CICE sea ice concentration data assimilation (COIN)

[jostbr]

Data assimilation of sea ice concentration using COIN method (by @keguangw). Allows for nudging towards an observed sea ice cover, e.g. some satellite product. For it to be used, the following is required:

1. A netcdf data file with observed sea ice concentration, error in observations both on the same grid as the model, i.e. an ncdump of the file would look something like this:

netcdf sic_20200304 {
dimensions:
	time = UNLIMITED ; // (1 currently)
	lon = 739 ;
	lat = 949 ;
variables:
	double time(time) ;
		time:long_name = "time" ;
		time:units = "days since 1970-01-01 00:00:00" ;
	float longitude(lat, lon) ;
		longitude:units = "degree East" ;
	float latitude(lat, lon) ;
		latitude:units = "degree East" ;
	float obs_sic(time, lat, lon) ;
		obs_sic:long_name = "sea ice concentration" ;
		obs_sic:units = "%" ;
		obs_sic:time = "time" ;
		obs_sic:missing_value = -999.f ;
	float obs_err(time, lat, lon) ;
		obs_err:long_name = "error of SIC" ;
		obs_err:units = "%" ;
		obs_err:time = "time" ;
		obs_err:missing_value = -999.f ;

2. A section in the ice_in namelist input file related to the COIN method (NB: THIS MOST BE PRESENT EVEN THOUGH YOU ARE NOT USING COIN IN YOUR RUN, I.E. IF FLAG da_ice is .false.):

&da_nml
    da_ice      = .true.
  , da_sic      = .true.
  , da_sit      = .false.
  , da_sno      = .false.
  , da_method   = 'coin'
  , da_data_dir = 'CICE_OBS_DIR'
  , Tobs        = 86400.0
  , da_insert   = .false.
  , corr_bias   = .false.
/