I was recently reading Zach Holman's blogpost on time and remembering that I've had to work with CMIP5 output that used a 360 day calendar, rather than a 365 day or a Greogrian (or Proleptic Gregorian) one.

Do models that store output in this calendar run on a more physically accurate timestep and then somehow interpolate to a different calendar, or are they actually simulating on a 360 calendar? If they're actually using a 360 calendar, how does that work? Is incoming solar radiation simply parameterised in an interpolated way across the year for 360 days of regular duration? I can't imagine that the days are stretched out to account for being 1.5% fewer.

Does any of this have a substantial impact on the results?


1 Answer 1


Models are set up with a calendar (length of year), and run using that calendar - there is, usually, no interpolation to any other calendar afterwards.

Yes, seasonal cycles in the forcing are adjusted to the relevant calendar - say 360 days.

  • $\begingroup$ Thanks! So does that mean, for example, that the days run longer to compensate? Does that have an effect on something like Diurnal Temperature Range due to more time for land heating over the day? $\endgroup$
    – jimjamslam
    Sep 29, 2021 at 5:30
  • 2
    $\begingroup$ No, the length of the day is the same in all these calendars - and as used in models. But the annual cycle length is adjusted to fit the years length. Your query applies there too of course - does the 360 year model have shallower annual cycles in e.g. ocean warmup? Is there a compensation for this? The answer is 'no compensation'. If it wasn't for all the other differences between climate models then the approx. 1.5% shorter model year should indeed make a small imprint on such things as the amplitude of the annual cycle in ocean heat uptake, for instance. Good question though! $\endgroup$ Oct 4, 2021 at 10:50
  • $\begingroup$ Thanks Peter, that's good to know! $\endgroup$
    – jimjamslam
    Oct 5, 2021 at 1:20

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