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Re: [ferret_users] MONTH_GREGORIAN climatological axis



Hi Patrick,

The basic issue comes down to: how does one define a "climatological February 29th?"

In the climatological_axes file, the climatological Gregorian year is defined so that 400 of them span the same time as 400 Gregorian years, which is the modulo span of the Gregorian calendar cycle. The average length of a Gregorian February is 28.2425 days -- so that is taken as the length of the climatological February.

As a result, you'll see gradual drifts and sudden jumps of the Gregorian climatological dates/times relative to the true Gregorian dates/times, reminiscent of the leap-shifting seen in the date of the solstices:

   http://en.wikipedia.org/wiki/Gregorian_calendar

The associated errors will be minor if you're working with monthly or annual means, but it could be problematic if you're looking at slow trends in the diurnal cycle. Unfortunately, the only "true" climatological cycle for the Gregorian calendar would be to define a climatological 400-year cycle -- complete with leap years every 4th year, except every century (unless the century is divisible by 400).

For observational applications (where the Gregorian calendar is most relevant) you won't have the many millennia required to define a robust climatological 400yr cycle. A way around this would be to (say) find the mean diurnal cycle for each month, and then analyze the behavior of that time series of "monthly-mean diurnal cycles".

Andrew

On Wed, 3 Nov 2010, Brockmann Patrick wrote:

Hi all,

Looking carefully  to the climatological axis of a gregorian calendar,
I don't understand why there an offset of 05:49:12  hours in
information given by TBOXLO
Though, TBOX is ok.

yes? use climatological_axes
yes? show axis/t MONTH_GREGORIAN
name       axis              # pts   start                end
MONTH_GREGORIAN TIME          12mi   16-JAN 12:00         15-DEC 17:49
T0 = 01-JAN-0000 00:00:00
 Axis span (to cell edges) = 365.2425 (modulo length = axis span)

     L     T                   TBOX      TBOXLO                TSTEP (days)
     1>  16-JAN 12:00:00       31         01-JAN 00:00:00         15.5
     2>  15-FEB 02:54:36       28.2425    01-FEB 00:00:00         45.12125
     3>  15-MAR 17:49:12       31         29-FEB 05:49:12         74.7425
     4>  15-APR 05:49:12       30         31-MAR 05:49:12         105.2425
     5>  15-MAY 17:49:12       31         30-APR 05:49:12         135.7425
     6>  15-JUN 05:49:12       30         31-MAY 05:49:12         166.2425
     7>  15-JUL 17:49:12       31         30-JUN 05:49:12         196.7425
     8>  15-AUG 17:49:12       31         31-JUL 05:49:12         227.7425
     9>  15-SEP 05:49:12       30         31-AUG 05:49:12         258.2425
    10>  15-OCT 17:49:12       31         30-SEP 05:49:12         288.7425
    11>  15-NOV 05:49:12       30         31-OCT 05:49:12         319.2425
    12>  15-DEC 17:49:12       31         30-NOV 05:49:12         349.7425

Any clarification is welcome.
Patrick




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