Chapter 4

Architecture of ocean monitoring and forecasting systems


CHAPTER
COORDINATORS

Avichal Mehra
CHAPTER
AUTHORS

Roland Aznar, Stefania Ciliberti, Laurence Crosnier, Marie Drevillon, Yann Drillet, Begoña Pérez Gómez, Antonio Reppucci, Joseph Sudheer, Marcos Garcia Sotillo, Marina Tonani, P. N. Vinaychandranand, and Aihong Zhong

4.3 Data Assimilation

Through data assimilation, OOFS combines observations and the numerical model solution with the scope of producing the best reconstruction of the ocean state to be used as initial condition of the forecasting cycle. According to Moore et al.(2019) and considering Figure 4.27, we can assume that a priori state estimate of the ocean computed from the numerical model (blue line in Figure 4.26) together with a priori direct but incomplete state estimate from ocean observations (black dots in Figure 4.26) produce a posteriori state estimate which “combines” all available information considering uncertainties in both model and observations (green line in Figure 4.26).

Figure 4.26. Data assimilation models (green) are helped by observations to produce more realistic forecasts, closerto real observations (source: MEDCLIC project, SOCIB-La Caixa Foundation).

Ocean data assimilation is then defined mathematically through a rigorous process that combines ocean observation statistics with statistics of ocean model behaviour to extract the most useful information, possibly from sparse observations of time-varying ocean circulation (Cummings et al., 2009). Broadening Step 1 in Figure 4.1, the main characteristics of the data assimilation modelling system can be presented as in Figure 4.27, which shows the major components of the data assimilation modelling system, which are defined by:

  • Access to observations;
  • Data quality control;
  • Data assimilation scheme.
Figure 4.27. Major components of a data assimilation modelling system.

Access to observations, quality, providers as well as examples have been presented in Section 4.2. Data quality control is performed by an automatic procedure, native in the assimilation scheme or performed in offline mode at the submission of the analysis cycle, which selects the best observational dataset from the one accessed. To do such selection, the procedure takes as input the quality flag value associated with each specific observation (see Figure 4.3): usually, observations with QC flag = 1 and/or 2 are selected and make eligible to be used by the data assimilation scheme.

Depending on the specific characteristics of the basin on which the system is working, the data quality control may include further checks to reject data which are not sufficiently good to be assimilated. Such criterion may be implemented in offline mode as pre-processing steps of the data access and management. This is the case, for example, of the Mediterranean Forecasting System (MedFS) delivered in the framework of Copernicus Marine Service: the system performs additional checks for Argo and SLA observations rejection based on specific criteria, which are listed in Table 4.8.

ARGO QC1 Check on the date and location quality flags: only the profiles with both flags equal to 1 are taken into account
ARGO QC2Out of the Mediterranean Sea region
ARGO QC3Retain only ascending profiles (descending are rejected)
ARGO QC4Check on the values of the quality flags of pressure, temperature and salinity for each depth: if one of the flags is not equal to 1, the layer is deleted
ARGO QC5 Check on the values of the temperature and salinity, data outside the following ranges are rejected: 0<T<35 ; 0<S<45
ARGO QC6Check on the thermocline: if distance between two subsequent measurements of temperature and salinity in the first 300 meters is larger than 40 m, the profile is rejected
ARGO QC7Measurement between 0 and 2 m are rejected
SLA QC1Check on the values of date, latitude, longitude, sea level anomaly and DAC: if one of these values is equal to missing value the measurement of sea level anomaly is rejected. Check on the quality flag of sea level anomaly: if the flag is not equal to 1 the measurement of sea level anomaly is rejected

Table 4.8. Quality control criteria adopted by the Mediterranean Analysis and Forecasting System (MedFS,🔗70) for in-situ (Argo) and SLA.

 

Data assimilation scheme is really the core of the system since it performs the mathematical work of combining model state and observations. Existing data assimilation methods are classified in 2 major groups (Bouttier and Courtier, 2002):

  • Sequential method, which considers past observations until the time of analysis: this is the case of NRT products (analysis);
  • Non-sequential method, which uses “future” observation: this is the case of the multi-year products (e.g.,reanalysis).

Another distinction can be made between continuous and intermittent assimilation in time:

  • Continuous assimilation: for a given period of time the observations are collected and the correction to the analysed state is smoothed over a specific assimilation window;
  • Intermittent assimilation: for a given period of time, the observations are collected within a specific assimilation window to compute a correction.

Carrassi et al. (2018) and De Mey (1997) detail more the nature of the assimilation schemes used in physical, biogeochemical, ice and wave forecasting systems, describing the formulation of the problem and numerical approximation. These concepts are detailed in the theoretical chapters from 5 to 9, which are dedicated to show how such methods are used for setting up an OOFS.

From the scheme in Figure 4.27, we can derive some key definitions at the basis of the assimilation cycle: the innovation, defined as the difference between the first guess (or forecast) and the observation. The data assimilation method tries to estimate with less uncertainty than either the model prediction or observation: it deals with the computation of the increment, defined as the analysis minus the model first guess. The data assimilation system itself has been used to monitor observations and data quality control (Hollingsworth et al., 1986) by computing statistics involving observations, such as observation increments used to setup the blacklisting; this is a list of observations that the data assimilation has rejected and represents valuable information to be shared also with data providers in order to fix potential issues or bugs in the observational datasets.

 

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Chapter 4

Architecture of ocean monitoring and forecasting systems

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