Units for define max, min and sampling frequency

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Luis Jesús Olvera Lazcano 2023-9-13

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评论： William Rose 2023-9-13

采纳的回答： William Rose

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I have been using a bandpass for my netcdf data, to have 30-80 day frequency.

Although Ive noticed until now that maybe Ive been putting the frequencies in the incorrect units:

frecuencia_min = 1 / 80; % Min frequency (cycle/day)
frecuencia_max = 1 / 30; % Max frequency (cycle/day)
% olr having 180x51x136, but for only an example olr has 1x1x136, being the
% last one the time dimension
olr_real(1,1,:) = bandpass(squeeze(olr_1(1,1,:)),[frecuencia_min frecuencia_max],1); % data is daily so fs=1
    

Are the units correct for the three frequencies?

Thanks! :)

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Answer 1

William Rose 2023-9-13

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@Luis Jesús Olvera Lazcano,

You call bandpass() as folows:

frecuencia_min = 1 / 80; % Min frequency (cycle/day)
frecuencia_max = 1 / 30; % Max frequency (cycle/day)
yfilt=bandpass(y,[frecuencia_min frecuencia_max],1);

This specifies that the sampling rate is 1 sample per unit time, and the filter cutoff frequencies are frecuencia_min and frecuencia_max cycles per unit time.

Therefore, if your data are sampled at 1 per day, then your cutoff frequencies are correctly specified.

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Luis Jesús Olvera Lazcano 2023-9-13

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Thank you very much!!

I have another question. Im doing the bandpass on 3D data as I mentioned before. Therefore I use a for cycle to filter each point of the grid as it follows

for i=1:length(lon)
    for j=1:length(lat)
        olr_real(i,j,:) = bandpass(squeeze(olr_1(i,j,:)),[frecuencia_min frecuencia_max],1); % data is daily so fs=1
        fprintf('[%d] y [%d]\n',i,j);
    end 
end