Royal Netherlands
Meteorological Institute (KNMI)
Ministry of Infrastructure and the
Environment
A drag parameterization
for extreme wind speeds
that leads to improved hurricane simulations
Gerrit Burgers
Niels Zweers
Vladimir Makin
Hans de Vries
EMS Annual meeting
Berlin, 12-16 September 2011
Zweers
Burgers
de Vries
Makin
Introduction
 The ability to forecast hurricane tracks has improved during years, while
hurricane intensity is still underestimated
 Underestimation of hurricane intensity in numerical weather prediction
(NWP) models is strongly related to the uncertainty in the computation of
the surface fluxes of momentum and heat
 Observational evidence points to reduced drag at extreme wind speeds
 We show that using reduced drag at extreme wind speeds lead to
improved hurricane forecasts
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Hurricanes and fluxes
 Emanuel (1995): tropical cyclones can only attain their energy
if the ratio of exchange coefficients of heat (Ck) and momentum (CD)
is sufficiently large:
(Ck/CD) ~ 1 – 1.5
 In most NWP models (Ck/CD) is much smaller for extreme wind speeds
 Two possible remedies
- enhancement of surface heat fluxes (spray parameterizations)
- stabilizing the magnitude of the wind drag for extreme winds
 This study: we examine hurricane intensity by focusing on the computation of
the momentum flux
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Drag and roughness
u*   / a
2
 Drag coefficient:
CD  (u* / U ( z))2
u* : friction velocity
U(z) : horizontal wind speed
U(z) = u*/κ log(z/zo)
 Most models use Charnock’s relation (1955) for the roughness length:
2
u*
z0  z*
g
The roughness and the drag increase with wind speed
Supported by observations for moderate and strong wind
 In models z* ~ [0.010, 0.035]
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Charnock vs. obs
However, observations for hurricane wind speeds
deviate from Charnock
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Makin (2005)
 Based on theoretical and observational evidence, Makin (2005) proposed a
drag parameterization that accounts for the observed reduction in the drag
coefficient
 Impact of spray on the stress through stratification
 reduced vertical mixing, reduced drag
 Later studies: in addition direct impact of spray
 spray force from rain of spray leads to reduced drag
We examine the impact of the Makin (2005) drag
parameterization in an NWP model on the prediction of the
10-meter wind speed, sea level pressure and hurricane track
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Formulation Makin
for drag coefficient
 The parameterization by Makin (2005):
z0  cl11/  c1z0/ u*2 / g
U10 < Ucrit:
Charnock relation
cz0: Charnock parameter
  min[1, acrit / u* ]
acrit: critical terminal fall
velocity
cl: constant
U10 > Ucrit :
reduced surface drag
 In this study, we choose z* = 0.025, because that is the default value of the
Hirlam model we use in the Hurricane simulations.
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Charnock vs. Makin
Drag coefficient
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
NWP model
simulations
 We test the parameterization in the NWP model HIRLAM* (High Resolution
Limited Area Model)
 Boundary conditions from ECMWF
 HIRLAM in the Gulf of Mexico, horizontal resolution 5km
 Two hurricanes: Ivan (2004) and Katrina (2005)
i. Forecasts, analyses every 6 hours (“analysis=previous forecast + data
assimilation”)
ii. Forecasts up to +96h
* http://hirlam.org
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
Sea level pressure
Katrina
with 6-hours
analysis cycle
932 hPa
916 hPa
(Zweers et al., 2010)
GRL
902 hPa
25/08/2005
10
29/08/2005
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
10-meter wind speed
77 m/s
Katrina
73 m/s
with 6-hours
analysis cycle
55 m/s
(Zweers et al., 2010)
GRL
25/08/2005
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29/08/2005
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
Sea level pressure
Ivan
with 6-hours
analysis cycle
11/09/2004
12
16/09/2004
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
10-meter wind speed
Ivan
with 6-hours
analysis cycle
11/09/2004
13
16/09/2004
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
10-meter wind speed
Ivan
+96h forecast
13/09/2004
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16/09/2004
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
Hurricane Track
Ivan
 Observed
15
Katrina
+48h, +72h, +96h forecasts
Modeled: Charnock relation New parameterization
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
Ivan
 Observed
16
Hurricane Track
Katrina
forecasts with analyses
Modeled: Charnock relation New parameterization
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Intermezzo
What happens
if cD decreases?
If the drag coefficient decreases, then
- stress decreases
- wind speed increases; boundary layer height decreases; depression deepens
- heat flux increases
- storm surge decreases
τ = cD U102 , but wind speed change partially cancels
effect uncertainty drag on stress
Typically: δU10/ U10  -0.25 δcD /cD
δτ / τ  0.5 δcD /cD
(Zweers et al., Natural Hazards, submitted)
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Effect on surge
• Surge depends on stress
• But through an integral over
space and time
 no simple relation
• Changes in drag result in
substantial differences (up to 1m)
Results from Delft-3D
storm surge model simulations
(Zweers et al., Natural Hazards, submitted)
Ivan, 16 September 15h00
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Conclusion
 We simulated two tropical cyclones - Ivan and Katrina – with
- Charnock’s relation, and
- new (Makin) drag parameterization with reduced drag for extreme winds
 Charnock’s relation:
- hurricane intensity is severely underestimated in forecasts
 New drag parameterization:
- hurricanes are much stronger in forecasts in terms of wind and pressure
- surge lower
- hurricane track nearly unchanged
- quite good agreement between model results and observed conditions
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
• Thank you for your attention
Further reading
• K.A. Emmanuel, Sensitivity of tropical cyclones to surface exchange
coefficients …, J. Atm. Sci. (1995)
• V.K. Makin, A note on the drag of the sea surface at hurricane winds, Bound.
-Layer Met. (2005)
• M.D. Powell et al., Reduced drag coefficient for high wind speeds in tropical
cyclones, Nature (2003)
• N. C. Zweers et al., A sea drag relation for hurricane wind speeds, GRL (2010)
• N.C. Zweers et al., On the influence of changes in the drag relation on
surface wind speeds and storm surge forecasts, submitted to Nat. Haz.
• N.C. Zweers et al., Reduced drag coefficients for hurricane winds in
atmospheric and storm surge simulations, in preparation
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Reduction from 32m/s
Drag coefficient
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Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
With reduced
Charnock coefficient
Drag coefficient
EGU
22 2011
Gerrit Burgers et al., A drag parameterization for extreme winds
Introduction – Theory – Aim – Methodology
– Results - ConclusionBBOS 2010
Zweers
- Zweers
et al.
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
10-meter wind speed
Katrina
with 6-hours
analysis cycle
25/08/2005
23
29/08/2005
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011
Results
10-meter wind speed
Katrina
with 6-hours
analysis cycle
25/08/2005
24
29/08/2005
Gerrit Burgers et al., A drag parameterization for extreme winds
EMS Annual Meeting | Berlin, 12-16 September 2011