The gray zone of
a physical process in numerical models is defined as the range of model
resolution in which the physical process is partly resolved, but yet needs to
be parameterized at the same time. Thanks to increase in computing power,
numerical simulations with the spatial resolution of a few kilometers or finer
have been realized and their advantages have been reported in recent decades,
while the problem of the gray zone problem in convective boundary layer (CBL)
has remained at the higher resolution.
In the first
half of this talk, a reference scale dependency is suggested for the
subgrid-scale (SGS) vertical transport in CBL, and its dependency on height,
variable, and CBL stability (i.e., u*/w*)
is discussed. The reference is considered as a true solution in the gray zone,
and scale-aware PBL parameterizations are expected to reproduce it.
In the second
half of this talk, a simple model to represent the SGS transport in the gray
zone is introduced, via a prescribed vertical transport profile and scale
dependency functions fitted to the Large Eddy Simulation (LES) results.