Modelling convective processes during the suppressed phase of a Madden–Julian oscillation: Comparing single-column models with cloud-resolving models

The role of convective processes in moistening the atmosphere during suppressed
periods of the suppressed phase of a Madden–Julian oscillation is investigated in
cloud-resolving model (CRM) simulations, and the impact of moistening on the
subsequent evolution of convection is assessed as part of a Global Energy andWater
Cycle Experiment Cloud System Study (GCSS) intercomparison project. The ability
of single-column model (SCM) versions of a number of state-of-the-art climate
and numerical weather prediction models to capture these convective processes is
also evaluated. During the suppressed periods, the CRMs are found to simulate a
maximum moistening around 3 km, which is associated with a predominance of
shallow convection. All SCMs produce adequate amounts of shallow convection
during the suppressed periods, comparable to that seen in CRMs, but the relatively
drier SCMs have higher precipitation rates than the relatively wetter SCMs and
CRMs. The relatively drier SCMs dry, rather than moisten, the lower troposphere
below the melting level. During the transition periods, convective processes act to
moisten the atmosphere above the level at which mean advection changes from
moistening to drying, despite an overall drying effect for the column. The SCMs
capture some essence of this moistening at upper levels. A gradual transition from
shallow to deep convection is simulated by the CRMs and the wetter SCMs during
the transition periods, but the onset of deep convection is delayed in the drier SCMs.
This results in lower precipitation rates for these SCMs during the active periods,
although much better agreement exists between the models at this time.