CMEMS L3S SST observations

Overview

This converter reads Sea Surface Temperature (SST) observations from the Copernicus Marine Environment Monitoring Service (CMEMS) Level-3S (L3S) blended SST product and converts them into a DART obs_seq file.

The input data consist of bias-corrected, multi-sensor SST observations from the ODYSSEA system. These products are distributed in netCDF format and include per-pixel uncertainty estimates and quality control flags.

The converter extracts:

  • SST observations (adjusted_sea_surface_temperature)

  • Observation uncertainties (sses_standard_deviation)

  • Quality flags (quality_level)

  • Observation time and location

and writes them into a DART observation sequence file using the SATELLITE_BLENDED_SST observation type. The unit of temperature is C.

For more information about the SST product, visit the Copernicus Marine Service: https://marine.copernicus.eu

Input data

The converter expects netCDF files from the CMEMS SST L3S product, e.g., SST_GLO_PHY_L3S_*.nc

Required variables:

  • adjusted_sea_surface_temperature (Kelvin)

  • sses_standard_deviation (Kelvin)

  • quality_level (0–5)

  • latitude

  • longitude

  • time

Only observations with quality level greater than 3 are retained.

Obs Error handling

The input uncertainty field sses_standard_deviation is used as the observation error.

To ensure numerical stability and prevent unrealistically small errors from dominating the assimilation, a lower bound is applied:

err = max(sses_standard_deviation, OBS_ERROR_SD_MIN)

with default:

OBS_ERROR_SD_MIN = 0.05 °C

NaN values in the uncertainty field are replaced with this minimum value. This lower bound represents a practical safeguard against over-weighting individual observations and does not replace more sophisticated error modeling or inflation during assimilation.

Namelist

The converter uses the following namelist:

&cmems_sst_to_obs_nml
   file_list         = 'sst_file_list.txt'  ! text file listing input netCDF files
   file_out          = 'obs_seq.sst'        ! output obs_seq filename
   avg_obs_per_file  = 500000               ! pre-allocation hint
   debug             = .true.               ! print diagnostic output
/

Usage

  1. Create a file list:

ls *.nc > sst_file_list.txt

  1. Edit input.nml to configure options

  2. Build the converter:

cd work
./quickbuild.sh

  1. Run:

./cmems_sst_to_obs

  1. Output:

obs_seq.sst