Monthly average from time series with daily values

I have 4 years of daily data in one file.

sla = ncread('data.nc','sla');
lon = ncread('data.nc','lon');
lat = ncread('data.nc','lat');
time = ncread('data.nc','time');

sla           181x101x1461            213668328  double              
lat           101x1                         404  single              
lon           181x1                         724  single
time         1461x1                        5844  single

I want to calculate the monthly mean of the sla. The time vector comes out as

           21916
           21917
           21918
           21919
           ...
           23376
  %days since 1950-01-01

I want January to have 31 days, February to have 28/29 days, and so on. So I guess(?) I have to use

datestr(21916)
  ans = 01-Jan-0060

I have tried to read through answers to similar questions, but that only made me more confused. Please explain on a basic level. Tnx!

Answer 1

[Jan]

DV = datevec(double(time) + datenum('01-Jan-1950'));  % EDITED

Now DV is a [1461 x 6] matrix. Now:

MonthIndex = DV(:, 1) * 12 + DV(:, 2)

gives an index for each month. accumarray will do the rest.

[K_r1]

The datevec function only accepts double arrays. I get error bc my time vector is a single array.

Can I make it a double array?

[Jan]

See [EDITED] code. Are you surprised that the command "double()" converts to double format? Searching in Matlab's docs is a good idea, they are excellent.

Answer 2

[Chad Greene]

With downsample_ts,

t_daily = datenum(1950,1,1,double(time),0,0); 
[sla_monthly,t_monthly] = downsample_ts(sla,t_daily);

Answer 3

[Peter Perkins]

K_r1, if you have access to MATLAB R2014b, there are some new data types that make this more straight-forward:

>> dt0 = datetime('1950-01-01')
dt0 = 
   01-Jan-1950
>> dt = dt0 + caldays([21916:250:23376]')
dt = 
   02-Jan-2010
   09-Sep-2010
   17-May-2011
   22-Jan-2012
   28-Sep-2012
   05-Jun-2013
>> months = between(dt0,dt,'months')
months = 
   720mo
   728mo
   736mo
   744mo
   752mo
   761mo