Nonhydrostatic ICosahedral Atmospheric Model (NICAM) is one of the most high-resolution global atmospheric models in the world. From the early 2000s, we performed global cloud-system resolving simulations without using convection parameterization. NICAM has been developed for more than fifteen years keeping pace with the development of supercomputers in Japan. Especially, the Earth Simulator (2003~) and the K computer (2011~) brought big computational resources to us and enabled larger-scale simulations. Integration of new components and optimization to new machines are continually going on. NICAM was chosen as one of the proxy application for evaluating Japanese next flagship supercomputer, post-K. The "co-design" effort for designs of the system is regarded as important in the post-K project. Through a performance evaluation of NICAM and the other proxy apps, parameters of the CPU architecture were considered. NICAM played an important role in the system software development such as compiler and the MPI implementation. In preparation for the coming exascale era, the development of NICAM is approaching a turning point. The transfer of a huge amount of data limits both the simulation and analysis. By introducing our efforts, the problems about capability, scalability, and performance portability are pointed out.