  The cold ($\sim 10^{4}\,{\rm K}$) component of the circumgalactic medium
(CGM) accounts for a significant fraction of all galactic baryons. However,
using current galaxy-scale simulations to determine the origin and evolution of
cold CGM gas poses a significant challenge, since it is computationally
infeasible to directly simulate a galactic halo alongside the sub-pc scales
that are crucial for understanding the interactions between cold CGM gas and
the surrounding ''hot'' medium. In this work, we introduce a new approach: the
Cold Gas Subgrid Model (CGSM), which models unresolved cold gas as a second
fluid in addition to the standard ''normal'' gas fluid. The CGSM tracks the
total mass density and bulk momentum of unresolved cold gas, deriving the
properties of its unresolved cloudlets from the resolved gas phase. The
interactions between the subgrid cold fluid and the resolved fluid are modeled
by prescriptions from high-resolution simulations of ''cloud crushing'' and
thermal instability. Through a series of idealized tests, we demonstrate the
CGSM's ability to overcome the resolution limitations of traditional
hydrodynamics simulations, successfully capturing the correct cold gas mass,
its spatial distribution, and the timescales for cloud destruction and growth.
We discuss the implications of using this model in cosmological simulations to
more accurately represent the microphysics that govern the galactic baryon
cycle.
