Firedrake [1] and Fenics [2] seem like interesting approaches to this. I know that firedrake is being explored for some climate modelling with fluidity project [3][4].
The value of any simulation platform is based on the value of the models and datasets in that platform. So the key question is, can you attract the people making the valuable models to your platform. Working with things that people are already using seems important, as does talking to those people about it. I've got a Google form asking questions currently to try to get more information about what people who currently make and use simulations need. [5]
[1] https://www.firedrakeproject.org/
[2] https://fenicsproject.org/
[3] http://fluidityproject.github.io/
[4] https://www.archer.ac.uk/community/eCSE/eCSE06-01/IC15-ecse-...
[5] https://docs.google.com/forms/d/e/1FAIpQLSfH-ns1CSQlyVyo7oj_...