Aquaplanet Slab Ocean Model Documentation

The aquaplanet SOM default configuration differs slightly from the standard SOM. First, the sea ice is removed by default by specifying a stub ice model in the compset definition. Second, the slab ocean is given a uniform 30 m depth in the default forcing file, following the TRACMIP specification [1]. Finally, the forcing file contains the bottom-of-the-slab Q-fluxes that are not computed from a fully-coupled simulation. Instead, the “test” cases (e.g., EC6AQUAPtest) contain a default forcing file with zero Q-fluxes; these may be replaced by user-defined forcing via the DOCN_SOM_FILENAME variable in the docn namelist. In the scientifically supported compsets (e.g., EC6AQUAP), a default forcing file is used that contains Q-flux values computed from a prescribed SST aquaplanet. The method is from Kiehl et al. (2006) [2], termed the “old way” in the SOM Forcing document. The equation that is used to derive the Q-flux is

\[\rho c_p h \frac{\partial \mathrm{SST}}{\partial t} = F_{\mathrm{net}} +\]

Q_{mathrm{flx}}

In the prescribed SST aquaplanet, however, the SST is constant, so the expression reduces to

\[F_{\mathrm{net}} = -Q_{\mathrm{flx}}\]

so the Q-flux is the inverse of the net ocean surface energy budget, which can be written

\[-F_{\mathrm{net}} = Q_{\mathrm{flx}} = R_{\mathrm{net}} - H - L_vE -\]

L_f rho_{mathrm{ice}} P_{mathrm{ice}}

where the rhs terms are the net downward radiative flux, the sensible heat flux, the latent heat flux, and the heat required to melt frozen precipitation, respectively. The aquaplanet Q-flux is derived from monthly output from a 5-year simulation, then averaged over time and longitude. An NCL script to calculate the Q-flux is provided on the CESM Simpler Models github site.

Other variables in the SOM forcing file are set to zero (e.g., U, V), global average values (e.g., S), or to aquaplanet-appropriate values (e.g., mask = 1 everywhere).

Aspects of the simulated climate in the SOM Aquaplanet are documented by Benedict et al. [3].

[1]	Voigt, A., et al., 2016: The tropical rain belts with an annual cycle and

a continent model intercomparison project: TRACMIP. Journal of Advances in Modeling Earth Systems, 8 (4), 1868–1891, doi: 10.1002/2016MS000748, URL http://dx.doi.org/10.1002/2016MS000748.

[2]	Kiehl, J. T., C. A. Shields, J.

J. Hack, and W. D. Collins, 2006: The climate sensitivity of the community climate system model version 3 (CCSM3). Journal of Climate, 19 (11), 2584–2596, doi:10.1175/JCLI3747.1, URL http://dx.doi.org/10.1175/JCLI3747.1.

[3]

Benedict, J. J., B. Medeiros, A. C. Clement, and A. Pendergrass, 2017: Sensitivities of the Hydrologic Cycle to Model Physics, Grid Resolution, and Ocean Type in the Aquaplanet Community Atmosphere Model. Journal of Advances in Modeling Earth Systems, submitted.