6.2.4. Gelato sea-ice scheme

Please read first 6.2.3. Sea-ice schemes

Report on implementing sea-ice schemes and Gelato in Surfex

Gelatos implementation in Surfex is the restriction to a 1D thermodynamical setting of the full Gelato sea-ice model (D.Salas y Melia, 2002) and was implemented in Surfex Version 8 . In the following, some namelist paramaters are quoted. See 6.2.5. Namelist NAM_SEAICEn for more details.

Gelato can optionnally be constrained by a Sea Ice Cover (SIC) field (either using explicit fields or through implicit values derived from SST - see XFREEZING_SST), and/or with Sea Ice Thickness fields. Gelato time step can be set as different from the SEAFLUX time step (see XSEAICE_TSTEP). The bathymetry prescribed by NAM_BATHY is used by Gelato.

Available sea-ice output fields when using Gelato sea-ice scheme :

Gelato diagnostic fields are described in the relevant section

Gelato prognostic fields are described in the relevant section

Constraining Gelato with Sea Ice Cover and/or Thickness

If you set the value of namelist parameter XSIC_EFOLDING_TIME (resp. XSIT_EFOLDING_TIME) to any value > 0, Gelato will use a provided (or computed) SIC field (resp. the provided SIT field) as a constraint; the namelist parameter is then interpreted as a relaxation time (expressed in days, possibly fractional) for damping toward this constraint; default value is 0. and means no relaxation.

Having these constraint fields evolve during the simulation is possible using namelist parameters CINTERPOL_SIC and CINTERPOL_SIT, with values "MONTH" or "ANNUAL", which means that a 3-values, quadratic time interpolation will be done using the 3 among 3 (resp 3 among 14) monthly fields found in PREP file. Feeding the data in the PREP files assumes the use of some binary such as 'updsst/updcli' (for PREP in FA format), external to the Surfex run(s). This time evolution scheme is based on the scheme devised for SST (see [->79])

Constraining Gelatos Sea-ice Cover using only Sea Surface Temperature fields

The sea-ice constraint field, if not provided as described above, will be derived from the SST field; hence :

  • This implies to define which provided SST forcing values do mean that the sea reaches its freezing point, and/or SIC is not zero.
  • Sea freezing point actually depends on sea surface salinity, which varies in space and time, but sea surface analysis data sets usually provide SST only, and represents that SIC is non-zero by using a constant, arbitrary SST on the relevant data points (as e.g. -1.8 Celsius in HadSST1 data).
  • Hence, the Surfex user will then have to set the value of namelist parameter XFREEZING_SST (in Celsius ) to tell Surfex which is this arbitrary SST, or more precisely to enforce that data points with time-interpolated SST <= XFREEZING_SST are considered to be at the freezing point for their current salinity value, and show a SIC value of 1 (i.e. 100 %).
  • Default value for XFREEZING_SST is -1.8 Celsius
  • The same scheme applies whatever the way SST does evolve in Surfex (either using the 1D ocean mixing model -with or without relaxation- or using one of the ways to provide forcing SST fields)