The recent "Neural General Circulation Models" pre-print [1] from Google Research indicates that the team built a spectral dycore from scratch using JAX; in Appendix A they note that it comes in at just over 20,000 lines of code! This model isn't quite "production grade" from the perspective of NWP (lacks microphysics and therefore any prognostic precipitation in the forecast), but it's the only project I can think of that has rapidly produced such a atmospheric model "from scratch" with a high degree of engineering rigor.

[1]: https://arxiv.org/abs/2311.07222