RMSE ( |
Var ( |
RMSE ( |
Var ( |
Resampling | |
Fine PF | 0.066 | 0.0035 | 0.34 | 0.15 | 226 |
Coarse PF | 0.79 | 0.0015 | 2.1 | 0.16 | 663 |
EnKF | 0.048 | 0.0018 | 0.32 | 0.12 | - |
Emu-PF ( |
0.13 | 0.00026 | 2.4 | 5.1 | 483 |
Many recent advances in sequential assimilation of data into nonlinear high-dimensional models are modifications to particle filters which employ efficient searches of a high-dimensional state space. In this work, we present a complementary strategy that combines statistical emulators and particle filters. The emulators are used to learn and offer a computationally cheap approximation to the forward dynamic mapping. This emulator-particle filter (Emu-PF) approach requires a modest number of forward-model runs, but yields well-resolved posterior distributions even in non-Gaussian cases. We explore several modifications to the Emu-PF that utilize mechanisms for dimension reduction to efficiently fit the statistical emulator, and present a series of simulation experiments on an atypical Lorenz-96 system to demonstrate their performance. We conclude with a discussion on how the Emu-PF can be paired with modern particle filtering algorithms.
Citation: |
Figure 1. Schematic for state dependence on parameters: we plot the state at eight different samples (1a), then apply a variety of interpolating schemes (1b) and lastly a statistical surrogate (1c). The shaded region in the rightmost plot shows one standard deviation in uncertainty. The second and third plot allow for the state to be estimated at a variety of parameter values
Figure 2.
Here we demonstrate how the GP example mapping from parameter space to state space (as in Figure 1) can be used in a particle filter update step. (A) The same GP mapping from parameter to state space is plotted (black line) along with the design points (blue dots along black line) used to fit that mapping and an observation (red dot and line) in state space along the left axis. A bi-modal prior distribution is plotted (light blue) along with samples from that distribution (
Figure 4.
Visualisation of the internal Emu-PF mechanisms over one assimilation step. Left column shows components of dimension
Figure 5.
Long term error statistics for the implementation of Emu-PF from fig. 4, compared to: a "coarse" PF that employs
Figure 6.
Error statistics for Experiment One,
Figure 8.
Error statistics for Experiment Three,
Figure 9.
Summary statistics for Experiment Four, long-time state estimation with
Figure 10.
RMSE against time for Experiment Five: dashed red lines plot the Fine PF (formulated under the Optimal Proposal), and solid blue lines plot the best-performing Emu-PF according to table 5. There is a clear improvement in skill in parameter estimation. State estimates are similar in skill (and, importantly, do possess some skill: the state RMSE is well below
Table 1. Summary statistics for twenty repetitions of experiment One. The 'Resampling' column counts how many resampling steps, out of a thousand, were performed by each algorithm.
RMSE ( |
Var ( |
RMSE ( |
Var ( |
Resampling | |
Fine PF | 0.066 | 0.0035 | 0.34 | 0.15 | 226 |
Coarse PF | 0.79 | 0.0015 | 2.1 | 0.16 | 663 |
EnKF | 0.048 | 0.0018 | 0.32 | 0.12 | - |
Emu-PF ( |
0.13 | 0.00026 | 2.4 | 5.1 | 483 |
Table 2. Summary statistics for twenty repetitions of experiment Two.
RMSE ( |
Var ( |
RMSE ( |
Var ( |
Resampling | |
Fine PF | 0.074 | 0.0043 | 0.7 | 0.61 | 173 |
Coarse PF | 0.49 | 0.0016 | 4.9 | 0.13 | 312 |
EnKF | 0.065 | 0.0027 | 0.78 | 0.66 | - |
Emu-PF ( |
0.38 | 0.00085 | 3.8 | 6.1 | 526 |
Emu-PF (PCA) | 0.27 | 0.00051 | 3.1 | 0.58 | 339 |
Table 3. Summary statistics for twenty repetitions of experiment Three.
RMSE ( |
Var ( |
RMSE ( |
Var ( |
Resampling | |
Fine PF | 0.062 | 0.0032 | 0.28 | 0.13 | 243 |
Coarse PF | 1 | 0.0012 | 3.4 | 0.11 | 739 |
EnKF | 0.045 | 0.0017 | 0.25 | 0.1 | - |
Emu-PF ( |
0.15 | 0.00034 | 2.4 | 5.1 | 590 |
Emu-PF (PCA) | 0.084 | 0.00075 | 1.5 | 0.085 | 334 |
Table 4. Summary statistics for Experiment Four
RMSE ( |
Var ( |
Resampling | |
Fine PF | 0.47 | 0.15 | 1706 |
Coarse PF | 5.1 | 0.16 | 9917 |
EnKF | 1 | 0.096 | - |
Emu-PF (Localized) | 0.83 | 0.31 | 3566 |
Table 5. Summary statistics for twenty repetitions of experiment Five.
RMSE ( |
Var ( |
RMSE ( |
Var ( |
Resampling | |
Fine OP-PF | 1.2 | 0.0075 | 1.9 | 3.3 | 226 |
Coarse OP-PF | 1.2 | 0.004 | 2.0 | 2.8 | 205 |
EnKF | 1.1 | 0.00042 | 1.5 | 1.8 | - |
Emu-PF ( |
0.5 | 0.0017 | 2.6 | 3.5 | 243 |
Emu-PF ( |
0.75 | 0.0035 | 2.0 | 3.7 | 232 |
Emu-PF (PCA) | 1.1 | 0.061 | 2.0 | 2.8 | 238 |
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