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pySTEPS / pysteps

Python framework for short-term ensemble prediction systems.

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Power spectral density (PSD) with "rapsd" function

ZHANGZ1YUE opened this issue · comments

commented

First of all, thank you very much for providing this tool. I very much like it!

Just quick question regarding one of the function rapsd from utils.
1: According to the discription, it computes the radially averaged PSD for a 2d array. However, if I want to compute the PSD for a normal cartesian coordinate array (rather than a radial one), how should I implement it?
2: Regarding the output of this function, I am not really sure about the unit and type of each axis. Could you please tell a bit more about this? (Ex. the following figure)
1

Thank you very much!!

commented

Hello @ZHANGZ1YUE

1: According to the description, it computes the radially averaged PSD for a 2d array. However, if I want to compute the PSD for a normal cartesian coordinate array (rather than a radial one), how should I implement it?

The input to utils.spectal.rapsd can be a 2d cartesian field. In this case, you must pass the fft_method argument, e.g. you could pass np.fft. If fft_method is None (the default), then is it assumed that the input array is the Fourier transform of the original field.

2: Regarding the output of this function, I am not really sure about the unit and type of each axis. Could you please tell a bit more about this?

You can set return_freq=True in the above method to get the coordinates of the x axis. The y axis depends on the units of the input array: e.g., if the original field in is "mm" and 1 grid point corresponds to 1 km, the PSD will be in units of mm^2 / km.

Note 1: there is utility function to plot 1D spectra: https://pysteps.readthedocs.io/en/stable/generated/pysteps.visualization.spectral.plot_spectrum1d.html

Note 2: you can have a look a this example on how we typically produce and plot PSD for precipitation fields: https://github.com/pySTEPS/pysteps-publication/blob/415d0bb69e70fa072a06138778085ed829260894/plot_cascade_spectra_SPROG_MEAN.py#L173

Finally, please let us know if you see any opportunity to improve the documentation of the methods above.

commented

Hi @dnerini
Your explanation and the provided example can't be more helpful. I might have more question in future regarding the details but so far, it does plot what I expected.

Finally, please let us know if you see any opportunity to improve the documentation of the methods above.

Definitely! Once I have a deeper understanding about these functions, I'll let you know if I have any suggestions for the documentation.

Thank you very much.