[RegCNET] RegCM Ensemble run

[Travis O'Brien]

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/<http://esd.lbl.gov/about/staff/travisobrien/index.html>
(510) 495-8047


On Mon, May 6, 2013 at 3:39 PM, James Ciarlo` <james.ciarlo at 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 at physics.org> wrote:
>
>> That is actually very helpful. Thanks :)
>>
>>
>> On 3 May 2013 16:16, Travis O'Brien <travis.obrien at 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<http://gforge.ictp.it/gf/download/docmanfileversion/54/973/UserGuide.pdf>,
>>> 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) <http://dx.doi.org/10.1007/s00382-010-0900-5>,
>>> 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/<http://esd.lbl.gov/about/staff/travisobrien/index.html>
>>> (510) 495-8047
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Fri, May 3, 2013 at 2:51 AM, James Ciarlo` <james.ciarlo at 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
>>>>
>>>> _______________________________________________
>>>> RegCNET mailing list
>>>> RegCNET at lists.ictp.it
>>>> https://lists.ictp.it/mailman/listinfo.cgi/regcnet
>>>>
>>>
>>>
>>
>
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