Within CAM5, there’s a handy namelist variable, “pertlim,” which is meant to apply random perturbations to the initial temperature field provided to the model. Typical values supplied should be on the order of 1e-14 (according to https://bb.cgd.ucar.edu/how-does-pertlim-work). Interestingly though, the perturbation is not “random” in the truest sense (also mentioned in the CGD Forum link). When trying to simulate multiple perturbed experiments, supplying the same value for pertlim in each will result in identical perturbations being applied, negating the effort. Instead, pertlim should be marginally incremented from one experiment to the next to get a truly diverse set of initial conditions.
[See my post on ensembles for an example array of pertlim values]
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