Extending the Modular Earth Submodel System (MESSy v2.54) model hierarchy: the ECHAM/MESSy IdeaLized (EMIL) setup

Abstract. As models of the Earth system grow in complexity, a need emerges to connect them with simplified systems through model hierarchies in order to improve process understanding.
The Modular Earth Submodel System (MESSy) was developed to incorporate chemical processes into an Earth System model. It provides an environment to allow for model configurations and setups of varying complexity, and as of now the hierarchy ranges from a chemical box model to a fully coupled chemistry–climate model. Here, we present a newly implemented dry dynamical core model setup within the MESSy framework, denoted as ECHAM/MESSy IdeaLized (EMIL) model setup.
EMIL is developed with the aim to provide an easily accessible idealized model setup that is consistently integrated in the MESSy model hierarchy. The implementation in MESSy further enables the utilization of diagnostic chemical tracers. The setup is achieved by the implementation of a new submodel for relaxation of temperature and horizontal winds to given background values, which replaces all other “physics” submodels in the EMIL setup. The submodel incorporates options to set the needed parameters (e.g., equilibrium temperature, relaxation time and damping coefficient) to functions used frequently in the past. This study consists of three parts. In the first part, test simulations with the EMIL model setup are shown to reproduce benchmarks provided by earlier dry dynamical core studies.
In the second part, the sensitivity of the coupled troposphere–stratosphere dynamics to various modifications of the setup is studied. We find a non-linear response of the polar vortex strength to the prescribed meridional temperature gradient in the extratropical stratosphere that is indicative of a regime transition. In agreement with earlier studies, we find that the tropospheric jet moves poleward in response to the increase in the polar vortex strength but at a rate that strongly depends on the specifics of the setup. When replacing the idealized topography to generate planetary waves by mid-tropospheric wave-like heating, the response of the tropospheric jet to changes in the polar vortex is strongly damped in the free troposphere. However, near the surface, the jet shifts poleward at a higher rate than in the topographically forced simulations. Those results indicate that the wave-like heating might have to be used with care when studying troposphere–stratosphere coupling.
In the third part, examples for possible applications of the model system are presented. The first example involves simulations with simplified chemistry to study the impact of dynamical variability and idealized changes on tracer transport, and the second example involves simulations of idealized monsoon circulations forced by localized heating. The ability to incorporate passive and chemically active tracers in the EMIL setup demonstrates the potential for future studies of tracer transport in the idealized dynamical model.



S1 User Manual: Set-up of an EMIL simulation
The "EMIL" dynamical core model is implemented as a set-up of ECHAM/MESSy, i.e. it can be run by modifying the namelist files and the initial files (i.e. no recompilation is necessary), as described in the following.

S1.1 Namelist set-up
The namelist set-up for an EMIL simulation is described in the following. An example set-up can also be found in the MESSy source code (under messy/nml/EMIL).
The following namelist files need to be modified: • switch.nml In the switch namelist, the only necessary submodel to be switched on is RELAX. Diagnostic submodels and submodels that control the set-up of tracers can optionally be used as usual.
• relax.nml The RELAX namelist file consists of a "coupling" (CPL) namelist, in which the options for temperature relaxation, wind damping and additional diabatic heating can be chosen and the according parameters that define the variables needed (e.g. wind damping coefficients, equilibrium temperature and inverse relaxation time scale) are set (see Sec. S1.2).
• ECHAM5.nml The set-up for the ECHAM-internal sponge layer is controlled here (DYNCTL namelist), and as the sponge is calculated in RELAX in the EMIL set-up, the ECHAM-sponge needs to be switched off (by setting spdrag = 0).
Furthermore, it is advisable to change the output via channel.nml, as many fields in the standard ECHAM output are meaningless in the EMIL set-up (see example under messy/nml/EMIL/channel.nml ).

S1.2 The RELAX namelist
As described in Sec. 2.1 of the main paper, the RELAX submodel incorporates functions for three processes: Newtonian cooling (newco), Rayleigh friction (rayfr ), and currently three different diabatic heating routines (tteh cc tropics, tteh waves, tteh mons). Each process can be switched on/off via namelist entry (see lines 10-14 in the example namelist given in Fig. S1), and for each process the variables to be used can be chosen.
For the Newtonian cooling routine, the equilibrium temperature (T eq ) to be relaxed to, as well as the inverse relaxation time scale (κ) have to specified, and for Rayleigh friction the wind damping coefficient (k damp ) has to be specified. As described in Sec. 2.1 of the main paper, the options for these variables are either constant values, pre-implemented functions or any field defined by a given channel and object pair.
The options with all parameters are summarized in Tables S1 to S2, with the meaning of the parameters explained in example namelists (Figs. S1 and S2) and in Sec. 2.1 of the main paper. For the equilibrium temperature with 'PK' set-up, there is an additional switch to "turn" the polar vortex off (l no polar vortex, line 111-112 in Fig. S2). If this switch is set to TRUE, the equilibrium temperature of the winter polar region is set to the standard US Atmosphere as for all other latitudes (i.e. the weighting function given by Eq. (5) of the main paper is set to zero for all latitudes).
The parameters for the diabatic heating routines described in Sec. 2.1.3 of the main paper, are also set by namelist entry, as summarized in Table S3.

S1.3 Initial files
Several modifications to the initial files are necessary for running the EMIL model set-up. For any set-up with the idealized model, the initial values for specific humidity need to be set to zero to obtain dry dynamics (in the ECHAM spectral initial file, set Q = 0 everywhere). Since there are no sources of water vapour, the humidity will remain zero throughout the simulation. To run the model with flat or idealized topography, both the surface geopotential in the surf input file, as well as the initial values for dynamical variables need to be modified. There are several solutions to set the dynamical variables divergence, vorticity and temperature to appropriate values, with one of them listed below.
Surface initial file: The topography is controlled by the variable surface geopotential (GEOSP ) within the "surface" initial file. For flat topography, set GEOSP = 0 everywhere. For idealized topography, the surface geopotential is set to the height of the chosen topography (e.g. wavenumber-2 mountain). All other variables in this file are not used by the idealized model.

Spectral initial file:
The initial values for vorticity, divergence and temperature in spectral coordinates and humidity in latitude-longitude coordinates are given in the spectral initial file. In all cases, the specific humidity has to be set to zero (Q = 0) everywhere (to obtain dry dynamics). For running flat topography, the following initial conditions can be used: temperature ST P = 0 for all wavenumbers > 0, and for wavenumber zero, ST P is set to a global mean temperature profile with height (e.g. taken from the initial file from a full ECHAM simulation), divergence SD = 0 and vorticity SV O = 1 × 10 −8 s −1 . The small value for vorticity is included to break the zonal symmetry (otherwise, the simulations will always remain in a zonally symmetric state). For running idealized topography, the initial values need to be modified to be not too far from the atmosphere's state. One way to achieve this is to modify the topography step-wise, e.g. for introducing a wavenumber-2 mountain, the amplitude of the mountain needs to be slowly increased (e.g. by steps of 500 m every year).