MATLAB: Calculate daily, monthly, annual averages from hourly data

daily averagedatenum

I have hourly data, I want to be able to create daily, monthly and annual averages while taking into account the bisexile years.

These are hourly data from a netcdf file. So I have a temperature variable and a time variable

Data are available from January 1, 2003 to June 30, 2017

For temperature and time variables

    Format:
           classic
Dimensions:
           time = 127080
Variables:
    tas_sirta
           Size:       127080x1
           Dimensions: time
           Datatype:   single
           Attributes:
                       units         = 'Â°C'
                       long_name     = 'air_temperature'
                       standard_name = 'air_temperature'
                       name          = 'tas'
                       dim           = 'time'
    Format:
           classic
Dimensions:
           time = 127080
Variables:
    time
           Size:       127080x1
           Dimensions: time
           Datatype:   single
           Attributes:
                       long_name     = 'date_time'
                       units         = 'hours since 1970-01-01'
                       actual_range  = '289272.0, 416351.0'
                       standard_name = 'time'
                       name          = 'time'
                       calendar      = 'standard'
                       dim           = 'time'
                       missing_value = NaN

I transform dates with datenum

timee = double(time)/24 + datenum('1970-01-01 00:00:00');
datestr(timee(1:7,1),'dd-mm-yyyy HH:MM:SS')

J'obtiens bien

    ans =
01-01-2003 00:00:00
01-01-2003 01:00:00
01-01-2003 02:00:00
01-01-2003 03:00:00
01-01-2003 04:00:00
01-01-2003 05:00:00
01-01-2003 06:00:00

but I do not know then to make the daily, monthly and annual averages taking into account all the specificities of a year (February with 28-29 days, month with 30, 31 days …)

thank you

Best Answer

The simplest way to do what you want is to convert your data into a timetable:

hourlytemp = timetable(hours(time) + datetime(1970, 1, 1), tas_sirta, 'VariableNames', {'air_temperature'});

Then retime that timetable to monthly, daily, yearly or whatever period you want:

monthlyaverage = retime(hourlytemp, 'monthly', 'mean')

Done!

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MATLAB: Timezone to UTC: How to add duration vector to datetime vector

Frank, what's up is Daylight Saving Time, which occurred on March 8th this year. I'm guessing you knew that, but got mixed up in the time arithmetic. Here's what's going on. You have this:

>> time = ['2015/03/08 00:00';'2015/03/08 01:00';'2015/03/08 03:00';'2015/03/08 04:00'];
>> Time = datenum(time,'yyyy/mm/dd HH:MM')
Time =
                    736031
          736031.041666667
                736031.125
          736031.166666667

datenums don't know anything about time zones or DST. Those times mean whatever you want them to mean. Since you then do this ...

>> date = datetime(Time,'ConvertFrom','datenum','Timezone','America/Montreal')
date = 
   08-Mar-2015 00:00:00
   08-Mar-2015 01:00:00
   08-Mar-2015 03:00:00
   08-Mar-2015 04:00:00

... I assume you mean that those datenums are to be interpreted as times in the America/Montreal time zone. But what you've left out is this:

>> date.Format = [date.Format ' z']
date = 
   08-Mar-2015 00:00:00 UTC-5
   08-Mar-2015 01:00:00 UTC-5
   08-Mar-2015 03:00:00 UTC-4
   08-Mar-2015 04:00:00 UTC-4

The second and third times are only an hour apart. Then this ...

>> [UTC_Offset,dst] = tzoffset(date);
>> Date = date - UTC_Offset
Date = 
   08-Mar-2015 06:00:00 UTC-4
   08-Mar-2015 07:00:00 UTC-4
   08-Mar-2015 07:00:00 UTC-4
   08-Mar-2015 08:00:00 UTC-4

... adds 5 hours to the first and second times, but adds only 4 hours to the third and fourth times. But you're doing the addition in the America/Montreal time zone. Adding 5 hours to the first two puts them after the DST shift @2am, which makes them look like you added 6 hours. In other words, your array is not UTC, it's shifted America/Montreal. The easiest thing to do to get UTC times is this:

>> date.TimeZone = 'UTC'
date = 
   08-Mar-2015 05:00:00 UTC
   08-Mar-2015 06:00:00 UTC
   08-Mar-2015 07:00:00 UTC
   08-Mar-2015 08:00:00 UTC

Hope this helps.

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.

Best Answer

Related Solutions

MATLAB: Timezone to UTC: How to add duration vector to datetime vector

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

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