Royal Netherlands Meteorological Institute KNMI Ministry of Infrastructure

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 Zweers Burgers de Vries Makin Gerrit Burgers et al. , A drag EMS Annual meeting parameterization for extreme Berlin, winds EMS 12 -16 September Annual Meeting | Berlin, 12 -16 September 2011

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 2 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 3 Gerrit Burgers et al. , A drag parameterization for extreme winds EMS Annual Meeting | Berlin, 12 -16 September 2011

Drag and roughness § Drag coefficient: 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: The roughness and the drag increase with wind speed Supported by observations for moderate and strong wind § In models z* ~ [0. 010, 0. 035] 4 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 5 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 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 6 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): cz 0: Charnock parameter acrit: critical terminal fall velocity cl: constant U 10 < Ucrit: Charnock relation U 10 > 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. 7 Gerrit Burgers et al. , A drag parameterization for extreme winds EMS Annual Meeting | Berlin, 12 -16 September 2011

Charnock vs. Makin Drag coefficient 8 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 5 km § Two hurricanes: Ivan (2004) and Katrina (2005) i. Forecasts, analyses every 6 hours (“analysis=previous forecast + data assimilation”) ii. Forecasts up to +96 h * http: //hirlam. org 9 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 h. Pa 916 h. Pa (Zweers et al. , 2010) GRL 902 h. Pa 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 11 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 +96 h forecast 13/09/2004 14 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 n Observed 15 Katrina +48 h, +72 h, +96 h 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 n 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 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 102 , but wind speed change partially cancels effect uncertainty drag on stress Typically: δU 10/ U 10 -0. 25 δc. D /c. D δτ / τ 0. 5 δc. D /c. D (Zweers et al. , Natural Hazards, submitted) 17 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 1 m) Results from Delft-3 D storm surge model simulations (Zweers et al. , Natural Hazards, submitted) Ivan, 16 September 15 h 00 18 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 19 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 20 Gerrit Burgers et al. , A drag parameterization for extreme winds EMS Annual Meeting | Berlin, 12 -16 September 2011

Reduction from 32 m/s Drag coefficient 21 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 - Conclusion BBOS 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