Sensitivity of High-resolution Tropical Cyclone Intensity Forecast to Surface Flux Parameterization Chi-Sann Liou, NRL Monterey, CA

Uncertainties of Surface Flux Parameterization at High Winds Current surface parameterization schemes are based upon the similarly theory fitted for wind speed less than 25 ms -1

Black and Chen (2006) CBLAST ** Powell Donelan Large & Pond NWP Model Drag Coefficient C D o+□  CBLAST HEXOS COARE

Surface Drag Coefficient C D Estimated from Ocean Observations (Jarosz et. al, 2007) 6 Moorings, TC Ivan 9/15/2004 C D Estimated from 3 Different r’sC D Estimated from r=0.02 Powell Moon et. al

C E /C D : (for TC growth) Black and Chen (2006) *** CBLAST HEXOS COARE NWP model Critical value (Emanuel, 1985)

Uncertainties of Surface Flux Parameterization at High Winds ==> Conduct numerical experiments to examine the sensitivities of TC intensity forecast to these uncertainties Surface Entropy flux (C E /C D at high winds?) Surface layer depth at high winds Impacts of SST cooling Surface Stress (level off at high winds) Current surface parameterization schemes are based upon the similarly theory fitted for wind speed less than 25 ms -1

Numerical Experiment Resolution: 45/15/5km, 30 levels Dynamics, Numerics: Nonhydrostatic, Fully Compressible (Klemp and Wilhelmson, JAS 1978) Sigma-Z Vertical Coordinate (Gal-Chen and Somerville, JCP 1975) Scheme C grid (Arakawa and Lamb, 1974) Multiple Nested Grids with Movable Inner Meshes (Liou and Holt, 2003) Precipitation Physics: Grid Scale – Explicit Moist Physics (Rutledge and Hobbs, JAS 1984) Convective – Kain and Fritsch (JAS 1990, JAM 2004) Boundary Layer: (Level 2.5 TKE Closure) Mixing length – Mellor and Yamada (RGSP 1982) Counter gradient flux – Therry and Lacarrère (BLM 1983 ) Surface Layer: Modified Louis (BLM 1979) Radiation: Harshvardhan (JGR 1987) Time-dependent Boundary Conditions: Davies (QJRMS 1976) Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS ® )

COAMPS ® Surface Flux Parameterization Uncertainties at high winds: Z 0 (or C d, or U * ) ?? Z 0h and Z 0q ?? Formula still good ??

Numerical Experiments Model Resolution: 45/15/5 km with moving third mesh Forecast length: 48h Initial conditions: 3D-Var analysis with 41 bogus data Six tropical cyclones (Cat 4 or higher): Lili (2002), Isabel (2003), Ivan (2004), Maemi (2003), Mindulle (2003), Indian Storm 5 (1999)

(Changes are for water points only) Set limit to U * when U 10  33 ms -1 : U *  1.45 for U 10 = 33 ms -1 U *  1.90 for U 10  51 ms -1 (  U 1/2 between 33 to 51 ms -1 ) Sensitivity Test 1 Sensitivity Test 2 Increase Z 0h and Z 0q by 10 times Sensitivity Test 3 Set Z 0h = Z 0q = Z 0 /10 as over land C E /C D

Sensitivity Test 5 When U 10  25 ms -1, SST is cooled at a rate:  SST = -0.5 o C/day, U 10 = 25 ms -1 = -3.0 o C/day, U 10  50 ms -1  U 2 between 25 to 50 ms -1 (Changes are for water points only) Sensitivity Test 4 When U 10  33 ms -1, assume surface layer is deeper than bottom layer and compute stress at the layer top as:  TB = (  sfc +  turb )/2

Averaged Impacts on COAMPS Track and Intensity Forecasts Impacts of Sensitivity Test 1: (Limiting Stress) (Control Run Scores)

Averaged Impacts on COAMPS Track and Intensity Forecasts Impacts of Sensitivity Test 2: (Z 0h *10.) C H /C D = 0.55, u = 40 m/s = m/s = m/s Impacts of Sensitivity Test 3: (Z 0h =0.1*Z 0 ) C H /C D = 0.77, u = 40 m/s = m/s = m/s

Averaged Impacts on COAMPS Track and Intensity Forecasts Impacts of Sensitivity Test 4: (  TB = (  sfc +  turb )/2) Impacts of Sensitivity Test 5: (  SST= -0.5 ~ -3.0 o C/day)

Impacts on Indian Storm-5 Forecasts TrackIntensity

Summary Tested uncertainties of surface flux parameterization at high winds have little impacts on the TC track forecast by COAMPS ® The limit on surface stress and increase of C E /C D have large impacts on the TC intensity forecast by COAMPS ® Their positive impacts on the intensity forecast almost as large as the negative impact by SST cooling The simple average of the stress at the top of the bottom layer has little help in the intensity forecast The accurate and balanced initial condition is another area that needs improvement for TC intensity forecast