I think the comments nearby about high communications bandwidth required by dense systems of equations are true -- basically they distinguish a tuned, integrated system sold by a vendor like Cray from a row of racks with commodity systems.<p>The point I wanted to add is that ECMWF, generally acknowledged to be the leader in NWP, also uses a pair of large Cray supercomputers (<a href="http:&#x2F;&#x2F;www.ecmwf.int&#x2F;en&#x2F;computing&#x2F;our-facilities&#x2F;supercomputer" rel="nofollow">http:&#x2F;&#x2F;www.ecmwf.int&#x2F;en&#x2F;computing&#x2F;our-facilities&#x2F;supercomput...</a>).<p>Note that the Cray XC30 uses Xeon processors and a custom interconnect.  The old ECMWF system used IBM Power chips.