Difference between revisions of "Slab ocean model"
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== A brief description of the slab ocean model == | == A brief description of the slab ocean model == | ||
| − | The slab ocean model is from Codron [2012]. This model uses the same horizontal grid as the GCM and is composed of two layers. The first layer (50 m depth) represents the surface mixed layer, where the exchanges with the atmosphere take place. The second layer (150 m depth) represents | + | The slab ocean model is from Codron [2012]. This model uses the same horizontal grid as the GCM and is composed of two layers. The first layer (50 m depth) represents the surface mixed layer, where the exchanges with the atmosphere take place. The second layer (150 m depth) represents the deep ocean. |
| − | the deep ocean. | ||
<span style="color:red"> In its current form in the Generic-PCM, the slab ocean model handles two key processes, (a) Heat transport by ocean circulation, and (b), Formation of oceanic ice:</span> | <span style="color:red"> In its current form in the Generic-PCM, the slab ocean model handles two key processes, (a) Heat transport by ocean circulation, and (b), Formation of oceanic ice:</span> | ||
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A = A$$^\rm{max}_\rm{ice}$$ – (A$$^\rm{max}_\rm{ice}$$ – A$$^\rm{min}_\rm{ice}$$ ) $$e^{\left ( \frac{–h_\rm{ice}}{h^0_\rm{ice}} \right )}$$ | A = A$$^\rm{max}_\rm{ice}$$ – (A$$^\rm{max}_\rm{ice}$$ – A$$^\rm{min}_\rm{ice}$$ ) $$e^{\left ( \frac{–h_\rm{ice}}{h^0_\rm{ice}} \right )}$$ | ||
| − | Here, A is the albedo, A$$^\rm{max}_\rm{ice}$$ = 0.65 is the maximal albedo, A$$^\rm{min}_\rm{ice}$$ = 0.2 is the minimal albedo, $$h_\rm{ice}$$ is the ice thickness (in m) and h$$^0_\rm{ice}$$ = 0.5 m. The albedo over the ice-free ocean is taken to be equal to 0.07. | + | Here, A is the albedo, A$$^\rm{max}_\rm{ice}$$ = 0.65 is the maximal albedo, A$$^\rm{min}_\rm{ice}$$ = 0.2 is the minimal albedo, $$h_\rm{ice}$$ is the ice thickness (in m) and h$$^0_\rm{ice}$$ = 0.5 m. The albedo over the ice-free ocean is taken to be equal to 0.07. These values for the maximal and minimal sea ice albedos are classical for <span style="color:red"> Earth-based </span> GCMs. <span style="color:red"> However, by definition, the Generic-PCM is meant to handle a more generalised case of planets. It is well known that the albedo of a material (for e.g., ice or snow) is spectral-dependent. Thus, the albedo value of ice will be different around a star that is unlike the Sun. For instance, the ice/snow albedo climate feedback will be significantly weaker for planets orbiting M-dwarf stars (for e.g., Proxima Centauri b) than for planets orbiting G-type stars like the Sun (see Joshi & Haberle [2012] and Shields [2014])).</span> |
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| − | <span style="color:red"> | ||
== Variables of the slab ocean model which can be written as outputs in diagfi.nc == | == Variables of the slab ocean model which can be written as outputs in diagfi.nc == | ||
Revision as of 12:11, 13 October 2022
Contents
A brief description of the slab ocean model
The slab ocean model is from Codron [2012]. This model uses the same horizontal grid as the GCM and is composed of two layers. The first layer (50 m depth) represents the surface mixed layer, where the exchanges with the atmosphere take place. The second layer (150 m depth) represents the deep ocean.
In its current form in the Generic-PCM, the slab ocean model handles two key processes, (a) Heat transport by ocean circulation, and (b), Formation of oceanic ice:
Heat transport by ocean circulation
The transport of heat by the ocean circulation is given by two components:
- First, the impact of subgrid-scale eddies is represented by horizontal diffusion, with a uniform diffusivity in both layers.
- Then, the mean wind-driven circulation is computed by calculating the Ekman mass fluxes in the surface layer from the surface wind stress, and taking an opposite return flow at depth. These mass fluxes are then used to advect the ocean temperature horizontally. In the case of divergent horizontal mass fluxes, the upwelling or downwelling mass flux is obtained by continuity. This simplified model reproduces the global meridional oceanic heat transport quite closely compared to
a full GCMan Oceanic-GCM (OGCM).
Formation of oceanic ice
Sea ice forms when the ocean temperature falls below –1.8°C (the freezing point of sea water) and melts when its temperature rises above freezing. The changes in ice extent and thickness are computed based on energy conservation, keeping the ocean temperature at –1.8°C as long as ice is present. A layer of snow can be present above the ice. The surface albedo is then that of snow, or that of bare ice:
A = A$$^\rm{max}_\rm{ice}$$ – (A$$^\rm{max}_\rm{ice}$$ – A$$^\rm{min}_\rm{ice}$$ ) $$e^{\left ( \frac{–h_\rm{ice}}{h^0_\rm{ice}} \right )}$$
Here, A is the albedo, A$$^\rm{max}_\rm{ice}$$ = 0.65 is the maximal albedo, A$$^\rm{min}_\rm{ice}$$ = 0.2 is the minimal albedo, $$h_\rm{ice}$$ is the ice thickness (in m) and h$$^0_\rm{ice}$$ = 0.5 m. The albedo over the ice-free ocean is taken to be equal to 0.07. These values for the maximal and minimal sea ice albedos are classical for Earth-based GCMs. However, by definition, the Generic-PCM is meant to handle a more generalised case of planets. It is well known that the albedo of a material (for e.g., ice or snow) is spectral-dependent. Thus, the albedo value of ice will be different around a star that is unlike the Sun. For instance, the ice/snow albedo climate feedback will be significantly weaker for planets orbiting M-dwarf stars (for e.g., Proxima Centauri b) than for planets orbiting G-type stars like the Sun (see Joshi & Haberle [2012] and Shields [2014])).
Variables of the slab ocean model which can be written as outputs in diagfi.nc
- tslab1/tslab2: temperature of the two ocean layers (K)
- pctsrf_sic: fraction of sea ice
- sea_ice: mass of sea ice (kg/m$$^2$$)
- tsea_ice: temperature of the sea ice surface (K)
- dt_hdiff1/dt_hdiff2: heating of the ocean layers by diffusion (K/s)
- dt_ekman1/dt_ekman2: heating of the ocean layers by Ekman transport (K/s)
Technical aspects
The slab ocean model is activated in callphys.def with: ok_slab_ocean = .true.
The sea ice model is activated in callphys.def with: ok_slab_sic = .true.
The horizontal heat transport is activated in callphys.def with: ok_slab_heat_transp = .true. The horizontal heat transport cannot be activated in parallel at the moment.
Values of A$$^{max}_{ice}$$ and A$$^{min}_{ice}$$ are fixed in slab_ice_h.F90
FOR SIDDHARTH: ADD NOTE ON SURFACE TYPES (RNAT)
