Congestus mode invigoration by convective aggregation in simulations of
radiative-convective equilibrium
Abstract
This study examines how the congestus mode of tropical convection is
expressed in numerical simulations of radiative-convective equilibrium
(RCE). We draw insights from the ensemble of cloud-resolving models
participating in the RCE Model Intercomparison Project (RCEMIP) and from
a new ensemble of two-dimensional RCE simulations. About half of the
RCEMIP models produce a congestus circulation that is distinct from the
deep and shallow circulation modes. In both ensembles, congestus
strength is associated with large-scale convective aggregation.
Aggregation dries out the upper troposphere, which allows moist
congestus outflow to undergo strong radiative cooling. The cooling
generates divergence that promotes continued congestus overturning (a
positive feedback). This mechanism is fundamentally similar to the
driving of shallow circulations by radiative cooling at the top of the
surface boundary layer. Aggregation and congestus invigoration are also
associated with enhanced static stability throughout the troposphere.
Changes in entrainment cooling are found to play an important role in
stability enhancement, as has been suggested previously. A modeling
experiment shows that enhanced stability is not necessary for congestus
invigoration; rather, invigoration itself contributes to midlevel
stability enhancement via its impact on the vertical profile of
radiative cooling. When present, congestus circulations have a large
impact on the mean RCE atmospheric state; for this reason, their
inconsistent representation in models and their impact on the real
tropical atmosphere warrant further scrutiny.