a drag parameterization for extreme wind speeds that leads to improved hurricane simulations gerrit...

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


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


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


Charnock vs. obs

However, observations for hurricane wind speeds deviate from Charnock


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


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


Charnock vs. Makin

Drag coefficient


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


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


Katrina

Results 10-meter wind speed


(Zweers et al., 2010)GRL

77 m/s

73 m/s

55 m/s

25/08/2005 29/08/2005


Results Sea level pressure


Ivan

11/09/2004 16/09/2004




Ivan

11/09/2004 16/09/2004



+96h forecast

Ivan

13/09/2004 16/09/2004


Observed

KatrinaIvan

Results Hurricane Track

Modeled: Charnock relation New parameterization

+48h, +72h, +96h forecasts


Ivan

Results Hurricane Track

forecasts with analyses

Observed Modeled: Charnock relation New parameterization

Katrina


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)


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


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


• 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


Reduction from 32m/s

Drag coefficient


With reduced Charnock coefficient

Drag coefficient

BBOS 2010 - ZweersEGU 2011 Zweers et al.Introduction – Theory – Aim – Methodology – Results - Conclusion


Katrina



25/08/2005 29/08/2005

a drag parameterization for extreme wind speeds that leads to improved hurricane simulations gerrit...

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