CLARIFICATION: Surface net freshwater flux

[arango]

Several changes were introduced in ​src:ticket:869 about how to process the surface/bottom tracer fluxes that are used as top/bottom boundary conditions to the vertical diffusion terms. A couple of additional changes are made here to remove any ambiguity about the surface freshwater flux needed for ROMS.

Notice that ROMS has the following vertical surface/bottom boundary condition for temperature and salinity:

In particular, the blue square shows what ROMS needs as a freshwater flux. Usually, E-P is in units of kilogram meter-2 second-1. Then, the flux is divided by freshwater density, which converts to meter second-1. In routine set_vbc.F such flux is multiplied by the surface salinity. Since the salinity does not have ​physical units, the vertical boundary condition is still in meter second-1.

I cleaned the metadata in varinfo.dat, now we have:

'ssflux'                                           ! Output
  'kinematic surface net salt flux, SALT*(E-P)/rhow'
  'meter second-1'                                 ! [PSS m/s]
  'surface net salt flux, scalar, series'
  'ssf_time'
  'idTsur(isalt)'
  'r2dvar'
  1.0d0

'swflux'                                           ! Input
  'data surface net freshwater flux, (E-P)/rhow'
  'meter second-1'                                 ! Input:  [m/s]
  'surface net freshwater flux, scalar, series'    ! [PSS m/s]
  'swf_time'
  'idsfwf'
  'r2dvar'
  1.0d0

'EminusP'                                          ! Input/Output
  'modeled surface net freshwater flux, (E-P)/rhow'
  'meter second-1'                                 ! computed by NLM ROMS
  'EminusP, scalar, series'                        ! bulk_flux.F or coupling
  'ocean_time'
  'idEmPf'
  'r2dvar'
  1.0d0

As specified in the preamble of varinfo.dat, the PSS information is added to indicate that the Practical Salinity Scale was used to determine conductivity, but it is not a physical unit. We are not using the latest UNESCO equation of state.

We have two different variables for input freshwater flux: swflux and EminusP. Historically, swflux has been used for available datasets while EminusP has been used when derived from parameterizations (BULK_FLUXES and atmospheric coupling). To avoid any confusion, the units and scale for swflux were changed to meter second-1 and 1.0d0, respectively. It is the standard unit for freshwater flux. If such field is in centimeter day-1, I recomend to add the scale_factor attribute to the NetCDF, so the conversion is done during reading in ROMS:

        float swflux(swf_time, eta_rho, xi_rho) ;
                swflux:long_name = "surface freshwater flux, (E-P)/rhow" ;
                swflux:units = "meter second-1" ;
                swflux:negative = "net evaporation" ;
                swflux:positive = "net precipitation" ;
                swflux:time = "swf_time" ;
                swflux:coordinates = "lon_rho lat_rho swf_time" ;
                swflux:scale_factor = 1.157407e-07f ;

We could use the Matlab script to adjust the NetCDF file:

nc_attadd('my_file.nc','scale_factor',0.01/86400,'swflux')

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• Corrected get_data.F and set_data.F processing of the freshwater flux when the SCORRECTION is activated.

• Corrected compiling issue in regrid.F. It was missing the MASKING conditional.