A shallow-deep unified stochastic mass flux cumulus parameterization in
the single column Community Climate Model
Abstract
Cumulus parameterization (CP) in state-of-the-art global climate models
(GCM) is based on the quasi-equilibrium assumption (QEA), which views
convection as the action of an ensemble of cumulus clouds, in a state of
equilibrium with respect to a slowly varying atmospheric large-scale
state. This view is not compatible with the organization and dynamical
interactions across multiple scales of cloud systems in the tropics and
progress in this research area was slow over decades despite the widely
recognized major shortcomings. Novel ideas on how to represent key
physical processes of moist convection-large-scale interaction to
overcome the QEA have surged recently. The stochastic multicloud model
(SMCM) CP in particular mimics the dynamical interactions of multiple
cloud types that characterize organized tropical convection. Here, the
SMCM is used to modify the Zhang-McFarlane (ZM) CP by changing the way
in which the bulk mass flux is calculated. This is done by introducing a
stochastic ensemble of plumes characterized by randomly varying
detrainment level distributions based on the cloud area fraction (CAF)
of the SMCM. The SMCM is here extended to include shallow cumulus clouds
resulting in a unified shallow-deep CP. The new stochastic multicloud
plume CP is validated against the control ZM scheme in the context of
the single column Community Climate Model of the National Center for
Atmospheric Research using six test-cases including both tropical ocean
and midladitude land convection. Some key features of the SMCM SP such
as it capability to represent the tri-modal nature of organized
convection are emphasized.