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In terms of uncoupled AGCM studies, I've seen people use periods within a decade (say, 5 years) as spin-up time, but I also see some people take tens of year as spin-up in some recent studies. So I'm thinking if it's related to the model resolution in different simulations. So does GCM spin-up time affected by the resolution they use? For example if the same model is run in different resolution, would the spin-up time be any different? Thanks!

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    $\begingroup$ Are these all global models? I'm not sure about resolution, but it would certainly be affected by the size of the domain. (I guess they must be global if climate models... I usually model a couple of hundred km at most...) $\endgroup$ – Semidiurnal Simon May 15 '14 at 9:50
  • $\begingroup$ @SimonW Climate models can be regional too (typically embedded in a larger global model imposing boundary conditions). $\endgroup$ – gerrit May 15 '14 at 15:10
  • $\begingroup$ @gerrit ah, makes sense :-) $\endgroup$ – Semidiurnal Simon May 15 '14 at 19:35
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Without spin-up time, our initial conditions would be blank or generic.

I guess the simple answer is "NO" spin-up time is not really affected by the resolution, unless the model resolution changes are so great that it takes much longer for diffusion / advection between cells to flush out. Though that wouldn't explain a 5 year change in spin up time unless you made drastic changes to a model with very large time steps. Increased model resolution could theoretically increase the necessary spin-up time by a small amount, if the changes in resolution were very large and the time steps used were coarse. However, in a reputable research setting the spin up time is chosen based on how long (in model time) it takes for things to get to an acceptable initial condition, and changes in resolution are usually not so extreme.

Complexities in the model can increase desirable spin-up time if they have long time-horizons before an acceptable initial condition is achieved. The spin-up time you choose for your model will influence the degree to which the initial conditions actually represent a realistic distribution of numerics across all parameters, assuming the model is adequately developed.

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  • $\begingroup$ Hi farrenthorpe, welcome to ES.SE! I'm not entirely sure how this answers the question... "more complex your model is, the more spin-up time you need to reach a realistic starting point" is the only part that seems relevant, but I'd be interested in the reasons for this. $\endgroup$ – Semidiurnal Simon Jun 4 '14 at 9:17
  • $\begingroup$ Hi Simon, what I was trying to get at is that increased model resolution may increase the necessary spin-up time by a small amount... but spin up time is really a function of what question you are trying to answer and how long (in model time) it takes for things to get to an acceptable initial condition. I guess the simple answer is "NO" spin-up time is not affected by the resolution, unless the model resolution changes are so great that it takes that much longer for diffusion / advection between cells to flush out. Though that wouldn't explain a 5 year change in spin up time. $\endgroup$ – farrenthorpe Jun 4 '14 at 19:40
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    $\begingroup$ Hi Simon, as for complexities in the model increasing spin-up time, that is really only true if those complexities have longer time-horizons before they even out into a nice value or cycle. The whole point of spin-up time is to get your model to an initial condition that best represents reality. Without spin-up time, our initial conditions would be blank. So, the longer the phenomena are that influence your study, the longer spin-up time might be needed. Hope that answers the question better! $\endgroup$ – farrenthorpe Jun 4 '14 at 19:44
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I would say that the biggest factor influencing spin-up time is whether the model includes a dynamically-coupled ocean component. A regional or even global atmosphere-only model, with a realistic initial state and forced by realistic boundary conditions, will reach equilibrium pretty quickly; certainly a decade or less. An ocean model may not reach equilibrium even after a thousand years, and in fact this can be seen in many of the CMIP5 model control runs.

It's probably also worth pointing out that while model drift is often attributed to incomplete spin-up in coupled models, often drift is more due to some kind of spurious energy 'leakage' in the model's energy balance, which the ocean component acts to restore. In this case, the model will never reach an equilibrium, 'drift free' state.

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