Revised Pressure-Wind and Radial Profile Models: Implications for Tropical Cyclone Archives...

Revised Pressure-Wind and Radial Profile Models: Implications for Tropical

Cyclone Archives

Mesoscale and Microscale Meteorology Division, ESSLNational Center for Atmospheric Research

NCAR is Sponsored by NSF

Greg Holland

1

• What Use Tropical Cyclone Archives?– The single most important action we can take;– Most Important Parameters.

• Pressure-Wind Relationships– Role and Archive Requirements;– Pressure-Wind or Vice Versa.

• Reconstructing Historical Surface Winds– Approach and Archive Requirements;

Summary

2Holland, IBTraCS Workshop Asheville 0509

• Forecast Statistics;• Statistical Forecasting;• Assessing Climate Variability and Change;• Insurance Premiums;• Building Code Development and

Engineering of Major Structures.

What Use Tropical Cyclone Archives?


What is the single most important action we can take?

• Intensity;• Overall Wind Structure;• Translational Speed.

What are the Most Important Damage and Engineering Factors?


R2 : Vm3=27%, Vt=49%, Rh-2 =70%

Willis Hurricane Index α Vm3, Vt, Rh-2

Pressure-Wind Relationships


Plotted are:• All known P-W Relationships (after Harper 2002);• Black dashed is AH;• Red triangles are Atlantic 1995-2005 observations

Generally take the form: x

mv a p e.g. Atkinson and Holliday: 0.6443.4(1010 )m cv p

Alternative is Dvorak Look-Up Table, which can be translated to:

0.6443.92(1015 )m cv p

What are the Implications for Historical Archives?

• The Good: – They introduced an important level of objectivity;– They can be reproduced, provided we know what was used.

• The Bad:– Widespread use of different approaches without adequate

documentation;– Mixing objective and subjective approaches.

• Further:– Historical relationships were 1-1 and could not possibly capture the true

relationship; however, they were easily reproduceable;– New relationships are multi-variate, which enable capture of some of the

observed scatter; however, this is at the cost of increased difficulty in reproducing what was done, especially if there is inadequate documentation.

Implications of P-W Relationships


• AH tends to over-estimate intense hurricane winds (Knaff and Zehr 2007), and so does Dvorak (Holland 2008);

Some Known Issues


• Ad-hoc application, including:– Lack of proper peer review of

many relationships;– Variations in PW relationships;– Sometimes wind as the

primary parameter, other times pressure.

Fortunately, many of these can be reversed later, given adequate documentation

Issues: Western Australia


0 20 40 60 80

0

20

40

60

80

100

120

Vm (m/s)

DP

(h

Pa

)

Period 1970-2005 Lat 0-30 Long 120-135 Dir All

ActualAH1minAH10min

Atkinson and Holliday relationship using 10 min winds (blue) and 1 min winds (black), showing the incorrect use of AH with 1 min winds.

Improved documentation needed

• Knaff and Zehr (2007):

• Holland (2008):

Recent Revisions for P-W Relations


0.6443.92(1015 ) ,m cv p

mxtv a bs c d p e p fv

5 2 xct

0.5

pb= -4.4*10 p + 0.01 p + 0.03 - 0.014 +0.15v + 1.0,

tp

x= 0.6 (1- ) , and215

( )m

bv p

e

Holland Model: Dependent NAT Data


• The variable b reproduces about half the maximum scatter in HURDAT.

• But the scatter experienced by most storms is captured:

• RMSD=3.5 m/s

• 85%<5 m/s

Note Dvorak overestimate

The Importance of RMW


Black: Holland (2008) model, left original, right including RMWRed: Hurricane model data courtesy of Yuqing Wang (f-plane, 5 km grid, no environmental flow)

20 30 40 50 60 70 80 90

20

40

60

80

100

120

Vm (m/s)

DP

(h

Pa

)

Wang Data 1 Type= 5

ActualDerived

20 30 40 50 60 70 80 90

20

40

60

80

100

120

Vm (m/s)

DP

(h

Pa

)

Wang Data 1 Type= 5

ActualDerived

Should we start with pressure and derive winds, or vice versa?

• Must archive both and document how they were derived;

• If either or both are directly observed, no issue, archive both and document this;

• However, >90% of archives use a combination of satellite inference and subjective judgment;

• Should we estimate winds and infer central pressure, or vice versa?

Maximum Winds/Central Pressure?


• For Hurricanes, wind responds to changes in the mass field (i.e. heating);

• Pressures are much more conservative:– Easier to observe;– Not subject to wind definition issues;– Not subject to as much high-frequency, “random”

variation;– Less impact from cyclone translation speed variations;– Not affected by supergradient wind issues.

Pressure vs Winds


e.g. Schubert et al (1980):

Winds Respond to Mass Changes


Hurricane Core

Wind responds to massMass responds to windE

ffic

ienc

y

Inverse of Deformation Radius

Deep TropicsLarge, weak systems

Mass adjusts to wind

Wind adjusts to mass

Wind vs Pressure Variation


Blue: cold SST= 26.3C

Red: warm SST=30.3C

Solid: dx = 1.67 km

Dashed: dx = 0.556 km

Wind “noise” at 556 m resolution is ~10 m/s

At 62 m resolution it is >20 m/s (gusts are explicitly simulated)

Model results from Rotunno et al (2008)

Roll Vortices at Landfall


• Hurricane Dean simulation using ARW• Vertical motion advects low-momentum air upward and

high momentum air downward.• Leads to low level wind variations of ~20 ms-1

Recommend pressure be the primary variable

Reconstructing the Surface Winds


CY CP L OT : I s ov e l s : V3 ( peak ) m/ s ; Vec t o r s : V3, T he t am Sc a l e : 1mm = 0 . 5 k m

Bur eau o f Me t eo r o l o gy : T C T r ac y _ 19 74Gr i d : C6 Run: T RACY _ 1974 T o t a l V3_ p eak ( max ) Env e l op e

25

30

30

35

35

40

40

45

45

50

55

60

65

70

75

80

85

8 5

85

85

90

90 90

90

9090

90 90

90

90

909

0

V 3 ( ma x ) =9 4 . 8 m/ s @ ( 9 3 , 1 1 6 ) ; v b =3 0 6 . 0 d e g , S t e p = 6 7 3 V e c t o r S c a l e : 2 0 . 0 m/ s

Da r wi n A MO Di ne s

S t a r t : 2 0 - D e c - 1 9 7 4 0 0 : 0 0 GMT E n d : 2 5 - De c - 1 9 7 4 1 1 : 5 0 GMTV 3 ( ma x ) : 2 4 - De c - 1 9 7 4 1 6 : 0 0 GMT

Hurricane Tracy Maximum Surface Wind Reconstruction Courtesy Bruce Harper

Needs: Track, Vm, Pc, Penv, RMWDesirable: Venv

Reconstructing the Surface Winds


Willoughby et al 1982

New approaches enable a good reconstruction of the surface wind field for historical storms, provided the archive includes information on: RMW, Pc, Vm, and Vne.

Vne is winds in the near environment and preferably 4 obs at orthogonal directions.

Also having an estimate of major secondary wind maxima would be useful, but not essential.

Examples of Profile Error Sensitivity


0 50 100 150 200 250 300-10

-5

0

5

10

Radius (km)

Azi

mu

tha

l W

ind

Dif

fere

nc

e (

m/s

)pn= 1005 pc= 950 bs= 18 vm= 56 vob= 56 RMW= 20 rob= 300

Error Range +5 to -5 m/s

a

0 50 100 150 200 250 300-15

-10

-5

0

5

10

15

Radius (km)

Azi

mu

tha

l W

ind

Dif

fere

nc

e (

m/s

)

pn= 1005 pc= 950 bs= 18 vm= 56 vob= 20000 RMW= 21 rob= 300

Error Range +5 to -5 km

b

0 50 100 150 200 250 300-10

-5

0

5

10

Radius (km)

Azi

mu

tha

l W

ind

Dif

fere

nc

e (

m/s

)


Error Range +5 to -5 m/s

c

0 50 100 150 200 250 300-10

-5

0

5

10

Radius (km)

Azi

mu

tha

l W

ind

Dif

fere

nc

e (

m/s

)


Error Range +100 to -100 km

d

Maximum Wind Error

External Radius ErrorExternal Wind Error

RMW Error

Needs: Track, Vm, Pc, Penv, RMW and Venv

Summary

Holland, IBTraCS Workshop Asheville 0509

• Tropical Cyclone Archives need to be customer driven, Climate/CAT-Modeling are very important customers:• Track, Pc, RMW, Vm, Penv, Venv,

Quality Flags;• Unless there are good quality direct

observations, Pc is recommended as the primary archive for intensity, with wind derived from a documented P-W relation and other knowledge of wind variations;

• Perhaps we should include an extra section in Annual Tropical Cyclone Reports, which discusses quality issues and uncertainty assessments for the season.

20

Revised Pressure-Wind and Radial Profile Models: Implications for Tropical Cyclone Archives...

Documents

Transcript of Revised Pressure-Wind and Radial Profile Models: Implications for Tropical Cyclone Archives...