In atmospheric models with kilometer-scale grids the resolution
approaches the scale of convection. As a consequence the most energetic
eddies in the atmosphere are partially resolved and partially
unresolved. The modeling challenge to represent convection partially
explicitly and partially as a subgrid process is called the convective
gray zone problem. The gray zone issue has previously been discussed in
the context of regional models, but the evolution in regional models is
constrained by the lateral boundary conditions. Here we explore the
convective gray zone starting from a defined global configuration of the
Met Office Unified Model using initialized forecasts and comparing
different model formulations to observations. The focus is on convection
and turbulence, but some aspects of the model dynamics are also
considered. The global model is run at nominal 5km resolution and thus
contributions from both resolved and subgrid turbulent and convective
fluxes are non-negligible. The main conclusion is that in the present
assessment, the configurations which include scale-aware turbulence and
a carefully reduced and simplified mass-flux convection scheme
outperform both the configuration with fully parameterized convection as
well as a configuration in which the subgrid convection parameterization
is switched off completely. The results are more conclusive with regard
to convective organization and tropical variability than extratropical
predictability. The present study thus endorses the strategy to further
develop scale-aware physics schemes and to pursue an operational
implementation of the global 5km-resolution model to be used alongside
other ensemble forecasts to allow researchers and forecasters to further
assess these simulations.