The Flexible Modelling Framework for the Met Office Unified Model (Flex-UM, part of the UM 12.1 release)

The Met Office Unified Model (UM) is a world-leading atmospheric weather and climate model. In addition to comprehensive simulations of the atmosphere, the UM is capable of running idealised simulations, such as the dry physics Held–Suarez test case, radiative convective equilibrium and simulating planetary atmospheres other than Earth. However, there is a disconnect between the simplicity of the idealised UM model configurations and the full complexity of the UM. This gap inhibits the 5 broad use of climate model hierarchy approaches within the UM. To fill this gap, we have developed the Flexible modelling framework of the UM – Flex-UM – which broadens the climate model hierarchy capabilities within the UM. Flex-UM was designed to replicate the atmospheric physics of the Geophysical Fluid Dynamic Laboratory (GFDL) idealised moist physics aquaplanet model. New parameterisations have been implemented in Flex-UM, including simplified schemes for: convection, large-scale precipitation, radiation, boundary layer and sea surface temperature (SST) boundary conditions. These idealised 10 parameterisations have been implemented in a modular way, so that each scheme is available for use in any model configuration. This has the advantage that we can incrementally add or remove complexity within the model hierarchy. We compare Flex-UM to ERA5 and aquaplanet simulations using the Isca climate modelling framework (based on the GFDL moist physics aquaplanet model) and comprehensive simulations of the UM (using the GA7.0 configuration). We also use two SST boundary conditions to compare the models (fixed SST and a slab ocean). We find the Flex-UM climatologies are similar to both Isca 15 and GA7.0 (though Flex-UM is generally a little cooler, with higher relative humidity, and a less pronounced storm track). Flex-UM has a single InterTropical Convergence Zone (ITCZ) in the slab ocean simulation but a double-ITCZ in the fixed SST simulation. Further work is needed to ensure that the atmospheric energy budget closes to within 1-2Wm−2 , as the current configuration of Flex-UM gains 9-11 Wm−2 (the range covers the two SST boundary conditions). Flex-UM greatly extends the modelling hierarchy capabilities of the UM and offers a simplified framework for developing, testing and evaluating 20 parameterisations within the UM. Copyright statement. The works published in this journal are distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other. 1 https://doi.org/10.5194/gmd-2021-193 Preprint. Discussion started: 12 July 2021 c © Author(s) 2021. CC BY 4.0 License.

model". The aquaplanet formulation of the UM was developed by Neale (1999), building on the earlier work of Swinbank et al. (1988), and later published in Neale and Hoskins (2000a, b). This provided the foundation on which the AquaPlanet Experiment (APE) Williamson et al., 2013) was built. For many modelling centers around the world, the aquaplanet configuration is a benchmark model for testing model performance (Stevens and Bony, 2013;Voigt and Shaw, 2015) and for implementing new parameterisations. Indeed the aquaplanet UM has been illuminating for studying tropical 5 convection, for example the Madden-Julian Oscillation Inness et al., 2001), the double-ITCZ (Talib et al., 2018), atmospheric tides and the diurnal cycle (Woolnough et al., 2004), and the sensitivity of convectively coupled equatorial waves to entrainment (Peatman et al., 2018).
At the most idealised end of the UM model hierarchy is the "idealised UM". This model configuration broadly describes a number of idealisations including the Held-Suarez Newtonian relaxation (Held and Suarez, 1994;Mayne et al., 2014b), 10 radiative convective equilibrium (Holloway and Woolnough), and exo-planet configurations (Mayne et al., 2014a;Lines et al., 2018;Boutle et al., 2017;Sergeev et al., 2020). The global configuration of the idealised model has also been broadly used for testing and developing the current dynamical core of the UM (ENDGAME) (Wood et al., 2014).
When viewing all the possible UM configurations through the climate model hierarchy lens -the idea that simple models are connected to sophisticated models via incremental steps -a large gap emerges between the simplified parameterisations 15 of the idealised UM (e.g. the Held-Suarez model), and the sophisticated parameterisations within the comprehensive UM.
Specifically, there is a gap in the intermediate complexity parameterisations of the atmosphere. The purpose of this paper is to narrow this gap in the UM climate model hierarchy using the Flexible framework for the UM (Flex-UM). In doing so, we will highlight the utility of Flex-UM and motivate possible directions for research and model development.
Flex-UM was designed so that it can be directly compared to a benchmark intermediate complexity climate model, the 20 Geophysical Fluid Dynamic Laboratory (GFDL) simplified moist physics aquaplanet (Frierson et al., 2006(Frierson et al., , 2007. Specifically, Flex-UM can be configured as a slab ocean aquaplanet, with a grey radiation scheme, simplified Betts-Miller convection scheme and simple boundary layer scheme, we describe these in detail in Section 2. Flex-UM is not an independent version of the UM. The reanalysis data and model configurations are described in Section 3. In Section 4 we compare the model climatologies for two different SST boundary conditions and then validate these against reanalysis. The first SST boundary condition we consider is the slab-ocean aquaplanet configuration in Section 4.1, where we validate Flex-UM against the similarly configured Isca climate model. The second SST boundary condition we consider is the fixed-SST aquaplanet configuration, where we compare 30 the behaviour of Flex-UM against the comprehensive UM in Section 4.2. We then evaluate the slab ocean configuration of Flex-UM against the comprehensive UM in Section 4.3. We then summarise the new features of Flex-UM in Section 5 and motivate possible use cases for the new capabilities within the UM model hierarchy. Flex-UM is a collection of parameterisations designed to fill the gap between the idealised UM and the full complexity of the UM. These parameterisations have been incorporated into the model in a modular way so that the model can be easily changed to build different configurations. Flex-UM is not a static model configuration, rather it is flexible to the design choices of the user. The inspiration for the default configuration of Flex-UM is the GFDL moist aquaplanet model (Frierson et al., 2006), Flex-UM is the model resolution (described in Section 3) and resolution dependent settings (e.g. the model timestep). 10 In developing Flex-UM, each parameterisation of GA7.0 was either turned off (as it is not needed), re-configured to be more idealised, or replaced with a new idealised parameterisation. These adaptations retain the existing code base and allow for easy transitions between idealised and more complex model configurations. There are five key components of the default Flex-UM configuration: radiation, large-scale precipitation, convection, boundary layer, and SST forcing. In the remainder of this section we will describe each key component and point to the relevant literature for further details.

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The default GA7.0 radiation scheme SOCRATES (Manners et al., 2021;Edwards and Slingo, 1996) is turned off in Flex-UM and a new grey radiation scheme has been developed. The grey radiation scheme uses an idealised optical depth to approximate the atmospheric water vapour structure and uses a two-stream approximation to provide an infrared cooling effect through the depth of the atmosphere, described in more detail in Section 2(b) of Frierson et al. (2006). In principal, SOCRATES could be adapted to mimic the grey radiation scheme, and this will be considered for later configurations of Flex-UM to explore the 20 impact of the radiation parameterisation on the model behaviour. The incoming solar radiation for Flex-UM is idealised, so that the radiation directly warms the surface and is a function of latitude only. There is no absorption of the solar radiation in the atmosphere, and there is also no seasonal or diurnal cycle. The details of this forcing are described in section 2(b) of Frierson et al. (2006). Unlike SOCRATES, the grey radiation scheme does not account for the radiative impact of aerosols, trace gases, and ozone. Furthermore, Flex-UM does not include clouds or their radiative impact.

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The second key component of Flex-UM is the resolved precipitation (also known as large-scale precipitation). The singlemoment microphysics scheme in GA7.0 is turned off and is replaced with a simplified large-scale condensation scheme, described in section 2(e) of Frierson et al. (2006). This scheme generates resolved precipitation when a grid-box is supersaturated. Precipitation falls out of the grid box immediately and is re-evaporated below. The resolved precipitation only reaches the surface if each subsequent layer below is also saturated.

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The Gregory-Rowntree convection scheme in GA7.0 is turned off and replaced with the simplified Betts-Miller scheme of Frierson (2007), an idealised version of the Betts-Miller convection scheme (Betts, 1986;Betts and Miller, 1986). The simplified Betts-Miller scheme is an adjustment style convection scheme, where an unstable atmosphere is stabilised via adjustment of the moisture and temperature profiles to reference profiles, see Section 2 of Frierson (2007) for details. The 4 https://doi.org /10.5194/gmd-2021-193 Preprint. Discussion started: 12 July 2021 c Author(s) 2021. CC BY 4.0 License. simplified Betts-Miller scheme that we developed for Flex-UM has already been included in a recent convection scheme comparison by Hwong et al. (2021).
The fourth key component of Flex-UM is the boundary layer scheme. The boundary layer scheme of GA7.0 is adapted to create a simplified Monin-Obukhov boundary layer, see Section 2(d) of Frierson et al. (2006) for details. The existing boundary layer code was modified, rather than replaced, to make use of the existing vertical diffusion code. The diffusion coefficients The SST is zonally symmetric, only varying in latitude, and is a maximum at the equator. Unlike the commonly used APE profiles, this SST profile is not set to 0 • C equatorward of 60 • in each hemisphere, rather the SST profile described above is bounded between 258 • K at the poles and 298.33 • K at the equator. In the remainder of the paper, this SST forcing profile 15 will be referred to as the "fixed-SST" configuration. The fixed-SST profile was used in early development of the Flex-UM parameterisations, as removing SST feedbacks simplified the debugging and validation process.
The second SST configuration is a slab ocean aquaplanet, an idealised single-layer ocean model where atmospheric fluxes interact with SST in the vertical but there is no horizontal ocean transport (i.e. no Q-fluxes). The slab ocean is incorporated into Flex-UM by adapting the land surface scheme JULES (Best et al., 2011). JULES computes the surface energy exchange over 20 the ocean when a coupled ocean is not in use. In GA7.0, the surface temperature is not allowed to vary from the sensible heat transfer. However, this option has been added to JULES to allow the surface temperature to vary while maintaining consistency with the UM boundary layer scheme. The initial SST profile for the slab ocean is the fixed-SST profile described above. After initialisation, the slab ocean is free to evolve. The implementation of the slab ocean is consistent with the slab ocean described in Section 2a) of Frierson et al. (2006). The slab ocean we developed for Flex-UM is already in use, for example for terrestrial 25 exoplanets (Boutle et al., 2017).
In summary, the Flex-UM default configuration has been designed following the GFDL moist aquaplanet model. Each of the parameterisations have been implemented in a modular way so that each variant can be used in any suitable UM configuration.
The development of Flex-UM enables a broader use of climate model hierarchies for the UM, allowing us to not only better understand the behaviour of the UM, but also to better understand the climate system. The adaptations and newly implemented  of the GFDL aquaplanet, sharing the spectral core and many of its parameterisations. In Section 4.1 we compare the slab-ocean 5 configurations of Flex-UM and Isca. In order to directly compare these two models, Flex-UM was configured to be as similar as possible to the Isca default configurations: an albedo of 0.31, no diurnal or seasonal cycle, constant incoming solar ration of 1360 W m −2 and a 2.5 m slab ocean depth. One difference is that Isca has solar absorption in the grey radiation scheme (consistent with Frierson (2007) while Flex-UM does not (consistent with Frierson et al. (2006). This is discussed in more detail in Section 4.1. the PC2 cloud scheme, and has non-orographic gravity waves. The Flex-UM and GA7.0 simulations use the same dynamical core, but have very different atmospheric physics. As such, their simulations are not expected to give the same model climatologies. These differences should not be viewed as undesirable, rather these are deliberate simplifications to the parameterisations 20 and the key motivation for developing Flex-UM, that is, to build a reduced complexity version of the UM.

Data
The Flex-UM model climatologies are compared to monthly ERA5 reanalysis climatologies (Hersbach et al., 2020;Copernicus Climate Change Service, 2017), the fifth generation of atmospheric reanalysis from ECMWF. The climatologies are computed over the January 1979 to December 2020 time period. The horizontal resolution of the data is 0.25 • × 0.25 • and all available 25 vertical levels are used between 1000-100 hPa (27 vertical levels). The ERA5 surface pressure was used to create a mask for discarding data that had been interpolated to the 1000 hPa pressure level if the surface pressure is less than 1000 hPa.
Interpolation is prone is errors in the data and this is especially important in computing the mass stream function. The horizontal and vertical resolutions are different between the reanalysis and models, and between the UM simulations as well. This was by design, as we ran the GA7.0 and Isca experiments at their standard resolutions. Flex-UM was run at a 5 similar resolution to Isca so that the two simulations are as similar as possible. This resolution choice was because the primary goal of Flex-UM is to implement the intermediate complexity parameterisations from Isca into the UM, and then compare the two models. We note that when comparing Flex-UM and GA7.0, the key differences in the climatologies are highly unlikely to be the result of resolution differences, as the Flex-UM model physics is very different to the GA7.0 case. All climatologies are plotted on their native resolution and data used to make difference plots have been linearly interpolated in the vertical to 10 12 common levels sets between 1000 hPa and 100 hPa, and horizontally regridded to a common resolution of 2.8 • × 2.8 • (the native resolution of Flex-UM).
The initial conditions for the Flex-UM and Isca simulations are an isothermal atmosphere with near-zero moisture. As such, their simulations require a suitable spin-up. A 10-year spin-up was discarded from the simulations, which is standard for these types of models, and the models were run for a further 10 years. The GA7.0 simulations do not need a long spin up as the 15 simulations start from initial conditions using a climate simulation that was spun-up for 30 years (as standard for GA7.0).
However, for consistency with the other simulations, a 20 year run was performed and the first 10 years discarded as spin up.
We note that the ERA5 dataset used in this study is 42 years long and the model runs are 20 years. In general it is good practice to use a common time period between datasets, however, as the purpose of this paper is to validate an idealised model, the simplifications in the model physics will dominate over any differences in the reanalysis time period. For this reason, we felt 20 there was no need to restrict the ERA5 data to the common 20 year period. The reanalysis and model simulations will be compared in Section 4. For each dataset, the climatological precipitation and zonal-means of temperature (T), relative humidity (RH), zonal wind (u), and the mass stream function (Ψ) will be shown. The RH is computed with respect to water above 0 • C for all datasets, ice below -23 • C for ERA5, ice below -20 • C for Isca and ice below 0 • C for the UM, and interpolated for temperatures in between for ERA5 and Isca. In this study, we also show the 25 atmospheric energy budget, which is a simple global balance between the radiation lost to space, heat absorbed by the surface, and the heat released at the surface, given by: how the LH is computed. It is common to use LH = L c P , where L c is the latent heat of condensation and P is precipitation (P = P rain + P snow ). However, it is more accurate to use LH = L c P rain + L s P snow or LH = L c P precip + L f P snow where L s and L f are the latent heat of sublimation and fusion, respectively. More discussion on this can be found in Pendergrass and Hartmann (2013). For ERA5, LH is computed by treating precipitation and snow separately. The LH in all model simulations is computed using precipitation only (LH = L c P ), as the Flex-UM and Isca models do not include snow.

Evaluating the Flex-UM Climatology
In this section we compare Flex-UM climatologies with ERA5 reanalysis, the simplified climate modelling framework Isca, Perhaps less well known is the atmospheric energy budget shown in Fig. 1(e), the area weighted global means are shown in the legend. The radiation lost to space (LWC in blue) peaks in the SH and the radiation gains from solar absorption (SWA in orange) peak in the NH near the equator. Both the LWC and SWA reduce to zero at the poles. The latent heat generated from 20 precipitation (LH in red) shows two peaks for the ITCZ and SPCZ. The sensible heat released from the surface (SH in green) is small at all latitudes. The residual atmospheric energy budget (i.e. the net in Equ. 1) is small but non-zero (-3.6 W m 2 ) and the shape of the residual zonal-mean mirrors the LH from precipitation.
In the following sections we directly compare Flex-UM with both Isca and GA7.0, and will make more general comparisons to ERA5. All of the model simulations in this study are aquaplanets, that do not attempt to model the full complexity of the 25 atmosphere, and so direct differences with ERA5 will not be presented. Rather we will make general comparisons to ERA5 to highlight the realism of the model simulations.

Slab ocean aquaplanet: Flex-UM vs Isca
The motivation for developing Flex-UM was to replicate the idealised parameterisations within the GFDL moist aquaplanet model. As such, we directly compare Flex-UM with the Isca climate model, which is based off the GFDL moist aquaplanet.

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The temperature and RH from the slab ocean aquaplanet simulations for Flex-UM and Isca are shown in Fig. 2  have higher RH at all latitudes. While the difference in RH between Flex-UM and Isca is large in the polar regions, the specific humidity difference is very small in these regions, where their zonal-mean difference is less than 0.5 gkg −1 (not shown), owing to the cooler polar temperatures where the specific humidity is small. So while the Flex-UM polar regions might be closer to saturation than the Isca polar regions, the moisture content is too small to make a meaningful difference in the two simulations. The precipitation and atmospheric energy budget from the slab ocean aquaplanet simulations for Flex-UM and Isca are shown in Fig. 4. The precipitation distribution of zonally symmetric aquaplanets with no seasonal cycle are not expected to 5 show the spatial distribution seen in ERA5. Rather, it is more common for the precipitation to peak on the equator (a single ITCZ), or for two peaks to occur on either side of the equator (double ITCZ). Isca has a single ITCZ and a second peak in precipitation in the storm tracks near 40 • in each hemisphere (seen in Fig. 4(b) and in the LH of Fig. 4(e). The peak equatorial precipitation for Isca is 18 mmday −1 and in the storm tracks 5 mmday −1 . Compared to Isca, Flex-UM has a broader ITCZ with less equatorial precipitation which peaks at 14 mmday −1 on the equator. The storm tracks are less pronounced in Flex-10 UM, where the subtropical precipitation reduces to 5 mmday −1 but does not have a local subtropical minimum or a local storm track maximum.
Compared to ERA5, the precipitation is higher in both Flex-UM and Isca, hence the larger LH component of the atmospheric energy budget Fig. 4(d-e). The shape of the zonal-mean LWC is similar in ERA5, Flex-UM and Isca. The Isca LWC is much larger than ERA5 and Flex-UM, which is due to more outgoing LW radiation (OLR) and a smaller net surface flux (not shown).

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The LWC in Flex-UM is similar to ERA5 but a little lower, and each LW radiative flux of Flex-UM is smaller than ERA5 and Isca (not shown). The SH flux is small in ERA5, Flex-UM and Isca. The SWA for Flex-UM is quite different to Isca and ERA5. The globally averaged incoming solar radiation is the same for ERA5, Flex-UM and Isca, while the outgoing SW  Flex-UM and peaks on either side of the equator, which is consistent with a double ITCZ structure (seen more clearly in the precipitation plots in Fig. 7). Flex-UM subtropical jet is more distinct from the eddy-driven jet compared to GA7.0. We also note that Flex-UM and GA7.0 are weakly superrotating aloft, where the equatorial wind is westerly instead of easterly. Superrotation in idealised models 10 is not uncommon, and has previously been found in aquaplanets, shallow-water and Held-Suarez models (Mori et al., 2013;Blackburn et al., 2013;Showman and Polvani, 2010;Lutsko, 2018).
The Hadley circulation in Flex-UM is narrower, shallower, weaker and pushed off the equator in both hemispheres, while GA7.0 is more like ERA5. As seen in the Flex-UM and Isca slab ocean mass stream function plots in Fig. 3d-  14 https://doi.org /10.5194/gmd-2021-193 Preprint. Discussion started: 12 July 2021 c Author(s) 2021. CC BY 4.0 License.
represent the polar cell. In GA7.0, the Hadley cell shape is more realistic, which we attribute to higher horizontal and vertical resolution, however the NH near surface Ψ remains unrealistic.
The precipitation and atmospheric energy budget from the fixed-SST aquaplanet simulations for Flex-UM and GA7.0 are shown in Fig. 7. The double ITCZ in Flex-UM is evident in the precipitation map Fig. 7a) and in the zonal-mean LH Fig. 7d).
There is also a small double peak of precipitation in GA7.0, though not separated enough to be considered a double ITCZ.

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The Flex-UM tropical precipitation peaks are shallower and broader than GA7.0, the storm tracks precipitation is higher, and the subtropical precipitation is higher than expected compared to the storm tracks. Compared to GA7.0, the LH in Flex-UM is considerably larger in the global mean, due to more rainfall (despite the shallower precipitation peaks at the equator). The SWA in Flex-UM is zero, discussed in more detail in Section 4.1, and in GA7.0 the SWA is similar to ERA5 although a little weaker. The LWC in Flex-UM and GA7.0 are also similar except within the ITCZ where the LWC is reduced, due to reduced The double ITCZ seen in the fixed-SST simulations for Flex-UM (and also very weakly in GA7.0) is a common structure seen in aquaplanets Rios-Berrios et al., 2020). The double ITCZ problem is also a well know model bias in comprehensive GCM where precipitation over the Pacific Ocean is too zonal, see for example Tian (2015); Zhang et al.

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(2015). The double ITCZ in aquaplanets is known to be sensitive to a number of model choices and is sensitive to feedbacks between the convection, cloud radiative effects and the large-scale circulation. Whether a single or double ITCZ occurs in an aquaplanet model has been shown to depend on the choice of convection scheme and the convection scheme parameters such as entrainment (Möbis and Stevens, 2012). However, the double ITCZ also occurs in comprehensive GCM simulations without convection schemes where only resolved precipitation occurs (Maher et al., 2018). The appearance of a single or double ITCZ 20 also depends on cloud radiative effects (Harrop and Hartmann, 2016;Popp and Silvers, 2017), and the energy balance near the equator (Kang et al., 2008;Bischoff and Schneider, 2016). It has also been shown the single and double ITCZ can result from no changes in the parameterisaitons but from changes in the model resolution and dynamics core (Landu et al., 2014).
Understanding the mechanisms that control the ITCZ and the occurrence of the double ITCZ are long standing problem and areas of active research. The zonal-mean zonal wind and mass stream function from the slab ocean aquaplanet simulations for Flex-UM and GA7.0 are shown in Fig. 9. The circulation in Flex-UM is more realistic in the slab-ocean case compared to the fixed-SST case, where the jet streams are more distinct, the zonal wind structure is broader, and the Hadley cells are stronger and closer together.  humidity in the slab ocean simulation which would increase the optical depth. The SWA in Flex-UM is zero, as discussed in Section 4.1. The Flex-UM slab ocean budget is closer to closing than the fixed-SST case, with a net of 8.8 W m −2 . In the slab ocean GA7.0 case, the budget terms are very similar to the fixed-SST case and the budget closed to within -1.5 W m −2 . The only shape differences in the budget terms is in the LH where only a single tropical peak is seen. boundary layer, and SST boundary conditions (using both a fixed-SST and slab ocean configurations). These parameterisations and surface conditions were developed by replicating the GFDL moist physics aquaplanet (Frierson et al., 2006;Frierson, 2007). We have implemented Flex-UM in a modular way, so that one or more schemes can be altered at a time. Hence Flex-UM is a framework rather than a specified model configuration.
We have compared Flex-UM to ERA5, the Isca modelling framework (based off the GFDL aquaplanet) and the compre- The double ITCZ is relatively common for aquaplanet configurations  and is a well known problem in comprehensive GCM (Tian, 2015;Zhang et al., 2015).
Unlike Isca and GA7.0, the Flex-UM atmospheric energy budget does not close to within a few W m −2 as we had hoped, 30 but rather it gains energy by 9 W m −2 in the slab ocean case and 11 W m −2 for the fixed-SST case. Further work is ongoing to close the energy budget. In the current implementation of Flex-UM, we have not included shortwave absorption within the grey radiation scheme. In Frierson's original development of the GFDL aquaplanet in Frierson et al. (2006) there was no shortwave absorption and this was later implemented in Frierson (2007). For the next configuration of Flex-UM, we will include shortwave absorption in the downward flux calculation consist with (Frierson et al., 2006). Perhaps this will improve the Flex-UM atmospheric energy budget, which will also be a focus for the next model release. A final feature to add to the next configuration of Flex-UM is to include a cloud scheme.
The goal for developing Flex-UM was to broaden the climate model hierarchy capabilities within the UM. Having achieved 5 this goal, the next natural question is to consider what can Flex-UM be used for? The slab ocean configuration has already been used for simulating terrestrial exoplanets (Boutle et al., 2017) and the simplified Betts-Miller convection scheme has recently been used for a convection scheme intercomparison by Hwong et al. (2021). The parameterisations of Flex-UM will play a role in developing and evaluating the next generation of dynamical core for the UM (named LFRic after Lewis Fry Richardson) and in evaluating the next generation of convection scheme within the UM (named CoMorph). In addition to supporting model 10 development and evaluation, Flex-UM can also be used to address bold science questions within an easier to understand setting.
Answering bold science questions is a fundamental motivation for using climate model hierarchies and Flex-UM enhances this capability within the UM.
Code and data availability. The UM code and it's configuration files are subject to Crown Copyright. A licence for the UM can be requested from https://www.metoffice.gov.uk/research/approach/collaboration/unified-model/partnership. The source code for Flex-UM will be integrated into the next model release of the UM (version 12.1) and will be available at https://code.metoffice.gov.uk/trac/um/browser (to access this link you first need to apply via the link above). The Flex-UM simulations in this study were completed using modifications to the UM version 11.7 and the GA7.0 simulations were completed using version 11.6. The simplified Betts-Miller scheme and slab-ocean code used in this study are already on the UM code trunk at version 11.7 as part of the Trac code management system (see link above). The grey radiation scheme, boundary layer code, and large-scale precipitation are currently under review and will be made available as part of the 20 https://doi.org /10.5194/gmd-2021-193 Preprint. Discussion started: 12 July 2021 c Author(s) 2021. CC BY 4.0 License.