Implementation of New Air-Sea Exchange Coefficients(Cd/Ch)

into the Operational HWRF Model:

Impact on Hurricane Intensity Forecast Skill

Young C. Kwon, Robert Tuleya, Hua-lu Pan, Vijay Tallaparagada, William Lapenta and Steve Lord

(EMC/NCEP/NWS/NOAA)

Motivation

The HWRF model showed the strong positive intensity bias during the 2008 and 2009 season

Because the main energy source and sink of hurricanes are enthalpy and momentum fluxes from the ocean, sensitivity tests of air-sea exchange coefficients are performed to improve the intensity forecast skill of HWRF by reducing the intensity bias

Test results show that the observation based Cd (Powell 2003) and Ch (CBALST 2007) combination produced the best performance

Cd profiles vs wind speed

y = 8E-11x4 - 2E-08x3 + 1E-06x2 + 5E-07x + 0.001

y = 3E-07x2 + 1E-05x + 0.0009

y = 6E-11x4 - 1E-08x3 + 5E-07x2 + 2E-05x + 0.0009

y = 3E-09x3 - 6E-07x2 + 4E-05x + 0.0008

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

0.005

0 20 40 60 80 100 120

wind speed (m/sec)

cd

new-lin

new-dec

new-inc

opr

powell

Linear (new-lin)

Poly. (new-dec)

Poly. (new-inc)

Poly. (opr)

Poly. (powell)

ch profile vs wind speed

y = 2E-07x2 + 4E-05x + 0.0011

y = -5E-07x2 + 5E-05x + 0.0011

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 20 40 60 80 100 120

wind speed (m/sec)

ch

lin

inc

dec

opr

jun

Poly. (inc)

Linear (lin)

Poly. (dec)

Linear (jun)

Linear (opr)

Cd Profiles vs wind speed

Ch Profiles vs wind speed

CBLAST

2003 Powell

operational

operational

Ratio of Ch to Cd for Operational and Modified

HWRF Configurations

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

10m wind speed (m/sec)

Ch

/Cd

ch_opr/ch_opr

ch_cblat/cd_Powell_03

Operational HWRF

Modified (test)

Experiment Design

Cases: Total of 154 runsHurricane Fay:

2008.08.15.18 – 2008.08.24.00 (33 runs)

Hurricane Gustav:

2008.08.25.12 – 2008.09.01.18 (30 runs)

Hurricane Hanna:

2008. 08.28.06 – 2008.09.07.00 (39 run)

Hurricane Ike:

2008.09.01.12 – 2008.09.14.06 (52 runs)

HWRF Configurations:1) H48N: Operational HWRF (blue)

2) H5_5: Modified Ch/Cd (purple)

Cd: 2003 Powell

Ch: CBALST (Jun Zhang et al, 2007)

Max. Improvement: ~ 22% (4.3kt)

Avg Intensity Error

Avg Track Error

Reduction of intensity error

No impact on track error

-4

-2

0

2

4

6

8

10

12

14

16

0 12 24 36 48 72 96 120

h48n

h5-5

0

5

10

15

20

25

0 12 24 36 48 72 96 120

h48n

h5-5

Intensity Bias

Standard Deviation

Reduction in HWRF intensity bias for longer lead time guidance

0

5

10

15

20

25

30

35

40

45

50

0 12 24 36 48 72 96 120

H48N

H5-5

Number of superior performance

Max. Improvement: ~ 35% (8.0kt)

Intensity Error (IKE)

Track Error (IKE)

Fay

Fay

Max. Improvement: ~ 24%

Intensity Error (FAY)

Track Error (FAY)

-8

-6

-4

-2

0

2

4

6

8

0 12 24 36 48 72 96 120

bias h48n

bias h5-5

0

2

4

6

8

10

12

14

16

18

20

0 12 24 36 48 72 96 120

h48n

h5-5

Intensity Bias

Standard Deviation

Summary

• The observation based Cd and Ch improve the intensity forecast skill of HWRF model about 20%

• The positive intensity bias of HWRF reduce about half at the 4-5 day forecast

• The track forecast skill remains the same with change of Cd and Ch

Future plan

1. Changing momentum Zo and thermal Zo instead of directly modifying Cd /Ch (Bob Tuleya, Issac Ginis)

Zom from wave mode ouput HWRF surface physics surface fluxes

2. Conduct experiments to improve the boundary layer physics of HWRF (J. Han GFS physics member)

Courtesy to Isaac Ginis

Coupled-Wave model output