DescriptionWith modern HPC machines becoming increasingly diverse in architecture and vendors, the numerical and climate modeling community faces numerous challenges. However, these new systems also offer exiting possibilities by allowing for the first time to do climate simulations on km-scales, which enables the direct simulation of important processes like storms and ocean eddies. The accompanying code-complexity increase needs to be tackled to scale the code-development process. Several national and international projects are working on the modernization of climate models such as the ICON and IFS-FESOM models. As a first step directive-based approaches are applied to enable the models to be run on current pre-exascale systems, which allows us to present performance results on the pre-exascale systems LUMI and JUWELS Booster. To pave the way for the future and towards weather and climate digital twins, it is important to mitigate vendor lock-in and favor (performance) portability and scalable development. Therefore, we also present multiple aspects of the modernization of complex, critical code-bases at the example of climate simulations. As such, we believe that this Mini-Symposium will be useful not only to the climate community, but to many other simulation and application codes which all face similar challenges.