tag:blogger.com,1999:blog-9959776.post7985989717755945376..comments2024-02-15T04:42:41.606+00:00Comments on James' Empty Blog: Decadal prediction stuff part 1James Annanhttp://www.blogger.com/profile/04318741813895533700noreply@blogger.comBlogger5125tag:blogger.com,1999:blog-9959776.post-54304099268455248312013-03-29T23:02:00.416+00:002013-03-29T23:02:00.416+00:00Steve, I gave an example of a subproblem of the c...Steve, I gave an example of a subproblem of the climate problem where we have participated in proving rather conclusively that failure to improve numerics led to confusing and inconsistent results. I've given you references that are readily available. But not the ones that are in press. Those are available to responsible scientists on a case by case basis. If you have a technical comment of substance I'd be happy to entertain it. Non specific derogatory comments are not helpful in this context. David Younghttps://www.blogger.com/profile/17029429374522399227noreply@blogger.comtag:blogger.com,1999:blog-9959776.post-41560079002579040862013-03-29T21:41:53.496+00:002013-03-29T21:41:53.496+00:00Oh look, David thinks he's discovered another ...Oh look, David thinks he's discovered another nail.Steve Bloomhttps://www.blogger.com/profile/12943109973917998380noreply@blogger.comtag:blogger.com,1999:blog-9959776.post-32184031803942912942013-03-29T18:00:20.914+00:002013-03-29T18:00:20.914+00:00James, A definite maybe is fine. Definitive evid...James, A definite maybe is fine. Definitive evidence would be provided as you say by doing grid refinement studies, even though even that is not definitive. An instructive example of this is the NASA drag prediction workshop in which such studies were done in about 20 or 30 CFD codes. Initially, the results had a much bigger spread than expected, due to the striking positive results bias in the literature. So, the problem was made easier for further workshops. Finally, it was a trivial case and with SPECIFIED grids and a fixed turbulence model there was reasonable consistency. The short summary: It was much worse than we thought. There is one figure in the long paper I sent you on this and some discussion.<br /><br />My problem here in climate science is that it looks a lot like CFD in 1996 before people really got beyond the "results look good" and "the codes are useful" syndrome and actually looked at quantitative measures of these things.<br /><br />Web site for drag prediction workshops: http://aaac.larc.nasa.gov/tasb/cfdlarc/aiaa-dpw/<br /><br />It is somewhat humorous to watch the evolution of the "story" from DPW1 to DPW5. Bottom line, small effects (and some large ones too) are not robustly predicted by RANS codes and the large knob of gridding is rather close to "dial a drag". Of course, drag is a "small" effect with tremendous implications, rather like the CO2 effect in climate.David Younghttps://www.blogger.com/profile/17029429374522399227noreply@blogger.comtag:blogger.com,1999:blog-9959776.post-1430861047599016602013-03-29T11:00:55.557+00:002013-03-29T11:00:55.557+00:00Well, I have to give a definite maybe to that. I s...Well, I have to give a definite maybe to that. I see the point, but if you are right, then you'd expect to see a substantial change in characteristics as resolution changes. I don't think this is seen (though to be honest, I haven't looked into this much). The models do reproduce dynamical structures that look pretty good in terms of synoptic weather - hurricanes are about the limit, but that is well below the scale of structures relevant here.<br /><br />Also, it should be straightforward to show such differences using much simpler dynamics-only models (without the full physics that costs so much time and effort). People here certainly worked on numerical schemes while developing high resolution models for the Earth Simulator, but I don't know what came of it.<br /><br />I realise I'm caricaturing the "efficient market" economist joke, but if numerical methods alone made such an important difference, I reckon someone else would have done it by now :-)James Annanhttps://www.blogger.com/profile/04318741813895533700noreply@blogger.comtag:blogger.com,1999:blog-9959776.post-12002456908584913472013-03-28T15:40:03.901+00:002013-03-28T15:40:03.901+00:00Yes, what this seems to me to show is exactly what...Yes, what this seems to me to show is exactly what one would expect from too course a grid or too much dissipation, the spatial variationl is "damped." Think back to your fluid dynamics days, James. Remember what the course grid results looked like compared to the finer grid results vs the data. And remember how it was impossible to see the "details" in a simulation with 1st order upwinding instead of 2nd order.David Younghttps://www.blogger.com/profile/17029429374522399227noreply@blogger.com