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Alternative model calendars

In the real world, the calendar year normally comprises 365 days and every fourth year 366 days because of the additional leap day on February 29. As the normal year is in fact 365.2425 days (365 days 5 hours 49 minutes 12 seconds) some further adjustments are done in the Gregorian calendar.

In the simplified world that climate models represent the Gregorian calendar of 365/366 days is not always used. For historic reasons some GCMs have been set up to have a ‘simpler’ calendar. Some models omit the leap day and use a calendar of 365 days. And a few models use a 360 day calendar in which each month is assumed to be 30 days.

From a technical point of view the GCM calendar is determined by the repetition period associated with the variation of solar radiation input. This repetition period can be set to any value but in this context the values 360 days, 365 days and 365.2425 days are relevant. The latter case results in that the GCM simulates the Gregorian calendar.

In a dynamical downscaling using an RCM the calendar of the GCM simulation is inherited.

Depending on the analyses one intends to carry out there are advantages and drawbacks with each of the model calendars. For a 360-day calendar analyses of monthly statistics are simplified but other analyses, such as comparing climate indices like the start of the growing season to observations, are more difficult. In some analyses the occasional leap day causes complication irrespective of whether it is model data or observations. And there is not standard best ways to handle these issues.

Read more:

  • Wikipedia on the Gregorian calendar.
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