  Planetary Climate Models (PCMs) are developed to explore planetary climates
other than the Earth. Therefore, the methods implemented need to be suitable
for a large diversity of conditions. Every planet with a significant atmosphere
has condensible cycles (e.g., hydrological cycle), which can play an essential
role in the planet's appearance and environment. We must accurately represent a
condensible cycle in our planet simulations to build a powerful planetary
climate predictor. OASIS is a 3D PCM capable of self-consistently representing
the main physical processes that drive a planet's environment. In this work, we
improve the representation of mass transport in OASIS, which is the first step
towards a complete and flexible implementation of a condensible cycle. We
implement an upwind-biased scheme on a piece-wise linear approximation with a
flux-limiter to solve the mass transport equation. We first benchmark the new
scheme on a 2D problem that confirms the superior properties of the new method
over the central finite-volume method in terms of performance, accuracy and
shape-preserving mass distribution. Due to the new scheme's less dispersive
nature, we do not have to apply any unphysical diffusion to maintain the model
stable. OASIS includes the new improved solver in the total mass and the
tracers (e.g., clouds/hazes and individual gas chemical species) transport. We
couple the new formulation with physical schemes and validate the new code on
two 3D simulations of an ocean Earth-like planet and an ocean tidally-locked
planet. The new OASIS simulations are robust and do not show any known problems
from the dynamics-physics coupling. We show that the two simulations capture
the main characteristics of ocean planet atmospheres and are easy to set up. We
propose these two simulations to be the first standard benchmark tests for
models built to explore moist planetary environments.
