MATLAB: Making a monthly mean

datameanmonthly

I have data for the geopotential for every 6 hours over a whole month. I want to get a mean over the data for the month. For example for January, I have 124 timesteps, each with their data, but I only want data for the whole month (1 dataset). I have tried, but I'm not sure if I've done it right? Pleas can anyone help me?

 lon = ncread(filename,'longitude') ; nx = length(lon) ; 
lat = ncread(filename,'latitude') ; ny = length(lat) ;
time = ncread(filename,'time') ; nt = length(time);
zmean = zeros([nx ny]);
for n = 1:nt
    z = ncread(filename,'z',[1 1 nt],[nx ny 1]); 
    zx(:,1:ny) = z(:,ny:-1:1);
    zmean = zmean + zx;
end
zmean = zmean/nt;

Here is (lon = 360×1), (lat = 181×1), (time = 124×1 int32), (z = 360×181)

Best Answer

Use datetime with retime . Upload data for details.

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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: Hi, i have two dates like 2013_12_28 and 2014_01_06, and i want to calculate the mean date (2014_01_01) which would be in the form of 2014.0014

md = mean(datenum({'2013_12_28', '2014_01_06'}, 'YYYY_mm_dd'));
dv = datevec(md);
days_into_year = md - datenum([dv(1),1,1]);

But once you have days_into_year you have to convert that into a year fraction, which requires careful definition of how long a "year" is. Is a "year" 365 days, always? Is a "year" 365.2421875 days? Is a "year" 365 days for non-leap years but 366 days for leap-years? If so then should it be 365 days for dates in the year before the leap day but 366 for dates on or after the leap day?

Converting to a specific date is much easier, and getting the result in terms of days is easy, but fraction of a year needs clarification.

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MATLAB: How to extract seasonal means

MATLAB: Hi, i have two dates like 2013_12_28 and 2014_01_06, and i want to calculate the mean date (2014_01_01) which would be in the form of 2014.0014

Related Question