  Large Eddy Simulation is a critical modelling tool for scientists
investigating atmospheric flows, turbulence and cloud microphysics. Within the
UK, the principal LES model used by the atmospheric research community is the
Met Office Large Eddy Model (LEM). The LEM was originally developed in the late
1980s using computational techniques and assumptions of the time, which means
that the it does not scale beyond 512 cores. In this paper we present the Met
Office NERC Cloud model, MONC, which is a re-write of the existing LEM. We
discuss the software engineering and architectural decisions made in order to
develop a flexible, extensible model which the community can easily customise
for their own needs. The scalability of MONC is evaluated, along with numerous
additional customisations made to further improve performance at large core
counts. The result of this work is a model which delivers to the community
significant new scientific modelling capability that takes advantage of the
current and future generation HPC machines.
