A drag parameterization for extreme wind speeds that leads to improved hurricane simulations
Gerrit BurgersNiels ZweersVladimir Makin Hans de Vries
EMS Annual meeting Berlin, 12-16 September 2011
Zweers Burgers de Vries Makin
Royal Netherlands Meteorological Institute (KNMI) Ministry of Infrastructure and the Environment
2 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, 12-16 September 2011
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
Introduction
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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
Hurricanes and fluxes
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Drag coefficient:
Most models use Charnock’s relation (1955) for the roughness length:
The roughness and the drag increase with wind speed Supported by observations for moderate and strong wind
In models z* ~ [0.010, 0.035]
2* ))(/( zUuCD
au /2*
Drag and roughness
u* : friction velocityU(z) : horizontal wind speedU(z) = u*/κ log(z/zo)
g
uzz
2*
*0
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Charnock vs. obs
However, observations for hurricane wind speeds deviate from Charnock
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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
Makin (2005)
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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The parameterization by Makin (2005):
In this study, we choose z* = 0.025, because that is the default value of the Hirlam model we use in the Hurricane simulations.
Makin (2005): cz0=0.010 Here: cz0=f(U10) from Makin (2003) (‘air flow separation’)
guccz zl /2*
/1/110 0
]/,1min[ *uacrit
Formulation Makin for drag coefficient
cz0: Charnock parameter
acrit: critical terminal fall velocitycl: constant
U10 < Ucrit:Charnock relation
U10 > Ucrit :reduced surface drag
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Charnock vs. Makin
Drag coefficient
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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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Katrina
Results Sea level pressure
with 6-hours analysis cycle
(Zweers et al., 2010)GRL 902 hPa
916 hPa
932 hPa
25/08/2005 29/08/2005
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Katrina
Results 10-meter wind speed
with 6-hours analysis cycle
(Zweers et al., 2010)GRL
77 m/s
73 m/s
55 m/s
25/08/2005 29/08/2005
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Results Sea level pressure
with 6-hours analysis cycle
Ivan
11/09/2004 16/09/2004
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Results 10-meter wind speed
with 6-hours analysis cycle
Ivan
11/09/2004 16/09/2004
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Results 10-meter wind speed
+96h forecast
Ivan
13/09/2004 16/09/2004
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Observed
KatrinaIvan
Results Hurricane Track
Modeled: Charnock relation New parameterization
+48h, +72h, +96h forecasts
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Ivan
Results Hurricane Track
forecasts with analyses
Observed Modeled: Charnock relation New parameterization
Katrina
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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 stressTypically: δU10/ U10 -0.25 δcD /cD δτ / τ 0.5 δcD /cD
(Zweers et al., Natural Hazards, submitted)
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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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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
Conclusion
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• 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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Reduction from 32m/s
Drag coefficient
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With reduced Charnock coefficient
Drag coefficient
BBOS 2010 - ZweersEGU 2011 Zweers et al.Introduction – Theory – Aim – Methodology – Results - Conclusion
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Katrina
Results 10-meter wind speed
with 6-hours analysis cycle
25/08/2005 29/08/2005
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Katrina
Results 10-meter wind speed
with 6-hours analysis cycle
25/08/2005 29/08/2005