2. [DocT] A brief introduction to Data Assimilation and Optimization¶

Data Assimilation is a general well established framework for computing the optimal estimate of the true state of a system, over time if necessary. It uses values obtained by combining both observations and a priori models, including information about their errors while simultaneously respecting constraints. This takes into account the laws of behavior or motion of the system through the equations of the model, and the way the measurements are physically related to the variables of the system.

In other words, data assimilation merges measurement data of a system, that are the observations, with a priori system physical and mathematical knowledge, embedded in numerical models. The goal is to obtain the best possible estimate, called “analysis”, of the system real state and of its stochastic properties. Note that this real state (or “true state”) cannot usually be reached, but can only be estimated. Moreover, despite the fact that the used information are stochastic by nature, data assimilation provides deterministic techniques in order to perform very efficiently the estimation.

Because data assimilation looks for the best possible estimate, its underlying procedure always integrates optimization in order to find this estimate: particular optimization methods are always embedded in data assimilation algorithms. Optimization methods can be seen in ADAO as a way to extend data assimilation applications. They will be introduced this way in the section Going further in the state estimation by optimization methods, but they are far more general and can be used without data assimilation concepts.

Two main types of applications exist in data assimilation, which are covered by the same formalism: fields reconstruction (see Fields reconstruction or measures interpolation) and parameters identification (see Parameters identification, models adjustment, or calibration). These are also referred to as state estimation and parameters estimation respectively, and one can if necessary elaborate joint estimation of both (see Joint state and parameter estimation in dynamics). In ADAO, some algorithms can be used either in state estimation or in parameter estimation. This is done simply by changing the required option “EstimationOf” in the algorithm parameters. Before introducing the Simple description of the data assimilation methodological framework in a next section, these two types of applications are briefly described. At the end, some detailed information allow Going further in the data assimilation framework and Going further in the state estimation by optimization methods, as well as Going further in data assimilation for dynamics and having An overview of reduction methods and of reduced optimization.

2.1. Fields reconstruction or measures interpolation¶

Fields reconstruction (or interpolation) consists in finding, from a restricted set of real measures, the physical field which is the most consistent with these measures.

This consistency is to understand in terms of interpolation, that is to say that the field we want to reconstruct, using data assimilation on measures, has to fit at best the measures, while remaining constrained by the overall field calculation. The calculation is thus an a priori estimation of the field that we seek to identify. One also speaks of state estimation in this case.

If the system evolves over time, the reconstruction of the whole field has to be established at each time step, taking into account the information over a time window. The interpolation process is more complicated in this case because it is temporal, and not only in terms of instantaneous field values.

A simple example of fields reconstruction comes from meteorology, in which one look for value of variables such as temperature or pressure in all points of the spatial domain. One have instantaneous measurements of these quantities at certain points, but also a history set of these measures. Moreover, these variables are constrained by evolution equations for the state of the atmosphere, which indicates for example that the pressure at a point can not take any value independently of the value at this same point in previous time. One must therefore make the reconstruction of a field at any point in space, in a “consistent” manner with the evolution equations and with the measures of the previous time steps.

2.2. Parameters identification, models adjustment, or calibration¶

The identification (or adjustment) of parameters by data assimilation is a form of state calibration which uses both the physical measurement and an a priori parameters estimation (called the “background”) of the state that one seeks to identify, as well as a characterization of their errors. From this point of view, it uses all available information on the physical system, with restrictive yet realistic assumptions about errors, to find the “optimal estimation” from the true state. We note, in terms of optimization, that the background realizes a “regularization”, in the mathematical meaning of Tikhonov [[Tikhonov77] [WikipediaTI], of the main problem of parameters identification. One can also use the term “inverse problem” to refer to this process.

In practice, the two observed gaps “calculation-measures” and “calculation-background” are combined to build the calibration correction of parameters or initial conditions. The addition of these two gaps requires a relative weight, which is chosen to reflect the trust we give to each piece of information. This confidence is depicted by the covariance of the errors on the background and on the observations. Thus the stochastic aspect of information is essential for building the calibration error function.

A simple example of parameters identification comes from any kind of physical simulation process involving a parametrized model. For example, a static mechanical simulation of a beam constrained by some forces is described by beam parameters, such as a Young coefficient, or by the intensity of the force. The parameters estimation problem consists in finding for example the right Young coefficient value in order that the simulation of the beam corresponds to measurements, including the knowledge of errors.

All quantities representing the description of physics in a model are likely to be calibrated in a data assimilation process, whether they are model parameters, initial conditions or boundary conditions. Their simultaneous consideration is greatly facilitated by the data assimilation framework, which makes it possible to objectively process a heterogeneous set of available information.

2.3. Joint estimation of states and parameters¶

It is sometimes necessary, when considering the two previous types of applications, to need to simultaneously estimate states (fields) and parameters characterizing a physical phenomenon. This is known as joint estimation of states and parameters.

Without going into the advanced methods to solve this problem, we can mention the conceptually very simple approach of considering the vector of states to be interpolated as augmented by the vector of parameters to be calibrated. It can be noted that we are in state estimation or reconstruction of fields, and that in the temporal case of parameters identification, the evolution of the parameters to estimate is simply the identity. The assimilation or optimization algorithms can then be applied to the augmented vector. Valid for moderate nonlinearities in the simulation, this simple method extends the optimization space, and thus leads to larger problems, but it is often possible to reduce the representation to numerically computable cases. Without exhaustiveness, the separated variables optimization, the reduced rank filtering, or the specific treatment of covariance matrices, are common techniques to avoid this dimension problem. In the temporal case, we will see below indications for a Joint state and parameter estimation in dynamics.

To go further, we refer to the mathematical methods of optimization and augmentation developed in many books or specialized articles, finding their origin for example in [Lions68], [Jazwinski70] or [Dautray85]. In particular in the case of more marked nonlinearities during the numerical simulation of the states, it is advisable to treat in a more complete but also more complex way the problem of joint estimation of states and parameters.

2.4. Simple description of the data assimilation methodological framework¶

We can write these features in a simple manner. By default, all variables are vectors, as there are several parameters to readjust, or a discrete field to reconstruct.

According to standard notations in data assimilation, we note the optimal parameters that is to be determined by calibration, the observations (or experimental measurements) that we must compare to the simulation outputs, the background (a priori values, or regularization values) of searched parameters, the unknown ideals parameters that would give exactly the observations (assuming that the errors are zero and the model is exact) as output.

In the simplest case, which is static, the steps of simulation and of observation can be combined into a single observation operator noted (linear or nonlinear). It transforms the input parameters to results , to be directly compared to observations :

Moreover, we use the linearized operator to represent the effect of the full operator around a linearization point (and we will usually omit thereafter to mention , even if it is possible to keep it, to mention only ). In reality, we have already indicated that the stochastic nature of variables is essential, coming from the fact that model, background and observations are all incorrect. We therefore introduce errors of observations additively, in the form of a random vector such that:

The errors represented here are not only those from observation, but also from the simulation. We can always consider that these errors are of zero mean. Noting the classical mathematical expectation, we can then define a matrix of the observation error covariances by the expression:

The background can be written formally as a function of the true value, by introducing the errors vector such that:

The background errors are also assumed of zero mean, in the same manner as for observations. We define the matrix of background error covariances by:

The optimal estimation of the true parameters , given the background and the observations , is then called an “analysis”, noted as , and comes from the minimisation of an error function, explicit in variational assimilation, or from the filtering correction in assimilation by filtering.

In variational assimilation, in a static case, one classically attempts to minimize the following function :

is classically designed as the “3D-Var” functional in data assimilation (see for example [Talagrand97]) or as the generalized Tikhonov regularization functional in optimization (see for example [WikipediaTI]). Since and covariance matrices are proportional to the variances of errors, their presence in both terms of the function can effectively weight the gap terms by the confidence in the background or observations errors. The parameters vector realizing the minimum of this function therefore constitute the analysis . It is at this level that we have to use the full panoply of function minimization methods otherwise known in optimization (see also section Going further in the state estimation by optimization methods). Depending on the size of the parameters vector to identify, and of the availability of gradient or Hessian of , it is appropriate to adapt the chosen optimization method (gradient, Newton, quasi-Newton…).

In assimilation by filtering, in this simple case usually referred to as “BLUE” (for “Best Linear Unbiased Estimator”), the analysis is given as a correction of the background by a term proportional to the difference between observations and calculations :

where is the Kalman gain matrix, which is expressed using covariance matrices in the following form:

The advantage of filtering is to explicitly calculate the gain, to produce then the a posteriori covariance analysis matrix.

In this simple static case, we can show, under an assumption of Gaussian error distributions (very little restrictive in practice) and of linearity, that the two variational and filtering approaches give the same solution.

It is indicated here that these methods of “3D-Var” and “BLUE” may be extended to dynamic or time-related problems, called respectively “4D-Var” and “Kalman Filter (KF)” and their derivatives. They have to take into account an evolution operator to establish an analysis at the right time steps of the gap between observations and simulations, and to have, at every moment, the propagation of the background through the evolution model. The next section provides information on Going further in data assimilation for dynamics. In the same way, these methods can be used in case of non linear observation or evolution operators. Many other variants have been developed to improve the numerical quality of the methods or to take into account computer requirements such as calculation size and time.

2.5. A schematic view of Data Assimilation and Optimization approaches¶

To help the reader get an idea of the approaches that can be used with ADAO in Data Assimilation and Optimization, we propose here a simplified scheme describing an arbitrary classification of methods. It is partially and freely inspired by [Asch16] (Figure 1.5).

A simplified classification of methods that can be used with ADAO in Data Assimilation and Optimization (acronyms and internal descriptive links are listed below)

It is deliberately simple to remain readable, the dashed lines showing some of the simplifications or extensions. For example, it does not specifically mention the methods with reductions (of which it is given hereafter An overview of reduction methods and of reduced optimization), some of which were variations of the basic methods shown here, nor does it mention the more detailed extensions. It also omits the test methods available in ADAO and useful for the study.

Each method mentioned in this diagram is the subject of a specific descriptive section in the chapter on [DocR] Data assimilation or optimization calculation cases. The acronyms mentioned in the diagram have the meaning indicated in the associated internal links:

2.6. An overview of reduction methods and of reduced optimization¶

Data assimilation and optimization approaches always imply a certain amount of reiteration of a unitary numerical simulation representing the physics that is to be treated. In order to handle this physics as well as possible, this elementary numerical simulation is often of large size, even huge, and leads to an extremely high computational cost when it is repeated. The complete physical simulation is often called “high fidelity simulation” (or “full scale simulation”).

To avoid this practical challenge, different strategies to reduce the cost of the optimization calculation exist, and some of them also allow to control the numerical error implied by this reduction. These strategies are seamlessly integrated into some of the ADAO methods or are the purpose of special algorithms.

To establish such an approach, one seeks to reduce at least one of the ingredients that make up the data assimilation or optimization problem. One can thus classify the reduction methods according to the ingredient on which they operate, knowing that some methods deal with several of them. A rough classification is provided here, which the reader can complete by reading general mathematical books or articles, or those specialized in his physics.

Reduction of data assimilation or optimization algorithms:

the optimization algorithms themselves can generate significant computational costs to process numerical information. Various methods can be used to reduce their algorithmic cost, for example by working in the most suitable reduced space for optimization, or by using multi-level optimization techniques. ADAO has such techniques that are included in variants of classical algorithms, leading to exact or approximate but numerically more efficient resolutions. By default, the algorithmic options chosen in ADAO are always the most efficient when they do not impact the quality of the optimization.

Reduction of the representation of covariances:

in data assimilation algorithms, covariances are the most expensive quantities to handle or to store, often becoming the limiting quantities from the point of view of the computational cost. Many methods try to use a reduced representation of these matrices (leading sometimes but not necessarily to reduce the dimension of the optimization space). Classically, factorization, decomposition (spectral, Fourier, wavelets…) or ensemble estimation (EOF…) techniques, or combinations, are used to reduce the numerical load of these covariances in the computations. ADAO uses some of these techniques, in combination with sparse computation techniques, to make the handling of covariance matrices more efficient.

Reduction of the physical model:

the simplest way to reduce the cost of the unit calculation consists in reducing the simulation model itself, by representing it in a more economic way. Numerous methods allow this reduction of models by ensuring a more or less rigorous control of the approximation error generated by the reduction. The use of simplified models of the physics allows a reduction but without always producing an error control. On the contrary, all decomposition methods (Fourier, wavelets, SVD, POD, PCA, Kahrunen-Loeve, RBM, EIM, etc.) aim at a reduction of the representation space with an explicit error control. Although they are very frequently used, they must nevertheless be completed by a fine analysis of the interaction with the optimization algorithm in which the reduced computation is inserted, in order to avoid instabilities, discrepancies or inconsistencies that are notoriously harmful. ADAO fully supports the use of this type of reduction method, even if it is often necessary to establish this generic independent reduction prior to the optimization.

Reduction of the data assimilation or optimization space:

the size of the optimization space depends greatly on the type of problem treated (estimation of states or parameters) but also on the number of observations available to conduct the data assimilation. It is therefore sometimes possible to conduct the optimization in the smallest space by adapting the internal formulation of the optimization algorithms. When it is possible and judicious, ADAO integrates this kind of reduced formulation to improve the numerical performance without reducing the quality of the optimization.

Combining multiple reductions:

many advanced algorithms seek to combine multiple reduction techniques simultaneously. However, it is difficult to have both generic and robust methods, and to use several very efficient reduction techniques at the same time. ADAO integrates some of the most robust methods, but this aspect is still largely the subject of research and development.

One can end this quick overview of reduction methods highlighting that their use is ubiquitous in real applications and in numerical tools, and that ADAO allows to use proven methods without even knowing it.

2.7. Going further in the data assimilation framework¶

To get more information about the data assimilation techniques, the reader can consult introductory documents like [Talagrand97] or [Argaud09], on-line training courses or lectures like [Bouttier99] and [Bocquet04] (along with other materials coming from geosciences applications), or general documents like [Talagrand97], [Tarantola87], [Asch16], [Kalnay03], [Ide97], [Tikhonov77] and [WikipediaDA]. In a more mathematical way, one can also consult [Lions68], [Jazwinski70].

Note that data assimilation is not restricted to meteorology or geo-sciences, but is widely used in other scientific domains. There are several fields in science and technology where the effective use of observed but incomplete data is crucial.

Some aspects of data assimilation are also known by other names. Without being exhaustive, we can mention the names of calibration, adjustment, state estimation, parameter estimation, parameter adjustment, inverse problems or inversion, Bayesian estimation, field interpolation or optimal interpolation, variational optimization, quadratic optimization, mathematical regularization, meta-heuristics for optimization, model reduction, data smoothing, data-driven modeling, model and data learning (Machine Learning and Artificial Intelligence), etc. These terms can be used in bibliographic searches.

2.8. Going further in the state estimation by optimization methods¶

As seen before, in a static simulation case, the variational data assimilation requires to minimize the goal function :

which is named the “3D-Var” objective function. It can be seen as a least squares minimization extended form, obtained by adding a regularizing term using , and by weighting the differences using and the two covariance matrices. The minimization of the function leads to the best state estimation. To get more information about these notions, one can consult reference general documents like [Tarantola87].

State estimation possibilities extension, by using more explicitly optimization methods and their properties, can be imagined in two ways.

First, classical optimization methods often involves using various gradient-based minimizing procedures. They are extremely efficient to look for a single local minimum. But they require the goal function to be sufficiently regular and differentiable, and are not able to capture global properties of the minimization problem, for example: global minimum, set of equivalent solutions due to over-parametrization, multiple local minima, etc. An approach to extend estimation possibilities is then to use a whole range of optimizers, allowing global minimization, various robust search properties, etc. There is a lot of minimizing methods, such as stochastic ones, evolutionary ones, heuristics and meta-heuristics for real-valued problems, etc. They can treat partially irregular or noisy function , can characterize local minima, etc. The main drawbacks are a greater numerical cost to find state estimates, and often a lack of guarantee of convergence in finite time. Here, we only point the following topics, as the methods are available in ADAO:

Secondly, optimization methods try usually to minimize quadratic measures of errors, as the natural properties of such goal functions are well suited for classical gradient optimization. But other measures of errors can be more adapted to real physical simulation problems. Then, an another way to extend estimation possibilities is to use other measures of errors to be reduced. For example, we can cite absolute error value, maximum error value, etc. The most classical instances of error measurements are recalled or specified below, indicating their identifiers in ADAO for the possible selection of a quality criterion:

• the objective function for the augmented weighted least squares error measurement (which is the basic default functional in all data assimilation algorithms, often named “3D-Var” objective function, and which is known in the quality criteria for ADAO as “AugmentedWeightedLeastSquares”, “AWLS” or “DA”) is:

• the objective function for the weighted least squares error measurement (which is the squared weighted norm of the innovation, with a coefficient to be homogeneous with the previous one, and which is known in the quality criteria for ADAO as “WeightedLeastSquares” or “WLS”) is:

• the objective function for the least squares error measurement (which is the squared norm of the innovation, with a coefficient to be homogeneous with the previous ones, and which is known in the quality criteria for ADAO as “LeastSquares”, “LS” or “L2”) is:

• the objective function for the absolute error value measurement (which is the norm of the innovation, and which is known in the quality criteria for ADAO as “AbsoluteValue” or “L1”) is:

• the objective function for the maximum error value measurement (which is the norm, and which is known in the quality criteria for ADAO as “MaximumError”, “ME” or “Linf”) is:

These error measures may be not differentiable for the last two, but some optimization methods can still handle them: heuristics and meta-heuristics for real-valued problem, etc. As previously, the main drawback remain a greater numerical cost to find state estimates, and often a lack of guarantee of convergence in finite time. Here again, we only point the following methods as it is available in the ADAO module:

The reader interested in the subject of optimization can look at [WikipediaMO] as a general entry point.

2.9. Going further in data assimilation for dynamics¶

We can analyze a system in temporal evolution (dynamics) with the help of data assimilation, in order to explicitly take into account the flow of time in the estimation of states or parameters. We briefly introduce here the problematic, and some theoretical or practical tools, to facilitate the user treatment of such situations. It is nevertheless indicated that the variety of physical and user problems is large, and that it is therefore recommended to adapt the treatment to the constraints, whether they are physical, numerical or computational.

2.9.1. General form of dynamic systems¶

Systems in temporal evolution can be studied or represented using dynamic systems. In this case, it is easy to conceive the analysis of their behavior with the help of data assimilation (it is even in this precise case that the data assimilation approach was initially widely developed).

We formalize the numerical simulation framework in a simple way. A simple dynamic system dynamic system on the state can be described in continuous time in the form:

where is the unknown state vector, is a known external control vector, and is the (possibly non-linear) operator of the system dynamics. It is an Ordinary Differential Equation (ODE), of the first order, on the state. In discrete time, this dynamical system can be written in the following form:

for an indexing of discrete times with . is the discrete evolution operator, symbolically obtained from by the discretization scheme. Usually, we omit the time notation in the evolution operator . Approximating the operator by introduces (or adds, if it already exists) a model error.

We can then characterize two types of estimates in dynamics, which we describe hereafter on the discrete time dynamical system: State estimation in dynamics and Parameter estimation in dynamics. Combined, the two types can be used to make a Joint state and parameter estimation in dynamics. In ADAO, some algorithms can be used either in state estimation or in parameter estimation. This is done simply by changing the required option “EstimationOf” in the algorithm parameters.

2.9.2. State estimation in dynamics¶

The state estimation can be conducted by data assimilation on the discrete time version of the dynamical system, written in the following form:

where is the system state to be estimated, and are respectively the computed (unobserved) and measured (observed) state of the system, and are the incremental evolution and observation operators, respectively, and are the evolution and observation noise or error, respectively, and is a known external control. The two operators and are directly usable in data assimilation with ADAO.

2.9.3. Parameter estimation in dynamics¶

The parameter estimation can be written a differently to be solved by data assimilation. Still on the discrete time version of the dynamical system, we look for a nonlinear mapping, parameterized by , between inputs and measurements at each step , the error to be controlled as a function of parameters being . We can proceed by optimization on this error, with regularization, or by filtering by writing the problem represented in state estimation:

where, this time, the choice of the evolution and observation error models and condition the performance of convergence and observation tracking (while the error representations come from the behavior of the physics in the case of state estimation). The estimation of the parameters is done by using pairs of corresponding inputs and outputs.

In this case of parameter estimation, in order to apply data assimilation methods, we therefore impose the hypothesis that the evolution operator is the identity (Note: it is therefore not used, but must be declared in ADAO, for example as a 1 matrix), and the observation operator is .

2.9.4. Joint state and parameter estimation in dynamics¶

A special case concerns the joint estimation of state and parameters used in a dynamic system. One seeks to jointly estimate the state (which depends on time) and the parameters (which here does not depend on time). There are several ways to deal with this problem, but the most general one is to use a state vector augmented by the parameters, and to extend the operators accordingly.

To do this, using the notations of the previous two subsections, we define the auxiliary variable such that:

and the operators of evolution and observation associated to the augmented problem:

With these notations, by extending the noise variables and appropriately, the joint state and parameters discrete-time estimation problem, using the joint variable , is then written:

where . The incremental evolution and observation operators are therefore respectively the augmented operators and , and are directly suitable for study cases with ADAO.

2.9.5. Conceptual scheme for data assimilation in dynamics¶

To complete the description, we can represent the data assimilation process in a dynamics specific way using a temporal scheme, which describes the action of the evolution ( or ) and observation ( or ) operators during the discrete simulation and the recursive estimation of the state (). A possible representation is as follows, particularly appropriate for iterative Kalman filtering algorithms:

Fig. 2.1 Timeline of steps for data assimilation operators in dynamics

with P the state error covariance and t the discrete iterative time. In this scheme, the analysis (x,P) is obtained by means of the “correction” by observing the “prediction” of the previous state. The concepts described in this diagram can be directly and simply used in ADAO to elaborate study cases, and are included in the description and the examples of some algorithms.