MATLAB: Grid data into monthly average

average monthlygrid datareshape

Hello All,

I have a grid data set for 32 years (384 months) in the form A=(180*360*384), I want to find monthly average for the January. that means average for 32 years for January. I tried to reshape the data however, its not working, can somebody help me out here please?

I am trying with this

R=reshape (A,12,32)

However, i am getting error.

Thanks

Best Answer

You are on right track:

Use this:

% Making some sample random data
A=rand(180,360,384);
% reshaping
Areshaped=reshape(A,180,360,12,32);
% Averaging along the 4th dimension (year)
Amean=mean(Areshaped,4);
size(Amean)
 180   360    12

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MATLAB: How to extract seasonal means

For those just coming here, you can see additional details and explanation here.

You seem to be grouping Jan, Feb and Dec from the same year together. I would be more inclined to group the winters as Dec yr1 with Jan yr2, Feb yr2. In doing so, I see the challenge being.

Grouping Dec from one year with Jan, Feb of the next
The first year will only have Jan, Feb while the last year will only have Dec.
If no data is removed, this means your will have data covereing 38 winters (Jan, Feb in last year as well as Dec in last year).

You asked what you were doing wrong. I don't think cumsum is doing what you want. The first value is itself. The second is the first plus the second. This pattern continues until the last value is the sum of all previous values plus itself. Instead, you want to sum each group separately.

Here's an approach that uses findgroups/splitapply

% Load data from nc file
lon=ncread('sstarc.nc','lon');
lat=ncread('sstarc.nc','lat');
sst=ncread('sstarc.nc','sst');
% Create vector of dates to use for extracting winter months
startDate = datetime(1982,1,1,"Format","MMM-uuuu");
endDate = datetime(2018,12,31,"Format","MMM-uuuu");
dates = startDate:calmonths(1):endDate;
winterMonth = month(dates) == 1 | month(dates) == 2 | month(dates) == 12;
% Extract winter data from sst
sstWinter = sst(:,:,winterMonth);
%% Create grouping variable for winter year 
% (treating consecutive Dec, Jan Feb as winter that year)
% Extract winter year as the year corresponding to January
winterYr = year(dates(winterMonth));
winterYr(month(dates(winterMonth))==12) = winterYr(month(dates(winterMonth))==12)+1;
% Find groups
G = findgroups(winterYr);
% Splitapply can't be used on 3D matrices. Reshaped to 2D to take the mean
sstTemp = reshape(sstWinter,size(sstWinter,1)*size(sstWinter,2),size(sstWinter,3));
f = @(x) mean(x,2,"omitnan");
tempYrMean = splitapply(f,sstTemp,G);
% reshape back to original number of rows, columns
sstYrMean = reshape(tempYrMean,size(sstWinter,1),size(sstWinter,2),[]);
%% Visualizing data
% Create variables for visualizing
[LAT, LON] = meshgrid(lat,lon);
winter1 = sstYrMean(:,:,1); % Change the index of the 3rd dimension to change the year
% Visualize the data for Winter 1982
geoscatter(LAT(:),LON(:),[],winter1(:))
colorbar(gca,"eastoutside")
title("Winter " + num2str(winterYr(1)))

MATLAB: Creating a monthly average from a matrix

Just

 avgAllYears = mean(KoppenA_prate, 3) ;

where 3 is the dimension along which the mean must be computed. With that you get a 90x180 array of means over all times.

EDIT:

 siz = size(KoppenA_prate) ;
 avg = zeros(siz(1), siz(2), 12) ;
 for mId = 1 : 12
    avg(:,:,mId) = mean(KoppenA_prate(:,:,mId:12:end), 3) ;
 end

EDIT2: or, more efficient but more complicated

 siz = size(KoppenA_prate) ;
 buf = mean(reshape(KoppenA_prate, siz(1), 12*siz(2), []), 3) ;
 avg = reshape(buf, siz(1), siz(2), []) ;

Note: both produce a 90x180x12 array of monthly averages, were e.g. avg(:,:,5) is the average of 5th months over all years.

Best Answer

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MATLAB: How to extract seasonal means

MATLAB: Creating a monthly average from a matrix

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