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, 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, 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

3 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011  Emanuel (1995): tropical cyclones can only attain their energy if the ratio of exchange coefficients of heat (C k ) and momentum (C D ) is sufficiently large: (C k /C D ) ~ 1 – 1.5  In most NWP models (C k /C D ) 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

4 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011  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] Drag and roughness u * : friction velocity U(z) : horizontal wind speed U(z) = u * / κ log(z/z o )

5 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Charnock vs. obs However, observations for hurricane wind speeds deviate from Charnock

6 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011  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

7 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011  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): c z0 =0.010  Here: c z0 =f(U 10 ) from Makin (2003) (‘air flow separation’) Formulation Makin for drag coefficient c z0 : Charnock parameter a crit : critical terminal fall velocity c l : constant U 10 < U crit : Charnock relation U 10 > U crit : reduced surface drag

8 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Charnock vs. Makin Drag coefficient

9 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 NWP modelsimulations  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 *

10 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Katrina ResultsSea level pressure with 6-hours analysis cycle (Zweers et al., 2010) GRL 902 hPa 916 hPa 932 hPa 25/08/200529/08/2005

11 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Katrina Results10-meter wind speed with 6-hours analysis cycle (Zweers et al., 2010) GRL 77 m/s 73 m/s 55 m/s 25/08/200529/08/2005

12 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 ResultsSea level pressure with 6-hours analysis cycle Ivan 11/09/200416/09/2004

13 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Results10-meter wind speed with 6-hours analysis cycle Ivan 11/09/200416/09/2004

14 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Results10-meter wind speed +96h forecast Ivan 13/09/200416/09/2004

15 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Observed KatrinaIvan ResultsHurricane Track Modeled: Charnock relation New parameterization +48h, +72h, +96h forecasts

16 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Ivan ResultsHurricane Track forecasts with analyses ObservedModeled: Charnock relation New parameterization Katrina

17 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Intermezzo What happens if c D 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 = c D U 10 2, but wind speed change partially cancels effect uncertainty drag on stress Typically: δ U 10 / U 10  δ c D /c D δτ / τ  0.5 δ c D /c D (Zweers et al., Natural Hazards, submitted)

18 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, 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

19 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011  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

20 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, 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

21 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Reduction from 32m/s Drag coefficient

22 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 With reduced Charnock coefficient Drag coefficient BBOS ZweersEGU 2011 Zweers et al.Introduction – Theory – Aim – Methodology – Results - Conclusion

23 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Katrina Results10-meter wind speed with 6-hours analysis cycle 25/08/200529/08/2005

24 Gerrit Burgers et al., A drag parameterization for extreme winds EMS Annual Meeting | Berlin, September 2011 Katrina Results10-meter wind speed with 6-hours analysis cycle 25/08/200529/08/2005