This section presents a twin experiment aimed for getting more understanding about the data assimilation under an idealized situation. In reality, model noise as well as the true state of a system is unknown. One needs to make certain assumptions in order to apply a data assimilation technique. A twin experiment allows us to study the effect of various assumptions on the behaviour of a data assimilation system. The goal of this first twin experiment is to check if the data assimilation can improve the estimate of all model state variables for the Yeongsan EFDC model in an ideal case where all error statistics are known.
Data assimilation setting
Model state
The following variables are included in the definition of the state vector, being updated directly by the filter:
- Algal Cyanobacteria
- Algal Diatom
- Algal Green
- Refractory PO Carbon
- Labile PO Carbon
- Dissolved Phosphorus
- Phosphate
- Refractory PO Nitrogen
- Labile PONitrogen
- DissolvedONitrogen
- Ammonia
- Nitrate
- Dissolved Oxygen
Noise model
In this experiment, the model uncertainty is assumed to come from the a number of model input parameters at four locations (see red circles on the figure below). The noise processes for each parameter are assumed additive, indepent from each other, and modelled by an AR(1) process with the following statistics:
- Algal Diatom: standard deviation 0.5 μg/L, correlation time 72 hours
- Algal Cyanobacteria: standard deviation 0.2 μg/L, correlation time 72 hours
- Algal Green Algae: standard deviation 0.2 μg/L, correlation time 72 hours
- Phosphate: standard deviation 0.05 μg/L, correlation time 72 hours
- Discharge: standard deviation 0.2 m3/s, correlation time 72 hours
- Global Radiation: standard deviation 0.5 W/m2, correlation time 24 hours
The above statistics apply for all the four locations.
Observation station
Synthetic observations are generated by running a stochastic model with the above mentioned noise specifications. We assume that the observation is perfect. One assimilation location is used (circled on the figure below) with four observed parameters: Algal Cyanobacteria, Algal Diatom, Algal Green, and Phosphate.
Results
At assimilation station
Discussion:
At validation stations
