As a major mode of intraseasonal variability, which interacts with weather and
climate systems on a near-global scale, the Madden – Julian Oscillation (MJO) is a
crucial source of predictability for numerical weather prediction (NWP) models.
Despite its global significance and comprehensive investigation, improvements in
the representation of the MJO in an NWP context remain elusive. However, recent
modifications to the model physics in the ECMWF model led to advances in the
representation of atmospheric variability and the unprecedented propagation of the
MJO signal through the entire integration period.
In light of these recent advances, a set of hindcast experiments have been designed
to assess the sensitivity of MJO simulation to the formulation of convection. Through
the application of established MJO diagnostics, it is shown that the improvements
in the representation of the MJO can be directly attributed to the modified
convective parametrization. Furthermore, the improvements are attributed to the
move from a moisture-convergent- to a relative-humidity-dependent formulation
for organized deep entrainment. It is concluded that, in order to understand the
physical mechanisms through which a relative-humidity-dependent formulation
for entrainment led to an improved simulation of the MJO, a more process-based
approach should be taken. T he application of process-based diagnostics t o t he
hindcast experiments presented here will be the focus of Part II of this study
