Extending the Modular Earth Submodel System (MESSy v2.55) model hierarchy: The ECHAM/MESSy idealized (EMIL) model set-up

While models of the Earth system gain more and more complexity, a need emerges to establish model hierarchies and to utilize simplified models to improve process understanding. The Modular Earth Submodel System (MESSy) was developed with the aim to provide an environment to allow for model configurations and set-ups of varying complexity, and as of now the hierarchy reaches from a chemical box model to the full coupled ECHAM/MESSy Atmospheric Chemistry (EMAC) Chemistry-Climate model. In the current study, we present and document the development of a new simplified set-up within the ECHAM/MESSy model, namely the dry dynamical core model set-up ECHAM/MESSy IdeaLized (EMIL). The set-up is achieved by the implementation of a new submodel for relaxation of temperature and horizontal winds to given background values (the RELAX submodel), which replaces all other physics submodels in the EMIL set-up. The RELAX submodel incorporates options to set the needed parameters (e.g. equilibrium temperature, relaxation time and damping coefficient) to functions used frequently in the past (given by Held and Suarez, 1994; Polvani and Kushner, 2002). Test simulations with the EMIL model set-up show that results from earlier studies with other dry dynamical core models are reproduced under same set-ups. Furthermore, modifications to the previously used set-ups are tested, with the following main findings: 1) lowering the equilibrium temperature in the lower stratosphere at winter polar high latitudes to more realistic values (i.e. below observed temperatures) results in high latitudes temperature profiles in the model closer to observations, and 2) when replacing the idealized topography to generate planetary waves by mid-tropospheric wave-like heating (as suggested in a previous study), the response of the tropospheric jet to changes in the equilibrium temperature is strongly damped, indicating that the wave-like heating has to be used with care. As application examples, we present simulations with simplified chemistry to study the impact of dynamical variability and idealized changes on tracer transport, and simulations of idealized monsoon circulations forced by localized heating. The ability to simulate dynamical systems and to incorporate passive and chemical active tracers in the EMIL set-up demonstrates the potential for future studies of tracer transport in the idealized dynamical model.