The analysis is performed on a global simulation of the Icosahedral Nonhydrostatic model (ICON) on a model grid R2B10 with an approximate horizontal resolution of 2.5 km. The data cover the period from 23 to 29 January 2020 with an output time step of 10 min. Due to the high resolution, no parametrisation of GWs is applied. We used the model output interpolated to the vertical level of 15 km.

For assessing the local effects, the data are divided into subdomains defined as the cells of the ICON R2B4 grid (160 km resolution). In this way, we are estimating the effect that would be missing if only a low-resolution model was applied instead of using the R2B10 grid, in the range of wavelengths admissible by the analysis and excluding the interaction of scales. Although we are missing some GWs even in the high-resolution simulation , these waves, as well as waves removed during the analysis due to the effective resolution limit, have short wavelengths and therefore we do not expect a significant influence on the results in the wavenumber ranges studied.

The computation of the three-dimensional spectra at the subdomains is done in two steps, by transforming the data first in wavenumber and then in frequency domain. For each subdomain, the data are first used to compute the time series of horizontal spectra. Afterwards, 1D Fourier transform is applied to the resulting time series of horizontal spectral coefficients, adding the frequency dimension to the spectra.

The computation of the horizontal spectrum is more involved because the data are defined on a triangular ICON grid and the subdomains are triangular as well. With some modification, we apply the procedure of Least-squares Fourier fitting , used for a similar procedure for physical orography in .

For each subdomain, the computation is done in the local Cartesian system with the origin in the circumcentre of the triangle, the x axis pointing to the east, and the y axis pointing to the north. The projection of the points from the high-resolution grid to this local system is constructed by projecting the points from the sphere to the plane defined by the vertices of the triangle.

In the first step, the field on the triangle is deplaned and tapered. For tapering, the 1D Tukey window with parameter α=0.2 and width being the height of the triangle is applied in the three directions perpendicular to the sides of the triangular subdomain, as illustrated in Fig. . This creates zero values on the sides and at the opposite apices. In the corners, the tapering is more effective, with the 1D Tukey window also applied with each adjacent side, which is very similar to standard 2D tapering methods in rectangular domains . The deplaning is done by subtracting plane fitted to the data in the triangle, using the linear approximation approach introduced in .

In the next step, the zonal and meridional wavenumbers of the spectra are defined. For this, the average horizontal resolution of the high-resolution data dx, as given by ICON, and the horizontal extent of the subdomains Lx and Ly, defined as the zonal and meridional sides of the smallest rectangle in which the subdomain can be inscribed, are used: 1ki=2πmiNxdx,mi=0,…,⌊Lx2dxMn⌋-1,2lj=2πmjNydx,mj=-⌊LydxMn⌋/2+1,…,⌊LydxMn⌋/2, where Nx=Lx/dx, Ny=Ly/dx and Mn is a parameter that allows the computation of only a part of the spectrum. Here, the symbol ⌊⋅⌋ represents the closest even number. To avoid computation of a large part of the spectrum below effective resolution, Mn=4 is used and we define the notation Ñx/2=⌊Lx/2dxMn⌋ and Ñy=⌊Ly/dyMn⌋. Since the data are real, we compute the horizontal spectrum only for positive zonal wavenumbers. After finding the horizontal spectrum with these wavenumbers, the part of the spectrum for the negative zonal wavenumber is completed by using the symmetry of the Fourier transform. In our data, Ñx and Ñy reach values around 20, based on the subdomain size.

With the wavenumbers defined by Eqs. () and (), the amplitudes of the Fourier transform are fitted, so that 3f(x,y)≈∑j=0Ñy/22f^0,j(r)cos⁡(ljy)-2f^0,j(i)sin⁡(ljy)+∑i=1Ñx/2-1∑j=-Ñy/2+1Ñy/22f^i,j(r)cos⁡(kix+ljy)-2f^i,j(i)sin⁡(kix+ljy)≡F[f^], where f^i,j(r) and f^i,j(i) are the real and imaginary components of the Fourier amplitudes f^=f^i,j(r)+if^i,j(i) and F is an operator mapping f^ to the approximation of the inverse Fourier transform, as defined by Eq. (). The fitting is achieved by minimisation of the functional 4J[f^]=‖f-F[f^]‖2+λ‖f^‖2. The regularizing term λ‖f^‖ is necessary to avoid overfitting. The minimiser can be obtained by evaluating the Gateaux derivative of the functional J and setting it to zero, which leads to the equation 5(FTF+λI)f^=FTf, with f^ being represented by a 1D vector composed of the individual values of f^i,j(r) and f^i,j(i) and F by a matrix containing terms 2cos⁡(kix+ljy) and -2sin⁡(kix+ljy) from Eq. ().

The regularization parameter λ is chosen as 0.1 times the average of the diagonal coefficients of the matrix FTF. Equation () describes the real Fourier transform and its equivalence to the formula for standard Fourier transform with both negative and positive wavenumbers up to the choice of truncation limits, is shown in .

The temporal spectrum is computed from the horizontal spectrum coefficients by 1D FFT 6f^^(k,l,ωn)=∑r=0Nt-1f^(k,l,tr)e-iωntr, where Nt is the number of time steps tk considered for the transform and ωn=-2πn/T with T being the length of the time series. For the daily spectra of the data with a time step of 10 min, we have Nt=144. Note the negative sign in the definition of ω that allows us to write 7f(x,y,t)=1Nt∑i=-Ñx/2+1Ñx/2-1∑j=-Ñy/2+1Ñy/2-1∑r=0Nt-1f^^(ki,lj,ωr)ei(kix+ljy-ωrt) with the inverse transform 8f^^(k,l,ω)=1ÑkÑl∑i=-Ñx/2+1Ñx/2-1∑j=-Ñy/2+1Ñy/2-1∑r=0Nt-1f(xi,yj,tr)e-i(kxi+lyj-ωtr), where we already included the modes with negative zonal wavenumber and, to simplify interpretation of the results, modified the truncation limit to get a symmetric range of meridional wavelengths. Using this definition of the Fourier transform, the Parseval theorem is 9∑xyt|f(x,y,t)|2=ÑkÑlNt∑klω|f^^(k,l,ω)|2.

However, the constants Ñk and Ñl are not as well defined as in the case of the standard Fourier transform since our data are not equidistant in a regular x–y grid: we do not have a clear relation between number of points and number of wavenumbers. In addition, we take only part of the spectrum to save computation time (owing to the parameter Mn in Eqs.  and ). To solve this issue, we introduce a scaling factor for each of the variable modifying its spectral amplitude based on the Parseval theorem, which is applied on the spectra uniformly across scales. The factor basically compares the left-hand side and the right-hand side of the Parseval theorem for the horizontal transform. For example, for zonal velocity u, the scaling factor αu is defined as 10αu2=ÑkÑl∑|u^(k,l)|2∑u′2, where u′ is computed by subsequently removing an interpolated plane from the data for a given triangle, tapering it and removing an interpolated plane again. The scaling factors are computed using a single time step only, since the median scaling factor evaluated from all the triangles in a single time step does not change. The factor is applied during the computation of the horizontal spectra and the results obtained from them (i.e., the 3D spectra and their projection to GW modes) are therefore modified as well.

The procedure above is applied for the velocity components (u, v, w), the buoyancy (b=-gθ′/θ‾, where θ‾ is the average potential temperature over the level at the given time and θ′=θ-θ‾) and the pressure p, which will be needed for the next steps. For simplicity, we will further denote the full 3D Fourier amplitudes by a single hat symbol as f^.

For each triangle, the separation of the gravity wave part from the spectra is done as follows:

The vertical wavenumber is first calculated using the dispersion relation for both acoustic-gravity waves (the derivation of the formula is shown in Appendix ) 11msw,gw±=±ω^2cs2+kh2N2-ω^2ω^2-f2-14H2. The intrinsic frequency is defined by ω^=ω-ku‾-lv‾, where u‾ and v‾ are the zonal and meridional velocities averaged over the triangle and over the time range used for the spectrum; kh=k2+l2, N is the Brunt–Väisälä frequency, and f is the Coriolis parameter. The scale height H is calculated from the averaged temperature (averaged over triangle and time) by H=RT‾/g and the speed of sound follows equation cs2=RT‾cp/cv with cp and cv denoting specific heat capacities at constant pressure and volume and R is the specific gas constant.

Next, we use the polarisation relations for gravity waves 12u^gw±=cs2N2(kω^+ilf)1Hθ-12H+imgw±ω2^-f2-cs2kh2b^gw±,13v^gw±=cs2N2(lω^-ikf)1Hθ-12H+imgw±ω2^-f2-cs2kh2b^gw±,14w^gw±=iω^N2b^gw±,15p^gw±=ρ0cs2N21Hθ-12H+imgw±(ω^2-f2)ω2^-f2-cs2kh2b^gw±. The parameter Hθ is computed as Hθ=Hcp/R and ρ0=ρ‾exp⁡z/H with ρ‾ evaluated from the averaged pressure and temperature as ρ‾=p‾/RT‾.

For vectors X1 and X2 of the shape X=(u^,v^,w^,p^,b^), we define the inner product 16X1,X2=12u^1u^2*+v^1v^2*+w^1w^2*+p^1p^2*cs2ρ‾2+b^1b^2*N2, where the asterisk ⋅* denotes the complex conjugate.

Next, we define a 5-dimensional vector Y=(u^gw±,v^gw±,w^gw±,p^gw±,b^gw±) and insert the polarisation relation to express it in the form Y=(Au,Av,Aw,Ap,1)b^gw±≡Ỹb^gw±, where Au, Av, Aw and Ap are the prefactors of b^gw± on the right-hand side of Eqs. ()–(). We can then project the computed spectra X=(u^,v^,w^,p^,b^) into the direction of this vector by 17Xproj=(X,Y)(Y,Y)Y=(X,Ỹ)(Ỹ,Ỹ)Ỹ, which ensures that the projected specra follow the polarisation relations for GWs. The theory behind the GW projection and the formula for the inner product is expanded in Appendix .

Only the first component of the resulting vector, which is the gravity wave part of the zonal velocity spectrum, is saved – the choice of the zonal velocity as a representative variable is arbitrary. The other components are connected by the polarisation relations.

We will aim at dividing the data into the geostrophic mode, two gravity wave modes, and the rest. The dispersion relation connected to the geostrophic mode is ω^=0, which means that the geostrophic modes can be separated by looking at the components of the spectra with this intrinsic frequency. Similarly, we can restrict the components to gravity waves by setting both the lower and upper limit on the intrinsic frequency ω^gw,min2≤ω^2≤ω^gw,max2 with 18ω^gw,min2=f2,19ω^gw,max2=12f2+cs2kh2+14H2-14f2+cs2kh2+14H22-cs2N2kh2+f24H2, as derived from the dispersion relation in Appendix  from the extremal vertical wavenumber values m→inf⁡ and m=0, respectively. As for the gravity waves, we further analyse parts of the spectra with the intrinsic frequency inside the limits only. In addition, we also include an upper limit on the vertical wavenumber 2π/2L, where L=453 m is the thickness of the model layer at the altitude studied. This ensures that we do not include in the analysis wavelengths that cannot be resolved by the model, although they can be obtained from the dispersion relation.

The study of spectra variability further focuses on the wave action density after interpolation to the klm grid. The total energy of the projected gravity wave modes can be defined as 20Egw=ρ‾4|u^gw|2+|v^gw|2+|w^gw|2+|b^gw|2N2, where the first three summands correspond to the kinetic energy and the last one to the potential energy. The wave action density is then expressed as 21N=Egwω^. The wave action density is conserved along wave propagation paths unless the wave dissipates or is being generated, and is therefore useful for predicting gravity wave propagation.

As the methodology of spectrum computation and projection on GW modes introduced in the paper work on the subdomains independently, we will first demonstrate it on zonal velocity component at 15 km altitude for two arbitrarily selected subdomains on 23 January 2020 (00:00 UTC for horizontal spectra).

First, we will look at a subdomain at 65° N and 47° E, a lowland region near the Kanin Peninsula, representing here a region without distinct orography that could cause significant GW activity (see terrain height in Fig. a). The zonal velocity component in the triangular region is depicted in Fig. b. Although there are no strong wave imprints, there are some perturbations that could represent wave crests and troughs with the orientation between the south-east and the north-west. This is then correctly captured in the horizontal spectrum in Fig. c, which highlights the NE-SW direction. Apart from the directionality, the spectrum contains larger values for longer wavelengths, decreasing towards the shorter wavelengths. When averaged over the bands of horizontal wavelength size across all directions, the slope of the spectrum is close to the -5/3 slope (Fig. d). However, it should be noted that the 1D spectrum in Fig. d is not averaged over time to account for the entire wave periods, as is usual for the 1D spectra, so some deviations from the slope are expected. The spectra averaged over larger regions are shown in Appendix .

If we take 24 h evolution of the horizontal spectra at the triangular subdomain and apply the Fourier transform in time, we arrive at the 3D spectrum with a cross-section for l=0 shown in Fig. e. There are again large values of spectral amplitude for long wavelengths and long periods, which is consistent with previous studies e.g.,. The quantity shown in the figure is the spectral density of the zonal velocity perturbation, the spectrum of kinetic energy density would be, however, very similar. The spectrum in Fig. e is symmetric with respect to the change of sign k,l,ω → -k,-l,-ω, which is a general property of the Fourier transform of real fields. The low values for k=l=0 are caused by the deplaning of the data.

The 3D spectrum is then projected to GW modes using the projection described in Eq. () and limited to the range of intrinsic frequencies described by Eqs. () and (). The spectrum projected onto the GW mode with positive wavenumber is shown in Fig. f. The basic structure of the spectrum is similar to that of the original 3D spectrum. However, due to the projection, the exact symmetry of Fig. e is no longer present. Since the spectrum is now a function of the vertical wavenumber through the selection of the gravity mode as well, there is a similar symmetry with respect to the change of sign k,l,m,ω→-k,-l,-m,-ω. This implies that the full spectrum for m>0 already contains all information, since the projection to the other mode can be obtained by the coordinate transformation, which makes it sufficient to study only the mode with positive vertical wavenumber. The representation by positive m and both signs of frequency and horizontal wavenumber is chosen to simplify discussion of the results. Other representations, such as using positive frequency and both signs of the wavenumbers, carry the same information and it is possible to switch to them using the symmetry of the spectra.

The projected spectrum is divided by two planes, ω^=±f, represented in Fig. f as the two slanted lines. As linear GW theory implies the limit on intrinsic frequency |f|<ω^ (Eq. ), there cannot be GWs between those two planes, hence the empty spots in the plot. Furthermore, since the sign of the vertical group velocity component of the GW corresponds to the sign of -m/ω^, we can see that the part of the spectrum where the intrinsic frequency is positive represents downward propagating waves and vice versa. In Fig. f, this means that the parts of the plot above and below the slanted lines are downward and upward propagating waves, respectively. By looking at the plot, it can be therefore noted that at least for the selected subdomain, altitude and day, the upward propagating waves are stronger compared to the downward propagating waves. Numerically, upward propagating waves in this case cause 58 % of the energy of the zonal velocity ∫|u^|2dkdldω. However, please note that this percentage is for the selected subdomain only. The global distribution of the percentage of upward propagating wave energy will be presented in Sect. .

Another region of missing values in the projected spectrum is the column of k=l=0, which also does not describe GWs, as the dispersion relation for these wavenumbers does not result in a real vertical wavenumber. Finally, the blank space in Fig. f is also around the ω^=±f planes for larger k and l. These values are removed because the vertical wavenumber given by the dispersion relation is in this region higher than what could be present according to the vertical resolution of the model.

The same plots for another subdomain are shown in Fig. . This subdomain lies on the Scandinavian Peninsula at 62° N and 7° E, which is known for exciting orographic GWs and thus provides a potentially more complex wave field. This is directly visible in Fig. b, where there are more small-scale perturbations of the wind compared to Fig. b for the previous subdomain. As in the previous case, the direction of the perturbations is visible in the directionality of the horizontal spectrum in Fig. c. The horizontal spectrum is more anisotropic compared to the previous case, although the 1D spectrum in Fig. d still points to a decreasing magnitude for decreasing wavelengths, with the slope oscillating around the slope -5/3.

The most noticeable difference between the spectra in the two subdomains is visible in the 3D spectra, especially after the GW projection (Fig. f). For the second subdomain, we do not see that the spectral values decrease with frequency and zonal wavenumber as in Fig. f due to much higher values around the extrinsic frequency ω=0. This means that the majority of the waves on the subdomain are stationary, which is typical for orographic GWs. Again, we see higher values in the right part of Fig. f, suggesting the dominant upward propagation of the waves. In this case, the percentage of energy for the upward propagating waves reaches 82 %, which is reasonable for the waves generated mostly by the orography.

The GW spectra presented in the previous section are informative about the wave type in individual subdomains. Now, we will extend our analysis to the global imprint of GWs.

In Fig. a, we show the distribution of the total energy computed from the GW spectra using Eq. () for a single day of the simulation. The most significant areas with high energy are the orographic regions (e.g., Andes, Rocky Mountains, Alaska Range, Greenland, Island or Himalayas) and the regions of the Southern Hemisphere connected to the subtropical convection. Apart from that, we also see GWs with other sources in the mid-latitudes and above the equator. The magnitude of the energy is, however, much weaker – note the logarithmic axis of the plot.

The energy can also be computed from the 3D spectra before the GW projection. Although such spectra should in principle describe all processes in the data, we again see only small scales because the size of the domains being around 200 km at most. Therefore, the global distribution of the energy is very similar to the distribution in Fig. a, mainly highlighting regions with orography.

The fraction between the energy computed from the GW spectra and the energy computed from the total 3D spectra is visualised in Fig. b. In the plot, we can distinguish between regions where the GW projection removes the majority of the spectra (low values of the fraction) and where the large part of the spectra projects well onto the GW modes (high values). The most noticeable result is the decrease in the spectra in the subtropical jet regions, especially in the Northern Hemisphere. At these regions, the jet introduces wind gradients that result in larger Fourier coefficient amplitudes at the subdomains, increase the 3D spectrum, but do not describe the GWs. However, the largest ratio between the energies is in the Alaska Range or Franz Josef Land. Nevertheless, the fraction is still not one, which, especially because of the logarithmic character of the gravity wave energy, means that we are discarding a noticeable part of the spectra by the GW projection. Apart from the wind perturbations at the subdomains that are not caused by GWs, the GW projection would also dismiss nonlinearities due to the applied linearized theory. This could be the reason for the decreased amplitudes after the projection, especially for the stronger orographic GWs.

As shown in Sect. , the GW projection method also allows us to distinguish between upward and downward propagating waves. For the day shown in Fig. a, the fraction of total energy of the upward propagating waves is visualized in Fig. . At most places, 70 %–80 % of the wave energy propagates upward, highlighting the importance of tropospheric wave sources. A lower percentage is present in two regions, where the values decrease even below 50 %. First, the fraction is generally lower above the tropical Pacific and Atlantic ocean. This is likely connected to the fact that the tropopause is in the tropics higher than the studied altitude and the spectrum can therefore be dominated by convective GWs coming from above. Second, the figure shows significant downward propagating waves at the northern-hemispheric polar regions, especially above the Chukotka region, possibly originating at the edge of the polar vortex in the absence of strong tropospheric sources.

In Fig. , we show the zonal mean distribution of the energy from Fig. a. The orographical GW hotspots are clearly visible as the strong peaks in the plot. The gravity wave energy is higher at the Northern Hemisphere, since the wind is stronger at the winter hemisphere (see the red line in the figure). Although the individual hotspots in the Northern Hemisphere produce lower GW energy compared to the southern Andes region, there is more orography in the Northern Hemisphere, increasing the total zonal-averaged energy. An interesting region is the band southward from the equator, where the zonal mean zonal wind is close to zero. In that region, there is no distinct orography. The local maximum of the GW energy is therefore most likely linked to the convectively generated GWs above the subtropics and near the Intertropical Convergence Zone (ITCZ).

Lastly, we will look at how the shape of the spectrum differs between the subdomains, which is a key factor in assessing how simplified a GW parametrisation can be. Due to the meridionally dependent wind structure and distribution of GW sources such as orography or convection, we anticipate differences among latitudes. Therefore, we divide the subdomains into 10 groups of 2048 subdomains based on latitude. As the equatorial region contains significantly more grid cells along the parallel lines, the groups closer to the equator are narrower compared to the groups closer to the poles. The latitudinal ranges for these groups are listed in Table  and visualised in Fig. .

In each group, we interpolate the GW spectra of the wave action density onto a constant klm grid and perform principal component analysis (PCA) to simplify these spectra. Based on the vertical wavenumber values computed from the dispersion relation, we select 12 positive and 12 negative vertical wavenumbers logarithmically spaced between the values corresponding to the vertical wavelengths 2.7 and 30.4 km. The PCA is applied log⁡(Ñ+1) for Ñ=N/(1 kg m² s⁻¹) instead of just N, as this focuses the analysis on the wavelengths for which the wave action density is largest while allowing changes in the order of magnitude. To have a sufficient number of samples for the PCA (2048 subdomains for 7 independent daily spectra), we reduced the number of points in k and l direction by half, so that the resulting klm grid contains approximately 2600 data points for each spectrum.

We ask the following questions:

Are there some differences in averages or principal components between the groups?

The first question gives an estimate on how the spectra launched by GW parametrisations should look like for different latitudes. The second question assesses the spatial variability of the spectra within the latitude ranges and the possibility to compress the spectra by PCA, so that the parametrisations can more easily encompass this variability.

First, we will describe the average spectrum of the wave action density. In general, the wave action density is maximal for long horizontal wavelengths and short vertical wavelengths (Fig. ). The wave action density maxima are slightly asymmetrical, with higher values for purely meridional waves compared to the solely zonal ones with the same horizontal wavelength.

When comparing the ten latitude groups, two different types of spectra are observed. For groups 4, 5 and 6, the spectra are relatively symmetric with respect to k and l (see group 4 in Fig. b). In contrast, the remaining groups are skewed in the direction of negative k (Fig. a, visible on the contours with lower wave action density). This is likely caused by the direction of the mean wind. The wind at the studied altitude at the higher latitudes is purely zonal, which influences the range of intrinsic frequencies observed in the data through the ku‾ term in the definition of intrinsic frequency. For the positive zonal wind u‾>0 and positive zonal wavenumbers k>0, the intrinsic frequencies ω^ decrease with increasing zonal wavenumber. This decrease then causes an increase of the vertical wavenumber following Eq. (). As a result, there are not enough subdomains with short zonal wavelength and long vertical wavelengths, causing the decrease in the values in the plot region.

Additionally, the skewness of the spectrum can be qualitatively explained by the distinct propagation behaviour of GWs from different sources. Since the propagation direction of orographic GWs is always generally opposite to the wind, more energy would propagate westward in the regions with orography and westerly winds. On the other hand, the mechanism of generation of convective GWs does not directly enforce a propagation direction, possibly leading to a more symmetrical spectrum.

The shape of principal components (PCs) from the PCA is also strongly influenced by the shape of the wave action density spectrum. The first three PCs for groups 1 and 4 are shown in Fig. . The equatorial cases with symmetric spectrum (group 4 in Fig. d–f) provide PCs with the most comprehensible meaning. The first two PCs determine the direction of the zonal and meridional wavenumber, respectively. The third component further modifies the distribution of the wave action density described by the second component. A similar representation of the first modes is also present for the other equatorial groups 5 and 6.

Regarding the extratropical groups, the situation is less clear, since the reduced number of available data in the regions of positive k effectively increases the variance there, which enforces the first few PCs to describe these spectral regions. However, in Fig. a–b, parts of the PCs modifying the horizontal spectra in the north-west and north-east directions can still be observed. Similar, usually oblique, horizontal directions are also observable in the remaining groups (not shown).

Most importantly, the PCA is effective in approximating the final shape of the spectra. Figure  shows how many PCs we need to explain a given percentage of variance for the individual groups. To obtain 80 % of the spectra variance, 50 PCs are sufficient for the majority of the groups. If the aim is to obtain 95 % of the variance, about 400 PCs are needed, which still means decreasing the number of values necessary to describe the spectra by a factor of 5.

Finally, the complexity of the GW spectra represented by the efficiency of the PCA in Fig.  also depends on the latitude. If we are interested in a basic shape of the spectra (80 % of the variance), the spectra show more complex structure in the middle latitudes, although they are relatively simpler at the poles and at the equatorial groups. The higher complexity in the mid-latitudes is most likely connected to the more complex, strong circulation caused by the subtropical jet. This is also in coherence with the fact that more PC are needed in the northern, winter hemisphere, where the circulation is also stronger. It should be noted here that the complexity does not depend on the strength of the GW field, as illustrated in Fig. , but rather on the zonal mean zonal wind. On the other hand, if the aim is to capture higher percentage of the variability, approximation of the polar and mid-latitude regions becomes more demanding. This implies that the polar group has highly variable spectra, although the general structure is likely determined by the wind.

The possible simplification of a selected spectrum from groups 1 and 4 using a limited number of modes is visualised in Fig. . When approximated with 20 PCs only (first column), the shape of the spectrum is already reasonably captured for both the examples. It is however smoother compared to the original spectrum, which can be improved by considering more PCs (second column for 200 PCs). The coarseness of the spectrum from group 1 complicates the approximation using PCA. Although the reconstructed spectra for both the groups do not look exactly the same as the original spectra even for the 200 PCs, the basic structure is preserved already by the first 20 modes and they could provide a reasonable approximation.