The Fourier transform can be used to detect regularities in your rainfall data along the time axis. Suppose your data are the rainfall amounts in a time span of about fifty years. If you are now looking at the absolute value of the Fourier transform abs(fft(data)) of these data and you find the highest peak in the x-axis at x=3.25 then this is telling you that a higher amount of rain (compared to other years) is falling every 3.25 years.

A tutorial of how to use the FFT can be found for example here: http://research.stowers.org/mcm/efg/Report/FourierAnalysis.pdf

By using MATLAB, you can get the output you want. The matter is simple, firstly, set the value of (t) using the step value, specify your data, then take the absolute value of the FFT of the proposed data. Finally, use stem function to obtain the output waveform with respect to the value of (t).

Matlap draw anything

I add that with the fourier transform you can change from the spatial to the wavenumber space. That means that you can analyse the (possible) periodicity and the spatial distribution of your data. See this paper Article RainFARM: Rainfall Downscaling by a Filtered Autoregressive Model

If it is time series data, you can use R.