MCM_UA Description Summary

The Manabe Climate Model (MCM) was developed in the 1980s and 1990s by S. Manabe and collaborators at NOAA’s Geophysical Fluid Dynamics Laboratory. A lower resolution model version was first developed in the mid-1980s and used a 4.5° latitude grid spacing in the atmospheric and oceanic components. The higher resolution model presented here has a ~2.25° (150 km) horizontal resolution, which is slightly greater than, but roughly comparable to many of the climate and ESMs used in the fifth phase of the Coupled Model Intercomparison Project (CMIP5) (See Table 6.11 in Ciais et al. 2013). While the grid spacing is similar to CMIP5 models, particularly in the oceanic component, most of the physical subgrid-scale parameterizations in all components are in general much simpler. This enables the model to be very inexpensive in terms of present-day computer resources, allowing many studies of past climates to be performed economically. Despite its simplicity relative to other climate and ESMs currently in use, MCM-UA has been shown to perform very well relative to other more sophisticated and state-of-the-science CMIP6 models in climatically important regions such as the Southern Ocean (Beadling et al. 2020).

The MCM-UA model contains general circulation models of the atmosphere, ocean, and sea ice. A simple land surface model is also used. The land surface model consists of a heat balance at the Earth’s surface assuming no heat storage in the land. The Manabe bucket model is used for hydrology (Manabe 1969). Whenever the bucket overflows, an a river routing scheme is used to transport the runoff into the ocean. When snow covers the land surface, the surface albedo is higher and snowmelt on land is determined from the surface heat budget.

The sea ice is advected by the ocean currents only. Convergence of sea ice is restricted if the ice thickness is greater than 4 m. Sea ice growth and melt is determined from a heat balance computation between the ice-atmosphere interface and the ice-ocean interface. A filter is used near the pole to control numerical instability arising from the convergence of the meridians. Model details are given in Delworth et al (2002, 2-degree model) and Manabe et al. (1991, 4-degree model) and references therein. The MCM 2-degree version used here, was state-of-the-art in the late 1990’s.

The oceanic component is a slightly modified version of the MOM1 ocean model (Pacanowski et al. 1991). The modifications allowed MOM1 to be the ocean component in an AOGCM. The horizontal grid is 192 east-west and 80 north-south, amounting to a nominal horizontal resolution of ~2.25°. The vertical grid uses 18 levels at fixed depths, with finer vertical grid spacing near the surface, and a 40 m deep surface layer grid box. The model formulation assumes a so-called rigid lid and uses a virtual salt flux to incorporate freshwater entering the ocean. This can lead to problems of exaggerating the water fluxes when the surface salinity is far from 35 PSU (Yin et al. 2010). Parameterized subgrid-scale mixing includes the Gent-McWilliams scheme (Gent and McWilliams 1990, Griffies 1998). Whenever vertical instabilities are found, the grid boxes are completely mixed, removing the instability.

The atmosphere component uses a spectral method to solve the equations of motion. The truncation used is rhomboidal at 30 waves. The transform grid is 96 east-west grid locations by 80 north-south. The atmospheric component uses 14 vertical sigma levels. Atmospheric convection is performed using the Smagorinsky et al. (1965) scheme of moist convective adjustment (MCA). Boundary layer mixing is achieved through a simple diffusive scheme, mixing potential temperature and water vapor.

The model topography is realistic given the limitations of the grids used. Coupling between the atmosphere-ocean/sea ice is performed once per day. Two ocean tracer/sea ice grid boxes underlie each atmospheric grid box east-west. The land-sea mask is determined by the atmospheric grid. These restrictions in the component grids allow the surface fluxes of heat and water to easily be conserved. Radiation is computed once per day – no diurnal variations are simulated by the model. The heat, freshwater fluxes, and wind stress are passed to the ocean once per day. The coupling is serial, where the atmosphere runs 1 day and then the ocean integrates that same day using the atmospheric fluxes and providing the sea surface temperature (SST) and sea ice to the atmosphere for its lower boundary at the start of the next day. Both heat and freshwater flux adjustments are used (Manabe and Stouffer 1988) to minimize climate drift in the control integration, while maintaining realistic SST and SSS distributions. These adjustments are a function of grid location and month of the year, but they have no variations on time scales longer than one year. The adjustments are computed prior to the start of the control integration and are exactly the same for all perturbation integrations. As shown in Manabe and Stouffer (1988) and Manabe et al. (1991), the use of these adjustment is quite successful.

This model was ported to the High Performance Computing (HPC) system at the University of Arizona in 2017 and renamed the Manabe Climate Model-University of Arizona (MCM-UA). The model executes about 20 model years per calendar day running single-threaded on only one processing unit on a single cpu. No tuning was performed either to improve the simulation or computer performance. The initial conditions ported were obtained from near the end of a 2000+ year control integration performed at GFDL in the middle 1990s. During the execution of the spin up after the port of the model to find an equilibrium for the CONTROL, the atmospheric time step was shortened from 18 minutes to 15 minutes which caused a slight warming of the model. The computer cost of the model is minimal on a machine with thousands of such processing units. However, storage costs are not negligible.


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