Understanding advances in the simulation of intraseasonal variability in the ECMWF model. Part II: the application of process-based diagnostics

摘要

In Part I of this study it was shown that moving from a moisture-convergent- touda relative-humidity-dependent organized entrainment rate in the formulation foruddeep convection was responsible for significant advances in the simulation of theudMadden – Julian Oscillation (MJO) in the ECMWF model. However, the applicationudof traditional MJO diagnostics were not adequate to understand why changing theudcontrol on convection had such a pronounced impact on the representation of theudMJO.ud In this study a set of process-based diagnostics are applied to the hindcastudexperiments described in Part I to identify the physical mechanisms responsible forudthe advances in MJO simulation. Increasing the sensitivity of the deep convectionudscheme to environmental moisture is shown to modify the relationship betweenudprecipitation and moisture in the model. Through dry-air entrainment, convectiveudplumes ascending in low-humidity environments terminate lower in the atmosphere.udAs a result, there is an increase in the occurrence of cumulus congestus, which actsudto moisten the mid troposphere. Due to the modified precipitation – moistureudrelationship more moisture is able to build up, which effectively preconditions theudtropical atmosphere for the t ransition t o d eep convection. R esults from this studyudsuggest that a tropospheric moisture control on convection is key to simulatingudthe interaction between the convective heating and the large-scale wave forcingudassociated with the MJO. ud