Vertical Structure and Diabatic Processes of the MJO: 20-Day Hindcast Intercomparison

The Madden-Julian oscillation (MJO) is the dominant mode of tropical sub-seasonal (30-60 day) variability. By modulating monsoon systems and influences modes of extra-tropical variability (e.g., the NAO), the MJO provides a key source of weekly and monthly predictability globally. Despite this, most weather and climate models exhibit large biases in their simulations of the MJO.

To understand and potentially alleviate these biases, a new inter-comparison project focuses on the vertical structure of processes associated with representations of tropical convection in models, particularly profiles of diabatic heating, moistening and momentum transfer. This project is a collaboration between the Years of Tropical Convection (YoTC) MJO Task Force and the GEWEX Cloud System Study (GCSS).

The inter-comparison has three components; this presentation focuses on daily 20-day hindcasts of two strong MJO events in winter 2009-2010 (YoTC cases E and F). This component bridges the gap between the other two components: 20-year simulations to understand the climatological MJO in models, and 2-day hindcasts to examine timestep-level detail in physical processes near initialisation.

This presentation will describe the design and initial results of the 20-day hindcast experiments. The skill of contributing models (nine to date) will be assessed; connections will be made between the drift in the models' predictions and climatological biases computed from the 20-year experiments. There are large variations in model skill across the inter-comparison, offering the opportunity to identify priorities for advances in physical parameterisation development. Vertical profiles of diabatic heating, moistening and momentum transfer from individual physics schemes will be compared to ECMWF YoTC analyses to determine the root causes of gains or losses in forecast skill. These results will improve the understanding of each model's drift from its initial conditions towards its climatology.

Companion presentations will discuss the other components of this intercomparison project.