Hi James,

That is a detail that we probably should have noted in our paper. The decision to perturb humidity was arbitrary.

I think that it would be reasonable to perturb any or all of the boundary condition (BC) fields by some small amount. In our case, we used humidity because it was the first field that we decided to perturb when trying to demonstrate that sensitivity experiments can produce large and coherent precipitation anomalies that are nothing more than 'model noise'. The humidity perturbation experiments demonstrated this thoroughly, so we never went beyond perturbing humidity in our study.

I'm not sure I have great advice for how to choose which field(s) to perturb. Since we were trying to demonstrate that RegCM is quite sensitive to boundary conditions, it served well to perturb only one BC field by a tiny amount. However, we had no a priori reason for choosing humidity. In retrospect though, I wonder if it might be a better choice to perturb all the BC fields by a small amount if you're trying to sample the range of model states that can result from a given set of (uncertain) boundary conditions. Then the problem would be to decide how large the perturbation should be for each BC field--this likely depends on the field and on the estimated uncertainty in the boundary conditions being used for the simulation. As far as I'm aware, this isn't something that has been rigorously studied.

Cheers,

-Travis-

Travis O'Brien

Lawrence Berkeley National Lab

http://esd.lbl.gov/about/staff/travisobrien/

(510) 495-8047

On Mon, May 6, 2013 at 3:39 PM, James Ciarlo` <james.ciarlo@physics.org> wrote:

Is there a reason why you chose to perturb only humidity and not temperature and or surface pressure for instance?

On 3 May 2013 17:31, James Ciarlo` <james.ciarlo@physics.org> wrote:

That is actually very helpful. Thanks :)

On 3 May 2013 16:16, Travis O'Brien <travis.obrien@gmail.com> wrote:

Hi James,

The ensemble_run flag is one of the parameters that activates the ensembling system. This is a relatively new feature that is described in the latest version of the RegCM UserGuide, Section 6.1.7.

When the ensembling system is activated, one or more of the boundary condition fields are perturbed by a small fraction each time they are read in. For example, the following settings would use the ensembling strategy of O'Brien et al. (2011), which is the main reference that describes the details of the ensembling method:

&globdatparam

ensemble_run = .true.

/

&perturbparam

lperturb_q = .true.

perturb_frac_q = 0.001d0

/

With these settings turned on, each time you run regcm, the output will be slightly different. So to create an ensemble of simulations, you might run RegCM five times, and each time change the parameter dirout to output.1, output.2, output.3, output.4, output.5, etc.

I hope this helps, and please let me know if I can clarify anything.

Kind Regards,

-Travis-

--
Travis O'Brien

Lawrence Berkeley National Lab

http://esd.lbl.gov/about/staff/travisobrien/

(510) 495-8047

On Fri, May 3, 2013 at 2:51 AM, James Ciarlo` <james.ciarlo@physics.org> wrote:

Dear RegCNET,

I am interested in running an ensemble expleriment with RegCM4. My intention was to choose different settings for the different simulations.

I have noticed the setting: ensemble_run = .false., in the globdatparam. This obviously needs to be set to true, but since I have never done an ensemble run before with RegCM4 I was wondering if there was anything I needed to know.

Regards,
James

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