Hybrid tangent linear model¶

OOPS’s hybrid tangent linear model (H-TLM) implementation, named HybridLinearModel in the code, can be used for any MODEL which has implemented a LinearModel in its interface.

The LinearModel is instantiated via a factory. To make this a HybridLinearModel, the name option in the linear model subconfiguration must be set to HTLM.

Coefficients can be generated either “on the fly” in a Variational run, or precalculated using GenHybridLinearModelCoeffs.

Assuming the coefficients have been precalculated, the HybridLinearModel can be configured using the following options within the linear model subconfiguration:

name: must be “HTLM” in order to instantiate a HybridLinearModel via the LinearModel factory
simple linear model: subconfiguration for the SimpleLinearModel wrapper:
- linear model: subconfiguration for the underlying MODEL-specific LinearModel
- geometry (optional): subconfiguration for the MODEL-specific Geometry of the MODEL-specific LinearModel when using SimpleLinearModelMultiresolution or SimpleLinearModelResidualForm
- residual form (optional, default false): boolean determining whether SimpleLinearModelResidualForm is used over SimpleLinearModelMultiresolution
update tstep: timestep of the HybridLinearModel, as used by CostFct4DVar/CostFctWeak (i.e. the time between HybridLinearModel updates using the coefficients); a duration in ISO 8601 format
variables: list of variables, as used by CostFct4DVar/CostFctWeak (i.e. the analysis variables); must be a superset of coefficients.update variables
coefficients: subconfiguration for the HybridLinearModelCoeffs class:
- input: subconfiguration for the reading of coefficients from file, consisting of base filepath (string) and one file per task (boolean; currently, this must be set to true)
- update variables: list of variables which are updated by the coefficients (having previously been trained on them)
- influence region size: number of vertical levels in column of influence (must be an odd number to allow centering on the level of interest)
- time window: subconfiguration for the TimeWindow over which coefficients are applied (i.e. the assimilation window)

Note that the configuration must match that used to precalculate the coefficients (with the exception of input, which would be set to output for the precalculation).

Here is an example configuration as would be used to run 4D-Var using the Variational application when coefficients have been precalculated:

# rest of Variational configuration
linear model:
  name: HTLM
  simple linear model:
    linear model:
      # MODEL-specific subconfiguration
  update tstep: PT1H
  variables: [x, y, z]
  coefficients:
    input:
      base filepath: path/to/coeffs
      one file per task: true
    update variables: [x, y]
    influence region size: 5
    time window:
      begin: 2024-01-16T06:00:00Z
      length: PT6H
# rest of Variational configuration

To instead generate coefficients “on the fly” (during the execution of Variational), more options are needed in the configuration. These options are the same as would be required to generate the coefficients using GenHybridLinearModelCoeffs, and are detailed on that page.