prediction of atlantic tropical cyclones with the advanced hurricane wrf (ahw) model jimy dudhia wei...

Prediction of Atlantic Tropical Cyclones with the Advanced

Hurricane WRF (AHW) Model

Prediction of Atlantic Tropical Cyclones with the Advanced

Hurricane WRF (AHW) Model

Jimy Dudhia

Wei Wang

James Done

Chris Davis

MMM Division, NCAR

Jimy Dudhia

Wei Wang

James Done

Chris Davis

MMM Division, NCAR

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

conditions (described later)

1D Ocean Mixed-Layer Model1D 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

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


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

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


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)

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)


T profile

Mixed layer

Stable layer

current

windSurface stress

Heat fluxes

SSTMixedlayer depth

mixing

Surface FluxesSurface Fluxes

Heat, moisture and momentum Heat, moisture and momentumH =ρcpu*θ* τ =ρu*u*E =ρu*q*

u* =kVr

ln(zr / z0 )−ψ m

θ* =kΔθ

ln(zr / z0h ) −ψ h

q* =kΔq

ln(zr / z0q)−ψ h

Subscript r is reference level (lowest model level, or 2 m or 10 m)z0 are the roughness lengths

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 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.

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 CoefficientDrag Coefficient

CD10 is the 10 m drag coefficient, defined such that

CD10 is the 10 m drag coefficient, defined such that

CD10 =k

ln(z10 / z0 )⎛

⎝⎜⎞

⎠⎟

2

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

τ ≡ρCD10V102

Enthalpy Exchange CoefficientEnthalpy 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

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

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 AHWModification 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 AHWModification to Ck in AHW

Cd - red, Ck - blueNew - dashedOld - solid

Cd - red, Ck - blueNew - dashedOld - solid

Dean track forecastsDean track forecasts

QuickTime™ and aBMP decompressor

are needed to see this picture.

Impact on Dean forecastsImpact 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

Hurricane FelixHurricane 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

Reached category 5 in Caribbean Minimum central pressure - 929 hPa Maximum wind - 165 mph Made landfall possibly as category 5 in

Nicaragua and Honduras

Felix track forecastsFelix track forecasts

Tropical Storm KarenTropical 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

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 Verification2007 Season Verification

From Weather Underground

2007 Atlantic Season2007 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)

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)

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 ComparisonModel 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

1.33 km WRF-2007 Track Forecast Error

0

100

200

300

400

500

600

700

0 12 24 36 48 60 72 84 96 108 120

Forecast Time (h)

Error (nm)

CLP5OFCLAVNONGPSUKMGFDLHWRFAHW

5

2017

16

11

8

22

Model Track ComparisonModel 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

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

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

Accuracy of track may be limited by global model boundary conditions

Weaker storms may still be over-intensified

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

prediction of atlantic tropical cyclones with the advanced hurricane wrf (ahw) model jimy dudhia wei...

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