prediction of atlantic tropical cyclones with the advanced hurricane wrf (ahw) model
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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