Neural network parameterisation of the mapping of wave–spectra onto nonlinear four–wave interactions Kathrin Wahle ∗ , Heinz Günther, Helmut Schiller GKSS Research Center, Max–Planck–Str. 1, 21502 Geesthacht, Germany Abstract A new approach to parameterise the exact nonlinear interaction source (snl) term for wind wave spectra is presented. Discrete wave spectra are directly mapped onto the corresponding snl–terms using a neural net (NN). The NN was trained with modeled wave spectra varying from single mode spectra to highly complex ones. The specification of training data was based on a classification of the wave spectra by cluster analysis. In course of the structuring of the NN the intrinsic dimensionalty of the spectra was estimated with an auto-associative neural net (AANN). The AANN might be used for a scope check of the method. Key words: Neural networks, Nonlinear wave–wave interaction, Wave–spectra, Wave models, Cluster analysis 1 Introduction Wave–models compute the dynamics of the sea–surface by solving the action balance equation describing the evolution of interacting weakly–nonlinear waves, e.g. Komen et al. (1994). Besides propagation of waves this involves growth of waves due to wind, the dissipation of wave energy due to whitecapping and the transfer of wave energy due to nonlinear wave–wave interaction. The computation of the last–mentioned process is by far the most time consuming step in the modelling. Operational wave–models therefore use approximations as the well known discrete interaction approximation (DIA) Hasselmann et al. (1985). The DIA is computational fast but has also known deficiencies. ∗ Corresponding authors. Email address: kathrin.wahle@gkss.de (Kathrin Wahle). Preprint submitted to Elsevier 2 April 2009 The further development of third generation operational wave–models requires a replacement of the approximative methods for the calculation of the nonlinear wave-wave interaction by fast and accurate methods. Attempts have been made with extensions of the DIA but they are so far not generally applicable since tuning for different classes of wave spectra is needed. The same problem occurs for methods based on diffusion operators. For a comprehensive discussion see Cavaleri et al. (2007). In this paper a neural net (NN) parametrisation of the mapping between wave–spectra and the corresponding nonlinear wave–wave interaction will be presented. The idea of applying neural networks in this context was first introduced by Krasnopolsky et al. (2001), Krasnopolsky et al. (2002). There the wave spectra as well as the nonlinear interaction source terms are assumed to be separable functions of frequency and direction which are approximated by expansion series, respectively. The neural network is used to map the two sets of expansion coefficients. This assumption was dropped in a successive work Tolman et al. (2005) where the authors used twodimensional Empirical Orthogonal Functions for the expansion of single peaked spectra. In contrast to this approach the feasibility of the direct mapping of the discretised wave spectra onto the nonlinear interaction source term will be demonstrated here. The study will not be restricted to single peaked spectra but will also include multi–modal ones. Two design decisions are crucial for the successfull construction of a NN: the choice of the training data and of the NN architecture, respectively. The wave spectra used for the training of the neural network (NN) are simulated spectra from a hindcast with the wave model WAM cycle 4 WAMDI group (1988). Although WAM uses the DIA the simulated spectra are suited to investigate the approach of a direct mapping. (Once the feasability has been shown one should make the effort of calculating all spectra with a wave model that uses the exact nonlinear interaction.) We expect that the training for wave spectra representing multi–modal wave systems will be more difficult then for single peaked spectra. Therefore the more complex cases should be well represented. To be able to enrich the number of multi–modal spectra in the training data set we perform a classification of the wave spectra. For the classification we use a cluster algorithm. To fix the NN structure it is important to have an estimate of the intrinsic dimensionality of the spectra, i.e. the minimum number of variables needed to represent the spectra. For this we constructed auto-associative neural nets (AANN) which map the spectra onto themselves while compressing them in between. The difference of the original spectrum with the reproduced one gives a quality measure of the reduced representation. This difference can be used to decide wether the compression was successfull for a given spectrum (wether 2 it is in the scope of the algorithm). The finding of the intrinsic dimensionality led to the decision about the actual NN structure for the mapping of the wave spectra onto the nonlinear interaction source terms. The results of the cluster analysis are given in section 2 whereas the technical details of the method are described in appendix A. Section 3 starts with a short introduction to neural nets and the special case of auto-associative neural nets. The concrete applications follow: in 3.1 some AANN architectures are tested in order to get an estimate of the intrinsic dimensionality of wave spectra. The design and training of the actual neural network for the nonlinear interaction source term is described in section 3.2. It includes the application of the AANN for outlier identification. We then discuss the relevant steps towards an operational usage of the procedure in an outlook given in section 4. Finally, the results are summarised in some concluding remarks in section 5. 2 The cluster analysis of wave–spectra The wave spectra used for the training of the neural network (NN) are simulated spectra from a hindcast with the wave model WAM cycle 4 WAMDI group (1988). The data span over 22,000 points in the north-atlantic for the one month period of january 1995. Spectra were saved every three hours at an equidistant resolution of 15◦ in direction and an equidistant relative resolution in frequency ranging from approximately 0.04 Hz to 0.4 Hz. The resultant dimensionality of the spectra is 600 (24x25). From this dataset of over five million spectra a representative subset containing all classes of spectra had to be selected. By random sampling the simple single– and bi–modal wave system cases would outweight any other class of spectra. As a result the neural networks would well map these cases but would badly perform for other classes. Thus a classification method should first of all be applied to the whole dataset. Important characteristics for the classification of the wave spectra are the number and shapes of wave systems in them. The classification method should identify these wave systems. This clearly is a pattern recognition problem which we solved by applying a cluster algorithm. As the spectra are represented using an azimuthal angle it happens that a wave system crossing 0◦ /360◦ is splitted artifically. To avoid this the wave spectra are preprocessed: the point of the origin of the direction is shifted if necessary to the direction with lowest energy. This is the only preprocessing needed here. Tolman et al. (2005) showed possiblities how to normalize the spectra further for other approaches. 3 Input to the cluster algorithm are the 600 points of a wave spectrum. Each of these points is defined by its coordinates (frequency and direction indices) and by the energy at this coordinate bin of the spectrum. The energies serve as weights of the points. The algorithm assigns the points into groups (called clusters) so that points from the same cluster are more similar (according to a distance measure) to each other than points from different clusters. The cluster algorithm CLUCOV Schiller (1980) is used which was adapted for the twodimensional problem of parametrising the wave spectrum. Each cluster is characterised by a twodimensional Gaussians. For each cluster its respective center and covariance matrix are estimated and the resulting Mahalanobis distance is used to assign points to the closest cluster. This allows for varying orientations of the different clusters. In the iteration procedure it is checked if the clusters are unimodal. If not, the cluster is split into two. The number of clusters also can decrease: if two clusters strongly overlap these clusters are combined. In the appendix A the algorithm is described in more detail. One can summarize the method as performing a point density function approximation with Gaussians as radial basis functions (RBF). Figure A.1 shows three examples. The left panel shows the input to the algorithm (original wave spectra). The right panel shows its parametrisation by the cluster algorithmus. It can be seen that the main characteristics are well reproduced. But also it should be noted that the number of clusters is often greater then the number of wave systems. In the first example the number of wave systems is two, but three clusters were found, the second example shows three wave systems and four clusters and the last spectrum has at least three wave systems in it and six clusters were found. This is due to the structure of the wave systems which are best approximated by two Gaussians, one around the peak and one for the tail (see again figure A.1). To summarize the cluster algorithm is well suited to classify wave spectra. The main characteristics are well reproduced. The number of clusters is often larger than the number of wave systems in a spectrum but the two numbers are highly correlated. High number of clusters indicate complex structures in the corresponding spectrum. Table 1 lists the result of the cluster analysis, i.e. the number of clusters found for each spectrum in the dataset. On this basis a representative subset of 791,570 spectra was choosen. The number of selected spectra increases with the number of clusters (more complex wave spectra). Above a number of clusters of five all spectra were taken and included twice into the dataset. Table 1 also quantifies the selection. This representative dataset was randomly subdevided in a training set (755,173 spectra) and a testing set (36,397 spectra) for the neural networks. 4 number of clusters number of spectra percentage selected 1 406,332 9.8 2 1,536,057 5.2 3 1,907,143 6.3 4 1,033,269 15.5 5 354,712 56.4 6–12 95,804 200 Table 1 Spectra used for the representative dataset. 3 Neural Networks Wave spectra and the nonlinear interaction source term are related via a six– dimensional Boltzmann integral. A computational efficient method to parametrise this complex functional relation is the usage of artificial neural networks (NN). This is possible since a NN with at least one hidden layer – a layer between the input and the output layer – is able to approximate any continuous function (Universal Approximation Theorem) as was shown by Haykin (1999). So a NN — in this context — is a computational tool for function approximation whose generalities will be described briefly (see Bishop (1995)). A NN is organized in layers: one input layer, one output layer and one or more hidden layers inbetween. Each layer consists of neurons. The number of neurons in the in– and output layer are given by the dimensions of the in– and output vectors. (In our application the two–dimensional wave spectra and nonlinear interaction source terms will therefore be arranged as vectors with 600 components, respectively.) The number of neurons in the hidden layer(s) are not preset and depend upon the problem. Each neuron in a layer is linked to each neuron in a neighboring layer with a weight. Neural nets work sequentially: each element of the input vector serves as entry for one of the neurons of the input layer. The output of the first hidden layer is computed by summation of the weighted inputs, shifting it by a bias and applying a nonlinear function (a sigmoid here). The procedure is repeated itself until the output layer is reached where the outcome of each neuron gives one element of the output vector. Weights and biases are the free parameters of the approximation. They are fixed during the training phase of the NN. To do so a dataset — the training set — is needed consisting of N pairs of input vectors (the 600 components of the wave spectra) and corresponding desired output vectors (the correspond5 ing 600 components of the exact nonlinear interaction source term). At the beginning of the training the outcome of the NN will differ largely from the desired output. Let ~y and y~′ be the m–dimensional output vectors as emulated by the NN and as given in the dataset respectively. Then the difference of the two will result in an error e defined by e= N 1 X ep N p=1 m y ′ i − yj 1 X ep = m i=1 max(y ′pi , p ∈ [1, N]) − min(y ′pi , p ∈ [1, N]) !2 . (1) This mean squared relative error per neuron e is iteratively minimized during the training by backpropagating it through the NN and adjusting the biases and weights according to a gradient descent scheme. It is measure for the quality of the function approximation by the NN. It is good practise to have an additional independent dataset — the testing set — which is needed to check the generalization–power of the NN after the training, i.e. to test if reasonable output is produced for input not included in the training. The training and testing phase is time consuming. But it needs to be done only once. Whereas the usage of a NN is very fast. The abovementioned holds true for an auto–associative NN which is a particular NN. Its especialness is that it maps the input vector onto itself. But at one stage of the mapping — in the so called bottleneck layer — the dimensionality is reduced. So when applying AANN’s with different degrees of dimensionality reduction to the 600 components of the wave spectra the number of variables needed to capture the information contained in the spectra can be determined. Technically speaking the number of neurons in at least one hidden layer in an AANN is less than the dimension of the in- and output vector ~x and x~′ . The AANN part m(~x) which maps the input vector onto the neurons in the bottleneck layer p~ is also called mapping part and the backprojection x~′ = d(~p) is called demapping part, accordingly (see figure A.2). The mapping part can be considered as performing a nonlinear generalization of the Principal Component Analysis (Nonlinear Principal Component Analysis, NLPCA) Kramer (1991). It retains the maximum possible amount of information from the original data, for a given degree of compression. Like PCA also NLPCA can serve important purposes, e.g. filtering noisy data, feature extraction, outlier identification, the compression of data, and the restauration of missing values Hsieh (2001), Schiller (2003). 6 3.1 Auto–associative neural network of wave–spectra In order to achieve the goal of directly mapping the wave spectra onto the corresponding nonlinear interaction source term it is essential to find their intrinsic dimensionality (the number of independent variables) to be able to fix an appropriate NN structure. The cluster analysis in the last section was not only useful for the creation of a representtive dataset but also it gives a hint about the intrinsic dimensionality of the wave spectra. This can be reasoned since the parametrisation of the wave spectra by gaussians is a first compressed representation. Since each spectrum can be parametrised by a few clusters and each cluster is characterised by six numbers (energy of the cluster, its center, its orientation and shape) the intrinsic dimensionality is expected to be of the order of a few tens. This will now be further investigated by means of auto–associative neural networks (AANN’s). To find an optimal AANN–parametrisation of the wave spectra AANN’s with different number of neurons in the bottleneck layer had to be trained. The optimization problem consists of balancing the loss of information with decreasing bottleneck neuron number and the usability of the NN. At optimum the compression is as loss–free as possible with additional bottleneck neurons giving only slight improvement. For the training the neural net program developed by Schiller (2000) was used. The AANN’s have 600 neurons in the in– and output layer since the wave spectrum is interpreted as a 600-dimensional vector. Each of the trained AANN’s had three hidden layers with 80 neurons in the first and third hidden layer and with differing number of neurons in the middle bottleneck layer. Figure A.3 shows the mean squared relative error per neuron e (see equation 1) of the AANN’s for the testing data set as a function of the number of bottleneck neurons. As expected the error e decreases with increasing number of bottleneck neurons. Clearly two different regimes are visible (the straight lines were fitted by linear regression): one for the number of bottleneck neurons below approximately 33 and one above. From this it was decided to choose the smallest AANN belonging to the second regime with 39 bottleneck neurons in it. Figure A.4 shows three examples of the performance of this AANN. The left panel shows the original wave spectra. The middle panel shows the output of the AANN — the mapping of the original wave spectra onto themselves. The right panel shows the directionally integrated wave spectra. The examples exhibit an increasing complexity of the wave spectra. The AANN output strongly resembles the original wave spectra throughout the spectral space. 7 The residual average error √ e is 1.4, 1.5 and 1.8% from top to bottom. To summarize the construction of different AANN’s has shown that the intrinsic dimensionality of the analysed wave spectra is about 40. An AANN with 39 bottleneck neurons gives a good parametrisation for all different classes of √ wave spectra with a residual average error of e =1.3%. 3.2 Nonlinear wave–wave interaction So far we have selected a dataset which incorporates all classes of wave spectra (see section 2) and we have found the typical number of independent variables describing each of these wave spectra (see section 3.1). After these preparatory works one can now start with the construction of a neural net (NN) for the direct derivation of the nonlinear interaction source terms from wave spectra. As a first approximation we decided to use the same architecture already used for the AANN: In– and output layers consist of 600 neurons as the discretized wave spectra and the snl–terms are interpreted as 600–dimensional vectors. Inbetween there are again three hidden layers with 80, 39 and 80 neurons, respectively. To train and test the NN the (exact) nonlinear interaction source (snl) terms corresponding to each of the spectra of the selected dataset of 791,570 wave spectra (see section 2) had to be calculated first. The method first suggested by Webb (1978) and known as the WRT method with further improvements by Vledder (2006) was applied for this purpose. The pairs of wave spectra and snl–terms were again subdivided √ into the training and testing dataset. The resulting residual average error e (see equation 1) of the resulting NN was 1.1% for both the testing– and training. Figure A.5 shows results of the performance of the NN. The top row shows the original wave spectra which served as input for the NN. The next row shows the corresponding exact snl–terms calculated with the WRT–method and below it the NN results (the WRT emulation by the NN) are shown. The bottom row shows the directionally integrated snl–terms. In all cases the NN emulation is very similar to the exact solution. The directionally integrated plots highlight the quality of the fit. The three wave spectra vary from single to three wave systems to demonstrate the broad applicability of the method to different classes of wave spectra. The choice of a represantative training dataset is crucial since a NN has good interpolation properties but produces unpredictable output when forced to extrapolate. The approach described here offers a ’in range’ check Schiller et 8 al. (2001), i.e. the AANN can be used to check if a given spectrum belongs to the class of spectra chosen for the training of the AANN. If the error e of the AANN is big for a given wave spectrum then it did not succeed to represent the spectrum through the bottleneck layer. Then it is questionable that the outcome of the snl–term NN will be in good agreement with the exaxt snl–term. Figure A.6 exemplifies the resulting √ outlier identification: A wave spectrum which gave a large of error (here e = 3.6%) when parametrising it with the AANN was choosen randomly (upper panel). The emulation of the corresponding snl–term with the NN is poorly (lower panel). The √ resultend relative error when comparing the exact with the NN solution is e = 10.2%. The NN method described here differs in mainly two points from the NN based method first presented in Krasnopolsky et al. (2001) and with latest results from Tolman et al. (2005): There the wave spectra as well as the snlterms are first decomposed and represented by a linear superposition of two sets of orthogonal functions. Two different approaches are used for the basis functions. The former approach is based on a mathematical basis where the frequency and direction dependence was separated (an assumption beeing not satisfied by spectra having more than one wave system at one frequency). The later one is based on principal components but it is restricted to single peaked spectra. In either case the neural net is used to map the expansion coefficients onto each other. The training and testing sets used were generated based on theoretical spectral descriptions. So firstly, the approach presented here is not restricted to special cases of wave spectra with e.g. only one wave system in it since a set of modeled spectra of a high variety of complexity has been used for its development. And secondly, no assumptions (as separabilty or convergence of an expansion series) about the wave spectra and nonlinear interaction terms are made. Finally an important advantage of any NN parametrisation over other implementations of complex physical processes is its computational efficiency. It is very time consuming to train a NN and to find an appropriate net architecture but its usage is fast. A runtime comparison of the exact WRT–method with the NN method presented here gives a speedup factor of roughly 500. 4 Outlook We have presented results of a neural net (NN) mapping directly wave spectra onto the corresponding exact nonlinear interaction source term. We now want to describe how we imagine to distinctly improve the method in order to 9 operationally incorporating it. To start with, a new training/testing dataset of wave spectra should be calculated with a model that uses the exact nonlinear interaction. The datasets should again contain all classes of complexity of wave spectra. With these datasets the training of the neural nets should be repeated and different net architectures (e.g. different mapping/demapping part) should be tried. In particular we assume it promising to train a AANN for both, the wave spectra and the nonlinear interaction source terms and to map the both bottleneck layers onto each other. This ansatz has some advantages: Firstly, more complex net architectures can be tried out for the NN which maps the two bottleneck layers onto eachother since the number of in– and output variables is one order of magnitude smaller than in the present NN. Secondly, with two AANN’s the ’in range’ check can be performed twice: one time for the wave spectra before applying the actual NN and one time afterwards for the emulated nonlinear interaction. Still, this is a necessary condition but not a sufficient one. Thirdly, when changing the resolution of the wave model only the two AANN’s have to be retrained, the NN for the actual mapping stays the same. This can be easily done, because the number of neurons in the bottleneck layers of the AANN’s is already known since the complexity of the spectra and the source terms does not change with resolution. Finally, it has to be shown that the NN emulation gives robust and accurate results when implemented in a numerical wave model. As suggested by Krasnopolsky et al. (2005), Krasnopolsky et al. (2007) parallel runs of the wave model with the original parametrization of the exact nonlinear interaction and with its NN emulation should be performed to do so. When operationally running the wave model a quality control block as suggested by Krasnopolsky et al. (2008) could determine whether the NN emulation will be used or not. 5 Conclusions The feasability of a neural network (NN) based method for direct mapping of discrete wave spectra onto the corresponding (exact) nonlinear wave–wave interaction source (snl) terms has been demonstrated. In a preparatory step the intrinsic dimensionalty of the wave spectra was estimated by means of an AANN to be about 40, i.e. 40 variables allow to capture the variability of the wave spectra with a residual average error of 1.3%. The composition of the training data set for the NN was based on automated classification of the wave spectra by means of a cluster analysis. 10 The NN for the mapping of the wave spectra onto the corresponding snl–terms shows good performance. It is able to emulate the WRT method calculations for single and multi mode wave spectra with a much higher accuracy then the approximations implemented in nowadays operational wave models. The quality of the emulation might be controlled using the corresponding AANN. 6 Acknowledgements The authors like to thank Gerbrant Ph. van Vledder for providing his improved version of the WRT–method for calculating the nonlinear interaction source terms and for offering his help on running it. Further, we thank the reviewers for their valuable comments on an earlier version of this paper. A The cluster algorithm CLUCOV: twodimensional case The CLUCOV algorithm is used assign the 600 points of a wave spectrum to clusters. The k–th cluster Gk is characterized by the moments of order zero, one and two of the distribution of the N points contained in this cluster. The points are defined by their frequency and direction coordinates X m = (Xfm , Xθm ) and their energies which serve as weights w m . Then the three moments are – the total weight I k of points X m contained in the cluster Gk k k I = N X wm m=1 – the centroid Qk of the cluster Gk Qki Nk 1 X w m Xim = k I m=1 with X m ∈ Gk , i ∈ {f, θ} – the covariance matrix C k of the cluster Gk Nk 1 X k Cij = k w m (Xim − Qki )(Xjm − Qkj ) with X m ∈ Gk , i, j ∈ {f, θ} I m=1 The eigenvectors of the covariance matrix point into the direction of the main axes of the ellipsoids by which the shape of the clusters is approximated. The square root of the eigenvalues of the covariance matrix denote the lengths of the main axes, and the square root of the determinant of the covariance matrix thus measures the volume of the clusters. 11 k All these three moments enter the definition of the distance fm of a point m m k at X from the k–th cluster G . Thus, each cluster builds its ‘own’ metric: The (Mahalanobis) distance ρkm of a given point X m from the centroid Qk of the k–th cluster enters the exponent of a Gaussian g k (X m ) g k (X m ) = ρkm 1 exp[− (ρkm )2 ] with 2 2π |C k | 1 q q = (X m − Qk )T (C k )−1 (X m − Qk ). The total weight I k of points in cluster k is included into the distance measure k fm as a linear weight factor, so that big clusters ‘attract’ further points k fm = I k · g k (X m ). In the direction of the main axes distances are measured in units of the square root of the corresponding eigenvalue of the covariance matrix (which is suggested by the quadratic form in the exponent of the gaussian). This distance measure is also invariant under linear transformations such as translation and rotation. k The determinant in the denominator of fm favours compact clusters against voluminous clusters of the same content. In the algorithm CLUCOV it is also possible to split and merge clusters, that is to change the number of clusters. To achieve this, a measure t for the overlap of two clusters Gk and Gl is defined: t= √ h0 hk hl The quantities hk and hl are superpositions of the gaussians f k (X) and f l (X) of the clusters Gk and Gl in their centroids Qk and Ql : hk = f k (Qk ) + f l (Qk ) and hl = f k (Ql ) + f l (Ql ) The quantity h0 is the minimum of this superposition along the distance vector (Qk − Ql ) between the clusters: h0 = min[f k (X) + f l (X)] 12 If for a pair of groups this measure t exceeds a limit tmerge (parameter of the merging procedure) this pair of groups is united into one group. 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Neural Networks 21, p.535–543 14 RBF parametrisation 0. 1 0.01 0.01 0.5 1 3 ° 0 0.1 .5 0.05 0.1 0. 05 0. 0.5 5 10 05 0. 0.1 0.5 3 1 0.05 0.06 01 3 5 6 1 0.41 5 0.1 0.0 0.10 0.16 0.26 frequency [Hz] 45 0 0.1 ° 5 1 1 0. 90 0.01 0.5 5 10 3 0.1 1 0.0 05 0.5 .1 0.01 0.05 0.1 0.5 1 1 00.05 .1 3 5 5 1 0.5 135 1 0.5 direction [ ] 180 0.01 .015 1 00. 3 5 10 6 6 3 0.5 225 10 5 0.01 1 1 .5 0..015 0 0 1 0.0 01 0.05 0.01 0.1 1 0.01 0.05 0.1 1 0.41 0. 0.5 0.5 3 6 0.10 0.16 0.26 frequency [Hz] RBF parametrisation 270 1 0.06 0.06 360 315 10 6 3 1 .5 0 0.1 0.05 0.01 1 3 5 01 0.5 1 3 5 0.41 0.5 1 0. 5 1 3 0.1 05 0. 3 direction [ ] 0.01 5 0.0 1 0.0 0.1 5 0.0 0 0.05 .01 0.5 0.1 1 3 6 0. 05 05 0. 1 45 0 0.01 3 0.5 0. 0 05 .0 1 1 0 .5 direction [°] 0.1 3 5 0.0 0.0 5 5 ° 1 0.5 0.05 direction [°] 0.0 5 0.5 0.0 0.1 5 0. 01 0.05 0.01 5 0. 0. 0. 05 1 0.01 ° 0.05 0.10 0.16 0.26 frequency [Hz] original wave spectrum 270 direction [ ] 5 0.1 135 0.01 6 0.1 0.05 0.01 0.0 0.06 360 315 45 0 1 0.5 1 3 0.5 0.5 0.1 1 0.0 05.1 0. 0 45 0 .1 90 5 90 5 05 0 180 01 0. 0.05 0.1 3 0. 0.41 0.01 0.0 0.1 5 0.5 1 225 1 0.1 0.5 05 0.15 0.0 0.0 0.1 5 0.5 0.5 1 0.01 135 0.01 0.05 0.1 0.0 direction [ ] 180 0.1 0.01 0.5 0.10 0.16 0.26 frequency [Hz] RBF parametrisation 0. 270 0.01 0.510.1 3 1 0.00 .501 1 0.0 5 0.01 0. 1 0.06 360 315 1 0. 1 1 0.1 225 0.41 1 0.5 00. .05 1 0.0 .05 0 0.1 0.5 0.05 270 45 0 5 0. 3 0.01 0.10 0.16 0.26 frequency [Hz] original wave spectrum 1 135 0.5 0.06 0.5 180 0.1 0.05 0.05 0.1 0.01 225 90 1 0.0 0.5 0.15 360 315 90 5 0.0 0.01 1 0. 45 0 135 0.1 0.01 0.05 0.1 0.1 0.1 90 180 5 0. 1 3 135 270 0.1 1 180 0.01 0.5 0.01 225 225 0 0.1 .05 0.05 0.1 0.01 0.01 270 360 315 1 00. .05 0.01 original wave spectrum 360 315 0.10 0.16 0.26 frequency [Hz] 0.41 Fig. A.1. Examples of cluster analysis: left panel shows original wave spectra, right panel shows corresponding RBF parametrisation. 15 Fig. A.2. Autoassociative NN with bottleneck: the input is mapped by p~ = m(~x) onto a lower dimensional (dim(~ p) < dim(~x)) space and is approximately reconstructed by the demapping NN part x~′ = d(~ p) ≈ ~x. 16 −4 squared relative error 4 x 10 3 2 1 0 10 20 30 40 50 number neurons in bottleneck 60 Fig. A.3. Squared relative error of the AANN’s as function of the number of bottleneck neurons. 17 original wave spectrum 0.5 AANN parametrisation 315 0.00.1 0.1 5 05 0. 0 3 0.1 0. 5 1 6 5 3 5 0.5 1 0.5 0.06 315 0.1 0.1 00.1 .1 .1 0.10 0.1 0.05 0.05 1 ° direction [ ] 0.1 1 1 0.5 00.0 .055 .1 0.41 05 0.05 0. original AANN 1.2 0.05 0.05 5 .05 00.0 1 0.0 1 0.10 0.26 0.01 0.01 1 0.8 0.6 0.4 0.2 0.01 0.01 0.26 .055 00.0 0.1 1 0.5 0.10 0.16 frequency [Hz] 1 05 0. 0.5 1 0. 45 0.10 0.16 frequency [Hz] 0.06 0.0 0.5 90 45 0.1 0.01 0.1 135 0.10.1 1 0.0 0.01 180 90 0.06 spectral energy density 0.01 ° direction [ ] 0.01 0 0. .05 1 5 0.0 0.01 225 55 0. 0. 055 0.0.0 0.5 0.1 0. 1 0.0 0.5 1 1 1 0.555 0.0 0.0 0.01 135 270 5 0.0 0.1 0.1 0.1 0.1 0.0 5 00.1 .1 .05 0.050 00. .005 5 055 0.0 0. 0.5 1 0.1.1 0 .1 00.1 0 0.01 0.5 1.4 00. 5.5 1 .05 1 directionally integrated 1 0.01.1 1.5 0 0.41 0.0 0.01 0.1 0.1 0.1 0.050.05 5.05 0.00 0.5 225 0.1 0.1 50.05 0 0.0 1 .5 1 00. .005 0.5 50.01 315 1 0.01 0.5 0.01 0.1 0.26 0.055 0.0 00.0 .055 0.01 original AANN 2 AANN parametrisation 0.0 270 0.0.55 0.10 0.16 frequency [Hz] 360 0.1 11 0.050.05 0.1 315 0.05 0.05 .1 00.1 0.06 .11 00. 0 0.41 0.5 0.5 5 5.0 0.00 0.26 original wave spectrum 360 1 11 45 0.1 0.0.55 00.5.5 1 0.1 0.10 0.16 frequency [Hz] 0.0 0.1 11 0.0 0.06 90 05 0. 1 0 135 0.0 1 90 0.1 1 0.41 directionally integrated 1 0.1 0.1 0.0 0.26 0.01 180 .1 00.1 0.0 5 135 05 0.1 5 05.0 0.0 055 0.0 0. .11 00. 01 0. 225 0.10 0.16 frequency [Hz] 5 0.0 0.0 0. 0.1 5 00.5. 1 0.1 0.06 2.5 0.1 1 05 0. 1 0. 5 5 0.0 0.0 0.0 180 0 0.41 0.1 0.0 0.1 05 0. 270 55 0.0 0.0 0.01 0.26 0.01 0.5 0.5 45 0.0 5 0.01 00.1.1 225 20 10 0.0 5 5 0.0 1 0.01 1 00. .005 5 0.05 0.05 270 ° 55 0.0 0.0 0.1 0.0 0.1 0.0 315 30 AANN parametrisation 360 0.05 0.05 1 0.1 0.10 0.16 frequency [Hz] original wave spectrum 360 direction [ ] 3 1 0.0 0.1 0.5 0 0.41 0.0 0.1 0.0 55 5 0.26 0.1 10 5 0.10 0.16 frequency [Hz] 5 .50 0.0 1 0.0 0.0 1 0.0 0.5 0.06 5 6 10 6 0.1 0.1 0.5 10 6 5 0.1 0 45 5 0.01 0.05 0.1 0.05 0.5 0.5 1 1 3 90 0.0 3 0.05 0.1 0.51 40 0. 05 1 1 11 11 0.5 05 0.01 135 10 3 0.1 5 6 180 0.5 0.01 00.0.0 55 0.1 3 1 0.0 6 0.1 0.5 11 0. 0.1 05 1 1 0.5 0.1 1 0.5 1 0.0 0.1 5 0. 3 0.1 0.1 0.5 1 0.5 ° ° 1 01 0. 0.01 0.05 0.05 5 0.0 0.01 0.05 225 0.1 0.0 0.0 0. 55 5 0.0155 00.0.0 135 direction [ ] 180 0. ° 50 0.5 0.5 0.1 0.01 direction [ ] 0.100.0 .15 1 0.00.1 5 90 direction [ ] original AANN 1 0.0 270 0.01 0.05 0.5 225 0 0.1 0.1.05 .005 0.01 270 180 0.5 0.5 spectral energy density 1 0.1 0.05 0.1 0.05 1 1 0.01 45 directionally integrated 60 1 0.0 0.1 .05 0 1 0.0 0.5 1 0. 0.0 315 05 360 1 1 0. spectral energy density 360 0.41 0 0.06 0.10 0.16 frequency [Hz] 0.26 0.41 0 0.06 0.10 0.16 frequency [Hz] 0.26 Fig. A.4. Examples of the performance of the AANN: left panel shows original wave spectra, panel in the middle shows corresponding AANN parametrisation, and right panel shows the directionally integrated spectra. 18 0.41 original wave spectrum original wave spectrum 360 ° 5 0.0 0 −1e −1e −1−08 1e 0 −08 1e−06 −0.0001 −1e−05 1e−10 1e−06 1e−05 0.0001 001 −0.0 −5e −1 −0 1e−10 1e−05 1ee−−055 1e−05 0−1e− 6 06 0.001 −1e 10−08 0−1e− 1e− 101e−06 5 1e−0 1e −1−0 e−80 6 1e− 8 08 0 e− 1e −1e−10 −06 −10 e−1e−06 −0 0 1e8−1 1e−06 0 6 −1 −0 1e 1e 6 1e−0 −0 1e 1e−10 6 135 1e−08 ° direction [ ] 1e −10 0 08 e− −1 06 1e− 1e−0 −180 1e−05 08 −1e−101e− −0.0005 6 0 1e 1e−−0 10 8 1e −0 8 1e−10 1e−10 ° direction [ ] 5 1e−0 0 1e −1 0 0 ° −1e− −1e−10 −1e− 0806 direction [ ] 8 e−− 10e8 −1 0 −1 1e −0 −5e−05 5e−06 01 0 −5e−06 .00 05 5e− 0.1 direction [ ] 0.1 5 8 −0 1e 8 1e−0 ° 1e−10 −0.0003 direction [ ] 0.00 03 5e−0.0002 −005 6 5e−05 06 10 0.1 0.01 0.0 ° direction [ ] 0.0 5 0.5 ° direction [ ] ° direction [ ] 5e− 5e−05 5e−06 1e−05 00 0 08 ° 068 −−0 11ee direction [ ] 0.05 −0.001 e8− 1−010 e1−e .00e05 −−016− −006 −1 −1e−05 −5e 1e−−05 0.0001 0 0.41 directionally integrated 1e−06 1e− 1e−05 05 1e−05 0.06 0.10 0.16 frequency [Hz] 0.26 x 10 exact NN 2 2 1 0 −2 0.41 directionally integrated −3 3 nonlinear transfer rate nonlinear transfer rate nonlinear transfer rate 0.05 0.1 0.5 1 0 −1 0 10 0.41 005 0.26 e− 0.10 0.16 frequency [Hz] 1e−05 −1 0.26 −1 0.06 0.0 1 −4 0.41 −0.0001 0 .0 exact NN 4 −2 1e −0 1088 10 −1e−08 e0−−0 −11e 1e− 180 45 1 0.10 0.16 frequency [Hz] 3 0 225 8 0 e− exact NN 2 0.000 0 −1 x 10 0.26 90 1e −5 4 1e−05 −1e 06 −1e− −0e− 10 1e−10 −1 80 1 00 0.0 0 10 directionally integrated −4 x 10 5 001 0.0 005 0.10 0.16 frequency [Hz] 08 8 0.06 1e−0 −0 1e− −0 0 1e−06 0 1e−05 1e 0.41 1e −1−e10 −−1 1e 06e− −010 08 8 0.0 −1e−10 0.06 1e− 0.26 08 0.10 0.16 frequency [Hz] 1e−06 1e−08 5e−05 1e−05 1e−060 1e−10 1e−08 −1e−10 −1e−08 −1e−06 1e−10 1e−06 05 1e− 1e−06 270 8 0 −0 −1e− 110 1e0− 45 0.06 e− 135 1 −5e− 0.0 05 −−011e 1ee−−−10 −0e0185−e− 080e− 5−1 6 10 5 1e 315 1e−06 0 −1e−06 −1e−08 −1e−10 1e−08 90 0.0001 45 0 135 1e −1e−05 5e−06−5e−06 1e−05 5e−05 5e−1e 06−05 1e−06 −1e−05 −1e−06 1e−08 −1e−10 1e−10 0−1e−08 1e−06 1e−05 5 1e−10 −1e−06 −1e−10 −1e−08 1e−08 −1e 1e−0 1e−0 0 −05 6 0 −1−e −1 −111−eee0 e− −−−61001e8180 05 −e0−61 0 0 360 −1e− 1e− 08 0810 −1e− −1 e− 05 −1e− −1106 ee−− 100 8 08 005 00 nonlinear interactions from NN −1 05 −5e− 5 −0 1e −5e−−06 1e−06 6 5 −0 1e−10 0 0 −1 10e−06 −1e e− −1 02 .00 −0 5e 06 −05 8 −0 10 −1e− 1e −0.0 5 −1e−0 −1e −1e −1e −06 1e− −08 −10 081e−1 1e−0 60 0 1e−06 1e− 1e−05 5e−06 −5e−06 −1e−05 −5e 180 0 05 0 05 −1e− 1e−05 3 00 0.0 0001 0. 1e− 225 1e−1 1e−05 1e−05 5e−06 05 180 0.41 1e−06 −0 270 e− 6 6 −0 1e 0.26 0 1e 270 −5 −0 nonlinear interactions from NN 315 6 5e5−0 e−0 6 5 −0 1e e−06 −5−05 −1e −5e−05 −0.0002 −0.0003 1e−08 08 0.10 0.16 frequency [Hz] 0 05 6 −0 1e 0.06 0.41 .00 05 00 0 0e.60−011 −0.001 00 1 0 8 −−1.0 e−05 −0 −0 1005 05 −08 1e 1e −.0 −5e−05 0 −0 −1e− −1e 0.0 −1e−06 −1e−08 1e−10 0 −1e−10 1e−08 1e−06 1e−08 1e−10 −1e−10 01e−10 −1e−08 −1e−10 0 −1e−08 8 225 45 0 315 5e−06 3 06 360 90 1 −1e− e− 01 1e 1−e1 −00 6 −0 0.41 0.26 1e 90 nonlinear interactions from NN 135 5 05 −1 90 360 5e−0 6 05 1e− 0. 5e−05 00 0.0001 02 0.0 002 5e−06 5e−05 1 1e−08 06 e− −5 0.26 0.0001 0 0.0 8 0 e− 270 08−1e 1e−0 1e−10 −1e−1 − 0 1e −1−1 e− 010 0 1e−08 −0 05 −0 135 0.0 0.1 5 1 0.10 0.16 frequency [Hz] −1e− 1e 0.01 0.5 3 5 1e−0 1e−0 6 −1e 1e−08 1e−10 0 −1e−08 −06 −1e−10 e− 0 8 −1 1e−0−1e−e08 −1 0 0 360 1e−10 −1e−10 −1e−08 0 −1e−08 −1e−10 0 1e−10 1e−08 45 0.10 0.16 frequency [Hz] 0.06 315 −1 8 10 0 e− 01e−1 −1 e− −1 10 0 180 −06 1e 1e−08 − 1e 1e−06 5 5 1e−10 0 10 6 6 −0 −08 1e 1e1e−0 − 0 −506 −1e−0 −5e 1e−0 5e−06 −5e−06 −1e−05 05 e− 225 1e−06 2 00 5 −5 −5e−05 5 −5e−06 1e−05 −1e−05 −0.0002 −1 e−0 5e−05 e− 5 5e 0.0001 05 1 −0−1− e− 6 e5−e0 05 −50 5e−05 6 5e−06 1e−0 0 0.41 5 6 10 1e−06 −1 1e−1 −1e−08 1e 6 1e−0 05 1e− −0 45 180 0.10.05 0.5 1 exact nonlinear interactions 08 0 1e−1 05 0.0 5e− 1e 06 05 065e− −1−e− 0.0001 1e− −5e 05 −0.0002−5e−05 0.26 1e− 0 5e−06 1e−05 6 1e−055e−0 −1−0.0 00 5e−0002 1e5 0.0 −0 5e− 5 05 225 5 0.0 0.10 0.16 frequency [Hz] 270 0.06 3 90 0.1 0.06 1e−1 270 0 01 0. 1 exact nonlinear interactions 315 06 0.1 0.0 45 0 0.41 315 5e− 0.5 0.1 0.01 0.01 0.26 360 90 6 90 exact nonlinear interactions 135 0.01 5 5 0.01 45 0.10 0.16 frequency [Hz] 1e−05e−06 5 5e−0 0.0 5 00 1 5 0.0 0.1 5 135 0.01 0.1 360 180 180 0.5 0.1 0.5 5 6 1 0.0 0.1 1 0.1 0.05 0.01 225 0.0 0.5 0.5 0.06 1 0.05 0.01 0.05 1 3 0.0 45 0 135 5 3 0.05 0.0 1 0.1 65 10 1 1 0. 90 1 3 3 00.0.5 5 1 0.0 0.5 1 6 180 1 0.05 0.1 10 0.1 0.5 0.1 56 135 3 0.01 0.1 1 225 0.5 3 0.05 0.01 180 1 5 1 5 0.0 0.0 0.5 225 0.01 0.5 1 0.01 0.1 0.05 10 1 270 0.5 0.01 0.1 0. 1 5 3 6 10 0.05 0.05 225 1 5 0.0 3 270 0. 05 0. 270 315 0.1 5 0.0 315 0.5 315 0.1 0.01 0.05 5 0.01 6 original wave spectrum 360 6 360 1 0 −1 −2 0.06 0.10 0.16 frequency [Hz] 0.26 0.41 −3 0.06 0.10 0.16 frequency [Hz] 0.26 Fig. A.5. Examples of the performance of the NN for emulating the WRT method: upper row shows original wave spectra, next row the corresponding exact snl–term and third row shows its emulation by the NN. In the plots of the snl–term the black areas correspond to most negative values and the white areas to the biggest positive values. The gray areas inbetween cover ranges of four orders of magnitude for the snl–terms for either sign. The lower row shows the directionally integrated snl–terms. 19 0.41 AANN parametrisation 360 0.1 5 0.1 0.0 0.0 5 0.1 0.0 1 1 00.5.5 0.0 1 5 0.0 90 4 3 2 0.06 6 −0 5e −1 nonlinear transfer rate 1e−08 1e− 07 0.06 5 −0 1e 1e−06 0.10 0.16 frequency [Hz] 0.41 x 10 0.26 0.41 0 −0.5 −1 −1.5 6 −0 0 −1e−05 −5e−06 −1e−06 −1e−07 0 1e−08 e−−071 1 e−10 1e 0.26 0.5 1e−06 6 5e−0 e− 1e−06 −10e6 −1e−06 1e−07 1e−08 −1e−07 −05 5e 0.41 1 0.10 0.16 frequency [Hz] directionally integrated −4 −5 1e−06 1e−10 0.26 07 1e− 45 1e−06 6 1e−0 1e−08 0 −1e−07 −1e−08 8 −0 6 1e−07 1e−10 1e−0 6 1e−0 6 5e− 06 −1e−10 1e−− 10e6− 08−1 −5e−05 e−0 7 −0 7 1e−07 1e 0 −1 1e ° direction [ ] 135 1e−1 1e 06 −1e−−06 0.1 ° direction [ ] 0.5 0.1 0.1 00.0 .05 5 5e −0 5 −05 1e−08 −1e−10 −5e−0 −1e 6 1e−05 1e −0 6 6 −0 − −1e 5e− −06 06 1e−10 1e−06 5e −5e−05 −0 6 1e 08 07 1e− 1e− −1 0 1e 180 6 −0 1e 1e − −−00081780 −11ee −1e−10 1 1 0. 05 0.5 0. 1 0.0 1 ° direction [ ] ° 5 0 −1e−07 07 −1e−08 −1e−1 1e−10 e− 0 −1 −06 1e−0 −1e 8 1e−07 08 e− −1 06e−07 1e−06 8−−1 −0−e50701−6e0 10 −1−e5e−− 6 1e −0 5e 225 08 e8− −0 −10 1e 1e−1 7 −0 1e direction [ ] 0.41 08 10 1e−10 −1e− −1e− 8 1e−0 1e−0 0.10 0.16 frequency [Hz] 0.1 270 −10 −11ee−10 07 −1 e 1e−10 −1e−10 1e−08 1e−07 1e−07 1e−08 −1e−10 1e−10 −1e−08 −1e−07 7 6 0.26 5 −0 e −1 07 e− −1e −1−08 315 0 0−71 1−e 1−e 5 −0 −0 1e 1e−0 0.06 360 6 −0 1e 0 1e−08 10 1e− 45 0 1e−1 08 −10 08 e−−11e− e 90 −1 7 06 e−−008 5e−70−81−0−11e 0−e 1−−e1 1e −5e−06 −1e− 6 nonlinear interactions from NN −1 −5e− e− 06 1e−07 1e−08 1e−10 05 0 135 0.10 0.16 frequency [Hz] 1e 0 7 1 0.01 0.05 0 1e−1 180 1e−1 06 e− 08 1e−07 1e−05 −07 −106 1e− −1e− 5e−06 −1e 7 0.1 5e−06 6 −0 0.50.5 0.06 1e−1 −0 06 1e 5 05 0. 0.1 0 0.41 0 1e−1 7 0−0608 ee1−−e 81−011− −e0− 1−e1 5e 5e− 08 0.0 06 0078 1−10 1e−e− −1−1e 1e− 0.01 0.26 −1 e− −1 0 −1e−10 −10 1e−08 e− 1e 1e−07 1e708 −1e−10 1e−07 1e−08 −1−07 e− 06 −1e−07 −1e−08 1e−06 5e−06 1e−05 −5 e− 5e−06 5e−0 1e−07 1e−10 06 5 −1e−05 5 −0 1e 1 0. 0.05 0.05 0.01 8 −0 1e 7 −0 1e e− 270 225 −5 e− 06 1e 1e−−0 087 1 1 1e−07 1e−08 −−11−0 −1e5e −06 ee−−60 1708 −1 − 1e .5 00.5 0.0 exact nonlinear interactions 10 315 0.1 135 90 0.10 0.16 frequency [Hz] 360 1 0.5 0.5 0.01 0.05 0.1 0.05 0.1 0.5 0.06 0.05 0.01 45 0.05 0.05 .1 0.5 00.1 11 5 0.0 0.1 0.0 0 180 0.01 1 0.0 0.01 5 0.1 0.1 05 0.05 0.05 0.105 1 . 0. 0 1 00.0 .055 45 1 0.0 225 original AANN 8 0. 0.1 1 90 0.5 135 0.0 0.1 0.5 1 1 0.5 0.5 0.0 0.5 5 0.5 270 .055 00.0 0.1 0.01 180 3 1 0.05 0.05 0.01 225 1 0. 01.0 50.1 0.0 0.1 270 315 0.05.5 0.1 05 0. 3 0.1 directionally integrated 9 0.1 5 1 0. 0.55 5 0.1 .5 00.5 315 10 6 .5 1 .5 00 0.1 55 00.0.0 1 0.0 1 spectral energy density original wave spectrum 360 exact NN 1e−06 1e−0 8 0.26 0.41 −2 0.06 0.10 0.16 frequency [Hz] Fig. A.6. The upper panel shows the orinal wave spectra and the output from the AANN for a case where the error of the AANN is large. The lower panel shows the corresponding exact and NN emulated snl–term (contours as in figure A.5). 20