|Title||Understanding advances in the simulation of intraseasonal variability in the ECMWF model. Part I: The representation of the MJO|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
|Authors||Hirons, LC, Inness, P, Vitart, F, Bechtold, P|
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 Cycle 32r3 of 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 Cycle 32r3 convective parameterization. Furthermore, the improvements are attributed to the move from a moisture-convergent- to a relative-humidity-dependent formulation for organised 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. The application of process-based diagnostics to the hindcast experiments presented here will be the focus of Part II of this study.