Re: 2 million lines of FORTRAN code
My (limited) understanding is that GPGPU hardware isn't good enough for interprocessor communication for high resolution weather forecasting to work well. This sort workload needs a lot of compute but also, inhernently, alot of comms between the compute. That's a major bottleneck.
You can bet it's been tried and tested, and it's an exciting avenue for research. For actual usage, not just yet, maybe next time.
Also, on FORTRAN, it's probalbly here to stay in HPC and scientific computing for a long time yet. A nice discussion is here: