In Part I of this study it was shown that moving from a moisture-convergent- to
a relative-humidity-dependent organized entrainment rate in the formulation for
deep convection was responsible for significant advances in the simulation of the
Madden – Julian Oscillation (MJO) in the ECMWF model. However, the application
of traditional MJO diagnostics were not adequate to understand why changing the
control on convection had such a pronounced impact on the representation of the
MJO.
In this study a set of process-based diagnostics are applied to the hindcast
experiments described in Part I to identify the physical mechanisms responsible for
the advances in MJO simulation. Increasing the sensitivity of the deep convection
scheme to environmental moisture is shown to modify the relationship between
precipitation and moisture in the model. Through dry-air entrainment, convective
plumes ascending in low-humidity environments terminate lower in the atmosphere.
As a result, there is an increase in the occurrence of cumulus congestus, which acts
to moisten the mid troposphere. Due to the modified precipitation – moisture
relationship more moisture is able to build up, which effectively preconditions the
tropical atmosphere for the t ransition t o d eep convection. R esults from this study
suggest that a tropospheric moisture control on convection is key to simulating
the interaction between the convective heating and the large-scale wave forcing
associated with the MJO