎝⎜

⎞

⎠⎟

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)

E ≡ρCE10V10Δq

CD and CkCD and Ck

From the works of Emanuel (1986), Braun and Tao (2001) and others the ratio of Ck to 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

From the works of Emanuel (1986), Braun and Tao (2001) and others the ratio of Ck to 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

Black et al. (2006)Black et al. (2006)

27th AMS Hurricane conference

Modification to Ck in AHW

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

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

Modification to Ck in AHW

Modification to Ck in AHW

Cd - redOld CB -

greenNew Ck - blue

dashedZ0q const -

blue solid

Cd - redOld CB -

greenNew Ck - blue

dashedZ0q const -

blue solid

Impact on Dean forecasts

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

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

2007 Season Verification2007 Season Verification

From Weather Underground

1.33 km WRF-2007 Intensity Forecast Error

0

10

20

30

40

50

60

70

80

0 12 24 36 48 60 72 84 96 108 120

Forecast Time (h)

Error (knots)

SHF5OFCLDSHPLGEMNCHGGFDLHWRFAHW

22

2017

1611

8

7

Model Intensity Comparison

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

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

SummarySummary 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

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