Observation and model comparison (OBS)

Authors: D. Lea, M. Martin, K. Mogensen, A. Vidard, A. Weaver, A. Ryan, ... $\$
The observation and model comparison code (OBS) reads in observation files (profile temperature and salinity, sea surface temperature, sea level anomaly, sea ice concentration, and velocity) and calculates an interpolated model equivalent value at the observation location and nearest model timestep. The resulting data are saved in a “feedback” file (or files). The code was originally developed for use with the NEMOVAR data assimilation code, but can be used for validation or verification of model or any other data assimilation system. The OBS code is called from nemogcm.F90.F90 for model initialisation and to calculate the model equivalent values for observations on the 0th timestep. The code is then called again after each timestep from step.F90.F90. To build with the OBS code active key_ diaobs must be set. For all data types a 2D horizontal interpolator is needed to interpolate the model fields to the observation location. For in situ profiles, a 1D vertical interpolator is needed in addition to provide model fields at the observation depths. Currently this only works in z-level model configurations, but is being developed to work with a generalised vertical coordinate system. Temperature data from moored buoys (TAO, TRITON, PIRATA) in the ENACT/ENSEMBLES data-base are available as daily averaged quantities. For this type of observation the observation operator will compare such observations to the model temperature fields averaged over one day. The relevant observation type may be specified in the namelist using endailyavtypes. Otherwise the model value from the nearest timestep to the observation time is used. The code is controlled by the namelist nam_obs. See the following sections for more details on setting up the namelist. Section 12.1 introduces a test example of the observation operator code including where to obtain data and how to setup the namelist. Section 12.2 introduces some more technical details of the different observation types used and also shows a more complete namelist. Section 12.3 introduces some of the theoretical aspects of the observation operator including interpolation methods and running on multiple processors. Section 12.4 describes the offline observation operator code. Section 12.5 introduces some utilities to help working with the files produced by the OBS code.