I'm running ROMS (svn 836) in a realistic application in southeast of Brazil and I've noticed what appears to be an overestimation of surface currents, possibly due to wind stress.
I'am using bulk fluxes and hourly fields from CFSR reanalyses as forcing and 1/12 degree resolution grid nested to Mercator Global, which provides initial conditions and daily boundary fields. These are my boundary conditions:
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Variable Grid West Edge South Edge East Edge North Edge --------- ---- ---------- ---------- ---------- ---------- zeta 1 Closed Chapman Imp Chapman Imp Chapman Imp ubar 1 Closed Flather Flather Flather vbar 1 Closed Flather Flather Flather u 1 Closed Rad + Nud Rad + Nud Rad + Nud v 1 Closed Rad + Nud Rad + Nud Rad + Nud temp 1 Closed Rad + Nud Rad + Nud Rad + Nud salt 1 Closed Rad + Nud Rad + Nud Rad + Nud tke 1 Closed Gradient Gradient Gradient
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Activated C-preprocessing Options: SSE ROMS SSE 112 ANA_BSFLUX Analytical kinematic bottom salinity flux. ANA_BTFLUX Analytical kinematic bottom temperature flux. ASSUMED_SHAPE Using assumed-shape arrays. BULK_FLUXES Surface bulk fluxes parameterization. CURVGRID Orthogonal curvilinear grid. DIFF_GRID Horizontal diffusion coefficient scaled by grid size. DJ_GRADPS Parabolic Splines density Jacobian (Shchepetkin, 2002). DOUBLE_PRECISION Double precision arithmetic. EMINUSP Compute Salt Flux using E-P. KANTHA_CLAYSON Kantha and Clayson stability function formulation. LONGWAVE_OUT Compute outgoing longwave radiation internally. MASKING Land/Sea masking. MIX_GEO_TS Mixing of tracers along geopotential surfaces. MIX_S_UV Mixing of momentum along constant S-surfaces. MPI MPI distributed-memory configuration. MY25_MIXING Mellor/Yamada Level-2.5 mixing closure. NONLINEAR Nonlinear Model. NONLIN_EOS Nonlinear Equation of State for seawater. N2S2_HORAVG Horizontal smoothing of buoyancy and shear. POWER_LAW Power-law shape time-averaging barotropic filter. PROFILE Time profiling activated . K_GSCHEME Third-order upstream advection of TKE fields. RADIATION_2D Use tangential phase speed in radiation conditions. RI_SPLINES Parabolic Spline Reconstruction for Richardson Number. !RST_SINGLE Double precision fields in restart NetCDF file. SALINITY Using salinity. SOLAR_SOURCE Solar Radiation Source Term. SOLVE3D Solving 3D Primitive Equations. SPLINES_VDIFF Parabolic Spline Reconstruction for Vertical Diffusion. SPLINES_VVISC Parabolic Spline Reconstruction for Vertical Viscosity. SPHERICAL Spherical grid configuration. TS_C4HADVECTION Fourth-order centered horizontal advection of tracers. TS_C4VADVECTION Fourth-order centered vertical advection of tracers. TS_DIF2 Harmonic mixing of tracers. UV_ADV Advection of momentum. UV_COR Coriolis term. UV_U3HADVECTION Third-order upstream horizontal advection of 3D momentum. UV_C4VADVECTION Fourth-order centered vertical advection of momentum. UV_QDRAG Quadratic bottom stress. UV_VIS2 Harmonic mixing of momentum. VAR_RHO_2D Variable density barotropic mode. VISC_GRID Horizontal viscosity coefficient scaled by grid size.
The problem can be seen in the attached pictures, which shows a snapshot of surface currents and SSH from ROMS and from global MERCATOR model, to which I nested my grid, a few days after initialization. A snapshot of wind velocity is also shown. I was expecting a geostrophic flow where currents vectors would follow more or less elevation gradients, as it happens on MERCATOR and in situ measurements show. However, currents on ROMS are stronger and presents a drift at the same regions where wind speed is also stronger, suggesting a possible overestimation of wind stress in my model.
At the beginning I thought this behavior would be restricted to the very near surface layer. However, the drift can be seen in deeper layers down to a ~ 50 m depth or more, specially on shallower regions.
The average flow seems to be overestimated as well, as shown in the pictures of averaged surface currents. ROMS currents are stronger than MERCATOR and other global models such as HYCOM NCODA, specifically along the slope in the domain of Brazil Current. I made a few tests altering vertical mixing scheme (MY25, GLS k-epsilon, LMD), increasing viscosity (from 4.0 to 25.0 m2/s) and changing CFSR hourly data to ERA INTERIM 6 hourly fields, but I'm not satisfied yet. Near surface currents remains stronger than expected for the region and I wouldn't like to have mesoscale features vanished by high viscosity values.
A few doubts I have:
1. Are my high velocities and currents drift due to strong wind stress or have I misinterpreted my results? What else could be causing this behavior?
2. I heard that bulk fluxes may overestimate wind stress, specially on gust situations, so what parameters could I tweak in order to reduce wind stress and obtain more consistent values of surface currents?
3. Should I increase viscosity and/or reduce wind momentum components?
4. Should I switch any of my boundary conditions?
5. Are my configuration parameters set properly (see below)? Is there anything weird in my setup?
Other information about my run are summarized below.
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Physical Parameters, Grid: 01 ============================= 219360 ntimes Number of timesteps for 3-D equations. 180.000 dt Timestep size (s) for 3-D equations. 30 ndtfast Number of timesteps for 2-D equations between each 3D timestep. 1 ERstr Starting ensemble/perturbation run number. 1 ERend Ending ensemble/perturbation run number. 0 nrrec Number of restart records to read from disk. T LcycleRST Switch to recycle time-records in restart file. 480 nRST Number of timesteps between the writing of data into restart fields. 1 ninfo Number of timesteps between print of information to standard output. T ldefout Switch to create a new output NetCDF file(s). 240 nHIS Number of timesteps between the writing fields into history file. 0 nQCK Number of timesteps between the writing fields into quicksave file. 5.0000E+00 nl_tnu2(01) NLM Horizontal, harmonic mixing coefficient (m2/s) for tracer 01: temp 5.0000E+00 nl_tnu2(02) NLM Horizontal, harmonic mixing coefficient (m2/s) for tracer 02: salt 5.0000E+00 nl_visc2 NLM Horizontal, harmonic mixing coefficient (m2/s) for momentum. T LuvSponge Turning ON sponge on horizontal momentum. T LtracerSponge(01) Turning ON sponge on tracer 01: temp T LtracerSponge(02) Turning ON sponge on tracer 02: salt 1.0000E-06 Akt_bak(01) Background vertical mixing coefficient (m2/s) for tracer 01: temp 1.0000E-06 Akt_bak(02) Background vertical mixing coefficient (m2/s) for tracer 02: salt 1.0000E-05 Akv_bak Background vertical mixing coefficient (m2/s) for momentum. 5.0000E-06 Akk_bak Background vertical mixing coefficient (m2/s) for turbulent energy. 5.0000E-06 Akp_bak Background vertical mixing coefficient (m2/s) for turbulent generic statistical field. 3.0000E-04 rdrg Linear bottom drag coefficient (m/s). 3.0000E-03 rdrg2 Quadratic bottom drag coefficient. 1.0000E-02 Zob Bottom roughness (m). 2.0000E+00 blk_ZQ Height (m) of surface air humidity measurement. 2.0000E+00 blk_ZT Height (m) of surface air temperature measurement. 1.0000E+01 blk_ZW Height (m) of surface winds measurement. 3 lmd_Jwt Jerlov water type. 2 Vtransform S-coordinate transformation equation. 4 Vstretching S-coordinate stretching function. 5.0000E+00 theta_s S-coordinate surface control parameter. 3.0000E-01 theta_b S-coordinate bottom control parameter. 40.000 Tcline S-coordinate surface/bottom layer width (m) used in vertical coordinate stretching. 1025.000 rho0 Mean density (kg/m3) for Boussinesq approximation. 14153.000 dstart Time-stamp assigned to model initialization (days). 19700101.00 time_ref Reference time for units attribute (yyyymmdd.dd) 3.0000E+01 Tnudg(01) Nudging/relaxation time scale (days) for tracer 01: temp 3.0000E+01 Tnudg(02) Nudging/relaxation time scale (days) for tracer 02: salt 3.0000E+01 Znudg Nudging/relaxation time scale (days) for free-surface. 3.0000E+01 M2nudg Nudging/relaxation time scale (days) for 2D momentum. 3.0000E+01 M3nudg Nudging/relaxation time scale (days) for 3D momentum. 3.0000E+01 obcfac Factor between passive and active open boundary conditions. F VolCons(1) NLM western edge boundary volume conservation. F VolCons(2) NLM southern edge boundary volume conservation. F VolCons(3) NLM eastern edge boundary volume conservation. F VolCons(4) NLM northern edge boundary volume conservation. 14.000 T0 Background potential temperature (C) constant. 35.000 S0 Background salinity (PSU) constant. 1.000 gamma2 Slipperiness variable: free-slip (1.0) or no-slip (-1.0). F LuvSrc Turning OFF momentum point Sources/Sinks. F LwSrc Turning OFF volume influx point Sources/Sinks. F LtracerSrc(01) Turning OFF point Sources/Sinks on tracer 01: temp F LtracerSrc(02) Turning OFF point Sources/Sinks on tracer 02: salt F LsshCLM Turning OFF processing of SSH climatology. F Lm2CLM Turning OFF processing of 2D momentum climatology. T Lm3CLM Turning ON processing of 3D momentum climatology. T LtracerCLM(01) Turning ON processing of climatology tracer 01: temp T LtracerCLM(02) Turning ON processing of climatology tracer 02: salt F LnudgeM2CLM Turning OFF nudging of 2D momentum climatology. T LnudgeM3CLM Turning ON nudging of 3D momentum climatology. T LnudgeTCLM(01) Turning ON nudging of climatology tracer 01: temp T LnudgeTCLM(02) Turning ON nudging of climatology tracer 02: salt T Hout(idFsur) Write out free-surface. T Hout(idUbar) Write out 2D U-momentum component. T Hout(idVbar) Write out 2D V-momentum component. T Hout(idUvel) Write out 3D U-momentum component. T Hout(idVvel) Write out 3D V-momentum component. T Hout(idTvar) Write out tracer 01: temp T Hout(idTvar) Write out tracer 02: salt T Hout(idUsms) Write out surface U-momentum stress. T Hout(idVsms) Write out surface V-momentum stress. T Hout(idTsur) Write out surface net heat flux. T Hout(idTsur) Write out surface net salt flux. T Hout(idSrad) Write out shortwave radiation flux. T Hout(idLrad) Write out longwave radiation flux. T Hout(idLhea) Write out latent heat flux. T Hout(idShea) Write out sensible heat flux. T Hout(idEmPf) Write out E-P flux. T Hout(idevap) Write out evaporation rate. T Hout(idrain) Write out rain rate. ... Metrics information for Grid 01: =============================== Minimum X-grid spacing, DXmin = 8.02032179E+00 km Maximum X-grid spacing, DXmax = 9.02554196E+00 km Minimum Y-grid spacing, DYmin = 8.02323505E+00 km Maximum Y-grid spacing, DYmax = 9.02110875E+00 km Minimum Z-grid spacing, DZmin = 1.11179822E-01 m Maximum Z-grid spacing, DZmax = 5.24095214E+02 m Minimum barotropic Courant Number = 7.89417705E-03 Maximum barotropic Courant Number = 2.30942340E-01 Maximum Coriolis Courant Number = 1.30899694E-02 Horizontal mixing scaled by grid size, GRDMAX = 9.02332508E+00 km ... Basin information for Grid 01: Maximum grid stiffness ratios: rx0 = 1.500002E-01 (Beckmann and Haidvogel) rx1 = 7.606897E+00 (Haney) Initial basin volumes: TotVolume = 6.3595735250E+15 m3 MinVolume = 1.1017982474E+07 m3 MaxVolume = 4.0811095033E+10 m3 Max/Min = 3.7040442866E+03